Prenatal Circulation

Despite the clinically significant alterations in fetal circulation inherent in TA, these do not seem to be detrimental to fetal somatic development. The systemic venous return is forced across the foramen ovale into the left heart. As a consequence, the usual PO ₂ differential present in a normally developing fetus is not present. The lowered PO ₂ to the heart and brain and the elevated PO ₂ to the lungs do not seem to produce any clinically significant anomalies. Almost all of the left ventricular output is ejected through the aorta in a fetus with type 1 defect; as a result the aorta is big and is rarely associated with arch obstruction. This is in contradistinction to type 2 defects, which predictably are associated with a higher prevalence of arch obstruction (30% to 50%).

Postnatal Circulation

An unrestrictive atrial septal defect (ASD) is crucial to allow mixing of the systemic and pulmonary venous blood and maintenance of cardiac output because there is a lack of communication between the RA and the RV. The pathophysiology of TA is dictated by the amount of pulmonary blood flow, which is determined by the anatomic obstruction to pulmonary blood flow (size of the BVF and degree of pulmonary stenosis [PS]), patency of the ductus arteriosus, and pulmonary vascular resistance (PVR). Patients may present with decreased pulmonary flow, increased pulmonary blood flow, or in the rare instance with a balanced circulation.

For patients with TA and normally related great vessels, blood from the LV enters the RV across the BVF to provide pulmonary blood flow. For those with concomitant PS or restriction at the BVF the pulmonary blood flow is provided via a patent ductus arteriosus (PDA). Patients may present with cyanosis due to reduced pulmonary blood flow or pulmonary overcirculation in the event that the BVF is nonrestrictive and there is not important PS. Cyanosis may worsen from progressive PS, narrowing of the BVF (there is a tendency for the BVF to decrease in size over the first 2 to 3 years of age), or closure of the PDA.

For patients with TA and transposed great vessels, systemic circulation is dependent on the size of the BVF, and the blood from the LV must traverse the BVF to reach the systemic great vessel. Systemic blood flow may become ductal dependent if there is severe subaortic stenosis, usually at the level of the BVF. Most patients present with pulmonary overcirculation, especially in cases in which there is a degree of obstruction to systemic blood flow (either at the level of the aortic arch or at the subaortic region). Patients with l-transposition of the great vessels (l-TGA) have a higher incidence of subaortic stenosis.

Systemic arterial desaturation is present in all patients with TA secondary to an obligatory admixture of systemic, coronary, and pulmonary venous blood in the left atrium (LA). The degree of desaturation depends on the pulmonary blood flow. A pulmonary-to-systemic blood flow ratio (Q _p :Q _s ) ratio of 1.5 to 2.5 : 1 usually results in adequate saturations, and higher shunt fractions will result in left ventricular volume loading, pulmonary overcirculation, and heart failure. Normal systemic ventricular function is one critical component of successful single-ventricle palliation, which eventuates in the Fontan operation.

Obstruction at the level of the interatrial communication is rare. This is evident when there is a pressure gradient of more than 3 mm of Hg across the ASD or giant a waves. Spontaneous closure of the BVF has an estimated prevalence of 38% to 48%. The mechanism involved is usually progressive muscular encroachment and fibrosis from endocardial proliferation. Echocardiographic studies have shown that a cross-sectional area of the BVF of less than 2 cm ² /m ² may predict later restriction at the BVF or even indicate prescient obstruction.

