ALTERATIONS OF CARDIOVASCULAR FUNCTION IN CHILDREN

Embryology

Cardiogenesis begins at approximately 3 weeks of gestation; however, most cardiovascular development occurs between the fourth and seventh weeks.¹ The heart arises from the mesenchyme and begins development as an enlarged blood vessel with a large lumen and a muscular wall (Figure 31-1, A). Initially, two lateral endocardial heart tubes fuse to form a single structure (Figure 31-1, B). During the fifth week of gestation, the midsection of this tube begins to grow faster than its ends. This single heart tube elongates and rotates to the right (D-loop formation), creating a bulboventricular loop by approximately the twenty-eighth day¹ (Figure 31-1, C). Also at this time the first fetal heart contractions occur. At this stage the primitive heart structures include a common atrium; common ventricle; the sinus venosus, which eventually evolves into the superior and inferior venae cavae; the bulbus cordis, which eventually evolves into the ventricular outflow tracts; and the truncus arteriosus, which eventually yields the main pulmonary artery (PA) and aorta (Figure 31-1, D). By the fourth week of gestation, cardiovascular septation, ventricular development, aortic arch evolution, and circulation begin.

Cardiac Septation

Separation first begins when collections of mesenchymal cells cause the endocardial lining of the heart to bulge into the internal lumen. These changes, known as endocardial cushions, are instrumental in closing the lower portion of the atrial septum, dividing the atrioventricular (AV) canals into the right and left AV orifices, and forming the upper portion of the interventricular septum. Altered formation of the endocardial cushions can result in ostium primum atrial septal defects, inlet ventricular septal defects (VSDs), malformation of the AV valves, or a complete AV defect (also known as atrioventricular septal defect).²

Atrial septation begins when two thin membrane-like structures, known as the septum primum and the septum secundum, grow toward the area of the endocardial cushions (Figure 31-2). The septum primum forms along the posterior wall of the common atrium and grows downward toward the center portion of the heart. The gap between the two structures, known as the ostium primum, normally closes by extensions from the endocardial cushions. At the time of closure, fenestrations or openings develop in the superior portion of the septum primum, creating the ostium secundum. Failure of the septum primum to fuse with the endocardial cushions results in an ostium primum defect in the atrial septum near the AV valve area.

The septum secundum is also a fenestrated, membrane-like structure located anteriorly that grows toward the endocardial cushions. During fetal development this structure does not completely fuse with the endocardial cushions to achieve complete atrial septal closure. The nonfused septum secundum and ostium secundum result in the formation of a flapped orifice known as the foramen ovale, which allows the right-to-left shunting necessary for fetal circulation. Altered development in any of these structures can lead to an atrial septal defect.

Ventricular septation develops when the muscular ridge located at the apex, the endocardial tissue, and the bulbar ridges in the bulbus cordis fuse (Figure 31-3). Closure of the interventricular septum ensures communication between the right ventricle (RV) and the PA and between the left ventricle (LV) and the aorta. Further evolution of the endocardial tissue gives rise to the membranous ventricular septum and the AV valves. The conal portion of the ventricular septum that separates the aorta from the PA forms from the bulbus cordis.

When the single primitive heart tube begins to form the D-loop, the venous and arterial poles of the heart are fixed, resulting in torsion within the anterosuperior region of the loop, known as the truncus arteriosus. This torsion creates a spiral ridgelike structure or septum within the truncus arteriosus that divides it into the PA and the aorta. The semilunar valves evolve from tubercles after this division is complete.

Before this division occurs, however, two large arteries form at the distal end of the truncus arteriosus. Over time they give rise to a series of arterial vessels, collectively called the six aortic arches. By the fifth week of gestation, the first two pairs disappear and the third eventually evolves into the common carotid artery, the external carotid artery, and part of the internal carotid artery. The fourth pair of aortic arches will form part of the true aortic arch and the proximal segment of the right subclavian artery. The fifth pair disappears; however, the sixth pair yields the proximal and branch pulmonary arteries within the lung parenchyma and the ductus arteriosus.

Swellings in the conal region at the base of the main trunk separate the right ventricular outflow (pulmonary outflow) tract from the left ventricular outflow (aortic outflow) tract. The conus also contributes to complete closure of the interventricular septum, and normal reabsorption of the subaortic conal region ensures rotation of the great arteries so that the aorta is posterior and to the right of the PA and the PA is anterior and to the left of the aorta. Despite division of the truncus arteriosus and separation of the right and left outflow tracts, a communication exists between the aorta and the PA known as the ductus arteriosus.

In order to deliver maximally oxygenated blood to the developing brain, fetal circulation differs from the adult pattern by the presence of alternate pathways known as fetal shunts (Figure 31-4). Fetal oxygenation occurs in the placenta instead of the fetal lungs. The fetal lungs are not aerated, although the fetus does make breathing motions.

In utero the fetus receives blood carrying oxygen and nutrients from the placenta through the umbilical vein. Fetal arterial oxygen tension is much lower than that found in the postnatal period—approximately 20 to 30 torr (mmHg pressure). Yet, despite this hypoxemic state, tissue hypoxia does not occur because of high fetal cardiac output and fetal hemoglobin. The blood travels to the liver, where a portion enters the portal and hepatic circulation; approximately half the flow is diverted away from the liver through the ductus venosus and into the inferior vena cava. Because the blood received from the inferior vena cava yields a higher pressure, blood entering the right atrium (RA) from the inferior vena cava is shunted through the foramen ovale and into the left atrium (LA) and is then pumped through the LV and into the aorta. Approximately two thirds of the blood flows to the head and upper extremities. Because this blood is mainly from the placenta, the brain and coronary arteries receive the blood with the highest oxygen concentration. The remaining blood flows into the descending aorta.

Less-saturated blood, with an oxygen tension of 15 to 19 torr, returns from the upper body, head, neck, and arms and travels from the superior vena cava into the RA. A small portion of this blood flows into the RV and out the PA and enters the nonfunctioning lungs. Most of the blood, however, bypasses the lungs by flowing through the ductus arteriosus and into the descending aorta. Blood from the descending aorta returns to the placenta through two umbilical arteries.¹

The nonaerated lungs and low oxygen tension induce vasoconstriction, creating high pulmonary vascular resistance. This is transmitted to the right side of the heart and the PAs. Conversely, fetal systemic resistance is low because of the large-volume placenta and ductus arteriosus. Therefore, because blood flow follows the path of least resistance, high pulmonary resistance diverts most of the blood flow into the PA, through the ductus arteriosus, and into the aorta. From there it travels into the low-resistance placenta.

Closure of Fetal Shunts

Once the umbilical cord is clamped, the umbilical arteries and vein, which comprise the cord, vasoconstrict and undergo fibrous changes. Therefore, blood flow through the ductus venosus falls instantly; absence of fetal shunting through this vessel usually occurs within the first 7 days of life. Once the ductus venosus closes, its remnants form the ligamentum venosum, or round ligament of the liver.

Increased pulmonary venous return and decreased inferior vena cava return cause functional closure of the foramen ovale within the first month of life. In the fetus the foramen ovale is held open by the blood flow from the high-pressure right side, reflecting pulmonary vascular resistance, to the lower-pressure area on the left side of the heart, reflecting systemic vascular resistance. At birth the pressure gradients reverse (left atrial pressure exceeds right atrial pressure by a small degree), causing the valve flaps of the foramen ovale to close. Functional closure occurs by the adherence of these flaps to the atrial septum. Anatomic closure occurs within the first month of life after deposition of fibrin tissue and cell products permanently seals the flaps closed. Until this occurs, any condition that stimulates an increase in the right-sided pressures or causes dilation of the RA can reopen the foramen ovale. Conditions in which a patent foramen ovale may continue past the first month of life include pulmonary hypertension, RV failure, and tricuspid atresia.

