Abstract
Ventricular assist devices (VADs) are invaluable tools for the management of end-stage heart failure in children of all ages. Although development of such devices in adults has advanced substantially in the past few decades through several generations of VAD enhancements, the development of such devices for children has lagged behind. However, the last 10 years have seen increased attention to VAD development and use in the pediatric population. Indeed, in the face of stable or only slightly increased numbers of pediatric donors, the number of pediatric heart transplants has increased significantly, in parallel with the increase in the number of children listed as heart transplant candidates. In part the ability to perform many more transplants with only slightly more donors is reflective of the use of VADs to bridge candidates to transplant who in the pre-VAD era would certainly have died. Nonetheless, of all patients on the waiting list for solid-organ transplantation in the United Sates, children listed for heart transplantation face the highest waiting list mortality, an effect that is especially pronounced in the youngest and smallest children, in whom VAD solutions are least satisfactory. It is anticipated that more children with congenital heart disease and cardiomyopathy-associated heart failure will require long-term VAD support in the coming decade as a bridge to transplantation or recovery.
Key Words
mechanical circulatory support, heart failure, ventricular assist device, heart transplant, congenital heart disease
History and Current State of Ventricular Assist Device Support
Ventricular assist devices (VADs) are invaluable tools for the management of end-stage heart failure in children of all ages. Although development of such devices in adults has advanced substantially in the past few decades through several generations of VAD enhancements, the development of such devices for children has lagged behind. The slower development of pediatric devices has been due to a number of factors, such as the inherent differences in physiologic parameters in children, the variable anatomy in children whose heart failure is secondary to congenital heart disease (CHD), and the relatively small population in need of such devices, which creates much less economic incentive for industry-sponsored device development. However, the last 10 years have seen increased attention to VAD development and use in the pediatric population. Indeed, in the face of stable or only slightly increased numbers of pediatric donors, the number of pediatric heart transplants has increased significantly, in parallel with the increase in the number of children listed as heart transplant candidates. In part the ability to perform many more transplants with only slightly more donors is reflective of the use of VADs to bridge candidates to transplant who in the pre-VAD era would certainly have died. Nonetheless, of all patients on the waiting list for solid-organ transplantation in the United Sates, children listed for heart transplantation face the highest waiting list mortality, an effect that is especially pronounced in the youngest and smallest children, in whom VAD solutions are least satisfactory. It is anticipated that more children with CHD and cardiomyopathy-associated heart failure will require long-term VAD support in the coming decade as a bridge to transplantation or recovery.
For many years extracorporeal membrane oxygenation (ECMO) was the only readily accessible form of mechanical circulatory support (MCS) for both short-term (i.e., postcardiotomy) and medium-term support. However, use of ECMO beyond 10 to 14 days was limited by substantial rates of complications, particularly stroke, bleeding, and infection, greatly hampering its utility in patients requiring more durable support. In North America there were limited options for other long-term assist devices in children smaller than adult size until 2000, when a Berlin Heart EXCOR pulsatile VAD was placed in a pediatric patient in the United States under compassionate use regulations. Subsequently an investigational device exemption (IDE) study was conducted in the United States and Canada, leading to Food and Drug Administration (FDA) approval in late 2011. In the past few years there has been wider use of the Berlin Heart device, increasing application of newer adult devices in ever smaller patients, “off-label” use of temporary devices for intermediate or longer-term support, and progress in initiatives to develop new devices for very small children.
Pediatric Heart Failure
The number of children with heart failure has been increasing, resulting in growing demand for transplant and therefore for MCS in the pediatric population. Possible explanations include better recognition of pediatric cardiomyopathy with earlier intervention with medical therapy and advancements in surgery and perioperative care for children with CHD, leading to increased long-term survival of this patient population. One of the inevitable consequences of improved survival of these patient groups will be an increased incidence of end-stage heart failure in children, adolescents, and young adults. A typical example of end-stage heart failure in the setting of operated CHD involves children whose morphologic right ventricle is sustaining the systemic circulation and who progress to failure of that systemic ventricle. Perhaps the largest group of patients who may become candidates for advanced heart failure management are those with single-ventricle physiology.
