Surgical Approaches to the Heart
A variety of incisions are used in cardiac surgery. The majority of pediatric cardiac surgeries are performed through two incisions: the median sternotomy and the posterolateral thoracotomy. Other incisions are occasionally used to accommodate atypical anatomy or allow for minimally invasive approaches.
Sternotomy
The most commonly used incision continues to be the median sternotomy ( Fig. 16.1 ), in which the sternum is divided along its length from top to bottom. Originally described in 1897, this incision came into widespread use after the advent of coronary artery bypass grafting in the 1960s. The overlying skin and subcutaneous tissues are divided with a combination of the knife and electrocautery, and the sternum is divided with a reciprocating saw ( Fig. 16.2 ). In neonates and young infants, scissors can be used to divide the bone. The reciprocating saw is used when opening the sternum for the first time. The blade has vertical movement through a short distance, and a protective shoe covers the tip of the blade. The shoe glides underneath the sternum, and prevents injury to the underlying cardiac structures. At the completion of the procedure, the sternum is closed with stainless steel wire, bands, or sometimes, in small children, heavy suture material. The incision provides excellent exposure, and is preferred for most intracardiac procedures. In addition to opening the sternum itself, the incision is extended for a short distance into the upper abdomen, separating the rectus muscle in the midline. Because there is minimal interruption of muscle, and because the sternum is solidly reconstructed at the end of the procedure, the incision is less painful than other commonly used incisions, such as the thoracotomy. There is minimal respiratory compromise. Early extubation, effective coughing and deep breathing are easily achieved with this incision. Long-term functional results are excellent, and lung function is minimally perturbed. This is in contrast to the situation found after thoracotomies, where a degree of restrictive lung physiology is predictable.
Reoperation is common for survivors of congenital heart surgery. A resternotomy requires careful assessment and planning because adhesions can develop between the underlying cardiac structures. Reopening the sternum can result in entry of the underlying heart chamber or great vessels and result in serious or even life-threatening hemorrhage. The diagnosis, previous operation, and careful review of operative reports can help identify patients at increased risk. Preoperative imaging studies should be reviewed with the resternotomy incision in mind to identify structures at risk. Additional imaging with computerized tomography and magnetic resonance imaging may be necessary to allow careful assessment of the relationship of cardiac chambers, great vessels, and conduits to the sternum because of the risk of hemorrhage with resternotomy. That said, the morbidity and mortality of these operations have gradually improved as more experience has been gained with these operations. In adults, the mortality risk increases with increasing number of reoperations. However, in children, resternotomy has not been associated with an increased mortality risk or an increased risk of morbidity such as hemorrhage or neurologic injury. Nevertheless, there are higher risk resternotomy patients. The presence of a right ventricle to pulmonary artery conduit is associated with an increased risk of resternotomy injury and each patient’s unique anatomy should be carefully evaluated prior to resternotomy.
Preparation for emergent volume support, and identification of alternative sites of access for cardiopulmonary support, are essential. Blood should be immediately available at the time of resternotomy. In adults and older children, the femoral vessels are frequently used for access. In older patients who have had multiple previous cardiac catheterizations and cardiac surgical procedures, ultrasonic interrogation or axial imaging of the femoral, iliac vessels, and distal aorta should be used to verify patency. The brachiocephalic artery, accessed through the sternal notch, and the axillary vessels provide alternative sites for emergency access. When a patient is identified as high risk for resternotomy injury, the axillary artery can be accessed through an infraclavicular incision prior to resternotomy. In older patients, use of a side graft to cannulate this vessel for cardiopulmonary bypass (CPB) is associated with decreased morbidity such as injury to the brachial plexus. The common carotid and internal jugular veins may be used for peripheral cannulation for emergent CPB in infants, children, teenagers, and young adults ( Fig. 16.3 ). Preparation should also be made for external cardioversion should the patient develop an arrhythmia before direct access to the heart is obtained. Resternotomy is accomplished using an oscillating saw (see Fig. 16.2 ). This device has a horizontal blade translating through a short distance that limits, but by no means eliminates, the risk of injury to underlying vascular structures. When these complex reoperations are completed, a formalized hemostasis checklist has been demonstrated to reduce the need for reoperations for bleeding in adult patients. They provide the clinician with a systematic approach to assess the potential for postoperative bleeding sites prior to closure of the mediastinum. Preparation for safe resternotomy begins at the completion of the previous operation and consideration for future operations should be taken into account in any patient in whom resternotomy is likely . A sheet of polytetrafluoroethylene (PTFE) can be placed over the heart at the time of closure in a child in whom subsequent reoperation is anticipated. The PTFE sheet has been demonstrated to aid in the prevention of cardiac injury during subsequent reoperations. More recently, bioresorbable adhesion barriers have been developed and early results showed promise in reducing the severity and extent of mediastinal adhesion during repeat sternotomy.
Infection of the sternal wound and mediastinitis are rare complications in children, but do occur. For the most part these complications are more easily managed than in older patients. In particular, osteomyelitis of the sternum is exceedingly rare, and sternal resection, a common necessity in adults with an infected sternal wound, is virtually never required in children. In children, reexploration, debridement, irrigation, and immediate closure over drains has been successful. In rare cases of primary sternal osteomyelitis, the sternum can be debrided leaving the posterior table intact for less aggressive infections. The wound is then closed over antibiotic-impregnated beads, and the patient treated with parenteral antibiotics for 6 weeks. A variety of techniques have been used to address more significant infections, including the use of vacuum dressings, omental flaps, and flaps of the rectus abdominis muscle. These techniques enhance drainage and neovascularization, improving control of infection and enhancing eventual closure.
The approach to the sternum considered at high risk for dehiscence is a subject unto itself. Though this has not been an area of significant concern in pediatric cardiac surgery, as our patient population ages, it may soon become a more important consideration. The time is nearing when up to half of those undergoing surgery for congenital cardiac disease may be adults. A significant portion of these procedures will necessarily be performed through a resternotomy. When the patient is obese or diabetic or has undergone previous chest irradiation, this sternum can be considered at increased risk for dehiscence. Many of these patients will have undergone multiple previous sternotomies. As a result, blood supply to the sternum may be significantly reduced. Indeed, a large portion of these patients may have lost both major blood vessels to the sternum, the internal mammary arteries, during previous explorations. In adult patients, the loss of both mammary arteries has been shown to increase the risk of deep sternal wound infection. As such, additional measures should be considered with sternal closure. Double sternal wires, steel bands, and sternal plates, which reduce movement and distribute stresses in the wound over larger areas, are strategies used to close the sternum at high risk of dehiscence.
As outcomes have improved after cardiothoracic surgery, interest in improving the cosmesis of incisions has emerged. A variety of minimally invasive techniques has been described . The longitudinal skin incision has been shortened significantly in some cases, even when the sternum is divided fully along its length. Another effort has involved abandoning the longitudinal skin incision altogether, in favor of a submammary or transverse skin incision with creation of skin flaps to permit a full median sternotomy incision. This particular incision has not been widely adopted secondary to fear of wound complications and compromised exposure, but some have reported excellent results with this incision and very low rates of complication. It is of note that the majority of the sensory supply to the chest wall enters laterally. If the submammary incision is appropriately performed, sensory supply to the breast should be uninterrupted. It is also important that this incision is made below the mammary tissue within the breast crease. Significant compromise to development of the breasts has resulted when the incision has been made too high.
Thoracotomy
Thoracotomy incisions are commonly used for ligation of the patent arterial duct, repair of aortic coarctation, placement of a pulmonary artery band, and construction of a systemic-to-pulmonary arterial shunt. A posterolateral thoracotomy is shown in Fig. 16.4 . The skin incision is made in a curvilinear fashion along the path of the ribs ( Fig. 16.4A ). In most instances, the latissimus dorsi muscle is divided ( Fig. 16.4B ), but the serratus anterior is spared ( Fig. 16.4C ). The intercostal muscles are divided between the ribs to be spread. At times, even the latissimus can be spared. In this instance, the approach to the thoracic cavity is made through a small space between the muscles, the triangle of auscultation. In cases where greater exposure is necessary, especially in older patients, it may be necessary to resect a rib to obtain ideal exposure. The skin incision can be limited compared to the length of the incision within the thoracic cavity. This incision is performed through a variety of intercostal spaces, depending on the level for which exposure is desired. The arterial duct and aortic arch are typically approached through the third or fourth intercostal space. Exposure for excision of pulmonary pathology typically involves an incision in the fourth or fifth intercostal space. Exposure for treatment of esophageal pathology can be made through the left fifth through eighth intercostal spaces, or the right fourth or fifth intercostal spaces. Exposure to the diaphragm is usually through the seventh intercostal space.
The transverse thoracosternotomy incision is used when there is need for extensive thoracic exposure ( Fig. 16.5 ). In this incision, both thoracic cavities are entered through bilateral anterolateral thoracotomies that are connected across the midline by a transverse sternotomy. This incision gives excellent exposure to all of the mediastinal structures anteriorly. It is most commonly used for bilateral sequential lung transplantation but has also been used for unifocalization of aortopulmonary collateral arteries. The skin incision can be made as indicated in the figure to preserve development of the breasts. This incision is sometimes complicated by sternal malunion, characterized by a malaligned sternum without ossification. Some have abandoned this incision for this reason, and prefer to approach the chest through bilateral anterolateral thoracotomies without dividing the sternum. The use of alternative wiring techniques or alternative materials such as cables for sternal approximation have been reported and may reduce the incidence of complications with the transverse sternotomy. Novel absorbable sternal pins for closure of the “clamshell” thoracosternotomy have recently been reported for use in patients that may not be suitable for bilateral anterolateral thoracotomies without sternal division.