The ductus arteriosus closes more gradually. Increased oxygen saturation in the systemic arterial blood is thought to be the major stimulus causing vasoconstriction of the ductus arteriosus. In addition, a decrease in the amount of endogenous prostaglandins promoting dilation and the release of vasoactive substances stimulate further ductal closure. Vasoconstriction of the ductal medial smooth muscle shortens and thickens the intima of the ductal wall within 15 to 18 hours after birth. Permanent closure is complete 10 to 21 days after birth. Fibrous tissue adheres to the remaining structure, and the ductus arteriosus eventually evolves into the ligamentum arteriosum. Conditions that involve low arterial oxygen saturations, such as cyanotic heart disease, decreased medial muscle layer within the ductus, or increased levels of circulating vasodilating substances in the blood, may delay or prevent ductal closure.³

Postnatal Hemodynamics

As stated, systemic vascular resistance begins to rise once the placenta is removed from the circulation. Normal levels in the infant range from approximately 10 to 15 Wood units × body surface area (in square meters) and gradually increase to 15 to 30 Wood units × body surface area (in square meters) by childhood.⁴ Likewise, the systolic pressure is low in the full-term newborn (approximately 39 to 59 mmHg), reflecting the decreased LV strength. As the systemic vascular resistance increases, the LV becomes more developed and the systolic pressure rises steadily until it equals adult levels once the child reaches puberty.

The heart rate of the newborn ranges from 100 to 180 beats/minute, which gradually decreases as the child grows. Similarly, the newborn’s cardiac output is high, which is a reflection of the fetal circulation described earlier. Oxygen consumption doubles at birth; to maintain adequate oxygen delivery, the cardiac output also remains high. These changes, however, cause minimal cardiac reserve in the newborn. Additional stressors could increase oxygen demands and result in acute deterioration. By 2 months of age, oxygen consumption decreases by half. As the newborn grows, stroke volume steadily increases while the heart rate decreases.²

Postnatal Circulation

Postnatal circulation allows the lungs to oxygenate the venous blood and allows saturated blood to be delivered to the systemic circulation. Desaturated blood returning from the superior vena cava, inferior vena cava, and coronary veins enters the RA and is pumped to the RV through the tricuspid valve. The RV then pumps the blood through the pulmonic valve to the PA; the blood flows to the lungs, where it is oxygenated. The oxygenated blood returns from the lungs through the pulmonary veins and enters the LA, which pumps blood to the LV through the mitral valve. The LV then pumps blood through the aortic valve and into the aorta. The coronary arteries receive the saturated blood along with delivery to the systemic circulation.

Heart Failure

Heart failure (HF), sometimes called congestive heart failure (CHF), is classified as an acquired condition. HF occurs when the heart is unable to maintain sufficient cardiac output to meet the metabolic demands of the body. HF can occur as the result of decreased myocardial function or excessive metabolic demands. The most common causes of HF in infancy and childhood are cardiomyopathy or the result of poor ventricular function. Table 31-5 lists the congenital heart defects that cause HF by age. Pulmonary overcirculation from large left-to-right shunts (mixing) is often called CHF but is not usually associated with decreased ventricular function and failure to meet metabolic demands. However the clinical manifestations are similar, such as failure to thrive (FTT), tachypnea, tachycardia, and respiratory infections.

PATHOPHYSIOLOGY In general, the pathophysiologic mechanisms of HF in infants and children are very similar to those in adults. The same compensatory mechanisms are activated in the face of inadequate cardiac output. An acute decrease in blood pressure stimulates stretch receptors and baroreceptors in the aorta and carotid arteries, which in turn stimulate the sympathetic nervous system. With the release of catecholamines and the stimulation of β-receptors, heart rate and the force of myocardial contraction increase. Venous smooth muscle tone also increases, which increases return of venous blood to the heart. Sympathetic stimulation also decreases blood flow to the kidneys, skin, spleen, and extremities so that maximum flow to the brain, heart, and lungs can be maintained. Decreased blood flow to the kidneys causes the release of renin, angiotensin, and aldosterone. If chronic, this cycle results in retention of sodium and fluid by the kidneys, which in turn increases volume in the circulatory system.

These neurohumoral and hemodynamic changes create abnormal ventricular wall stress and cause the myocardium to hypertrophy. The myocardial fibers also stretch to accommodate the increased volume. Hypertrophy and fiber stretch temporarily increase contractility and hence the force of ventricular contraction. These mechanisms eventually fail to maintain cardiac output as HF progresses. A review of the Frank-Starling law of the heart (see Chapter 29) is useful for an understanding of the cycle of compensation and decompensation that occurs in HF.

CLINICAL MANIFESTATIONS Symptomatic HF in children has many causes. It is not usually necessary to determine if it is right- or left-sided HF. When assessing a child with HF, a combination of symptoms generally is present. Pulmonary overcirculation is the predominant cause associated with congenital defects.

HF in infants is manifested as poor feeding and sucking, often leading to FTT. Dyspnea, tachypnea, and diaphoresis may be accompanied by retractions, grunting, and nasal flaring. Wheezing, coughing, and rales are rare even with significant HF. Common skin changes, such as pallor or mottling, are often present.

Hepatomegaly (enlargement of the liver) is atypically attributable to systemic venous congestion. In infants the normal liver is soft, sharp-edged, and palpable 1 to 2 cm below the costal margin. However, the absence of hepatomegaly does not rule out HF.

Periorbital edema and weight gain without caloric increase are uncommon manifestations of right ventricular failure in infants. Peripheral edema, which is a common finding in adults, is rare in infants and young children and more often signifies renal disease rather than cardiac disease. The clinical manifestations of HF are listed in Box 31-2.

EVALUATION AND TREATMENT A thorough physical examination with an emphasis on cardiac and pulmonary findings often will reveal the degree of HF. Plotting the child’s growth (height, weight, head circumference) is an important method for monitoring a child’s health. Infants with HF and pulmonary overcirculation usually have low weight with normal length and head circumference. Failure to thrive (FTT) is usually the result of increased metabolic expenditure relative to caloric intake. An electrocardiogram (ECG) should be performed to determine the presence of dysrhythmias or hypertrophy. A chest radiograph is useful in assessing the presence of cardiomegaly and signs of increased pulmonary circulation.

Treatment is aimed at decreasing cardiac workload and increasing the efficiency of heart function. Medical management initially consists of diuretics, such as furosemide. Depending on the degree of HF, other diuretics can be used in combination with furosemide to counteract potassium losses. Agents that reduce afterload, such as angiotensin-converting enzyme (ACE) inhibitors and beta-blockers, have recently been used to further manage severe HF.^2,3 Caloric supplementation is routinely prescribed.

1. Lesions that cause right ventricular outflow tract obstruction and shunting from the right side of the heart to the left side, as in tetralogy of Fallot (see p. 1223)

2. Defects involving the mixing of saturated and unsaturated blood within the heart chambers, as in a univentricular heart (also referred to as a single ventricle)

3. Defects in children with transposition of the great arteries (see p. 1231), in which two parallel circulations exist and survival depends on the existence of a patent ductus arteriosus or septal defect

Patent Ductus Arteriosus

The patent ductus arteriosus (PDA) is a vessel located between the junction of the main and left pulmonary arteries and the lesser curvature of the descending aorta, usually just distal to the left subclavian artery. During fetal circulation the PDA allows blood to shunt from the PA to the aorta. At birth, once the placenta is removed and the lungs are expanded, the PDA will start to constrict within the first hours of life. Closure of the PDA in full-term infants is usually noted between 15 hours of life and 2 weeks of age.⁷ As an isolated defect, PDA occurs in 5% to 10% of all congenital cardiac defects. In premature infants, studies have shown that the incidence of PDA is as high as 45% in newborns less than 1750 g.⁸

PATHOPHYSIOLOGY Failure of the PDA to close results in persistent patency of the ductus arteriosus. The hemodynamic effects of PDA depend on the size of the lumen and the resistance in the pulmonary and systemic circulations. At birth the pulmonary and systemic vascular resistances are almost equal and are reflected in the PA and aorta, respectively; therefore, shunting is minimal. However, as pulmonary vascular resistance falls, a reversal of fetal shunting occurs. Blood now begins to shunt left to right, from the aorta to the PA. The hemodynamic effect is increased pulmonary blood flow, resulting in increased pulmonary venous return to the LA and LV with increased workload on the left side of the heart. The increased workload is caused by increased pulmonary venous return to the LA and, potentially, an increase in right ventricular pressure if pulmonary vascular changes occur in response to the increased blood flow, leading to an increase in pulmonary vascular pressure (Figure 31-5, C).

