Congenital Heart Disease

The overall incidence of congenital heart disease (CHD) is approximately 8 per 1000 live births and is usually divided into two categories: cyanotic (the defect contains a right-to-left shunt) and acyanotic (the defect may contain a left-to-right shunt). The most common cyanotic lesions, in order of decreasing frequency, are pulmonary stenosis (PS) , transposition of the great arteries (TGA) , tetralogy of Fallot (ToF), tricuspid atresia (TA), and pulmonary atresia with intact ventricular septum (PA/IVS). The most common acyanotic lesions, in order of descending frequency, are ventricular septal defect (VSD) , atrial septal defect (ASD) , aortic stenosis (AS), coarctation of the aorta (CoA) , persistent ductus arteriosus (PDA), and complete common atrioventricular canal (CCAVC).

Pathophysiology of Congenital Heart Disease

Anesthesia providers caring for children with CHD must fully understand the anatomic components of the lesion and how the blood flows through the heart and lungs. Because of the complexity of the lesions and subsequent repairs, this can often be confusing. Therefore, a structured approach should be used, with a focus on determining the relative ratios of pulmonary and systemic blood flow. These ratios may ultimately determine the important aspects of anesthetic management. This structured approach involves the following steps:

1.

Determine whether blood flow is obstructed in any part of the heart. Right-sided obstructions decrease blood flow to the lungs and result in low PaO ₂. Left-sided obstructions decrease blood flow to the body, resulting in decreased tissue perfusion, metabolic acidosis, and shock.
2.

Determine whether blood is being shunted from one side of the heart to the other. If blood is shunted from the right side to the left side (e.g., ToF), it does not go through the lungs and results in cyanosis. Left-to-right shunting (e.g., VSD) will result in volume and pressure overload on either or both ventricles and may lead to CHF. In its advanced form, overcirculation of the pulmonary bed leads to pulmonary hypertension and, if untreated, irreversible pulmonary vascular obstructive disease. This results in a reversal of the shunt (to right-to-left) and causes hypoxemia and cyanosis (sometimes known as Eisenmenger syndrome ). On a basic level, it may seem that the direction of shunting is determined by the location of the defect and obstruction. However, in many cases the resistance in the pulmonary and systemic circuits will determine the direction of shunt. Specialists in CHD like to refer to the ratio of pulmonary vascular resistance (PVR) to systemic vascular resistance (SVR), also expressed as pulmonary blood flow (Qp) and systemic blood flow (Qs), respectively. This ratio will determine whether the patient has a right-to-left shunt (Qp:Qs <1), a left-to-right shunt (Qp:Qs >1), or both at different times during the cardiac cycle. Other factors, such as ventricular failure or dilation and severe valvulopathy, may also contribute to shunting.
3.

Determine whether there is a volume load or a pressure load on the heart. When a ventricle is overburdened by excessive volume overload (e.g., large VSD) or obstruction to forward flow (e.g., right ventricular outflow tract obstruction), the ventricle can begin to fail. In general, the right ventricle responds with dilatation, and the left ventricle with concentric hypertrophy. In either case, when the load exceeds the ventricular capacity, CHF develops. Left-sided CHF often results in pulmonary manifestations and/or systemic hypoperfusion while right-sided CHF can lead to hypoperfusion, hepatomegaly, liver dysfunction, and peripheral edema.

Once these three key points are determined, the anesthesia provider can begin to formulate a plan to safely anesthetize the child with CHD with respect to choice of anesthetic drugs, ventilation strategy and plan for responding to intraoperative cyanosis or hypotension. In the next section, we will take a closer look at the anatomy and physiology of some of the more common causes of CHD, starting with the acyanotic defects.

Acyanotic CHD

Ventricular Septal Defect

The most common congenital heart defect (approximately 25% of all congenital cardiac lesions) is the ventricular septal defect (VSD). There are five types of VSD, based on the anatomic location of the defect:

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Muscular: occurs in the posterior, apical, or anterior muscular portion of the septum and can be single or multiple.
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Inlet: occurs in the part of the septum underneath the septal leaflet of the tricuspid valve.
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Conoseptal: occurs in the outflow tract of the right ventricle beneath the pulmonary valve.
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Conoventricular: occurs in the membranous portion of the septum.
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Malalignment: results from a malalignment of the infundibular part of the septum.