The anterolateral thoracotomy is shown in Fig. 16.6 . The right anterolateral thoracotomy has been used for repair of a variety of congenital cardiac malformations, with good results. The anterolateral thoracotomy can be placed in the mammary crease and has been used as an alternate, more cosmetically appealing approach for simple intracardiac operations such as closure of atrial septal defects. There have been isolated reports, however, of compromised or asymmetric development of the breast with this incision, and of increased pain. The left anterolateral thoracotomy is often used on the hemodynamically unstable victim of thoracic trauma with suspected damage to the thoracic structures. If sufficient access is not possible with this incision, it can be extended across the midline for further access to cardiac structures.
Minimally Invasive Approaches
In continued attempts to improve the cosmetic results after surgery, a variety of minimally invasive techniques have emerged. Originally applied to adults, they are now frequently applied to children. Among these techniques are partial upper and lower sternotomy, video-assisted thoracoscopic surgery, the mini-thoracotomy, the subxiphoid approach to the heart, and robotic techniques. Use of these techniques is controversial. Opponents cite the potential for compromised exposure, and the accompanying increased risk of the procedure. Proponents cite the psychosocial benefits of smaller incisions. The potential for limited exposure must be balanced against the complexity of the case, and the likelihood of future reoperation. Partial sternotomies can consist of a partial superior sternotomy or a partial inferior sternotomy. The partial upper sternotomy has been used for such complex procedures as the arterial switch operation. The inferior partial sternotomy has been commonly used for many years for procedures such as placement of epicardial pacemaker leads, where access to the anterior portion of the heart or atrium is needed. Atrial septal defects have been repaired through this incision. More recently, a broader range of cardiac operations have been performed through this incision including closure of ventricular septal defects, repair of tetralogy of Fallot and atrioventricular septal defect with common junction, and procedures on the mitral valve. In small children with more pliable sternums, a similar variety of procedures has been accomplished through a subxiphoid incision. The vertical infra-axillary thoracotomy has also been reported as a minimally invasive alternative to median sternotomy for closure of atrial septal defects. In addition to the cosmetic result, the risk of injury to developing breast tissue in female patients is avoided with this approach.
Video-assisted thoracic surgery has become a mainstay of general thoracic surgery during the last decade. In this type of procedure, one large incision is replaced by two to four smaller incisions ( Fig. 16.7 ). A thoracoscope is placed through one incision. Other ports are used to place stapling devices, thoracoscopic scissors, or instruments for dissection and retraction. The technique has been used for ligation of the arterial duct, closure of interatrial communications, and division of vascular rings. In some series, pain and postoperative stay are significantly reduced. When used for closure of interatrial communications in adult patients, video-assisted thoracoscopic surgery has been shown to be safe, with a shorter length of intensive care unit stay compared to full open sternotomy. The majority of adult patients (more than 60%) undergoing this technique for interatrial communications closure are extubated in the operating room. In pediatrics, video-assisted thoracoscopic surgery has been predominantly employed for division of vascular rings. In a large single institution series over 25 years, there seemed to be waning enthusiasm for this approach in the more contemporary surgical era. This lack of enthusiasm may be related to the risk of life-threatening, difficult-to-control bleeding in patients with a patent double aortic arch.
Open Sternum
After completion of a long, complex procedure, there may be significant accumulation of extravascular fluid in the heart, lungs, chest wall, and even the peritoneal cavity. Reapproximation of the sternum can result in a significant reduction in cardiac output due to compression of the heart by adjacent structures, combined with the diastolic dysfunction that is the result of myocardial edema. This acute restrictive physiology is similar to that seen with tamponade, and has been termed pseudotamponade physiology. Because a period of decreased cardiac output is predictable following complex procedures, even the patient with initially acceptable hemodynamics may display pseudotamponade physiology within the first 12 hours following termination of CPB. Delayed sternal closure has been used commonly as a means of avoiding or managing low cardiac output syndrome in neonates and small infants undergoing complex procedures. Other indications for delayed sternal closure include ongoing bleeding that cannot be easily controlled. Occasionally, the sternal edges may be stented open to increase available space. Indications for an open sternum strategy and timing of closure vary greatly. For some programs, all patients undergoing the first stage of palliation for hypoplastic left heart syndrome undergo delayed sternal closure. For others, the strategy is applied on an individual basis. Closure is undertaken when hemostasis has been achieved and the patient has diuresed and been weaned from the majority of inotropic support. Some have advocated waiting until the patient has diuresed to the preoperative weight. While the sternum is open, the mediastinum is covered with either Gore-Tex or a silicone sheet. This can be secured with adhesive, saving the skin edges from the injury of sutures.
Materials
The job of the congenital cardiac surgeon often requires reconstruction of nonexistent structures. The materials available for this task include biomaterials, synthetic materials, and combinations thereof.
Sutures
Sutures are necessary for any surgical task. Materials are usually classified as absorbable or nonabsorbable, and as monofilament or polyfilament. The choice of sutures depends on the specific application and stresses expected on the suture line or anastomosis. Vascular anastomoses are constructed most commonly with monofilament suture. The suture glides through the tissue easily, and this results in minimal damage to the tissue and improved hemostasis. For tasks such as fascial closure, absorbable braided sutures are commonly used because they have a lower likelihood of tearing through tissue, and will be reabsorbed, minimizing the risk of infection.
Absorbable sutures persist in the body for a limited period of time. Among the earliest absorbable sutures is gut, created from the intestines of cows or sheep. Used less commonly than in the past, the suture may be used untreated, or after treatment with chromium salts, a process that increases durability. More modern absorbable sutures include polygalactic acid (Vicryl), and polyglycolic acid (Dexon). These polyfilament braided sutures are absorbed within 60 to 90 days. They are commonly used for closure of fascia, subcutaneous tissue, and skin. Absorbable sutures are also available as monofilament. Polyglyconate (Maxon), and poliglecaprone (Monocryl), have excellent strength when compared with other absorbable sutures, and are frequently used to close the skin. In addition to use in closing wounds, they have been used for vascular anastomoses. Polydioxanone (PDS) is a monofilament suture that has a long duration of absorbability and high tensile strength, with the additional advantage of maintaining its integrity in the face of infection. It appears to induce less fibrosis than polygalactic acid. As a result, it is commonly used for surgery of the airway.
Among nonabsorbable sutures, one of the oldest is silk. It is considered nonabsorbable and polyfilament, though it is ultimately degraded. Although in the past silk was used for vascular anastomoses, today it is used for retraction or to ligate small vessels. Braided polyester sutures with Teflon coating (Tevdek, Tycron) have minimal memory, hold knots well, and are used frequently to secure patches used to close ventricular septal defects, or to secure prosthetic valves. Polypropylene (Prolene) is a monofilament suture that may last from 2 to 6 years. Its long durability, easy maneuverability, and tendency to glide easily though tissue, make it the suture of choice for cardiac reconstruction and vascular anastomoses. The Gore-Tex suture, made of PTFE, is a monofilament nonabsorbable suture also used for vascular anastomoses. The suture is frequently used with Gore-Tex grafts to improve hemostasis of the suture line.
Patch Material and Valveless Conduits
One of the easiest materials to use is fresh autologous pericardium. The material is easily harvested at the time of a primary operation, though it is less easily harvested during reoperation. The material is pliable, sterile, and has no likelihood of inciting an immunologic reaction. Some prefer for the pericardium to have firmer texture to increase ease of handling, in which case it is fixed in glutaraldehyde. Glutaraldehyde fixation results in cross-linking of collagen, making the pericardium stiffer. Such fixation may reduce aneurysmal formation when pericardium is used for vascular reconstruction. When native pericardium is not available, glutaraldehyde-treated bovine pericardium can be used. Though this tissue is readily available, and has minimal to no risk of transmission of disease, it does undergo calcific degeneration. Photo-oxidized bovine pericardium is a newer preparation that uses dye-mediated photo-oxidation to cross link pericardial collagen rather than the chemical fixation associated with glutaraldehyde. The handling characteristics of this type of bovine pericardium is similar to that of native pericardium and there appears to be a reduced risk of calcification.
Homograft aortas or pulmonary arteries—in other words, cadaveric tissues—can also be used for vascular reconstruction. Such material is commonly used during reconstruction of the pulmonary arterial tree and confluence, or to relieve isolated pulmonary stenoses. They are also commonly used for reconstruction of the aortic arch in the neonate. The material is hemostatic and easy to handle. It is, however, expensive, and it requires time for thawing, carries a risk of calcification, and comes in varying thicknesses that are often difficult to predict. In addition, homograft material can incite an immune response resulting in a nonspecific increase in antihuman lymphocyte antibodies, potentially complicating future transplantation.
Dacron, a synthetic polyester material, has been a mainstay in vascular reconstruction since its introduction in the 1950s. It is used in its tubular form to reconstruct vascular segments. It can also be used as a patch, and is commonly used for closure of ventricular septal defects. The Dacron material can incite a fibrous reaction. This is an advantage for closure of ventricular septal defects, in that such fibrous ingrowth may allow closure of the tiny residual defects at the edges of the patch. Fibrous ingrowth is even further enhanced when the Dacron is covered with velour. Caution, however, should be used when Dacron is placed close to an arterial valve, as fibrous ingrowth can impair motion of the valvar leaflets.
When used as a conduit, the porosity of Dacron poses a problem for hemostasis. In order to improve hemostasis, grafts can be preclotted by soaking them in either autologous blood or albumin, and then heating the grafts in an autoclave or applying fibrin glue (fibrinogen plus thrombin) to the grafts on the operating room table. In order to make the grafts easier to use, they can be manufactured with sealants already applied to the grafts. One example is the Hemashield Dacron graft (Medox Medical), in which formaldehyde cross-linked collagen is used to decrease porosity.