CLINICAL MANIFESTATIONS If pulmonary vascular resistance has fallen, infants with PDA will characteristically have a continuous-machinery type murmur heard best at the left upper sternal border throughout systole and diastole. If the PDA is significant, the infant also will have bounding pulses, an active precordium, a thrill upon palpation, and signs and symptoms of pulmonary overcirculation. Infants with a small PDA will usually remain asymptomatic.

EVALUATION AND TREATMENT Chest radiograph will reveal cardiomegaly and increased pulmonary vascular markings. An ECG may demonstrate ventricular enlargement, particularly on the left, but in most cases it is within the normal range. Echocardiography and auscultation confirm the diagnosis based on the characteristic continuous-machinery type of murmur.

PDA closure in asymptomatic children is recommended by 2 years of age because of the risk of subacute bacterial endocarditis. Premature infants who develop respiratory distress are initially given indomethacin, a prostaglandin inhibitor, to close the duct. If this is unsuccessful, surgical ligation and division may be warranted.

Historically the most widely used method for PDA closure is surgical closure involving ligation and division of the ductus with complete closure in nearly 100% of cases. Mortality associated with surgical intervention nears 0%; however, there continues to be some morbidity associated with the approach through a left thoracotomy incision.

Several other options for PDA closure are available depending on the size of the child and the PDA. Many specialists perform interventional closure of the PDA during catheterization. The catheter is advanced into the ductal opening whereby multiple coils or other devices are placed into the lumen that prohibit flow through the duct. The greatest advantages to this procedure are the avoidance of a surgical procedure and thoracotomy pain and a brief observation stay in the hospital.

Another option is closure through video-assisted thoracoscopic surgery. This procedure involves making three small incisions in the left lateral wall, through which a probe is inserted. A clip is then placed around the vessel to occlude it. An advantage of this procedure over surgery is that there is less associated morbidity because of the avoidance of a thoracotomy incision.⁷

Atrial Septal Defect

An atrial septal defect (ASD) is an abnormal communication between the atria (Figure 31-6, A and B). Although it is an isolated lesion, it is the fourth most common congenital heart defect, occurring in 5% to 10% of all congenital cardiac defects. The three major types are an ostium primum defect, an opening found low in the septum that may be associated with AV valve abnormalities, especially mitral insufficiency; an ostium secundum defect, an opening in the center of the septum (this is the most common type of atrial defect); and a sinus venosus defect, an opening that occurs high up in the atrial septum near the superior vena cava and RA junction. This defect is often associated with partial anomalous pulmonary venous connection.

PATHOPHYSIOLOGY Although the pressure difference between the two atria is minimal, the ASD allows blood to be shunted from left to right because of the slightly higher pressure of the left atrial chamber and lower pulmonary vascular resistance as compared with systemic vascular resistance. Right atrial and ventricular enlargement develops as a result of left-to-right shunting. Children with ASD are generally asymptomatic and rarely display signs of pulmonary overcirculation. Moderate to large ASDs allow an increase in pulmonary blood flow and, over time, pulmonary vascular changes can occur that may, although rarely, result in pulmonary hypertension.

CLINICAL MANIFESTATIONS Because most children with ASD are asymptomatic, diagnosis usually is made during a routine physical examination by the auscultation of a crescendo-decrescendo systolic ejection murmur that reflects increased blood flow through the pulmonary valve. The location of the murmur is between the second and third intercostal spaces along the left sternal border. A wide fixed splitting of the second heart sound is also characteristic of ASD, reflecting volume overload to the RV, causing prolonged ejection time and delay of pulmonic valve closure.

EVALUATION AND TREATMENT In most cases an echocardiogram is sufficient to confirm the diagnosis of an ASD. A chest radiograph may reveal cardiomegaly and increased pulmonary vascular markings in an asymptomatic child. An ECG may demonstrate right axis deviation and diastolic overload of the RV manifested as right ventricular hypertrophy.²

ASD closure, generally before the child reaches school age, results in improved health later in life. If left unrepaired, right ventricular compliance decreases with age, and pulmonary hypertension and right ventricular hypertrophy may occur, placing the person at risk for the development of HF, atrial dysrhythmias, or embolic events later in life. Surgical closure is the corrective method of choice and involves a pericardial patch or suture closure of the defect, depending on the size of the opening. Repair is done through a midsternal approach with the use of cardiopulmonary bypass. Minimally invasive techniques are being used on near-adult sized patients. Sinus venosus defects require a slightly different approach that consists of a synthetic patch to close the opening and baffle the anomalous right pulmonary venous drainage to the LA. Operative mortality associated with ASD closure is near 0%, with minimal morbidity.^7,9,10 Device closure performed in the catheterization laboratory is becoming a routine alternative to surgical closure.⁷

Ventricular Septal Defect

A ventricular septal defect (VSD) is an abnormal communication between the ventricles (Figure 31-7, A). VSDs are the most common type of congenital heart lesion and account for 25% to 33% of all congenital heart defects. The four types of VSDs are based on location in the septum. The perimembranous type, which occurs in the outflow tract on the LV immediately below the aortic valve, is the most common type, accounting for up to 80% of all VSDs that require treatment. Muscular VSDs, which occur low in the ventricular septum between the trabeculae, are most likely to close spontaneously and are difficult to close surgically because of their location low in the ventricular apex. Most muscular VSDs are hemodynamically insignificant and require no medical or surgical treatment. Supracristal VSDs occur in the right ventricular outflow tract or infundibulum, below the pulmonary valve. AV canal or inlet VSDs occur posterior and inferior to the membranous system, beneath the septal cusp of the tricuspid valve and inferior to the papillary muscles of the conus.

PATHOPHYSIOLOGY The direction of shunting in a child with a VSD is from the high-pressure left side to the lower-pressure right side. The amount of shunting depends on the size of the defect and the degree of pulmonary vascular resistance. Small VSDs present increased resistance to shunting and limit blood flow through the defect; thus the degree of pulmonary vascular congestion and ventricular chamber enlargement is minimal (Figure 31-7,C).

After 1 to 2 weeks of life, when pulmonary vascular resistance has decreased, moderate-size to large VSDs allow a large amount of shunting from left to right. The shunted blood goes directly out the RV outflow tract and into the PA rather than remaining in the RV cavity (Figure 31-7,D). Therefore, the main PA, LA, and LV all enlarge. LV hypertrophy occurs to effectively pump the additional volume. Pulmonary overcirculation accounts for the symptoms associated with a large VSD in most cases.

Over time the pulmonary bed also undergoes changes because of increased pulmonary blood flow caused by the left-to-right shunting. The smooth muscle layer in the arteriolar walls thickens and proliferation of the intimal layer occurs. The effect of these changes is a decrease in the diameter of the pulmonary vessels, which increases the resistance to blood flow. If the pulmonary vascular resistance is severely increased these changes eventually become irreversible, and pulmonary vascular resistance continues to rise. In some cases it exceeds systemic vascular resistance, causing the shunt through the VSD to reverse direction. Deoxygenated blood now flows into the systemic circulation, and cyanosis occurs, a phenomenon known as Eisenmenger syndrome.

CLINICAL MANIFESTATIONS Clinical manifestations in children with VSDs depend on the age of the child, size of the defect, and level of pulmonary vascular resistance. Newborns with small VSDs are relatively asymptomatic. Initially no murmur is present because the newborn’s high pulmonary vascular resistance causes equalization of the pressures between both ventricles. Once pulmonary vascular resistance has dropped, left-to-right shunting occurs, creating a murmur. Infants with large VSDs display symptoms of HF and poor weight gain. Adults who develop pulmonary vascular obstructive disease as a result of unrepaired VSD will be cyanotic and have clubbing.

On physical examination a loud, harsh, holosystolic murmur and systolic thrill can be detected at the left lower sternal border. The intensity of the murmur reflects the pressure gradient across the VSD. An apical diastolic rumble may be present with a moderate to large defect, reflecting increased flow across the mitral valve.