A VSD may be isolated or occur in conjunction with other lesions ( Fig. 3.1 ). The type of VSD does not usually influence anesthetic management; however, when it is excessively large or when associated with other anatomic defects, hemodynamic changes may occur during anesthesia.

The clinical features of a VSD are determined by its size and direction of blood flow. If the VSD is relatively small, there are usually no clinical symptoms. A large VSD allows unrestricted blood flow, the direction depending on the PVR to SVR ratio. In almost all children with a VSD, SVR is higher than PVR, and blood flows from left to right through the VSD. If untreated, over time this will result in CHF as the right ventricle becomes overloaded (from the normal venous return plus the extra volume from the left ventricle returning through the VSD). The excessive pulmonary blood flow eventually leads to pulmonary hypertension and reversal of the shunt (right-to-left flow leading to cyanosis).

Treatment of CHF may include digoxin, diuretics, and an angiotensin-converting enzyme (ACE) inhibitor while waiting for natural or surgical closure. Small muscular and conoventricular VSDs close naturally (40% by age 3 years, 75% by age 10 years); however, large VSDs should be closed surgically before pulmonary vascular changes become irreversible. Children with a previous VSD repair may occasionally demonstrate myocardial dysfunction, arrhythmias, or right bundle-branch block.

Atrial Septal Defect

Atrial septal defect (ASD) accounts for about 7.5% of CHD. There are multiple types based on the anatomic location of the defect ( Fig. 3.2 ).

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Ostium secundum: occurs in the midportion of the atrial septum and is the most common form of ASD.
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Ostium primum: occurs low in the atrial septum.
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Sinus venosus: occurs at the junction of the right atrium and the SVC or IVC.
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Coronary sinus: refers to a hole in the wall of the coronary sinus as it traverses the left atrium.
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Patent foramen ovale (PFO): occurs when there is inadequate fusion of the septum secundum and the septum primum.

Nearly all small secundum type ASDs close spontaneously during the first year of life. However, large secundum ASDs, or those with significant shunting, will require surgical repair or placement of a closure device via cardiac catheterization.

Ostium primum, sinus venosus, and coronary sinus ASDs do not close spontaneously and must be closed surgically. PFOs occur in about 20% to 30% of the general population. Children are usually asymptomatic after ASD repair. Although there are no unique anesthetic considerations for noncardiac surgery, careful attention must be paid to de-bubbling the intravenous lines. Air bubbles that enter into the venous system may cross over to the arterial system and cause a clinically significant air embolus in the cardiac or cerebral arteries. The specific type of ASD does not influence anesthetic management unless it causes physiologic abnormalities.

Complete Common Atrioventricular Canal

Complete common atrioventricular canal (CCAVC, also referred to as an endocardial cushion defect ) consists of an ostium primum ASD and a nonrestrictive inlet VSD, and often occurs in children with trisomy 21. There is usually a left-to-right shunt at the atrial and ventricular levels which can result in CHF during infancy. Pulmonary hypertension may develop from the increase in pulmonary blood flow.

Surgical repair of CCAVC is usually performed in the first year of life. Complete heart block occurs in 5% of patients undergoing repair, and residual mitral insufficiency may be seen.

Patent Ductus Arteriosus

Before birth, blood bypasses the lungs and travels from the main pulmonary artery to the descending aorta through the ductus arteriosus ( Fig. 3.3 ). Normally, there is physiologic ductus closure in the first days of life (because of pressure differences in the aorta and pulmonary artery) and anatomic closure in the first months of life. In certain conditions such as prematurity, the ductus remains patent indefinitely and serves as a source of left-to-right shunt and right-sided pulmonary overcirculation. Patent ductus arteriosus (PDA) represents approximately 7.5% of congenital heart disease.