Management of the Newborn With Decreased Pulmonary Blood Flow

The subgroups of neonates with decreased pulmonary blood flow ( Fig. 65.5 ) require some form of a shunt to increase pulmonary blood flow. These patients are dependent on the PDA for their pulmonary blood flow. Ductal closure would lead to severe hypoxemia, metabolic acidosis, and death, therefore necessitating continuous infusion of prostaglandin E ₁ (PGE ₁ ) to maintain ductal patency. Pulmonary overcirculation is a condition in which there is increased blood flow to the lungs compromising systemic perfusion and adequate tissue oxygen delivery. Arterial oxygen saturation, as measured by pulse oximetry, is surrogate to the Q _p :Q _s (shunt fraction). For example, an oxygen saturation of 75% approximates a Q _p :Q _s of 1 : 1, 85% of approximately 2 : 1, and 95% of 5 : 1. The serum lactate levels are a good indicator of the systemic cardiac output and systemic oxygen delivery. Lactate levels are elevated (>2) when the tissue perfusion is inadequate. Lactate levels are not a reflection of Q _p :Q _s except that inadequate Q _p might result in significant hypoxemia, leading to inadequate tissue oxygen delivery, and excessive Q _p (in relation to Q _s ) might correlate with inadequate Q _s (as more output is shunted toward the pulmonary circulation) and inability to sustain necessary systemic cardiac output. A Q _p :Q _s of 1 : 1 requires a “double” cardiac output from the single ventricle (one output for the pulmonary circulation and one for the systemic circulation), and a Q _p :Q _s of 3 : 1 requires a quadruple cardiac output! So it is understandable how the demands of increasing cardiac work from unlimited pulmonary blood flow can overwhelm the capability of the heart to provide essential systemic flow. Optimal Q _p :Q _s is obtained by medical management with adjustments in pulmonary and systemic vascular resistance. This includes correction of acidosis, maintenance of normal blood glucose and calcium levels, normothermia, and manipulation of the blood PO ₂ and CO ₂ levels. Mechanical ventilation and sedation may be required to maintain physiologic Q _p :Q _s . The systemic vascular resistance can be altered by using vasodilators and vasoconstrictors. It is critical to realize that the Q _p :Q _s is a ratio and not an absolute number for cardiac output. Lactic acidosis is an excellent parameter to reflect whether or not systemic oxygen delivery is adequate in the face of the Q _p :Q _s presented by the patient. A patient with a systemic oxygen saturation of 95% and normal lactate levels may have a high Q _p :Q _s but is clearly maintaining adequate systemic oxygen delivery (e.g., has a normal cardiac output). A patient with an oxygen saturation of 80% (Q _p :Q _s 1.5 : 1) and a lactate level that is at 3 and rising may have less overall cardiac work but is also showing signs of inadequate cardiac output.

Echocardiogram demonstrating a patient with tricuspid atresia and a very restrictive bulboventricular foramen and decreased pulmonary blood flow requiring a shunt in the neonatal period.

As mentioned previously, the first priority for management is to ensure a stable source of pulmonary blood flow. Flow generated across a BVF may be adequate initially, but the proclivity of the BVF to close mandates close follow-up of oxygen saturations and echocardiograms with willingness to place a shunt if the flow into the pulmonary arteries across the BVF becomes inadequate. If the baby is ductal dependent for pulmonary blood flow (common), then the initial management might require choosing between a ductal stent or a surgically placed aortopulmonary shunt.

The Modified Blalock-Taussig Shunt

Creating an aortopulmonary shunt is used to provide a stable source of pulmonary blood flow. The classic Blalock-Taussig shunt (which entailed division of the subclavian artery and anastomosis of the subclavian artery to the PA), Potts shunt (direct connection between the left PA and the descending aorta), and Waterston shunt (direct connection between the posterior portion of the ascending aorta and the right PA where it courses behind the aorta) remain mostly of historical interest only. At present the modified Blalock-Taussig shunt remains the choice for augmenting pulmonary blood flow in the neonate. Introduced by de Leval et al. in 1975, this aortopulmonary shunt is a modification of the original description of the Blalock-Taussig shunt in 1944. It involves interposing an expanded polytetrafluoroethylene (ePTFE; Gore-Tex) graft between the innominate or subclavian artery and the ipsilateral PA ( Fig. 65.6 ). The goal of the operation is to provide pulmonary blood flow to obtain a systemic oxygen saturation of approximately 75% to 85%. This provides a Q _p :Q _s of 1 to 1.8 : 1. Depending on the size of the neonate and on whether there is an additional source of pulmonary blood flow (such as across a restrictive BVF), the most commonly used shunts are 3.5 mm or 4.0 mm in diameter. As shown by the Congenital Heart Surgeons Society study, an indexed shunt size of 0.8 to 1.0 results in more favorable outcomes (mortality and successfully transitioning to a cavopulmonary shunt). The provision of a stable source of pulmonary blood flow allows for symmetric growth of the PAs without overcirculation, thus allowing favorable anatomic conditions for future cavopulmonary anastomosis.