EVALUATION AND TREATMENT ECG and chest radiographs reflect the amount of shunting through the defect. The ECG of an individual with a small VSD may be normal, whereas an ECG of an individual with a large VSD may reveal biventricular hypertrophy and LA enlargement. Chest roentgenographic findings are significant for cardiomegaly and increased pulmonary vascular markings; again, the severity is directly related to the magnitude of shunting. An echocardiogram identifies the position, size, direction of shunting, and dimensions of the LA, LV, and RV chambers. It also can provide an estimate of PA and RV pressures. Cardiac catheterization may be performed to determine hemodynamics and, in some instances, the location of other VSDs.

Many VSDs spontaneously close during the first year of life.² Infants with symptoms of HF and poor weight gain despite medical management should have their VSD corrected as soon as possible. Left-to-right shunting with a pulmonary flow/systemic flow (Qp:Qs) ratio of greater than 2:1 or evidence of elevated pulmonary vascular resistance are indications for closure. Closure of the VSD at this time is to prevent the development of pulmonary vascular obstructive disease.

Placement of a PA band to decrease the amount of pulmonary blood flow was initially used as a palliative procedure but is now rarely used unless the presence of an additional lesion makes complete repair difficult. Patch closure, using a synthetic material such as Dacron, is accomplished through a sternotomy and with the use of cardiopulmonary bypass. A transatrial approach is preferable to a right ventriculotomy because of the increased incidence of conduction disturbances associated with ventriculotomy. Contraindications for VSD closure include evidence of pulmonary vascular obstructive disease or Eisenmenger syndrome.^2,7 Occlusion devices for VSD closure performed in the cardiac catheterization laboratory are now in early clinical use.^7,11

Atrioventricular Canal Defect

An atrioventricular canal (AVC) defect results from nonfusion of the endocardial cushions during fetal life, yielding abnormalities in both the atrial and ventricular septa and AV valves (Figure 31-8). This defect accounts for as many as 5% of all congenital heart defects, and approximately 30% of AVC defects occur in children with Down syndrome.⁷ The three types of AVC defects are based on the cardiac components involved. Complete AVC (CAVC) defects consist of an inlet VSD, a primum type of ASD, and defects in both the mitral and tricuspid valves. Partial AVC (PAVC) defects consist of a primum type of ASD and a cleft in the septal or anterior leaflet of the mitral valve. Transitional AVC (TAVC) defects involve partial fusion of the endocardial cushions, resulting in variable AV valve abnormalities.⁷

PATHOPHYSIOLOGY Hemodynamic abnormalities seen in AVC defects depend on the components of the lesion and the level of pulmonary vascular resistance. Shunting is minimal during the neonatal period when pulmonary vascular resistance is high. However, once pulmonary vascular resistance drops, left-to-right shunting occurs through the septal defects, resulting in increased pulmonary blood flow and HF.

PAVC defects mimic the hemodynamics of secundum ASD in which the left-to-right shunting through the primum ASD causes RA and RV dilation and increased pulmonary blood flow. The mitral regurgitation that occurs, caused by the cleft mitral valve, is not usually hemodynamically significant.

CAVC defects reflect the hemodynamics of an ASD and a VSD, resulting in biatrial and biventricular enlargement. RA and RV volume overload occurs because of shunting through the primum ASD and tricuspid regurgitation. Likewise, LA and LV volume overload occurs because of shunting through the VSD, increased pulmonary venous return, and mitral regurgitation.

CLINICAL MANIFESTATIONS Children with PAVC defects are generally asymptomatic. Findings on physical examination are similar to those of secundum ASD except for the systolic regurgitant murmur of mitral regurgitation at the apex. At 4 to 12 weeks of age, when pulmonary vascular resistance drops, children with CAVC defects usually begin to show symptoms of HF. Physical findings are similar to those found in individuals with VSDs with the addition of a holosystolic murmur radiating to the back and apex, reflecting mitral regurgitation. A mid-diastolic rumble at the left lower sternal border or apex reflects relative stenosis of the mitral or tricuspid valve from increased flow. Infants with CAVC may have signs of HF and frequent respiratory infections.

EVALUATION AND TREATMENT The ECG generally demonstrates a superior left axis deviation, first-degree AV block, and RV hypertrophy or right bundle branch block. The ECG of CAVC defects also may show LV hypertrophy. Chest radiograph shows cardiomegaly, increased pulmonary vascular markings, and a prominent main PA. Echocardiography allows visualization of the components of the defect, including continuity between the AV valves, their sizes, and chordal attachments. Cardiac catheterization may be electively performed and can confirm the location of septal defects, AV valve abnormalities, degree of left-to-right shunting, and presence of pulmonary hypertension.

Timing of surgical repair depends on the severity of symptoms, degree of shunting, and level of pulmonary vascular resistance. The trend is to perform complete repair between 3 and 6 months of life to avoid the development of pulmonary vascular changes. Surgical repair is performed through a midsternotomy implementing a one- or two-patch repair to close the septal defects and repair the involved AV valves. Mortality has declined below 10% unless the child is a newborn, has severe AV valve incompetence, or has a small LV (unbalanced AV canal). Postoperative complications include heart block, dysrhythmias, or mitral regurgitation requiring further surgical intervention or valve replacement.^7,12

Tetralogy of Fallot

Tetralogy of Fallot (TOF) consists of four defects: a large VSD that is high in the septum, an overriding aorta that straddles the VSD, pulmonary stenosis (PS), and RV hypertrophy (Figure 31-9,A). It is the most common cyanotic congenital heart defect and accounts for 10% of all defects.²

PATHOPHYSIOLOGY TOF develops during two phases of embryologic growth: (1) during the division of the truncus arteriosus by the spiral septum in the third or fourth week of gestation and (2) during the division of the ventricles between the fourth and eighth weeks of gestation. Normally as these events progress, the truncal septum fuses with the bulbar ridges and in turn with the endocardial cushions. The membranous portion of the interventricular septum grows upward to meet the endocardial cushions, and ultimately all of these tissues come together to complete the interventricular septum.

The embryologic error that causes TOF is not known for certain, but two theories have been proposed.¹ The first is that the truncus arteriosus divides unevenly, resulting in great vessels of unequal size. The second theory proposes that infundibular overgrowth in the RV is the major developmental anomaly. Defects in ventricular septation also occur, producing the large VSD, which allows the aorta to override the VSD.

The pathophysiology associated with TOF varies widely, depending primarily on the degree of pulmonary stenosis, the size of the VSD, and the pulmonary and systemic resistance to flow. Because the VSD is usually large, pressures are equal in the RV and LV. Therefore, the major determinant of shunt direction through the VSD is the difference between pulmonary and systemic vascular resistance (see Figure 31-9,C). Infants who have little or no right-to-left shunting are acyanotic and are known as “pink tets.” They may have a net left-to-right shunt similar to a large VSD. If pulmonary vascular resistance is higher than systemic resistance, the shunt is from right to left. Because many factors can alter the balance between pulmonary and systemic resistance, shunt direction is not necessarily constant.

Pulmonary stenosis decreases blood flow to the lungs and, consequently, the amount of oxygenated blood that returns to the left heart. If blood also shunts from right to left through the VSD, deoxygenated blood mixes with the oxygenated blood returning from the lungs. The result is low oxygen saturation (hypoxemia) in the systemic circulation. The body attempts to compensate for chronic hypoxemia by producing more red blood cells (thereby causing polycythemia) and by increasing blood flow to the lungs through collateral bronchial vessels in long-standing cases.

CLINICAL MANIFESTATIONS In cases with decreased pulmonary flow through the right ventricular outflow tract as long as the ductus arteriosus remains open, the newborn’s pulmonary blood flow may be adequate. As the ductus closes, however, cyanosis becomes apparent. Chronic hypoxemia causes clubbing of the fingers and toes (see Chapter 33).

A rare manifestation of TOF is the sudden onset of dyspnea, cyanosis, and restlessness, sometimes called a hypercyanotic spell or a “tet spell,” that generally occurs with crying and exertion. The cause of these episodes is unknown, but it is theorized that the RV outflow tract goes into spasm or the systemic resistance drops suddenly. In either case the relative or actual increase in pulmonary vascular resistance increases the right-to-left shunt and the cyanosis. Hypercyanotic spells are often the event that initiates surgical intervention. If the spells are frequent or do not terminate spontaneously, they are considered a medical-surgical emergency.⁷

Infants with TOF may have difficulty with feeding because the exertion required increases hypoxia, and therefore they experience slow growth and FTT. Most infants with TOF grow normally.