A variety of factors tend to contribute to patency of the ductus arteriosus, such as hypoxemia, respiratory or metabolic acidosis, and persistent pulmonary hypertension of the newborn. The direction of shunted blood through a large PDA depends on the ratio of PVR to SVR. In a nonrestrictive PDA, a left-to-right shunt occurs if SVR is greater than PVR. Newborns with a large PDA and left-to-right shunt may show signs of pulmonary overcirculation and CHF, which include a widened pulse pressure, a continuous murmur, and an inability to wean ventilatory parameters. Treatment usually consists of diuretics until the PDA can be closed either medically, with administration of indomethacin, or surgically with an open or video-assisted catherization device closure (coil embolization).

It is crucial to identify ductal-dependent lesions in the newborn, in which patency of the ductus arteriosus is not only favorable but required for survival. These include cyanotic lesions such as pulmonary atresia/stenosis, tricuspid atresia/stenosis, and transposition of the great arteries, and some acyanotic lesions, such as coarctation of the aorta, hypoplastic left heart syndrome, critical aortic stenosis, and interrupted aortic arch. As soon as a ductal-dependent lesion is discovered, a prostaglandin E1 (PGE1; alprostadil) infusion is started at 0.05 to 0.1 mcg/kg/min. Infants should be monitored for apnea during administration of PGE1, and are maintained at relatively low concentrations of oxygen to encourage ductal patency.

Aortic Stenosis

Aortic stenosis (AS) represents up to 5% of CHD. It ranges in severity from mild to severe, or complete aortic atresia as seen in hypoplastic left heart syndrome (see below). The neonate with critical AS relies on their PDA for systemic blood flow; if the PDA closes, circulatory shock will occur. Most cases of mild AS are detected later in childhood by the presence of a murmur.

The clinical manifestations of AS will depend on the degree of stenosis and the ventricular function. Significant stenosis produces a large pressure gradient between the left ventricle and the aorta resulting in left ventricular hypertrophy with subsequent decreased ventricular compliance and function.

Hemodynamically significant AS requires surgical intervention, which is accomplished by balloon valvuloplasty or open surgical valvotomy. In some cases, treatment of AS causes aortic regurgitation, which may eventually require aortic valve replacement. In some children, a Ross procedure (pulmonary autograft) is performed, in which the child’s own pulmonary valve is moved into the aortic position, and a right ventricle-to-pulmonary artery homograft conduit is placed.

Coarctation of the Aorta

Coarctation of the aorta (CoA) represents about 8% of all congenital heart defects, of which approximately 80% also have a bicuspid aortic valve. It usually occurs distal to the origin of the left subclavian artery at the insertion site of the ductus arteriosus. The coarctation narrows the aorta, thus increasing left ventricular afterload. CHF develops in about 10% of cases in infancy. There is a 15% to 20% risk for having CoA in girls with Turner syndrome (45, XO).

Neonates with severe CoA need their PDA to provide blood to the systemic circulation. If the PDA closes, the infant goes into circulatory shock. Therefore, PGE1 is administered to keep the ductus open until the CoA is repaired. More commonly, CoA presents during childhood. Typically, it is diagnosed during investigation of a new heart murmur, accompanied by hypertension of the upper extremities and decreased or absent femoral pulses. Left ventricular hypertrophy and CHF can result from chronic pressure overload.

The CoA can be treated by balloon dilation angioplasty, stent placement, surgical end-to-end anastomosis, subclavian flap repair, or graft placement. In many patients hypertension persists throughout childhood; the duration of postoperative hypertension correlates with the duration of hypertension before the repair.

Cyanotic Congenital Heart Diseasae

D-Transposition of the Great Arteries

D-transposition of the great arteries (TGA) accounts for about 5% of CHD and is the most common form of cyanotic CHD in the neonatal period. In TGA, the great vessels are transposed, which means that the aorta arises from the right ventricle, and the pulmonary artery rises from the left ventricle. Thus, circulation exists as two separate parallel circuits unless a communication (PDA, VSD, or PFO) can mix the blood to maintain survival. Infants with TGA will appear cyanotic shortly after birth when the PDA functionally closes. As soon as the diagnosis is made by echocardiogram (or even before), PGE1 is administered to maintain ductal patency, and the infant is considered for emergent balloon atrial septostomy in the cardiac catheterization lab to allow more complete mixing at the atrial level through an unrestricted communication.