PTFE, with the trade name Teflon, can be expanded or stretched such that it carries a pore size of 20 to 30 µm, a size that has been determined to be the optimal pore size for healing. The result is expanded PTFE, commonly known as Gore-Tex. It is used extensively for systemic-to-pulmonary artery shunts and for arterial reconstruction. Gore-Tex is thrombogenic and a pseudointimal layer will develop over time that is the result of a combination of thrombus and cellular ingrowth. This pseudointimal layer will decrease the caliber of small grafts and can result in critical reduction of flow. Aspirin has been shown to improve patency of small caliber Gore-Tex grafts. Gore-Tex grafts can be heparin bonded to reduce the risk of thrombosis and loss of luminal patency. When used for peripheral arteriovenous prosthetic access in adult patients, there is improved patency of the graft with a significantly lower risk of early thrombosis during the first 5 months of follow up.
Valves and Valved Conduits
The decision regarding valve replacement in children is complex. Unlike adult patients, the choice of a prosthesis is influenced by growth of the patient, the need for durability, and the potential for avoidance of anticoagulation in active children where the injury profile is not insignificant. Four broad categories of prosthesis exist: mechanical, xenobioprosthetic, homograft, and autograft.
Mechanical Valves
The currently manufactured mechanical valves have a bileaflet design ( Fig. 16.8 ). All mechanical valves are thrombogenic and require anticoagulation and carry the risk for thromboembolic and bleeding complications. Anticoagulation is accomplished with vitamin K antagonists such as warfarin, acenocoumarol, or phenprocoumon commonly combined with antiplatelet agents (typically aspirin), and dipyridamole. The risk of thromboembolic and bleeding complications is present, even in patients with optimally managed anticoagulation, and is influenced by factors specific to the patient, along with the position of the valve. The annual rate of thrombosis with a mechanical prosthesis ranges from 0.1% to 5.7%. A lower thromboembolic risk is observed in patients with aortic valve replacement compared to mitral valve replacement and patients with both aortic and mitral valve replacement have the greatest risk. Preserved ventricular function is associated with a lower risk of thromboembolic complications. In a single-center report, children undergoing aortic valve replacement had 93% freedom from thromboembolic complications at 20-year follow-up and little anticoagulation-related hemorrhage. The risk of complications related to anticoagulation is greatest for mechanical valves placed in mitral or left-sided atrioventricular valve position. Three separate reports summarizing individual institutional experiences in children totaling 115 patients reported 10-year freedom from thromboembolic complications above 92%, and the risk of complications related to bleeding at between 76% and 97%. The true incidence of thromboembolic complications in children undergoing replacement of the mitral valve with a mechanical prosthesis is probably underestimated by single-center reports. A multicenter study found that 4 of 102 survivors of mitral valve replacement required re-replacement for thrombosis at a mean follow-up of 6.0 years. Prospective studies of adults, with combined enrollment of over 1000 patients, showed that bileaflet mechanical valves in the mitral position were associated with an incidence of both thromboembolic events and complications due to bleeding at a frequency between 1% and 3% per patient per year. Freedom from thromboembolic complications after 10 years was 85.5%, and freedom from bleeding was 81.7%. The most widely used prosthetic valve of the last two decades is the St. Jude valve. It is a low profile, bileaflet valve constructed of pyrolytic carbon. In its most recent iteration, it has a rotatable valvar mechanism. Competing bileaflet designs include the ATS, On-X, and Carbomedics valves. There has been considerable interest of late in the potential of utilizing the On-X valve with aspirin as the only antithrombotic agent. The manufacturer claims that the lack of silicon coating on the carbon used to construct the valve makes it less thrombogenic. Controlled trials (PROACT) are underway to evaluate the risk of thromboembolic complications using a less aggressive anticoagulation goal (international normalized ratio [INR] 1.5 to 2.0). The interim results from the trial showed a modest 1.8% per year reduction in the risk of major bleeding with the lower INR target and no significant increase in thromboembolism. There are also isolated reports of aspirin-only anticoagulation for other mechanical valves such as the St. Jude prosthesis.
Mechanical valves have been produced for adults but recently, smaller mechanical prostheses in the 15-mm and 17-mm size range, suitable for many small children and even infants and newborns, have become available. The HALO IDE trial sponsored by the Food and Drug Administration in conjunction with St. Jude Medical will investigate the efficacy of the 15-mm prosthesis in patients 5 years of age or less. While the study is ongoing, it is not currently recruiting participants. Despite the availability of smaller sizes of prostheses, annular enlargement such as the anterior enlargement of Konno, or the posterior enlargement of Nicks or Manouguian may be required when the diameter of the native aortic root does not permit placement of a valve sufficiently large to meet the hemodynamic needs of the patient and allow for growth. In the mitral position, the anatomy does not lend itself as well to annular expansion. Other techniques must be utilized. Valves can be placed in the supra-annular position, or at angles. The techniques can permit placement of a modestly larger prosthesis. Significant oversizing of mitral prostheses should be avoided, as this can result in subaortic obstruction and heart block.
Xenobioprosthetic Valves
Xenobioprosthetic valves include porcine aortic valves and valves manufactured from bovine or equine pericardium. Xenobioprosthetic material has typically been treated with glutaraldehyde in order to decrease immunogenicity and improve durability ( Fig. 16.9A ). Xenobioprosthetic valves have low thrombogenicity and generally do not require chronic anticoagulation. In children, xenobioprosthetic valves placed in the systemic circulation undergo rapid and unpredictable calcific degeneration, limiting their use ( Fig. 16.9B ). Recent efforts have been directed at newer techniques for preservation that limit this process of calcification with mixed results. Xenobioprosthetic valved conduits have been used in rare circumstances where anticoagulation must be avoided, but in general, xenobioprosthetic valves are rarely used in children in the systemic circulation. Xenobioprosthetic valves are commonly used in children in the pulmonary and tricuspid position. In the lower pressure right-sided circulation the valves are more durable and chronic anticoagulation is not required.
Homograft Valves
Valved homograft material can also be used for valvar replacement and reconstruction ( Fig. 16.10 ). The homograft has commonly been used for replacement of the aortic valve and aortic root in the face of endocarditis. Small homograft valved conduits are ideal for complex reconstruction in neonates and small infants, such as repair of common arterial trunk, and tetralogy of Fallot with pulmonary atresia. In North America, however, homografts in sizes suitable for neonates and small infants are becoming less and less available. The use of homografts to join the right ventricle to the pulmonary arteries is discussed more fully when considering selection of valves.
Pulmonary Autograft Valves
The pulmonary autograft can be harvested from the right ventricular outflow tract and used as a replacement of the aortic valve (the Ross procedure). The native outflow tract from the right ventricle is then reconstructed with another conduit, typically a homograft. The pulmonary autograft is particularly attractive for infants and small children as the native, viable tissue will grow with the patients. Modern results with the Ross procedure in the pediatric age group have been encouraging. Multicenter registry data from the Netherlands and Germany shows that among 263 patients less than 17 years of age at the time of the Ross procedure the 10-year freedom from aortic valve reintervention was 95%. Although neonates and infants are at increased risk of early mortality, among survivors freedom from aortic valve reintervention was 98% at 10 years. Although the autograft does allow for growth, late dilatation of the neoaortic root with resultant aortic insufficiency has been identified in a subgroup of patients undergoing the Ross procedure. At least two mechanisms resulting in regurgitation appear to explain dysfunction of the autograft. Patients undergoing the Ross procedure for isolated aortic incompetence have been shown to have an increased risk for development of incompetence, primarily due to dilation of the left ventriculo-aortic junction. Patients undergoing the Ross procedure for congenital aortic stenosis have an increased incidence of ascending aortic dilation, with dilation of the sinutubular junction that also results in aortic insufficiency. Efforts to limit development of autograft dilation and insufficiency include the use of annuloplasty sutures, and placement of the autograft within a Dacron tube graft. Both of these techniques limit the potential for growth, and are only suitable in older patients. The risk of autograft dilatation has led to renewed interest in the subcoronary implantation technique in teenagers and adults. Utilizing this technique for autograft implantation, the mean peak gradient in a series of 347 patients was less than 10 mm Hg. There was 95% freedom from autograft reintervention at 10 years. Although it is generally acknowledged that the pulmonary homograft placed during the Ross procedure has greater longevity than that used for reconstruction of the right ventricular outflow tract for other forms of congenital cardiac disease, presumably due to the orthotopic position, normal pulmonary arteries, and pulmonary vascular resistance, recent data indicate that replacement still will be necessary. Additional studies have indicated that even a mild, and apparently acceptable, gradient across the right ventricular outflow tract will increase importantly during exercise, and that exercise-induced arrhythmias are common following the Ross procedure.
Selection of Valves
No consensus has been reached on selection, but some generally accepted guidelines are presented. For use in the aortic position in neonates, infants, and small children, the Ross procedure is commonly chosen because it accommodates growth and anticoagulation is not required. Furthermore, the size of the autograft matches the size of the normal left ventricular outflow tract, and lesser degrees of enlargement of the aortic root are required compared to mechanical valves. Homografts can be used in the aortic position in infants and small children, either as a first choice, or if the pulmonary valve is deemed unsuitable for use in the aortic position. For older children, either the Ross procedure or mechanical valves have acceptable outcome. Xenobioprosthetic valves within valved conduits have been used as conduits from the left ventricular apex to the aorta for relief of severe and complex obstruction in the left ventricular outflow tract. Because of the potential for rapid and unpredictable degeneration, xenobioprosthetic valves are generally not used for aortic valve replacement in children or young adults.