Although pediatric VAD therapy has been in recent rapid evolution, there has also been significant refinement of VAD therapy in adults over the last decade. The most significant change to have an impact on patient management strategy was the emergence of durable intracorporeal (implantable) continuous-flow devices such as the Thoratec HeartMate II (Abbott, Abbott Park, IL) and HeartWare HVAD (Medtronic, Dublin, Ireland). Owing to the impressive outcomes in patients supported with these devices, with relatively low-morbidity profiles, the indications for device placement have evolved, making the early institution of VAD therapy a reasonable option in preference to escalating medical management. In adults VADs are used as a bridge to transplant, as a bridge to recovery, or as destination therapy (for patients who opt against transplantation or for whom transplantation is not an option). Although the use of destination therapy in pediatrics is evolving, the vast majority of pediatric VADs are currently still used as a bridge to transplant.
The applicability of the intracorporeal devices in the pediatric population is limited by body size, and therefore they are most commonly used for larger children and adolescents. However, there have been case reports of patients successfully implanted with HeartWare devices with a body surface area (BSA) of 0.6 m. The advent of three-dimensional computed tomography mapping has also facilitated fit testing of such devices in children.
Although miniaturized intracorporeal devices for smaller children are on the horizon, the Berlin EXCOR, a paracorporeal pulsatile device, is the only currently FDA-approved available option for infants.
Patient Selection
For patients with advanced heart failure, VAD implantation is indicated when the benefits of the device are deemed to outweigh the risks. Because patients, devices, and clinical settings represent infinitely variable (and changeable) combinations of risk and benefit, appropriateness and timing of device implantation are determined for each patient by the multidisciplinary team, the family, and where appropriate, the patient. Special consideration should be given to not only medical circumstances, but also social aspects. VAD selection should also be influenced by the institutional experience.
Beyond small size, other important confounding issues for children in whom VAD implantation is being considered may include the increased operative difficulty and potential complications inherent in patients who have had prior operations and have structurally abnormal hearts. Examples of the latter include anomalous anatomy intrinsic to their CHD (abnormalities of atrial situs, ventricular situs, great vessel arrangement, or systemic venous drainage). As well there may be “iatrogenic abnormalities” that must be accounted for during implantation (e.g., systemic to pulmonary shunts, cavopulmonary connections, and prior atrial septectomy). In addition to careful attention to the anatomy and circulatory physiology, a thorough understanding of the unique pathophysiologic features of pediatric heart failure is an absolute prerequisite to a successful outcome with VAD therapy.
At present, long-term VAD support in children generally requires candidacy for heart transplantation, or at least “candidacy for candidacy.” When considering contraindications to VAD therapy in children, extreme prematurity, very low body weight (<2.0 kg), significant preexisting neurologic injury, a constellation of congenital anomalies with poor prognosis (unlikely survival beyond childhood), and major chromosomal aberrations are generally accepted contraindications for any form of MCS. Multisystem organ failure may be a relative contraindication but does not necessarily exclude patients from MCS if reversal of organ function is predicted with the achievement of hemodynamic improvement. Indeed, it has been well documented that liver, renal, and pulmonary dysfunction frequently improve after restoration of hemodynamic stability with MCS.
Device Selection
Because pediatric patients are so divergent in terms of size and cardiac physiology, appropriate device selection and good understanding of each device are keys to success.
Device selection for initial MCS in children with heart failure is ideally limited to cardiac support with VADs, which support the left ventricle (left ventricular assist device [LVAD]), right ventricle (right ventricular assist device [RVAD]), or both (biventricular assist device [BiVAD]), though pure RVAD support is extraordinarily rare. However, some children with acute decompensated heart failure also have significant pulmonary dysfunction, which is most often reversible and may require cardiopulmonary support. In such cases temporary support with ECMO may be indicated before LVAD implantation. In some cases of RVAD or BiVAD support an oxygenator can be added to the RVAD for temporary pulmonary support, and later removed with lung recovery. Moreover, if the patient is in cardiopulmonary arrest with ongoing cardiopulmonary resuscitation, then ECMO is the initial support of choice because this can be rapidly initiated (peripherally) and will provide support to both right and left heart as well as the lungs ( Fig. 40.1 ).
Extracorporeal Membrane Oxygenation
ECMO is covered separately in Chapter 39 , so we will limit the discussion here to the major differences between ECMO and VAD support.
ECMO is a temporary support strategy, and should be confined to short-term support for heart failure. Although it has previously been used as a bridge to transplant, ECMO is rarely used for this purpose in current practice given expectation of waiting times, which are likely to be measured in months. There is a significant survival benefit of long-term VAD support over ECMO support for patients waiting for heart transplantation. In addition, posttransplant survival is higher in patients supported with VAD compared with those who had ECMO support, irrespective of diagnosis.