Squatting is a spontaneous compensatory mechanism used by older children to alleviate hypoxic spells. Squatting and its variants increase systemic resistance while decreasing venous return to the heart from the inferior vena cava. The decrease of systemic return makes relatively more oxygenated blood available to the body. The increase of systemic resistance also reverses the shunt through the VSD to a left-to-right shunt, which has the effect of increasing pulmonary blood flow. Through both of these mechanisms, squatting temporarily decreases the degree of hypoxemia. It is uncommon to witness this because most cases are surgically corrected in early infancy.

The typical heart murmur of TOF is a pulmonary systolic ejection murmur caused by the obstruction in the outflow tract, which creates turbulence during systole. More obstruction to flow (e.g., smaller orifice for blood to flow through) produces a louder murmur. This explains why the murmur often disappears during a hypoxic spell, when obstruction increases and pulmonary blood flow decreases to a minimal amount. The second heart sound seems to be single, but in fact it is not. The pulmonary component is very soft and delayed and usually is not heard, although it is present. The enlarged RV may cause the left side of the chest to be more prominent, and a “heave” also may be palpated.

EVALUATION AND TREATMENT The ECG indicates RV hypertrophy. Chest radiographic examination shows that the heart is shaped like a boot (upturned apex because of a small main PA) and that pulmonary vascular markings are decreased. Echocardiograms and angiograms enable the clinician to see the size and position of the VSD, the stenotic pulmonary infundibulum or valve, the smaller-than-normal PA, and the overriding aorta. Measurements made during cardiac catheterization (electively done) demonstrate equal systemic pressure in the RV and LV, decreased pressure in the PA distal to the obstruction, and low oxygen saturation in the aorta if there is right-to-left shunting.

The current practice is to repair TOF before 1 year of life. Triggers for repair include increasing cyanosis and hypercyanotic spells. Palliative procedures include the placement of a pulmonary-to-systemic artery shunt known as the Blalock-Taussig shunt to increase pulmonary blood flow or a modification of the shunt using prosthetic graft material placed from either the subclavian or innominate artery to the PA. These shunts may cause PA distortion but may be necessary in a very small symptomatic child. Corrective repair involves patch closure of the VSD, resection of infundibular or valvular stenosis, and patch augmentation of the RV outflow tract. The procedure is done through a median sternotomy on cardiopulmonary bypass. The operative mortality is less than 5%. Complications include dysrhythmias and occasionally heart block.¹³

Tricuspid Atresia

Tricuspid atresia consists of an imperforate tricuspid valve, resulting in no communication between the RA and RV (Figure 31-10). This defect accounts for 2% to 3% of congenital heart defects and is the third most common cyanotic heart defect. Tricuspid atresia is a combination of defects, including the imperforate tricuspid valve as well as a septal defect, hypoplastic or absent RV, enlarged mitral valve and LV, and varying degrees of pulmonic stenosis. Tricuspid atresia also may be associated with transposition of the great vessels. The most common type of tricuspid atresia involves a hypoplastic RA with decreased pulmonary blood flow, ASD, VSD, and normally related great vessels.²

PATHOPHYSIOLOGY Systemic blood returns through the superior and inferior venae cavae to the RA. Venous return flows through the ASD into the LA, mixing with blood returning from the pulmonary circulation. The blood then enters the LV. Most of this blood goes out into the systemic circulation through the aorta, but varying amounts pass through the VSD into the hypoplastic RV and to the lungs. Pulmonary circulation depends on the presence of a VSD and the presence of a functioning RV of reasonable capacity. If the RV is absent, the pulmonary valve is usually imperforate as well. If this is the case, a PDA is necessary to ensure that some blood flows into the pulmonary circulation.⁷

Pulmonary circulation also depends on the relationship between pulmonary and systemic vascular resistance. As long as pulmonary resistance is lower than systemic resistance, blood flows through the VSD from left to right, feeding the pulmonary circulation. If pulmonary resistance rises above systemic resistance, pulmonary blood flow will be significantly diminished.

CLINICAL MANIFESTATIONS Some degree of central cyanosis is common in tricuspid atresia, depending on the amount of pulmonary blood flow. Growth failure also is common. Children experience exertional dyspnea, tachypnea, and hypoxemia. Long-term effects of hypoxia are polycythemia and clubbing. These children also may display hypercyanotic spells. Hepatomegaly may be present if the ASD is restrictive or CHF occurs as a result of increased pulmonary blood flow.

The murmur heard with tricuspid atresia may have several components. The VSD causes a systolic regurgitant murmur; the larger the VSD, the softer and shorter the murmur is likely to be. A narrowly split second heart sound caused by decreased pulmonary blood flow may be present, or the second heart sound may be single if there is pulmonary atresia.

EVALUATION AND TREATMENT Chest radiographic examination shows a heart size that is normal or slightly increased. ECG usually shows RA, LA, and LV hypertrophy with left axis deviation. Echocardiography and cardiac catheterization, if performed, depict left-to-right shunting at the ventricular level, inability of blood flow to enter the RV, and the presence of associated defects.

Newborns with ductal dependent pulmonary blood flow are immediately given prostaglandins to maintain adequate pulmonary perfusion. Initial surgical intervention involves the placement of a Blalock-Taussig shunt (or its modification). If the ASD is restrictive, a Rashkind procedure (balloon atrial septostomy) may be performed during catheterization. Children who experience increased pulmonary blood flow may require the placement of a PA band. Corrective repair involves closing the septal defects, taking down the previous shunts or band, and connecting the superior and inferior venae cavae to the PA to separate the pulmonary systemic circulation (Fontan procedure and its modifications). Postoperative complications include pleural effusions, elevated pulmonary vascular resistance, LV dysfunction, and dysrhythmias.¹³

Coarctation of the Aorta

Coarctation of the aorta (COA) is a narrowing of the lumen of the aorta that impedes blood flow. This defect accounts for 8% to 10% of all congenital heart defects. COA is almost always in a juxtaductal position, although it can occur anywhere between the origin of the aortic arch and the bifurcation of the aorta in the lower abdomen. About 50% of individuals with COA have a bicuspid aortic valve (Figure 31-11).²

PATHOPHYSIOLOGY COA may develop because of an abnormal contractile ductal tissue that constricts at the time of ductal closure. COA causes a condition in which there are higher pressures proximal to the site of stenosis and lower pressures distal to the site. In preductal COA the RV acts as a systemic pump, sending unoxygenated blood through the ductus into the descending aorta below the coarctation (Figure 31-12). In postductal COA the RV cannot pump enough blood through the ductus to the descending aorta because of pressure caused by the narrowed aorta. Systolic pressures increases in the ascending aorta and LV and decreases in the descending aorta beyond the COA (Figure 31-13). Long-standing COA, collateral circulation, which involves small arteries arising from the subclavian arteries, joins intercostal arteries that flow into the descending aorta. These collateral vessels bypass the COA and supply blood to the lower extremities. The direction of shunting through the ductus, if present, depends on the pressure difference between the PA and aorta and the location of the ductus. When blood pressure is greater in the aorta than in the PA, blood flow through the ductus will be left to right toward the lungs, resulting in increased pulmonary venous return to the left side of the heart. This may place an additional strain on the LA and LV, leading to increased volume and work. With time LV hypertrophy develops because of increased afterload and obstruction to flow caused by the coarctation. HF also may develop.

CLINICAL MANIFESTATIONS Clinical manifestations vary depending on the severity of the coarctation and age of presentation. In newborns the onset of symptoms depends on the timing of ductal closure after a fall in pulmonary vascular resistance, the location of the COA, and the presence of associated defects. The newborn usually presents with CHF symptoms. Once the ductus closes, these infants will deteriorate rapidly from the development of hypotension, acidosis, and shock. Older children may not be diagnosed until hypertension is noted. Hypertension is noted in the upper extremities with decreased or absent pulses in the lower extremities. Children may have cool mottled skin and occasionally leg cramps during exercise. A systolic ejection murmur, heard best at the left interscapular area, is caused by rapid blood flow through the narrowed area.