Mechanical valves are commonly used for the mitral (or systemic atrioventricular junction) position. Despite the need for anticoagulation, these valves have the necessary durability. Mortality after such replacement continues to be high in younger children. Results, however, seem to be improving, with one study showing reduction in operative mortality from 31% to 3.6% when comparing children receiving operations at the same institution before and after 1990. A recent report showed an early mortality rate of 13% in children younger than 2 years undergoing replacement of the mitral valve, a figure to be compared with a mortality rate as high as 52% in a report from 1990. Xenobioprosthetic valves are rarely used in the mitral position due to the high rates of calcification and failure. Exceptions can be made, however, in extreme circumstances, such as hematologic disorders or pregnancy, when there is a need to avoid anticoagulation. There are advocates for the use of the pulmonary autograft in the mitral position, a procedure now known as the Ross II. The autograft is placed within a Dacron tube graft. The short-term results are good, and there is no need for anticoagulation, but follow-up is limited. The Melody valve (Medtronic Corporation) is a bovine jugular vein valve placed in an expandable stent that was developed specifically for use in the pulmonary valve position ( Fig. 16.11 ). This valve has been adapted for use in infants and small children for mitral valve replacement. The valve remains competent even when not fully expanded. After placement the valve can be serially dilated to permit patient growth. Experience is limited to small single-institution series and long-term outcome is not known.
For reconstruction of the right ventricular outflow tract, biologic valves, including homograft valved conduits and xenobioprosthetic valves, are most commonly used. Biologic valves have longer durability in the lower pressured pulmonary position compared to any position in the systemic circuit, favoring the use of biologic over mechanical valves. Furthermore, the risk of anticoagulation is avoided. Homograft conduits have excellent handling qualities, conform to the anatomy, and facilitate achievement of hemostasis. Limited durability, however, can be a problem. Early valvar insufficiency and obstruction have been reported. In some children, homografts undergo severe calcification, with accompanying shrinkage, that can result in obstruction. Calcification appears to be more accelerated in younger children. Immunologic incompatibility between donor and recipient have been proposed as contributing factors to this calcification and failure of the graft.
An alternative to homografts is use of the bovine jugular venous conduit (Contegra, Medtronic) ( Fig. 16.12 ). The Contegra graft has been used in reconstruction of the outflow tract in patients with common arterial trunk, tetralogy of Fallot, the Ross procedure, and pulmonary atresia. Early and mid-term hemodynamic results are favorable, with one study showing valvar regurgitation to be absent in almost half of patients at a mean follow-up of 26 months. In one of the largest series, with a mean follow-up of 2.1 years, there was no relevant gradient detected at the level of the valves and minimal valvar insufficiency. In a prospective multicenter study conducted by the Congenital Cardiac Surgeon’s Society, the bovine jugular vein fared well, with a lower probability of progressing to more severe forms of severe regurgitation than other types of conduit. Bovine jugular vein conduits are susceptible to aneurysmal dilatation, a particular tendency for distal stenosis, and an increased risk of endocarditis.
The use of small conduits is not surprisingly a risk factor for failure, but interestingly, oversizing the valve by more than a z score of 2.7 has also been shown to be a risk factor for early failure. This trend was confirmed in the study coordinated by the Society of Congenital Heart Surgeons, where outcomes were better when sizes with z scores between 1 and 3 were chosen. There is general agreement that a bioprosthetic valve is the best option in the tricuspid position when the native valve cannot be repaired. Bioprosthetic valves appear to fair better in this position than in any other position. The freedoms from reoperation is reported to be 97.5% ± 1.9% and 80.6% ± 7.6% at 1 and 5 years, respectively.
Catheter-delivered bioprosthetic valves placed within expandable stents are increasingly used for reintervention on the right ventricular outflow tract after repair of congenital heart disease such as tetralogy of Fallot. The catheter-delivered valves are used both to relieve stenosis and/or regurgitation in previously placed valves and conduits as well as in the native pulmonary root. The obvious advantage is to avoid a cardiac reoperation. Results have been excellent with a procedural success rate of over 95% and mortality rate of 1.5%. Complications included conduit rupture of 4.1% and coronary artery compromise of 1.3%.
Valve Repair
As discussed in the preceding section, options for replacement of valves are limited during childhood. Repair, if feasible, preserves the potential for growth, avoids anticoagulation, and also the need for valvar re-replacement. The disadvantages of repair include residual lesions, such as stenosis and insufficiency, and limited durability. Decision-making regarding the suitability of a lesion for repair is complex, and must take into account the pros and cons outlined above, as well as the specific lesion and the ability of the surgeon. Repair of the mitral valve is commonly performed and uses techniques that borrowed from the experience in adults, as well as techniques that have been developed from repair of atrioventricular septal defects. Repair of the mitral valve is frequently successful and durable. In contrast, replacement of the mitral valve necessitates a repeated replacement in almost three-quarters of patients. Mitral valvopathy amenable to surgical intervention may be the result of rheumatic heart disease, acquired and congenital cardiomyopathies, Marfan disease, Shone’s complex, and congenital mitral stenosis. Techniques for repair vary depending on the etiology of the pathology. For repair of mitral stenosis, techniques include commissurotomy, valvoplasty, cordal fenestration, and splitting of papillary muscles. For mitral insufficiency, techniques include repair of clefts, resection, shortening or augmentation of leaflets, cordal shortening, annuloplasty, and creation of a double orifice. In general, mortality is low with repair in the setting of biventricular circulations. In those with a functionally univentricular circulation, in contrast, repair carries a higher mortality.
Repair of the aortic valve is less well accepted than that of the mitral valve, albeit experience is accumulating. Over the last decade, experience and success with repair have improved. Techniques include repair of valvar perforations, suspension of prolapsed leaflets, annuloplasty, and extension of leaflets with pericardium. In particular, extension of the leaflets has been used with success in patients with rheumatic aortic valve disease, with 90% of patients free from valvar related complications at 7 years. Extension has also been applied to patients with congenital pathology with improving results. More contemporary approaches to aortic valve repair include the Ozaki technique. This approach relies on the use of autologous pericardium to construct a stentless bioprosthetic valve in the operating room after excision of the native aortic valve. It has been applied to a wide variety of aortic valve morphology. In a series of over 400 reconstructions, the freedom from reoperation was 96.2% at 4 years of follow-up, and the mean residual gradient was 13.8 ± 3.5 mm Hg.
Strategies for Cardiopulmonary Bypass and Perfusion
Surgical intervention inside the heart or on the great vessels normally requires significant interruption of flow of blood in regions of the surgical field to achieve adequate visualization ( ). To permit a more controlled surgical approach, extracorporeal circulation and gas exchange was developed and first used successfully in the early 1950s. During CPB, venous blood from the great veins or right atrium is diverted to an artificial lung and then reinfused into systemic artery, most commonly the aorta. A variety of specific techniques are used for cannulation and perfusion. These are intended to deliver blood from which carbon dioxide has been removed and oxygen added into the patient at a rate sufficient to fully support the function of the bodily organs for the duration of the surgical repair. Such techniques have permitted the development of extraordinary surgical reconstructive procedures. Optimal strategy permits extensive surgical intervention, with largely predictable freedom from permanent injury to the organs. Planned and unplanned modifications of techniques, however, may place organs at the risk of ischemia. Additionally, the nature of the interactions of blood with artificial surfaces, the effects of associated alterations in temperature, and nonpulsatile perfusion during bypass make the technique a pathway for direct inflammatory and ischemic injury.
Circuitry of Cardiopulmonary Bypass
One task of the perfusionist is to tailor the CPB circuit to the specific needs of the individual patient. The variability of size, anatomy, and pathophysiology necessitates the use of a great number of products. Large extracorporeal surface areas and prime volumes have been identified as potential contributors to complications following CPB. Multiple sizes of oxygenators, heat exchangers, reservoirs, and other components have been designed to address these issues. Much of the research and development for the child has focused on reduction in surface area, prime volume, and biologic incompatibility.
It is advisable for the circuits at a given institution to be organized in the same manner, facilitating the ability of perfusionists to provide safe and consistent service to all patients. A common configuration for bypass is to use bicaval cannulation with a single venous line for drainage into a hard-shell venous reservoir with an integrated cardiotomy reservoir ( Fig. 16.13 ). A typical strategy for a cardiac cannulation is shown in Fig. 16.14 . A roller pump is used to pump deoxygenated blood from the reservoir through a hollow fiber oxygenator with an integrated heat exchanger and filter. Blood exiting the oxygenator returns to the patient via the arterial cannula placed in the ascending aorta.
Additional roller pumps provide active suction for use as field suckers, a left ventricular vent, or a vent in the aortic root. A variety of pressure transducers, level detectors, bubble detectors, and inline blood gas analyzers are used for enhanced precision and safety.
Oxygenators
Hollow fiber membranes are manufactured to mimic the pulmonary capillary bed by packing together microporous fibers in a spiral or crosshatched fashion. The flow rate and composition of gases delivered to the oxygenator can be manipulated to control the transfer of oxygen, carbon dioxide, and anesthetic vapors into the perfusate, taking into account the surface area of the membrane, and the diffusion and blood-gas solubility coefficients of different gases. Surfaces exposed to the blood can be coated with some form of a biomimetic treatment. Several types of treatments are available, all having the goal of increasing the biocompatibility of the circuit, reducing damage to blood and minimizing the impact of bypass on the inflammatory response.
Reservoirs
Two types of reservoirs are utilized, venous and cardiotomy. The former is a collection chamber only for the venous blood and may be in the form of a bag or hard-shell reservoir. The latter collects all shed blood returning from the operative field via cardiotomy suction and vents. Both reservoirs are filtered. The cardiotomy filters are designed to remove debris such as tissue, fat, macrothrombi and suture material. Many reservoirs are available that combine the venous and cardiotomy suction as a single unit. In this configuration, the separation of the chambers is made internally. After the filtration process, their volumes combine into a single outlet, which simplifies connections and permits visualization of air and the level of fluid in the reservoir. According to the latest published survey, 97% of pediatric institution respondents in North America and 90% of international respondents use hard-shell venous reservoirs.