Right modified Blalock-Taussig shunt. In this illustration the patent ductus arteriosus has been ligated, but that is not always required.

The postoperative management of the newborn with an aortopulmonary shunt can be challenging. Optimum management requires meticulous attention to the Q _p :Q _s and often involves titration of inotropic medications to ensure adequate systemic perfusion combined with ventilator management to minimize overcirculation. The goals are to maintain adequate shunt flow, arterial oxygen saturation (SaO ₂ ) between 75% and 85%, and adequate ventricular function. If ventricular function is adequate to manage the increased cardiac output demands of increased Q _p :Q _s , many babies tolerate oxygen saturations of 85% to 90% quite well. Use of near-infrared spectroscopy (NIRS) has become standard with these neonates, and trends in both the cerebral and somatic NIRS may offer early warning signs of poor systemic perfusion. The differential diagnosis for low SaO ₂ includes hypotension, shunt stenosis ( Fig. 65.7 ) or occlusion, lung disease (pulmonary venous desaturation), and impaired ventricular function (or low cardiac output). Although many surgeons favor subtherapeutic “low-dose” unfractionated heparin drip in the immediate postoperative course, which can be later transitioned to enteral aspirin to maintain shunt patency, there are no data to support the use of subtherapeutic heparin, and the efficacy of long-term use of aspirin may depend on platelet response to aspirin as measured by current assays. Some studies have shown that a significant number of neonates and infants are refractory to aspirin with respect to platelet effect. More recently, some surgeons have favored using ePTFE grafts that are infused with heparin, although the benefit of these grafts is also unproven. Perhaps the most important factor for graft patency is the technical adequacy of the anastomosis and the quality of the vessels to which the graft is connected.

Angiogram performed after a modified right Blalock-Taussig shunt. (A) Proximal and distal shunt stenosis is demonstrated. (B) Appearance after stent placement.

Excessive pulmonary overcirculation may be manifested by high SaO ₂ levels, pulmonary edema, and failure to wean from mechanical ventilation. This often requires manipulation of the PVR by judicious use of milrinone, permissive hypercapnia, and selective inotropic support to improve the ability of the heart to maintain the increased cardiac output demands of a high Q _p :Q _s . In extreme circumstances, persistent pulmonary overcirculation may also require operative revision to reduce shunt size (often by “clipping” or banding the shunt). An important factor in this type of circumstance is whether or not the ductus was left open at the time of shunt creation. Many surgeons leave the ductus alone and allow it to close by discontinuation of prostaglandin infusion. This has the advantage that if a shunt occludes in the postoperative period, the intensive care unit (ICU) team can reinstitute prostaglandin in hopes of restoring ductal patency. (The logic behind this is simply that in the era of classic BT shunts, performed via a thoracotomy, the PDA was never ligated). Occasionally, however, a PDA does not close after the cessation of prostaglandins and can create a significant additional source of pulmonary blood flow that may need surgical attention. Although this is unusual, it is important for the ICU team to know whether or not the ductus was ligated at the time of surgical shunt placement.