Treatment for TGA requires the arterial switch operation usually within the first 2 weeks of life. Survival exceeds 95%. Left ventricular function usually remains good throughout childhood, although supravalvular pulmonary stenosis may remain and require intervention. Occasionally, children will demonstrate atrial and ventricular tachyarrhythmias.

Hypoplastic Left Heart Syndrome

Hypoplastic left heart syndrome (HLHS) is the second most common type of cyanotic CHD. It presents in the first week of life, and is the most common cause of death from CHD in the first month of life. HLHS consists of hypoplasia of the left ventricle, aortic valve stenosis or atresia, mitral valve stenosis or atresia, and hypoplasia of the ascending aorta with a discrete CoA. The result is the lack of blood flow through the left heart, causing an obligatory left-to-right shunt at the atrial level and a right-to-left shunt through a PDA. Systemic flow becomes completely dependent on the PDA, and coronary perfusion is retrograde in the presence of aortic atresia or critical aortic stenosis. The diagnosis of HLHS is often made in utero or in the first few days of life when the PDA closes and the infant presents in heart failure and shock. Clinical signs include tachycardia, tachypnea, pulmonary rales (from pulmonary edema), hepatomegaly, and poor peripheral pulses with diminished distal capillary refill. PGE1 is started immediately upon diagnosis to maintain ductal patency, and the infant is prepared for urgent surgery.

In most centers, treatment consists of a three-stage surgical correction performed over the first several years of life. In the first week of life, a Norwood procedure is performed to create a “neo-aorta” to establish unobstructed systemic blood flow. This allows the majority of neonates with HLHS to survive infancy. The single right ventricle provides systemic blood flow, and pulmonary blood flow is provided by placement of a modified Blalock–Taussig (subclavian-to-pulmonary artery) shunt (see below) or a right ventricle-to-PA conduit (Sano shunt). An atrial septectomy (or permanent ASD creation) is performed to create an unobstructed atrial communication to allow oxygenated blood flow to return from the lungs and reach the systemic circulation. After this initial stage the patient’s oxygen saturation is usually 60% to 75%.

The second stage is usually performed between 4 and 6 months of age. The procedure is known as a hemi-Fontan (also called bidirectional Glenn or Norwood 2 ). The SVC is anastomosed to the right PA, so that blood returning from the head bypasses the right ventricle and flows passively into the pulmonary circulation. This procedure is delayed until the patient’s PVR, which continues to drop after birth, decreases to the point at which the lungs are able to accept the additional blood flow. The second stage also decreases the effective blood flow load on the single ventricle. At this age, the patient does not require their BT shunt, as the newly created SVC-to-PA anastomosis serves as the source of pulmonary blood flow. After this stage, the patient’s oxygen saturation is usually 70% to 85%.

The third stage, performed at approximately 2 to 3 years of age, is the completion Fontan, in which the IVC is joined directly to the pulmonary artery. After this procedure, all venous blood returning to the heart bypasses the single ventricle heart and flows passively into the lungs, while the single right ventricle serves to pump oxygenated blood returning from the lungs to the body. After this stage, the patient’s oxygen saturation now approaches normal values for the first time. Patients with a fenestrated Fontan (a small hole or connection between the IVC-PA connection and the atrium) may serve as a source of right-to-left shunt reducing the patient’s normal saturation. Shunting across the fenestration occurs during times of elevated pulmonary pressure.

The circulation that remains is referred to as Fontan physiology . Blood flow to the lungs becomes dependent on the transpulmonary gradient, which is the pressure difference between the Fontan circuit (systemic veins and pulmonary arteries) and the pulmonary venous atrium. Thus, any condition that increases PVR will decrease blood flow through the lungs and cause hypoxemia ( Table 3.1 ).

Table 3.1

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