Pumps
Although many types of pumps have been described in the literature, there are primarily two types of arterial pumps in use today, namely roller and centrifugal. Roller pumps contain a length of tubing located inside a curved raceway and have gone largely unchanged in their function since their original use by Gibbon in the early 1950s. This raceway is placed at the travel perimeter of rollers mounted on the ends of rotating arms positioned opposite each other. These rollers are mounted in such a way that one roller is compressing the tubing at all times. By compressing a segment of the blood-filled resilient tubing, the pump pushes blood ahead of the moving roller, producing continuous flow. The output of the roller pump is determined by the revolutions per minute of the pump and the volume displaced with each revolution. This stroke volume depends on the internal diameter of tubing and the circumferential length of the raceway. The roller pump head is reusable. The flow rate is simple to determine by multiplying stroke volume by the revolutions each minute. Multiple sizes of tubing can be used in the same pump, making it applicable to patients of all sizes. These pumps, however, do have disadvantages. The pump displaces any tubing contents so it can pump both air and blood, necessitating the need for a system to detect bubbles. Because it is an occlusive pump, pressure transducers must be connected to the system to detect excessive pressures, reducing the risk of particulate microembolization from tubing spallation, shear-induced blood damage, or circuit rupture. Adjustment of occlusion, or spacing between the roller and back wall of the raceway, is imperative for the accurate calculation of flow rate and to minimize blood trauma.
Centrifugal pumps consist of an impeller arranged with either vanes or a nest of smooth plastic cones inside a plastic housing. The sterile, disposable impeller is coupled magnetically with an electric motor spinning in the drive console. When the impeller rotates rapidly, it generates a pressure differential, causing blood to flow. Centrifugal pumps are nonocclusive and pressure dependent. They generate increased flow when either the preload increases or the afterload decreases. The nonocclusive nature of the pump eliminates tubing wear, spallation, and excessive line pressures associated with roller pumps. However, since they are pressure dependent, a flow transducer is necessary to determine accurate rates of flow. Due to their nonocclusive design, if the pump slows or stops, reversal of flow can occur. Centrifugal pumps are more expensive compared to the roller pump and are not reusable. As a result of the described limitations of centrifugal pumps, a recent survey found that roller pumps were the predominant pump device in pediatric centers.
Filters and Hemoconcentrators
In addition to the cardiotomy and venous reservoir filters mentioned previously, all but approximately 8% of domestic and international pediatric institutions use arterial line filters as a last line of defense against gaseous (GME) and particulate microembolization. They come in a variety of sizes, with preestablished limits to rates of flow and are available in either inline or integrated designs. Inline arterial filters require additional volume to prime, but are excellent gross bubble traps. Because they are microporous filters, they are susceptible to obstruction and manufacturers recommend placing a clamped bypass line around inline filters that can be opened to prevent the interruption of CPB if the filter becomes obstructed. Integrated arterial line filters have recently been developed. These filters are wrapped around the hollow fiber membrane and are held within the oxygenator housing. Advantages of this design include ease of setup and lower prime volumes; however, they cannot be bypassed if they become obstructed.
Hemoconcentrators allow the perfusionist to remove water and other electrolytes, such as potassium, from the blood. They contain hollow fibers similar to those within a dialysis filter. Blood passes through the inside of hollow fibers and vacuum may be applied on the outside to encourage water removal. Everything smaller than the size of the pores of the semipermeable membrane will be extracted, including water, electrolytes, some cytokines and drugs. Everything larger than the pores will remain in the blood stream, including red cells, platelets and most plasma proteins. Significant hemoconcentration can be achieved. Some heparin will be removed; thus adequacy of heparinization must be monitored regularly. The hemoconcentrator can be used at any time during the case, provided there is sufficient volume in the venous reservoir. This is the same type of filter used to perform modified ultrafiltration (MUF), zero balance ultrafiltration, or dilutional ultrafiltration (DUF). MUF is utilized after CPB to concentrate the hematocrit as well as clotting factors and platelets where zero balance ultrafiltration or dilutional ultrafiltration techniques are performed during CPB to reduce inflammatory mediators.
Conduct of Cardiopulmonary Bypass
Rates of Flow
Although the determinants of delivery of oxygen, namely concentration of hemoglobin, saturation of oxyhemoglobin, and rates of flow, are more easily measured during CPB than at any other time in the life of a neonate or infant, the adequacy of delivery of oxygen should always be continuously monitored and adjusted to avoid overt or occult injury to the organs throughout the perioperative period. Rates of flow have typically been guided by nomograms based on body weight or surface area and Fick’s principles of delivery of oxygen and metabolism. The regional distribution of blood during CPB is related to host biology, anesthetic and vasoactive milieu, and technical factors during CPB. The probability of adequate flow to the whole body or organs is related to the total rate of flow. Typically, full flow refers to a perfusion index of 2.8 to 3.6 L/m 2 per minute, which corresponds to 150 to 200 mL/kg per minute in a neonate. Low flow is delivered at varying hypothermic conditions affording metabolic protection and typically refers to rates of between one-quarter and half of full flow. The rates of flow to the whole body necessary to maintain adequate cerebral perfusion range from 30 to 80 mL/kg per minute. Isolated perfusion of organs during bypass is governed by the relative distribution of vascular resistance. Reduction in temperature to 16°C to 20°C allows termination of flow for a limited amount of time to permit unobscured access to the surgical field, a condition referred to as deep hypothermic circulatory arrest (DHCA). The safe duration of DHCA in any individual patient is unknown and highly related to the determinants of delivery of oxygen. The deleterious effects of lowered flow may be more pronounced after hypothermic arrest, when a higher perfusion pressure is necessary to reestablish cerebral flow. Because of variability between patients and techniques, it is advisable to measure indicators of cerebral oxygenation such as near-infrared spectroscopy (NIRS). Manipulation of the independent determinants of delivery of oxygen, such as hemoglobin and partial pressures of carbon dioxide, can be used to restore cerebral oxygen delivery.
Hemodilution
Hemodilution has almost universally accompanied CPB because of the desire to prime extracorporeal circuitry with products other than blood. Rheologic considerations for microvascular flow during hypothermia have supported this approach and are thought to outweigh the reduction in delivery of oxygen associated with the anemia produced by hemodilution. The weight of evidence, however, supports limiting hemodilution in neonates and children, targeting a hematocrit of at least 30% even during deep hypothermia. While variations in prime solutions are mainly targeted at manipulation of electrolytes, oncotic pressure and hemoglobin, the effects on prothrombotic, procoagulant and anticoagulant factors should also be recognized as important effects of hemodilution and calculated based upon estimated blood volume and circuit volume for each patient.
Temperature Regulation
Hypothermia reduces both the cerebral metabolic rate and the availability of oxygen for transfer to the brain. The effects on cerebral metabolism are complex. The metabolism of brain and other tissue is reduced with reduction in body temperature. Most data suggest an inverse exponential relationship, with as much as a 3.5-fold reduction in metabolism for a reduction of 10°C in temperature, referred to as the Q10. Others have found a Q10 as low at 2.3. The bulk of evidence suggests a nearly inverse exponential reduction in metabolism is reduced by an average of 2.8 fold for a 10°C change in temperature. The result of metabolic suppression with hypothermia is that cerebral oxygen extraction is reduced, while flow of blood is still autoregulated, whether measured by saturations in the jugular bulb or NIRS. Because temperature affects the solubility of oxygen and carbon dioxide in solution, and their interaction with hemoglobin, changes in temperature are coupled with changes in gas tensions and pH. In a broad range of studies, the independent effect of pH is small compared to the effect of temperature with a small reduction in oxygen consumption in more acidotic environments. Because pH responsiveness of the cerebral vasculature remains in effect at hypothermia, however, control and manipulation of pH is a critical part of temperature management.
The coupling between cerebral metabolism and blood flow seems to be reasonably maintained in the temperature range of 32°C to 37°C. Below 30°C, however, uncoupling is commonly demonstrated, regardless of pH, such that the metabolism is reduced more than blood flow. The solubility of oxygen in plasma and the affinity of hemoglobin for oxygen are both increased with hypothermia, such that availability of oxygen in the tissues is reduced at any given rate of flow, the Bohr effect. The increased ratio of cerebral flow to metabolism with hypothermia is commonly viewed as cytoprotective from an energetic viewpoint, but the decreased availability of oxygen may negate this apparent metabolic protection. The result of increased solubility and leftward oxyhemoglobin shift is that the fall in cerebral oxygenation with ischemia is not attenuated by hypothermia, even though saturations of hemoglobin are better maintained. Altogether the cytoprotective effects of mild hypothermia exceed measurable metabolic effects and likely involve other mechanisms including alterations in gene expression.
Strategies for Cooling
Surface cooling can begin with induction of anesthesia, which promotes loss of heat and impairs thermoregulatory responses. A reduction in temperature to 35°C is generally well tolerated, and may confer protection against ischemia in the period prior to bypass by metabolic suppression and alteration of responses to cellular injury. Further cooling on bypass is targeted based on the anticipated level of flow required to complete the surgical repair. If DHCA is anticipated, a nasopharyngeal temperature of 18°C is generally the target, with evidence for increased complications at significantly higher and lower temperatures. Active cooling should be accompanied by measures of the adequacy of uniform cerebral cooling, for which measurements of surface temperature are inadequate. Other indicators include jugular venous saturation, the electroencephalogram, and NIRS, from which evidence of metabolic suppression can be more directly ascertained. At least 20 minutes of cooling is associated with improved outcome if hypothermic circulatory arrest is utilized. A high-flow hard-cooling pump strategy is necessary to raise the jugular venous saturation above 95%. Measures that increase cerebral blood flow, such as a pH-stat strategy, can improve brain cooling as previously discussed.