As mentioned earlier in this chapter, the management of patients with shunt physiology is potentially complex. Although the Q _p :Q _s is an important determinant, it is merely a ratio and not an actual number. It is also important for the management team to assess the overall systemic oxygen delivery as a reflection of cardiac output. For example, if there is truly excessive pulmonary overcirculation, there may be a low systemic cardiac output, and this drives down the mixed venous O ₂ saturation. If the mixed venous O ₂ saturation is 40%, and there is a 5 : 1 shunt, but the pulmonary venous saturation is 90% due to pulmonary edema, then the O ₂ saturation might be 81% (five parts 90 = 450 and one part 40 gives a mixing in the heart of 490, which divided by 6 provides a mixed saturation of 81). Although this may seem ideal, it clearly is not and will become manifest by rising lactate levels. Conversely, a patient may have a saturation of 93% and normal lactate levels. This would reflect that the heart is capable of maintaining excessive additional cardiac output (as much as six outputs to the lung for every output to the body), and, although this may not be a good long-term situation, if it is tolerable, it can be managed with systemic afterload reduction, permissive hypercapnia, and, if necessary, investigation into whether or not there is an additional source of pulmonary flow, such as across a patent ductus or BVF. The point to illustrate is that patients with shunts have abnormal circulation, and the astute management team takes into account a variety of factors beyond the Q _p :Q _s in determining whether or not the patient is well palliated.

Hypoxemia in the immediate postoperative period represents an emergency. Absence of a “shunt murmur” (in a hypoxemic patient) represents shunt thrombosis until proven otherwise. Confirmation by echocardiography is important, and this usually requires emergent return to the operating room for revision or may even require institution of extracorporeal membrane oxygenation support if the operating room is not available. In instances in which the ductus was not ligated, rapid reinstitution of PGE ₁ infusion may be lifesaving. Less acute and severe hypoxemia can sometimes be managed in the catheterization laboratory by balloon angioplasty or stenting of the shunt (see Fig. 65.7 ). Other diagnoses in the differential of postoperative hypoxemia include low cardiac output, lung disease, atelectasis, and phrenic nerve paralysis.

Management of the Newborn With Increased Pulmonary Blood Flow

Infants with increased pulmonary blood flow who present with physiologically important “pulmonary overcirculation” will demonstrate significant clinical effects such as lactic acidosis, poor renal function with rising blood urea nitrogen or creatinine level, and even pulmonary edema—which may actually affect oxygenation leading to hypoxemia, despite the excessive pulmonary blood flow (which is why Q _p :Q _s is a guideline to be used in conjunction with the overall clinical picture) ( Fig. 65.8A ). Patients with transposed great vessels and TA may actually have significant systemic outflow obstruction (either at the subaortic level or at the level of the aortic arch and isthmus or both). This combination of defects creates the dual problem of decreased systemic perfusion and pulmonary overcirculation; and when there is aortic arch hypoplasia or a prominent coarctation, these infants may be ductal dependent for systemic perfusion and require, despite their “pulmonary overcirculation,” the institution of PGE ₁ for maintenance of ductal patency to perfuse the body (when the aorta is obstructed). These are complex circumstances. When pulmonary overcirculation is simply created by flow across a large BVF (VSD) into normal pulmonary arteries (without aortic obstruction), then consideration can be given to placing a PA band. Strict control of the pulmonary blood flow is required to protect the lung bed and prevent lung disease because the ultimate goal of palliation is a Fontan. PA banding was originally introduced by Muller in 1952. Providers must be cognizant of the fact that PA banding may accelerate narrowing of the BVF over time with ensuing subaortic obstruction and cyanosis. Further, PA distortion and ventricular hypertrophy represent additional risks inherent to PA banding. Patients who receive a PA band to control excessive pulmonary blood flow are at risk for developing rapidly progressive cyanosis that may necessitate further intervention in a short-term time frame.

Infant with type 1 tricuspid atresia and increased pulmonary blood flow. (A) Before pulmonary artery (PA) bands, chest x-ray showing pulmonary edema. (B) After PA bands. Lung fields have cleared.

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