EVALUATION AND TREATMENT A chest radiograph shows an enlarged heart with congested lung fields in newborns. Rib notching between the fourth and eighth ribs may be seen in children older than 5 years, reflecting erosion of the ribs from enlarged collateral vessels from the ascending aorta to the descending aorta, bypassing the coarctation. An ECG may be normal or reveal LV hypertrophy. An echocardiogram will confirm the diagnosis and rule out other intracardiac defects. Cardiac catheterization and/or magnetic resonance imaging (MRI) is performed only if the echocardiogram is inconclusive.

The first step in treatment of the symptomatic infant is stabilization, which may require prostaglandin administration, mechanical ventilation, and inotropic support to maintain adequate cardiac output. Once this is achieved, surgical intervention is indicated. Surgical repair for infants younger than 1 year consists of either a subclavian flap aortoplasty technique to enlarge the constricted area or resection with end-to-end anastomosis of the arch segments. Depending on the arch morphology, a modification of this procedure enlarges the aorta beyond the area of constriction. For children older than 1 year, surgical repair consists of resection with end-to-end anastomosis.¹³ Cardiopulmonary bypass is not required because of the extracardiac nature of the lesion, and the approach is accomplished through a left thoracotomy.

Postoperative complications include recoarctation and paradoxical postoperative hypertension. Residual permanent hypertension requiring continued medical therapy is related to age at repair; therefore, surgical intervention is recommended at the time of diagnosis. Operative mortality for infants is less than 5%, and for children older than 1 year, it is less than 1%. Balloon dilation angioplasty in newborns has been successfully performed. However, aortic aneurysm formation and restenosis have been noted; therefore, surgical repair remains the correction of choice for the newborn.^13–16

Aortic Stenosis

Aortic stenosis (AS) is a narrowing of the aortic outflow tract (Figure 31-14). The lesion accounts for 5% of all congenital heart defects.² Valvular stenosis is caused by malformation or fusion of the cusps. It is the most common type of AS, tends to be progressive, and, in rare cases, can lead to sudden death as a result of low cardiac output or myocardial ischemia. For children with mild AS, no exercise restrictions may be needed. For those with moderate AS, some exercise limitations may be advised. Severe AS is an indication for limiting exercise until the repair is accomplished. Less common forms of AS are subvalvular stenosis caused by a constricting fibrous ring below the valve and supravalvular stenosis that occurs above the valve.⁷

PATHOPHYSIOLOGY Obstruction to blood flowing out of the aorta causes an increased workload on the LV, resulting in left ventricle hypertrophy (LVH). LV failure may develop, leading to an increase in LA pressure and a backup in the system, eventually resulting in pulmonary vascular congestion and pulmonary arterial hypertension. LVH can decrease coronary artery perfusion, resulting in myocardial ischemia and it can alter the LV papillary muscle, causing mitral insufficiency.

CLINICAL MANIFESTATIONS Most children with mild to moderate AS are asymptomatic. Signs of exercise intolerance may not appear until preadolescence. Syncopal episodes, epigastric pain, and exertional chest pain may occur in more severe forms of AS. A systolic ejection murmur at the right upper sternal border that transmits to the neck and left lower sternal border is produced by blood flow through the stenotic area. An ejection click may be heard with valvular AS. Severe forms of AS, especially critical AS in the newborn, result in shock and require immediate intervention.

EVALUATION AND TREATMENT Diagnosis may be made based on previous medical history and physical findings. Chest radiographic examination may reveal a dilated ascending aorta or LV enlargement. Increased pulmonary vascular markings may be seen in severe forms. In mild cases the ECG is normal. LVH with strain pattern (features of LVH with inverted T waves in the lateral precordial leads) may be seen in severe forms. An echocardiogram may reveal a thickened and poorly functioning LV with abnormal closure of the damaged bicuspid aortic valve. Cardiac catheterization may be performed to augment echocardiographic data or intervention and can determine the location, cause, and severity of obstruction.

The presence of ST-segment changes on ECG, severe CHF, and evidence of discrete severe stenosis at the aortic outflow tract are indications for intervention. Balloon aortic valvuloplasty is a palliative procedure performed for valvular AS; however, it is associated with complications, including aortic insufficiency and dysrhythmia.¹⁷ Aortic valvotomy, under inflow occlusion or cardiopulmonary bypass, is performed for valvular AS. Operative mortality remains high in infants (up to 20%), although older children have a mortality close to 0%.¹³ As many as 25% of individuals require a second surgery within 10 years for restenosis, at which time valve replacement may be the procedure of choice.

Subvalvular AS and supravalvular AS require surgical repair involving excision of the area causing the constriction. For subvalvular AS involving a small LV outflow tract and aortic annulus, a Konno procedure may be done to enlarge the LV outflow tract and aortic annulus with a patch.^13,18

Pulmonary Stenosis

Pulmonary stenosis (PS) is the narrowing of the pulmonary outflow tract. This may be in the form of abnormal thickening of the valve leaflets or narrowing of the arterial (supravalvular) or ventricular (subvalvular) side of the valve (Figure 31-15,A). Pulmonary atresia is the severe form of PS and involves complete fusion of the commissures, allowing no blood flow out of the RV to the PA. PS accounts for 5% to 8% of all congenital heart defects.²

PATHOPHYSIOLOGY PS creates resistance to blood flow from the RV to the PA. The narrowed orifice (valve) produces increased resistance (afterload) to ejection. In order for the RV to maintain adequate cardiac output, the myocardium hypertrophies. If the RV outflow tract obstruction is severe, blood may back up into the RA, causing dilation. This may result in reopening of the foramen ovale with resultant unoxygenated blood shunting to the LA, causing cyanosis (see Figure 31-15,B).

CLINICAL MANIFESTATIONS Clinical manifestations depend on the severity of PS. A systolic ejection murmur at the left upper sternal border reflects obstruction to flow through the narrowed pulmonary valve. A systolic ejection click is present with valvular stenosis at the upper left sternal border. A thrill also may be palpated at the upper left sternal border. Children with moderate PS may have exertional dyspnea and fatigability because of the inability of the body to increase pulmonary blood flow to meet demands for increased cardiac output. Severe PS will produce cyanosis and HF.

EVALUATION AND TREATMENT A chest radiograph shows a normal-size heart with a prominent main PA caused by poststenotic dilation. An ECG is usually normal but may reveal right axis deviation and RV hypertrophy with moderate PS. Echocardiography confirms the diagnosis and detects associated defects. Cardiac catheterization, if performed for interventional purposes, further demonstrates PA anatomy.

Mild to moderate PS will not likely require intervention but should be observed closely with prophylaxis for subacute bacterial endocarditis. Most mild PS is not progressive. Treatment is indicated when a significant pressure gradient is detected across the RV outflow tract.

Critical (severe) PS must be addressed immediately. The treatment of choice is balloon angioplasty. This procedure is considered highly effective in decreasing the pressure gradient across the pulmonic valve and is noted to have few associated complications.¹⁹ Surgical correction involves a pulmonary valvotomy incising the fused commissures. Operative mortality is less than 1%.² Both valvotomy and balloon angioplasty may result in some pulmonary valve incompetence, and long-term follow-up may reveal the need for further intervention.

Hypoplastic Left Heart Syndrome

Hypoplastic left heart syndrome (HLHS) refers to the abnormal development of the left-sided cardiac structures, resulting in obstruction to blood flow from the LV outflow tract. HLHS involves underdevelopment of the LV, aorta, and aortic arch, as well as mitral atresia or stenosis (Figure 31-16). Therefore, infants with HLHS must have a well-functioning RV and the presence of a PDA and atrial septal communication for survival. HLHS accounts for 1% of all congenital heart defects and is considered the most complex congenital defect.³

PATHOPHYSIOLOGY Because of the high pressures caused by LV outflow tract obstruction, saturated blood enters the LA and mixes with desaturated blood in the RA through an atrial septal communication. Blood flow follows the normal pathways through the right side of the heart. Exiting the PA, the mixed-saturation blood flows through the ductus and to the descending aorta. The amount of blood flow that travels to the pulmonary and systemic circulations depends on vascular resistance in the respective systems. Retrograde blood flow through the hypoplastic ascending aorta provides coronary and cerebral blood flow.