Recent evidence-based reviews cite no advantage to hypothermia in either neurosurgery or open heart operations. Since many operations can be completed without significant interruption in flow of blood, this finding may be unsurprising. These meta-analytic reviews, nonetheless, fly in the face of overwhelming laboratory and clinical evidence of protection from ischemic injury with hypothermia in global ischemia. Because the metabolic benefit of cooling and hypothermia is lost during rewarming, which may be superimposed on a period of reduced delivery of oxygen, a greater risk of ischemia to both heart and central nervous system occurs with rewarming. Given the multiple factors that may cause unexpected disruption in perfusion at full flow, some emergent in nature, most centers continue to use mild or moderate hypothermia as a protective adjunct to CPB without planned reduction of flow or circulatory arrest. While the overall perioperative inflammatory response, although reduced during hypothermia, does not seem to be altered by strategies depending on temperature, moderate hypothermia probably induces cellular adaptations at the transcriptional and translational level that result in survival programming.
In practical terms, schemes for cooling are relatively standardized in most institutions. The complexity of the defect to be corrected or palliated dictates the strategy for the temperature used during bypass, albeit compounding anatomic features such as the presence of aortopulmonary collateral arteries may influence the strategy. Typically, mild hypothermia, at 37°C to 32°C, will be employed for simple defects such as atrial and ventricular septal defects. Moderate hypothermia, between 32°C and 28°C, is used for more complex lesions such as atrioventricular septal defect or tetralogy of Fallot. Deep hypothermia, from 28°C down to 18°C, is reserved for the most complex lesions requiring a period of circulatory arrest, such as palliation of hypoplasia of the left heart, repair of interrupted aortic arch or correction of discordant ventriculoarterial connections. While the described cooling practice is common, some have begun using warm CPB for even the most complex procedures, cooling only to a mild hypothermic temperature of 34°C. Apparent acute benefits of this approach include decreased CPB times and a reduction in perioperative bleeding, but longer-term measures of end-organ preservation including neurologic function are necessary.
Acid-Base Management
The management of blood gases during CPB is intertwined with that of temperature and has been widely investigated and debated. The complexity ensues because metabolic rate, the solubility of gases in blood, the ionization of water and therefore the pH of electroneutrality, the ionization of intracellular buffer, and the affinity of both oxygen and carbon dioxide for hemoglobin are all dependent on temperature. There are two strategies for pH management. A pH-stat strategy maintains normal levels of carbon dioxide and hydrogen ions when measured at hypothermia or temperature corrected. An alpha-stat strategy maintains normal gas tensions and acid-base balance when measured at normothermia or temperature uncorrected. The alpha-stat approach is associated with minimal metabolic suppression and represents the physiologic situation in homeotherms with temperature gradients across parts of the body but with thermoregulation maintained. The pH-stat approach is associated with metabolic suppression and more closely mimics the metabolic milieu of hibernation with induction of metabolic suppression.
The pH affects the ratio of flow of blood to metabolism. While levels of adenosine triphosphate in the brain are maintained during alpha-stat cooling, with pH-stat cooling there is evidence of luxury perfusion. At temperatures below 30°C blood flow is pressure-passive over a wider range of metabolism with over-perfusion evidenced by the appearance of edema. The increased flow with pH-stat strategy is widely utilized to increase uniformity of cerebral cooling, oxygenation, and metabolic suppression. There is evidence of improved outcome in children subjected to DHCA or low-flow bypass when using the pH-stat strategy. Evidence also exists for improved myocardial function with pH-stat techniques. The effects of pH on noncerebral tissue are also important in determining the distribution of flow on CPB. A pH-stat strategy directs more blood to the brain in the presence of aortopulmonary collateral connections. Approaches that combine pH-stat strategy for cooling with alpha-stat strategy for maintenance of high-flow hypothermic perfusion may represent a compromise between inadequate delivery of oxygen and metabolic suppression and over-perfusion–related formation of edema and postacidotic increased cerebrovascular resistance.
Cerebral Protection and Anesthesia
Suppression of cerebral consumption of oxygen occurs with both vapor- and barbiturate-based anesthesia and hypoxia tolerance, based on lactate production, is enhanced. The suppression of metabolism by anesthetic vapors is accompanied by maintenance of high energy phosphates indicating desirable energetic balance. Because vapor agents are also cerebral vasodilators, the ratio of cerebral flow to metabolism is higher with these agents and the increase in cerebral flow may be maintained for hours. Suppression of thermoregulatory responses to hypothermia may be an important role for the salutary effect of lower-stress anesthetic strategies on survival in complex repairs. Inhibition of potassium–adenosine triphosphate (K-ATP) channels by vapors may induce preconditioning, reduce reperfusion injury, and reduce apoptosis in ischemic models. The vasodilatory effects of vapor anesthetics can be expected to improve the uniformity of cerebral cooling and warming. Withdrawal of anesthetic vapor is likely to induce cerebral vasoconstriction in a fashion parallel to the vasodilation seen on acute introduction. Because the neonatal brain is particularly vulnerable to apoptosis via excitotoxic injury, vapor anesthetics might be particularly indicated. However, all anesthetic agents with the exception of dexmedetomidine and opioids have been implicated in enhanced apoptotic change in vulnerable developing and previously injured brains.
Pathways of Cerebral Injury Related to Bypass
Cerebral injury is deterministically related to the delivery of oxygen with irreversible necrotic cell death resulting from a sustained reduction in delivery below 20% of normal. Rates of delivery above half baseline typically do not result in injury, while delivery rates of one-quarter to half of the normal range result in cellular injury whose outcome is modifiable by other factors, such as temperature and free-radical scavenging, even when applied after the insult. Apoptotic cell death ensues hours to days after subnecrotic hypoxic-ischemic injury in susceptible populations of cells and there exists a spectrum of ischemic and apoptotic death in both focal and global models of ischemia. Amplification of injury through excitatory amino acid neurotransmitter-related calcium-dependent cascades of uncontrolled neuronal depolarization may play a role in both necrotic and apoptotic cell death. Modification of excitotoxicity can be demonstrated with glutamate receptor antagonists such as ketamine and dextromethorphan, receptor agonists such as anesthetic vapors and barbiturates, magnesium, and hypothermia. Although therapeutic trials have generally been disappointing in profound ischemia, an incremental effect is likely in more moderate injury.
Although hypothermic circulatory arrest represents an obvious example of global ischemia, it is likely that regional partial ischemia exists during many phases of bypass and the perioperative period. In an animal model of tissue oxygenation during changing conditions of bypass, a range of levels of oxygen in the tissues was demonstrated using the phosphorescent quenching technique, even during high-flow bypass. More hypoxic regions appear during low flow and hypothermic circulatory arrest. The cerebral circulation is susceptible to hypoxic injury throughout the perioperative period and partial ischemia is possible even during high-flow bypass with neuronal fate modifiable by postoperative factors.
Myocardial Protection and Cardioplegia
Initiation of CPB often has myocardial protective effects if the heart is properly unloaded by enhancing the availability of oxygen delivered through the coronary arteries and reducing consumption of oxygen. Hypothermia is a core component of myocardial protection. It further decreases consumption of oxygen and preserves stores of high energy phosphates. Myocardial work can be further reduced by inducing a hyperkalemic arrest via the administration of cardioplegia. The coronary arteries are isolated from the distal aortic circulation by placement of an aortic cross-clamp distal to the cannula used to deliver cardioplegia. Cold solutions are then immediately delivered at 4°C. Aortic valvar competence is necessary to ensure the cardioplegia flows to the coronary arteries and not into the left ventricle. If the aortic valve is not competent, the aortic root can be opened and cardioplegia can be delivered directly into the orifices of the coronary arteries. Cardioplegia can also be delivered in a retrograde fashion via a catheter placed in the coronary sinus, as long as the sinus is not receiving blood from a left superior caval vein. Retrograde cardioplegia is often used as a supplementary method of cardioplegia even when antegrade cardioplegia is possible.
Localized myocardial hypothermia can be achieved with cooling jackets and placement of ice slush in the pericardial space. This technique should be judiciously used, however, as it can result in injury to the phrenic nerves. As an adjunct to hypothermia, the high concentration of potassium in the cardioplegia results in myocardial electromechanical silence and diastolic arrest. The initial arresting dose of cardioplegia at our institution is 20 mL/kg of del Nido cardioplegia solution in a 1 : 4, blood to cardioplegia solution, ratio. Maintenance doses of 10 to 20 mL/kg are given every 90 to 120 minutes thereafter until the repair is complete.
A crucial component of successful myocardial protection is ventricular decompression. Myocardial consumption of oxygen and impedance to subendocardial flow of blood is significantly reduced by lowering the ventricular mural tension. Decompression is accomplished most commonly with the use of a catheter introduced through the right superior pulmonary vein and the left atrium. The catheter passes through the left atrium, across the mitral valve into the left ventricle. Constant suction is applied to the catheter with the use of a designated roller pump and an inline, one-way, over-pressure relief valve on the bypass machine. Blood returning from the venting catheter is returned to the systemic circulation via the cardiotomy reservoir as discussed previously.