Controversy exists, however, regarding the best mode of MCS if anticipated support duration is short (<2 weeks). Many pediatric heart centers use ECMO irrespective of the cause of heart failure. In addition to extracorporeal cardiopulmonary resuscitation, other potential applications of ECMO for circulatory support include the presence of significant pulmonary hypertension, hemodynamic instability due to septic shock, or severe pulmonary edema resulting from ventricular dysfunction. Thus ECMO is often preferred when the right heart is unable to provide “adequate” flow to fill the left heart (and therefore the systemic circulation). Suboptimal right heart output can be due either to inherent right ventricular dysfunction (e.g., severe cardiac allograft rejection), intractable ventricular arrhythmias, or pulmonary hypertension.
The advantages of short-term VADs compared with ECMO include the simplicity of the circuit and, more importantly, better decompression of the failing left ventricle, which may be crucial for optimizing recovery from pulmonary edema. The lack of an oxygenator and the simpler circuit configuration induce less inflammation and are likely less thrombogenic, which results in a lower level of anticoagulation requirement. Better ventricular decompression is critical in patients with acute heart failure in whom there is a reasonable chance of cardiac recovery (e.g., acute myocarditis). Short-term VAD support with a centrifugal pump provides excellent decompression of the left heart (or systemic ventricle), with immediate impact on left atrial pressure, pulmonary venous hypertension, pulmonary edema, and lung function. It is clear that short-term VADs that directly drain the left heart provide better decompression of a failing left ventricle than does a simple venoarterial ECMO strategy that has only indirect effect on the left heart. Left heart decompression during ECMO support can be enhanced if there is an adequate atrial septal defect (or if one can be created with possible stent implantation) either by transseptal flow or the placement of a transseptal drainage cannula. Conversely, with LVAD support, inflow comes from a cannula placed directly into the left side of the heart. Hence short-term VAD support may provide a better chance of pulmonary and cardiac recovery than ECMO support with or without direct left-sided decompression.
Other Temporary/Short-Term Ventricular Assist Devices
Left Ventricular Assist Device
Indication.
Short-term VADs may be used in children with heart failure secondary to (1) acute, potentially reversible processes (e.g., acute myocarditis and acute rejection of a cardiac allograft), (2) acute exacerbation of chronic heart failure (e.g., acute worsening of dilated cardiomyopathy due to superimposed infection), or (3) postoperative ventricular dysfunction (e.g., following reimplantation of anomalous left coronary artery arising from the pulmonary artery and late arterial switch operation with deconditioning of the left ventricle). If the causes are acute processes, it is reasonable to anticipate that the cardiac function may recover with adequate left heart decompression, which would warrant a trial of temporary support.
Temporary Ventricular Assist Device Cannulation
The decision regarding when to discontinue or transition to other type of support warrants clinical judgment and varies considerably between individuals. Outcomes for ECMO support in children with presumed acute myocarditis include survival rates of 63% in the Extracorporeal Life Support Organization registry and 70% to 80% in single-center reports. Most patients with acute myocarditis can be supported reasonably well with either mode of temporary MCS (ECMO or short-term VAD). The difference in left heart decompression, however, could make a difference in patients with the most severe form of the disease, those with such severe left ventricular (LV) dysfunction that the aortic valve does not open (fortunately a small proportion of the entire patient population).
In contrast to the goal of temporary VAD therapy in acute heart failure, which is recovery and explant, the goal of short-term VAD support in acute-on-chronic (AOC) heart failure is more modest. In the AOC setting the temporary VAD is used to optimize candidacy for placement of a durable (long-term) VAD and for ultimate transplantation. This approach is sometimes called a “bridge to decision,” in that candidacy for durable VAD implant or transplantation may be indeterminate at the time of evaluation based on potentially reversible end-organ dysfunction. The use of a temporary VAD in this setting allows time for the clinical situation to stabilize and the determination of candidacy to be made conclusively. The rationale for this approach in children has been demonstrated in a recent multicenter analysis of children undergoing durable VAD implantation. In this analysis, patients who were the most ill at the time of VAD implantation, Pedimacs category 1 ( Box 40.1 ), had markedly worse survival than those in Pedimacs categories 2 and 3. Thus for the AOC patient in category 1 the implantation of the temporary VAD allows stabilization and “downcategorization,” potentially resulting in improved prognosis.
Pedimacs 1: “Critical cardiogenic shock” describes a patient who is “crashing and burning,” in which a patient has life-threatening hypotension and rapidly escalating inotropic pressor support, with critical organ hypoperfusion often confirmed by worsening acidosis and lactate levels.