CLINICAL MANIFESTATIONS Newborns with HLHS generally are born full term and initially appear healthy. As the ductus closes, systemic perfusion is decreased, resulting in hypoxemia, acidosis, and shock. Usually no heart murmur is detected. The second heart sound is loud and single because of aortic atresia.

EVALUATION AND TREATMENT A chest radiograph shows cardiomegaly and increased pulmonary venous congestion. ECG shows RV hypertrophy and diminished left-sided forces. Echocardiography reveals the components of the defect with a diminutive LV cavity, hypoplastic aortic valve and arch, and hypoplastic or absent mitral valve with an enlarged RV cavity. Interventional cardiac catheterization with balloon septostomy may be necessary if the atrial communication is inadequate. This procedure is very high risk.⁷

Prostaglandin infusion to maintain patency of the ductus arteriosus is essential for newborn infant survival. Immediate correction of acidosis, inotropic support for adequate cardiac output, and ventilatory manipulation to balance systemic and pulmonary blood flow prevent further deterioration and achieve stabilization.

Surgical intervention includes a three-stage approach that classically begins with a Norwood procedure. The Norwood procedure consists of an atrial septectomy, placement of a pulmonary-to-systemic artery shunt to maintain adequate pulmonary blood flow, creation of a permanent communication between the RV and aorta, and patch augmentation of the hypoplastic aorta. The Sano modification utilizes a graft between the RV and the PA to provide stable pulmonary blood flow rather than a pulmonary to systemic arterial shunt.²⁰ Postoperative complications include imbalance of systemic and pulmonary blood flow, leading to inadequate cardiac output and persistent HF. In most centers, survival after the initial procedure is now averaging greater than 70%.

The second stage is the bidirectional Glenn procedure, which is performed between 2 and 9 months of age, depending on the child’s clinical status, pulmonary vascular resistance, and ventricular function. This involves joining the superior vena cava to the PA and take-down of the shunt to the lungs. Complications include superior vena cava obstruction, pleural effusion, and low cardiac output.

The third stage is the Fontan procedure, described earlier, which separates the systemic from the pulmonary circulation. Timing for surgical repair depends on the child’s ventricular function, presence of AV valve regurgitation, and pulmonary vascular resistance. Most surgeons perform the Fontan procedure when the child is approximately 2 to 4 years of age.^7,13

Cardiac transplant also may be an option for these newborns. Most surgical centers offer the three-staged palliative surgeries as an initial approach, with an option for cardiac transplantation later in life.^7,21

Transposition of the Great Arteries

Transposition of the great arteries (TGA) refers to a condition in which the aorta arises from the RV and the PA from the LV (Figure 31-17,A). The result is two separate, parallel circuits in which unoxygenated blood circulates continuously through the systemic circulation and oxygenated blood circulates repeatedly through the pulmonary circulation. This condition is incompatible with extrauterine life unless a communication exists between the two circuits to provide the necessary oxygen to the body. Communication is accomplished through mixing of pulmonary and systemic circulations through a PDA, ASD, or VSD (see Figure 31-17, B). Dextro-transposition of the great arteries (d-TGA) is the most common cyanotic congenital heart defect and accounts for 10% of all congenital heart defects; “dextro” refers to the aorta remaining to the right of the PA.²

Two factors allow newborns with complete transposition to survive long enough to be treated. First, blood from the two closed systems can mix through the ductus arteriosus for a short time after birth if pulmonary vascular resistance remains high. Some mixing also may occur through the foramen ovale. If the child has a VSD, mixing occurs through that opening as well.

PATHOPHYSIOLOGY It is not known precisely which embryologic events lead to transposition, but researchers have proposed that the fault lies in the development of conal tissue in the fibrous skeleton of the heart.¹ The conus is a segment of muscle that separates the AV (tricuspid and mitral) valves from the semilunar (aortic and pulmonic) valves. (The fibrous skeleton and heart valves are described and illustrated in Chapter 29; see Figure 29-4.) The interventricular septum is intact in about 60% of cases of transposition; a VSD is present in the remaining 40%. PS is associated with the transposition in about 4% to 6% of children with intact septa and in 28% to 31% of children with VSDs.^2,7

The discussion that follows is limited to the pathophysiology of complete transposition with an intact interventricular septum.

CLINICAL MANIFESTATIONS The degree of mixing permitted by fetal structures determines the type and severity of clinical manifestations. Cyanosis may be mild shortly after birth and worsen during the first day because of functional closure of the ductus arteriosus. Low oxygen levels in the blood (hypoxemia) cause metabolic acidosis, tachycardia, and tachypnea. The presence of a PDA or large ASD allows for more mixing and results in only mild cyanosis, but the infant may develop CHF.

The first heart sound is normal, and the second sound may be heard as a single sound even though both the aortic and pulmonic valves are functioning. The single S₂ may occur because transposition places the aortic valve closer to the chest wall than the pulmonic valve. No murmur is noted with transposition of the great arteries with an intact ventricular septum.

EVALUATION AND TREATMENT On chest radiograph the heart has a characteristic shape—like an egg on its side—and pulmonary vascular markings are increased. The heart may be enlarged if the infant is a few weeks old and has a VSD. ECG findings reveal a right-axis deviation and some RV hypertrophy. Echocardiography confirms the diagnosis of transposition of the great arteries. Cardiac catheterization may be necessary to define the coronary anatomy; measure ventricular ratios; and, if need be, perform a balloon septostomy. Recent reports have raised concerns about the relationship between balloon septostomy and stroke; therefore, balloon septostomy is done only if the situation is critical.²²

Surgical repair during the newborn period involves the arterial switch operation that moves the great arteries. The coronary arteries are removed from the aorta before the arterial switch is performed and reimplanted without torsion or kinking into the aorta. This establishes normal blood flow with the LV as the systemic pump. Results are approaching 100% survival. Complications include narrowing at the sites of the great artery anastomoses and, rarely, coronary insufficiency.^7,22

Mustard and Senning operations (the creation of an intra-atrial tunnel to baffle the systemic venous blood flow to the mitral valve and the pulmonary venous blood flow to the tricuspid valve) are no longer the procedures of choice because the RV must perform as the systemic pump. Long-term follow-up of children with Mustard and Senning operations revealed significant rates of RV failure and dysrhythmias.⁷

The Rastelli procedure is used with children with transposition, VSD, and severe PS. This procedure involves closing the VSD with a baffle by rerouting LV blood through the VSD to the aorta. The pulmonary valve is closed, and an RV-to-PA prosthetic or homograft valve conduit is placed. This procedure requires prosthetic conduit replacement as the child grows and is associated with ventricular failure and dysrhythmias in the postoperative period.

Total Anomalous Pulmonary Venous Connection

Total anomalous pulmonary venous connection (TAPVC), or total anomalous pulmonary venous return, occurs when the pulmonary veins abnormally connect to the right side of the heart either directly or through one or more systemic veins that drain into the RA (Figure 31-18). An ASD generally is present also. This defect is extremely rare, accounting for only 1% of all congenital heart defects. The four types of TAPVC are based on the site of drainage. Supracardiac TAPVCs are the most common form (50%) and drain to the superior vena cava through the vertical or innominate vein. Cardiac TAPVCs (20%) drain directly into the RA or through the coronary sinus. Infracardiac TAPVCs (20%) traverse the diaphragm and drain into the portal or hepatic vein or the inferior vena cava. Mixed TAPVCs (10%) are a combination of the various types. Partial anomalous venous connection is a condition in which only one or a few of the pulmonary veins, usually the right-sided veins, drain into the RA or one of its tributaries.²

PATHOPHYSIOLOGY Physiologically TAPVC can be differentiated into two groups: nonobstructive and obstructive, depending on the absence or presence of obstruction to pulmonary venous drainage. The hemodynamics of the nonobstructive group involve the RA receiving the oxygenated blood that would normally flow into the LA. The amount of blood shunted into the LA versus the volume entering the RV depends on the size of the ASD and compliance of the RV. Therefore, if the ASD is restrictive and RV compliance approaches normal, more blood will enter the RV than the LA, resulting in RA and RV enlargement, as well as increased pulmonary blood flow. This causes increased pulmonary venous blood return and larger amounts of saturated blood. If the ASD is unrestrictive and the RV does not thin out to increase compliance, the majority of mixed saturated blood is shunted from the higher pressure RA to the LA.