Anticoagulation
Although other anticoagulants have been used in special situations, heparin is overwhelmingly the most commonly administered anticoagulant. Heparin has a rapid onset of action, is easily reversed with protamine, and has important antiinflammatory effects. Dosing regimens include simple weight-based schemes, titration to a functional endpoint depending on activated clotting time, and measurement of concentrations with more complicated predictions. Convincing evidence for superiority of approaches is lacking. The gold standard of determining adequate anticoagulation suitable for bypass has been the activated clotting time. This test, however, does not take into account the effects of volume of blood, previous exposure to heparin, deficiency of antithrombin III, hypothermia, or hemodilution. The Hepcon heparin management system (Medtronic) uses a prebypass titration of protamine to determine a patient-specific concentration of heparin to be maintained throughout the bypass run. While on bypass, samples of blood are taken every 30 minutes to determine both concentrations of heparin and activated clotting times. Additional need for heparin is based on maintaining an adequate concentration regardless of an extended activated clotting time. The Hepcon device calculates the dose of protamine needed to reverse heparin based on the circulating concentration of heparin at the end of the bypass run. The thrombotic, embolic, and inflammatory complications of CPB are increased with lower heparin effect. Adequate anticoagulation is crucial, otherwise intravascular coagulation, thrombosis, oxygenator dysfunction, and consumption of clotting factors may occur. Heparin-induced thrombocytopenia, still an unusual complication in infants and children, may be more difficult to recognize.
Deep Hypothermic Circulatory Arrest
DHCA was first developed as a neuroprotective strategy when continuous perfusion could not be maintained. Currently, there is intense debate over the degree of protection offered by hypothermic circulatory arrest compared to hypothermic perfusion. The Boston circulatory arrest trial demonstrated that a strategy utilizing hypothermic circulatory arrest compared to even low-flow bypass is associated with more immediate cerebral brain injury. Patients undergoing hypothermic circulatory arrest had more seizures, an increased tendency to have abnormal electroencephalograms, and lower developmental performance at 1 year. Both groups underperformed at 8 years. Prolonged hypothermic circulatory arrest, greater than 40 minutes at 18°C, is associated with impaired neurodevelopmental outcome. The relationship between a shorter duration of circulatory arrest and outcome is uncertain, but examination of the data from the Boston trial reveals a range of outcomes across the range of circulatory arrest intervals, indicating a multiplicity of determinants of outcome, including individual biologic susceptibility. The safe duration of hypothermic circulatory arrest is related to the rate of use of oxygen from available stores and can be predicted on the combination of hemoglobin, temperature, pH, and time. The transition to anaerobic metabolism during DHCA can be detected by a reduction in the rate of cerebral desaturation as measured by NIRS. The distribution of tensions of oxygen in the brain during hypothermic circulatory arrest is higher and apoptotic regulators are lower with a pH-stat strategy for cooling, predicting a longer safe time for hypothermic circulatory arrest.
In an individual, the use of NIRS can guide the safe duration of hypothermic circulatory arrest by limiting the time of low cerebral oxygenation. With optimal cooling to 18°C and a circulatory arrest time of less than 30 minutes, cerebral injury is unlikely. Moderate and profound hypothermia may initiate protective proapoptotic mechanisms that off-set the deleterious effects of sublethal ischemic times. During longer periods of circulatory arrest, intermittent reperfusion at intervals of 15 to 30 minutes has been demonstrated to maintain cytoarchitecture, cerebral distribution of oxygen, and indicators of excitotoxicity in animals. In humans, avoidance of hypoxic ischemia using a strategy of intermittent reperfusion during DHCA can be a rational strategy for prolonged, complex repairs.
Selective Antegrade Cerebral Perfusion
Because of the variability in time necessary to complete repair and the limited duration of DHCA, strategies have been employed to maintain continuous delivery of oxygen to the brain. Selective antitrade cerebral perfusion via the brachiocephalic artery has become widely used. The optimal strategy for this technique remains poorly characterized because measurements of cerebral blood flow are not readily available and autoregulation may be altered during cold CPB and selective perfusion. pH-stat cooling to a target of 20°C to 26°C, as in anticipated circulatory arrest, followed by direct perfusion of the brachiocephalic artery, is common practice. At our institution we currently cool to 18°C with alpha-stat blood gas management and perfuse via a cannulated graft to the innominate artery. Flow rates of 10 to 80 mL/kg per minute have been described. Flow rates of less than 30 mL/kg per minute, however, are not likely to provide adequate cerebral blood flow to open all capillary beds. Moreover, the increased affinity of hemoglobin at hypothermia may limit availability of oxygen during perfusion, partially off-setting the anticipated reduction in metabolism. Limited data from adults undergoing elective arch reconstruction suggest that moderate hypothermia may be an alternative strategy with acceptable neurologic outcome. However, the optimal temperature is not known, and is highly related to flow strategies and the likelihood of interruption. Noncerebral beds remain poorly perfused during selective cerebral perfusion techniques, and thus are still susceptible to ischemic injury. For these reasons, a target of deep hypothermia should be maintained in any operation in which prolonged selective perfusion or a period of circulatory arrest might be necessary. Techniques to monitor adequacy of cerebral flow during this technique include transcranial Doppler and NIRS.
Experimental models of continuous cerebral perfusion compared to hypothermic circulatory arrest show improved cerebral oxygenation, better postperfusion hemodynamics, reduced apoptosis with less ischemic injury, and improved outcomes. A recent comparison of cerebral perfusion versus hypothermic circulatory arrest in neonates undergoing reconstruction of the aortic arch showed no difference in neurodevelopmental outcome at 1 year. The technique for cerebral perfusion in this study, however, used a rate of flow of only 5 to 20 mL/min and measures of cerebral oxygenation were not reported. Because the rates of flow in those receiving cerebral perfusion were not likely to result in adequate delivery of oxygen, the results are not surprising, showing no difference between complete ischemia and inadequate perfusion. Outcomes utilizing an alternative approach of high-flow perfusion of 50 to 70 mL/kg per minute show no evidence of ischemic injury on postoperative magnetic resonance imaging and good neurodevelopmental outcomes.
Because bypass exposes the body to a significant inflammatory stimulus, there may be a relative disadvantage of prolonged cerebral perfusion compared to circulatory arrest of shorter duration. At present, there exists no direct comparison between cerebral perfusion at rates of flow with measured adequate cerebral oxygenation and DHCA. The optimal pH strategy for continuous selective perfusion is also debatable and conflicting, with some evidence favoring an alpha-stat approach, and other evidence favoring a pH-stat approach. Because of the inherent risk of prolonged ischemia and the unpredictable delayed effects of hypothermic cerebral perfusion and circulatory arrest on postoperative flow of blood to the brain, we favor strategies that rely both on measurement and maintenance of cerebral oxygenation throughout the operative period.
Pharmacologic and Mechanical Adjuncts
Corticosteroids
Pretreatment with corticosteroids is widely used with broad but conflicting evidence for alteration of outcome. Pretreatment in adults reduces postoperative levels of tumor necrosis factor-α, interleukin-6, the incidence of atrial fibrillation, and markers of myocardial ischemia. Evidence exists for both exacerbation and amelioration of hypoxic and ischemic cerebral damage. It was shown that two doses of 30 mg/kg methylprednisolone may ameliorate the inflammation-related delayed reflow and cerebral metabolism after hypothermic circulatory arrest. The membrane-stabilizing effect may reduce excitatory neurotoxicity and perivascular edema may be reduced, but necrotic cell death appeared to be unaffected and apoptotic cell death may be increased. Corticosteroids are commonly used in CPB pump primes; however, a recent study out of Children’s Hospital of Philadelphia revealed a lack of clinical benefit as well as potential contributions to incidents of wound infections and as a result, they have discontinued the use of corticosteroids in their pump prime. A recent analysis of linked databases also found no evidence for improved outcomes with corticosteroid administration.
α-Adrenergic Blockade
The distribution of cardiac output is strongly influenced by the sympathetic nervous system, mainly through α-adrenergic mechanisms. Although deep anesthetic strategies can alter the neurohumoral stress response to surgery, evidence exists for high levels of sympathetic response during cardiac surgery regardless of the anesthetic regimen. We have shown an improvement in whole-body economy of oxygen using phenoxybenzamine that permits and necessitates a strategy of bypass at high rates of flow. In the presence of milrinone, α-adrenergic blockade is more effective than nitrovasodilators in improving flow both on and off bypass. Although outcomes seem improved with this approach, a randomized outcome trial is lacking. The relationship between hemodynamics, oxygen delivery, and outcomes in neonates undergoing arch reconstruction using α-adrenergic blockade has been recently reviewed.
Control of Glucose
Evidence for a beneficial effect of glycemic control in pediatric cardiac surgery is distinctly lacking. The neonatal brain and heart are poorly tolerant of hypoglycemia, which has been related to seizure activity and poor outcome in a large retrospective study of intraoperative glycemic patterns and neurologic outcome. In contrast to findings in adults, posthypoxic supplementation of glucose may reduce neurologic injury in the developing brain. Insulin has antioxidative, antiapoptotic prosurvival cell programming effects independent of its glycemic effects. Because deficiency of insulin is rare in infants and children, the role for exogenous insulin is not yet established.
Ultrafiltration
MUF is a technique to remove excessive water from the body after restoration of the native circulation but before removal of the cannulas used for bypass. As an incremental strategy in adults, MUF reduces morbidity and postoperative use of resources. In children, benefits include improved hemodynamics, pulmonary mechanics, and cerebral metabolism. Some of these effects may be achieved by other strategies to reduce the deleterious effects of bypass-related hemodilution and activation of blood products, including conventional ultrafiltration and strategies for reduction of the prime. The direct effect of MUF on inflammatory mediators and on ultimate clinical outcome is uncertain. Extreme reduction in the volume of prime volume miniaturization of the circuit may eventually replace ultrafiltration. It is likely that a combination of ultrafiltration and incremental alterations in components of the circuit to improve biocompatibility can also improve outcomes. MUF has been established as a class 1A recommendation as a component of blood conservation strategies.