Pedimacs 2: “Progressive decline” describes a patient who has been demonstrated “dependent” on inotropic support but nonetheless shows signs of continuing deterioration in nutrition, renal function, hepatic function, respiratory function, fluid retention, tachyarrhythmia, or other major status indicator. Patient profile 2 can also describe a patient with refractory volume overload, perhaps with evidence of impaired perfusion, in whom inotropic infusions cannot be maintained due to tachyarrhythmia, clinical ischemia, or other intolerance.
Pedimacs 3: “Stable but inotrope dependent” describes a patient who is clinically stable on mild-moderate doses of intravenous inotropes (or has a temporary circulatory support device) after repeated documentation of failure to wean without symptomatic hypotension, worsening symptoms, or progressive organ dysfunction (usually renal). It is critical to monitor nutrition, renal function, fluid balance, and overall status carefully in order to distinguish between a patient who is truly stable at Patient Profile 3 and a patient who has unappreciated decline rendering them Patient Profile 2. This patient may be either at home or in the hospital. Patient Profile 3 can have modifier A, and if in the hospital with circulatory support can have modifier TCS
Pedimacs 4: “Resting symptoms” describes a patient who is at home on oral therapy but frequently has symptoms of congestion at rest or with activities of daily living (ADLs). He or she may have orthopnea, shortness of breath during ADL such as dressing or bathing, gastrointestinal symptoms (abdominal discomfort, nausea, poor appetite), disabling ascites or severe peripheral edema (extremity or facial). This patient should be carefully considered for more intensive management and surveillance programs, which may in some cases reveal poor compliance that would compromise outcomes with any therapy.
Pedimacs 5: “Exertion Intolerant” describes a patient who is comfortable at rest but unable to engage in any activity, living predominantly within the house or housebound. This patient has no congestive symptoms, but may have chronically elevated volume status, frequently with renal dysfunction, and may be characterized as exercise intolerant.
Pedimacs 6: “Exertion Limited” also describes a patient who is comfortable at rest without evidence of fluid overload, but who is able to do some mild activity. ADLs are comfortable and minor activities outside the home such as visiting friends or going to a restaurant can be performed, but fatigue results within a few minutes of any meaningful physical exertion. This patient has occasional episodes of worsening symptoms and is likely to have had a hospitalization for heart failure within the past year.
Pedimacs 7: “Advanced NYHA Class 3” or “Ross Class III” describes a patient who is clinically stable with a reasonable level of comfortable activity, despite history of previous decompensation that is not recent. This patient is usually able to walk more than a block. Any decompensation requiring intravenous diuretics or hospitalization within the previous month should make this person a Patient Profile 6 or lower.
Temporary VAD support may be initiated via either peripheral access (either percutaneous or open surgical access) or centrally, via sternotomy. The advantage of the former approach is the rapidity with which support can be initiated in critically ill patients. The disadvantages include relatively large cannula size, which precludes this approach in small children, as well as the difficulty in providing biventricular support. The advantages of central cannulation include universal applicability, regardless of size. The disadvantages include the need to perform a sternotomy, or in some cases a redo sternotomy, which would obviously take longer than a peripheral approach. For both central and peripheral cannulation, flow is provided in a continuous fashion by either a centrifugal pump for a two-cannula system (see later) or an axial pump (see later) with one-cannula systems. For a two-cannula system an oxygenator can be inserted into the system to provide pulmonary support. This is not possible with a single-cannula system.
For peripheral cannulation, LVAD support may be accomplished with a single arterial cannulation site using the Impella system ( Fig. 40.2 ) or with a left atrial (transvenous, transseptal) cannula in combination with arterial cannulation using the TandemHeart system (see Fig. 40.2 ). The Impella system is positioned retrograde across the aortic valve, under fluoroscopic (and often echocardiographic) guidance with the inflow to the system at the distal portion of the cannula and the outflow positioned above the aortic valve. The inflow cannula for the TandemHeart is positioned across the atrial septum with either echocardiographic or fluoroscopic guidance with the arterial cannula being inserted into a femoral artery. Neither system has been used extensively in children, because of cannula size limitations (for TandemHeart 21 French [Fr] venous, 17 Fr arterial; for Impella 14 Fr arterial). A recent comparison in an experimental animal model of acute heart failure suggested that the TandemHeart system may provide better left ventricular decompression than the Impella system at equivalent flow rates. For RVAD support a right-sided version of the Impella has been developed, the so-called Impella RP. This has demonstrated significant efficacy in temporary right-sided support after durable LVAD implantation and right ventricular myocardial infarction, but its use has not yet been reported in children.