The hemodynamics of obstructed TAPVC cause pulmonary venous hypertension because of resistance caused by the obstruction resulting in an elevation in pulmonary vascular and RV pressures. Pulmonary edema occurs from hydrostatic capillary pressure exceeding the osmotic pressure of the blood and eventually contributing to the development of HF. This group has a strong association with the infracardiac type of TAPVC and is a surgical emergency.

CLINICAL MANIFESTATIONS The predominant clinical manifestation in infants with TAPVC is cyanosis caused by mixture of oxygenated and deoxygenated blood entering the systemic circulation. The degree of cyanosis is inversely related to the amount of pulmonary blood flow. Children with unobstructed TAPVC may be asymptomatic until pulmonary vascular resistance drops, at which time pulmonary blood flow will increase, resulting in signs of pulmonary overcirculation, particularly growth retardation and frequent pulmonary infections, in addition to mild cyanosis. Obstructed TAPVC results in cyanosis and rapid deterioration necessitating immediate surgical correction, or death will occur.

Physical examination may reveal a systolic murmur at the left upper sternal border and a mid-diastolic murmur at the left lower sternal border. A murmur may be absent in obstructed TAPVC. A characteristic quadruple rhythm, consisting of S₁, widely split S₂, and S₃ or S₄, or a gallop rhythm also is present.

EVALUATION AND TREATMENT The ECG shows a right-axis deviation, RV hypertrophy, and occasionally RA hypertrophy. The chest radiograph of unobstructed TAPVC reveals cardiomegaly, increased pulmonary vascular markings, and a snowman or figure-eight appearance in the supracardiac type. A chest roentgenogram of obstructed TAPVC shows a normal-size heart and a ground-glass appearance of the lung fields, reflecting pulmonary venous congestion or edema. The echocardiogram reveals the abnormal pulmonary venous connections.

Surgical repair varies with the type of TAPVC and whether the defect is obstructed or unobstructed. Obstructed lesions are repaired at the time of diagnosis, whereas the unobstructed type generally is repaired during infancy. The procedure is performed on cardiopulmonary bypass and involves anastomosis of the common pulmonary vein to the LA; ligating the common pulmonary vein; and closing the ASD, as in the supracardiac and infracardiac types. Repair of the supracardiac type involves baffling the pulmonary venous drainage to the LA. This repair has the highest success rate because of the low technical difficulty, whereas infracardiac repair is associated with a high mortality (up to 25%) and morbidity. Potential complications include reobstruction; atrial dysrhythmias, including sick sinus syndrome; PA hypertension; and LV dysfunction.¹³

Truncus Arteriosus

Truncus arteriosus is the failure of the large embryonic artery, the truncus arteriosus, to divide into the PA and the aorta. This results in a single vessel arising from both ventricles, providing blood flow to the pulmonary and systemic circulations (Figure 31-19,A). This common trunk straddles the VSD (always present) and has a single valve with three or four leaflets, which may result in stenosis or regurgitation, or both. The incidence is 2% of all congenital heart defects, and a right aortic arch is present 50% of the time. There are four types of truncus arteriosus. Type I is the most common (60%) and involves the main PA arising from the truncus and then dividing into the right and left PAs. Type II is less common (20%) and involves the PAs arising from the posterior aspect of the truncus. Type III is the least common (10%) and involves the PAs arising from the lateral aspect of the truncus. Type IV, also known as pseudotruncus, is now considered a severe form of TOF with the bronchial arteries arising from the descending aorta to supply the lungs.²

PATHOPHYSIOLOGY Blood flow from the RV and LV is pumped into the main truncus, resulting in mixing of the pulmonary and systemic circulations (see Figure 31-19,B). The differential flow out to either the pulmonary bed or the systemic circulation depends on the pulmonary and systemic vascular resistances. Generally the pulmonary vascular resistance is less than the systemic vascular resistance, resulting in the majority of blood flow traveling to the lungs. This may be altered, however, because of PS, small pulmonary arteries, or increased pulmonary vascular resistance. Pulmonary vascular disease develops early with this defect because of increased pulmonary blood flow.

CLINICAL MANIFESTATIONS Physical findings depend on the amount of pulmonary blood flow and the presence of other cardiac anomalies. If PS is present, a newborn will present with cyanosis, caused by already elevated pulmonary vascular resistance, but no HF. Conversely, if PS is not present, the newborn initially will have mild to moderate cyanosis that worsens with activity. Once pulmonary vascular resistance drops, the pulmonary bed will receive preferential flow and the infant will have signs of HF. A wide pulse pressure with bounding pulses also may be present, caused by increased pulmonary blood flow. A harsh systolic regurgitant murmur is present along the left sternal border as a result of the VSD, and a systolic click at the apex and left upper sternal border may be present, reflecting opening of the truncal valve. An apical rumble with or without a gallop rhythm also may be present because of increased pulmonary blood flow. If truncal valve insufficiency exists, an early diastolic, high-pitched decrescendo murmur may be present.

EVALUATION AND TREATMENT An ECG generally reveals biventricular hypertrophy and occasionally LA enlargement. A chest radiograph reveals cardiomegaly with biventricular and LA enlargement, as well as increased pulmonary vascular markings. When PS is present, the heart size is normal and the pulmonary vascular markings are decreased. Echocardiography and cardiac catheterization, if performed, determine the type of truncal defect, competency of the truncal valve, and differential blood flow.

Surgical repair in early infancy is recommended to prevent the sequelae of severe HF and pulmonary vascular disease. The definitive repair consists of a modified Rastelli procedure involving VSD patch closure to divert the blood flow from the LV outflow tract into the truncus. The pulmonary arteries are excised from the aorta and connected to the RV through a tissue homograft—namely, aortic and PA segments or cadaver tissue that is specially preserved. Synthetic conduits may be used but tend to calcify and develop narrowing within the lumen, leading to obstruction and the need for early replacement. Mortality varies depending on the type of truncal anomaly (20% to 50%). Postoperative complications include HF, residual VSD, dysrhythmias, and pulmonary hypertension. The RV to PA homograft requires replacement because it becomes inadequate for somatic growth.¹³

Stage I (days 1 to 12): Small capillaries, arterioles, and venules become inflamed, as does the heart itself.

Stage II (days 13 to 25): Inflammation spreads to larger vessels, and aneurysms of the coronary arteries develop.

Stage III (days 26 to 40): Medium-size arteries begin the granulation process, causing coronary artery thickening; inflammation resolves in the microcirculation; and there is increased formation of thrombi.

Stage IV (day 41 and beyond): Vessels develop scarring, intimal thickening, calcification, and stenosis of coronary arteries.

KEY TERMS

Aortic stenosis (AS) 1228

Atrial septal defect (ASD) 1219

Atrioventricular canal (AVC) defect 1222

Bulbus cordis 1211

Coarctation of the aorta (COA) 1226

Complete AVC (CAVC) defect 1222

Conus 1231

Cyanosis 1217

Ductus arteriosus 1211

Eisenmenger syndrome 1217

Endocardial cushion 1209

Foramen ovale 1211

Heart failure (HF) 1216

Hypoplastic left heart syndrome (HLHS) 1230

Kawasaki disease 1234

Ligamentum venosum 1212

Ostium primum 1210

Ostium secundum 1210

Partial AVC (PAVC) defect 1222

Patent ductus arteriosus (PDA) 1218

Pulmonary atresia 1229

Pulmonary stenosis (PS) 1229

Septum primum 1210

Septum secundum 1210

Systemic hypertension 1235

Tetralogy of Fallot (TOF) 1223

Total anomalous pulmonary venous connection (TAPVC) 1232

Transitional AVC (TAVC) defect 1222

Transposition of the great arteries (TGA) 1231

Tricuspid atresia 1225

Truncus arteriosus 1233

Ventricular septal defect (VSD) 1220