Preconditioning
Ischemic preconditioning, or induction of tolerance to prolonged ischemia by brief exposure to ischemia, has been observed in the myocardium, brain, liver, kidney, intestine, and skeletal muscle, and probably is a feature of all mammalian cells. The effect was first described by the observation of reduced size of infarcts after 40 minutes of coronary arterial occlusion if preceded by cycles of 5 minutes of occlusion. The early protective window occurs within minutes, and fades within a few hours. A later second window of protection, or delayed preconditioning, opens about 24 hours after some preconditioning stimuluses.
Early protective responses to sublethal ischemia induce alteration in the flow of blood and metabolism. Within hours, signaling systems involving hypoxia-inducible factors and heat shock proteins confer resistance to apoptotic transformation. Features of this characteristic response can be induced by hypoxia-ischemia, hyperthermia, hypothermia, hypoglycemia, a range of drugs including K-ATP channel blockers, erythromycin, volatile anesthetics, opioids, acetylsalicylic acid, glutamate, and erythropoietin, the latter having received much attention in recent clinical trials. The mediators of this response favoring survival can be released remotely from the target organ, and improved myocardial function has been demonstrated in humans after minimal ischemia induced by a tourniquet placed on the leg. Induction of a common signaling pathway for survival of cells, as opposed to apoptosis, has been suggested as the underlying mechanism for these findings.
Considerations at the End of Bypass
Altered Cerebral Flow of Blood and Metabolism
The increased relative flow of blood to the brain induced by hypothermia can result in a postoperative increase in cerebrovascular resistance and cerebral edema, although responsiveness to carbon dioxide is maintained. Some of this impaired flow may be due to microvascular occlusion and can be ameliorated by treatments that reduce aggregation and adhesion of platelets, such as donors of nitric oxide, antagonists of thromboxane, and antiplatelet drugs. MUF also seems to improve cerebral blood flow and metabolism, presumably by reduction of inflammatory mediators.
Hypothermic circulatory arrest results in both loss of autoregulation and delayed reflow, with a prolonged suppression of flow of blood and uptake of oxygen. This delayed recovery of cerebral metabolism may exacerbate the neurologic injury related to circulatory arrest. Suppression of metabolism during hypothermic circulatory arrest using a pH-stat strategy and maintenance of a higher postoperative hematocrit can, in piglets, partially ameliorate this metabolic derangement occurring after circulatory arrest. Postoperative arterial hypoxemia exacerbates the arrest-related injury and maintenance of delivery of oxygen to the brain with mechanical assistance may be preferable to inadequate postoperative delivery. We have demonstrated that, following hypothermic circulatory arrest in neonates, inadequate postoperative economy for oxygen is related to poor late neurodevelopmental outcome and postoperative hypercapnia is protective in this setting. Continuous cerebral perfusion does not eliminate the risk of postoperative cerebral desaturation and the interaction between intraoperative management and postoperative flows of blood remains complex.
Postconditioning
Ischemic postconditioning describes modification of injury by interventions applied just at the time of reperfusion, and was first observed with intermittent reperfusion. This may be a form of modified reperfusion, and has been observed with a variety of agents, including erythropoietin, insulin, and isoflurane, which are commonly administered in the operating room around the time of CPB and other ischemic challenges. While hypothermia and volatile anesthetics amplify the protection afforded by preconditioning for tolerance of prolonged ischemia, postconditioning seems to have limited effect after prolonged ischemia. Remote postconditioning has been described via brief occlusion of leg or kidney perfusion just at the time of reperfusion of myocardium, yielding a reduction in the size of infarcts 48 to 72 hours later. While the application of these techniques may seem simple and associated with minimal risk, the clinical utility remains to be elucidated. Inadvertent conditioning may result from aspects of stimulations providing pre- and postconditioning commonly present throughout the operative period. The technique of intermittent reperfusion during prolonged hypothermic circulatory arrest may be a paradigm for both preconditioning and postconditioning. An acidotic perfusion milieu at the beginning of reperfusion may not only enhance flow but may also be important in promoting ischemic postconditioning and antiapoptosis.
Postoperative hyperthermia is common after CPB and has been associated with poorer neurologic outcome. Attention to the control of temperature during rewarming and immediately following bypass can reduce the incidence of undesired hyperthermia. Limiting the arterial outflow blood temperature from the bypass circuit to 37°C is recommended. Induction of postoperative hypothermia should be strongly considered for patients experiencing uncontrolled or prolonged perioperative ischemia. Hypothermia may afford protection independent of its effects on cerebral blood flow and metabolism. The effects of hypothermia postinjury are likely due to reduction in apoptotic cell death and thus both intraoperative and postoperative hypothermia may provide antiapoptotic programming. Reduction in apoptotic cell death has been demonstrated using cerebral perfusion as a supportive strategy as opposed to hypothermic circulatory arrest. Postoperative mild hypothermia and administration of albumin are simple clinical interventions commonly applied that may alter outcome after incomplete ischemia. Evidence of focal ischemia from gas embolism or other causes should prompt consideration of hyperbaric treatment with oxygen.
Monitoring
Because a range of conditions can affect central hemodynamics, regional cerebral perfusion, cerebral metabolism, and flow metabolism coupling during cardiac surgery, cerebral oxygenation is likely altered in both predictable and unpredictable ways. Cerebral hypoxia, measured by jugular venous saturation or NIRS, has been shown experimentally to be related to injury during ischemia with similar findings reported in humans. Hypoxic-ischemic conditions cannot reliably be identified by standard hemodynamic monitoring. Because aggressive prevention of overt and occult hypoperfusion improves outcomes, measurement of global and regional oxygenation is recommended as a method to prevent and treat unanticipated and unappreciated hypoxic-ischemic conditions. This is especially crucial in the immediate perioperative period when intervention to improve outcome is possible.
Intraoperative Echocardiography
Transesophageal echocardiography has become a mainstay of intraoperative management of the patient undergoing cardiac surgery. Specific guidelines have been developed as to the indications for its use in children. There is general agreement that the technique is indicated in every child over 3 kg. Some centers use the system in any infant over 2.5 kg in whom a probe is easily placed. An echo is generally performed at the beginning of every case, both to confirm the anatomy previously detected by using transthoracic windows and to obtain real-time orientation to dynamic structures. Intraoperatively, echocardiography can be important to verify that air has been cleared from cardiac chambers prior to allowing cardiac contractions and emergence from cardiopulmonary support. It can also be very useful in delineating the source of failure when a child is not able to be weaned from bypass (i.e., residual lesions). For instance, echocardiography is very useful when differentiating hypovolemia from functional compromise and residual structural deficits. At the termination of the procedure, echocardiography is critical in verifying the adequacy of many repairs.
Postoperative Extracorporeal Support
Short-term mechanical support is occasionally necessary following complex cardiac operations. The use of extracorporeal membrane oxygenation (ECMO) carries a reasonable expectation of recovery for small patients in cardiopulmonary failure. In the current era, the technique is an essential component of programs undertaking complex congenital cardiac surgery. The circuit used by most centers is a direct descendant of the circuit initially developed for support of the neonate with pulmonary hypertension and persistent fetal circulation and has the advantage of being standardized and well understood by the specially trained staff who manage the circuit. Key to its success was the development of the silicone membrane oxygenator, which extended the safe duration of extracorporeal support from hours to days, providing enough time for recovery of most neonates with respiratory failure. Silicone membranes were followed by hollow fiber membrane oxygenators and both the oxygenator and the circuit can be heparin bonded. As a result the requirement for heparin is reduced, which can be helpful in controlling bleeding. More recently, polymethylpentene membrane oxygenators have been developed that all but eliminate issues with plasma leakage associated with the hollow fiber membrane. This advantage allows for longer-term use of an oxygenator without the need to exchange it due to decreased gas exchange. ECMO is still of value only in the short term, but because myocardial dysfunction is likely to recover within 96 hours, this duration of support is suitable for the postoperative patient. Survival to discharge from hospital for patients who required extracorporeal support subsequent to the operation is in the range of 40%. Not surprisingly, among patients requiring such support, a lack of residual lesions favors survival. Patients with a functionally univentricular circulation suffering acute thrombosis of a shunt are noteworthy for a high rate of survival. In the early 2000s, many centers developed a rapid response system to permit rapid cannulation to salvage patients who sustained unexpected cardiac arrest. More recently and as a part of cardiopulmonary resuscitation to ECMO processes and protocols, ECMO circuits are maintained for the emergent initiation of ECMO. The technique, however, is still suitable only for relatively short periods of treatment. Bridging to transplantation requires long-term, low-morbidity support.
Centrifugal pumps have recently been put to use supporting patients with isolated cardiac dysfunction requiring temporary support. The Thoratec Centrimag and Pedimag (Thoratec) and the Maquet Rotaflow (Getinge Group) have been used in patients with both biventricular and single ventricle circulation.
Providing mechanical support for the failing heart over longer periods has become a clinical reality in adults, and permits both support of the circulation as well as the opportunity for rehabilitation and decrease in pulmonary vascular resistance. In children, long-term support is more challenging due to challenges of small size and congenital heart disease anatomy. The Berlin Heart (Berlin Heart AG) is a paracorporeal pneumatic displacement pump with chambers and cannulas specifically designed for pediatric use and it was approved in the United States in 2011. The pumping ventricle is available in a variety of sizes from 10 to 80 mL. The smallest size is suitable for support of infants ( Fig. 16.15 ). Because of the paracorporeal or external position of the pump and the range of available cannulas, the device accommodates a range of anatomic variances. It is even possible to support patients with functionally univentricular circulations. Thrombus within the device can be identified by visual inspection and the external position permits changing of the pump without reoperation. The downside of the Berlin Heart is the high rate of embolic stroke, up to 29%, but recent experience with the use of bivalirudin is promising and may prove to be a safer anticoagulation strategy.