For patients in whom central cannulation is elected, standard bypass cannulas are typically employed. For isolated LVAD support the inflow cannula may be placed via the left ventricular apex or via the left atrium. In the event that the latter approach is taken, care must be taken to avoid injury to the mitral valve in the event that recovery is hoped for. The outflow cannula for the LVAD may be placed directly into the aorta or attached to a graft sewn to the ascending aorta or one of its branches. If RVAD support is necessary, the inflow cannula is usually placed in the right atrium with the outflow cannula placed directly in the main pulmonary artery or into a graft anastomosed to the main pulmonary artery. Because the main pulmonary artery is quite short, great care must be taken to avoid rendering the pulmonary valve incompetent with cannula placement or positioning the cannula tip too distally so as to preferentially perfuse one lung (typically the left) at the expense of the other. In the event that pulmonary support is required in the setting of biventricular temporary VAD placement, an ECMO type of oxygenator can be inserted in the RVAD circuit, sometimes colloquially termed RECMO. With pulmonary recovery the oxygenator can easily be removed. There are rare instances in which isolated RVAD support with an oxygenator may be necessary, such as in the case of severe pulmonary failure with right ventricular failure, as a means to unload the failing RV and allow recovery or bridge to lung or heart/lung transplantation. Whether LVAD, RVAD, or BiVAD temporary support is elected, the sternum can typically be closed, and the patient may be able to be removed from mechanical ventilation in most cases within a relatively short time frame. For central cannulation one of several commercially available centrifugal pumps is employed depending on institutional preference and patient size, such as Thoratec PediMag or CentriMag (St. Jude Medical, St. Paul, MN), Rotaflow (Maquet, Wayne, NJ), or TandemHeart (Cardiac Assist, Inc./TandemLife, Pittsburgh, PA) ( Table 40.1 ; see Fig. 40.2 ).
| Device | Size Restrictions | Duration of Support | Type of Support | Pump Flow | Approved for Use by US FDA | Pathway of US FDA Approval | Labeled Excluding Use in Children | Studied Protectively in Children |
|---|---|---|---|---|---|---|---|---|
| Temporary | ||||||||
| ECMO | None | Days | Biventricular | Continuous | Yes | 510K | No | No |
| CentriMag | None | Days-weeks | Univentricular or biventricular | Continuous | Yes | 510K | No | No |
| Rotaflow | None | Days-weeks | Univentricular or biventricular | Continuous | Yes | 510K | No | No |
| TandemHeart pVAD | N/A | Days | Univentricular | Continuous | Yes | 510K | No | No |
| Impella 2.5/5.0 | N/A | Days | Univentricular | Continuous | Yes | 510K | No | No |
| Durable | ||||||||
| Berlin Heart EXCOR | Wt > 3 kg | Months-years | Univentricular or biventricular | Pulsatile | Yes | HDE | Labeled for children | Yes |
| HeartWare HVAD | Wt > ≈ 15 kg | Years | Univentricular or biventricular | Continuous | Yes | PMA | No | No |
| HeartMate II | Wt > ≈ 30 kg | Years | Univentricular | Continuous | Yes | PMA | No | No |
| SynCardia Total Artificial Heart | BSA > 1.7 m 2 | Years | Biventricular | Pulsatile | Yes | PMA | No | No |
Intraaortic Balloon Counterpulsation
Although not technically a VAD, the intraaortic balloon pump (IABP) is discussed here for completeness. This device, which can be inserted peripherally or centrally, consists of a long balloon positioned in the descending aorta (see Fig. 40.2 and Fig. 40.3 ). The balloon is inflated and deflated with timing synchronized to provide inflation during ventricular diastole and deflation before ventricular systole. Thus the pump provides both enhanced coronary perfusion (diastole) and reduced ventricular afterload (systole). Though the IABP has greatly facilitated the management of adult patients with heart failure, its application in the pediatric population has been very limited for several reasons. An important constraint is the size of the balloon (smallest currently is 7.5 Fr), making it too large for all but larger children and adolescents. Furthermore, heart failure in children is virtually never related to coronary insufficiency, rendering the enhancement of coronary perfusion by the IABP superfluous. Last, the compliance of the juvenile aorta is remarkably higher than in middle-aged and elderly adults, so the degree of diastolic unloading that the IABP can generate in children is minimal. Thus, although they may be reasonable for short-term support in larger children, IABPs have a very limited role in long-term mechanical support or bridging to transplantation. In the event that a single arterial support apparatus is elected, the support provided by the Impella device is likely to be significantly superior in children to that provided by the IABP, though this proposition has been evaluated only in adults.
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