Porth's Essentials of Pathophysiology, 4e

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Disorders of Cardiac Function

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are largely asymptomatic and are diagnosed by the presence of a systolic murmur. Moderate or greater ste- nosis has been shown to progress over time, particularly before 12 years of age, so these children require careful follow-up. Critical pulmonary stenosis in the neonate is evidenced by cyanosis due to right-to-left atrial-level shunting and right ventricular hypertension. These infants require prostaglandin E 1 to maintain circulation to the lungs through the ductus arteriosus. 84 Pulmonary valvotomy is the treatment of choice for all valvular defects with pressure gradients from the right ventricle to the pulmonary circulation greater than 30 mmHg. Transcatheter balloon valvuloplasty has been quite successful in this lesion. Stenosis in the peripheral pulmonary arteries can also be effectively treated with balloon angioplasty, with or without stent placement. 84 Tetralogy of Fallot. Tetralogy of Fallot is the most common cyanotic congenital heart defect, accounting for approximately 5% to 7% of all congenital heart defects. 85 As the name implies, tetralogy of Fallot consists of four associated defects: (1) a ventricular septal defect involving the membranous septum and the anterior por- tion of the muscular septum; (2) dextroposition (shifting to the right of the aorta) so that it overrides the right ventricle and is in communication with the septal defect; (3) obstruction or narrowing of the pulmonary outflow channel, including pulmonic valve stenosis, a decrease in the size of the pulmonary trunk, or both; and (4) hyper- trophy of the right ventricle because of the increased work required to pump blood through the obstructed pulmonary channels 86 (see Fig. 19-21C). Variations of the defect can include complete atresia of the pulmonary valve or absence of pulmonary valve tissue altogether. Most children with tetralogy of Fallot display some degree of cyanosis that is caused by a right-to-left shunt across the ventricular septal defect. The degree of cyano- sis is determined by the restriction of blood flow into the pulmonary bed. Right ventricular outflow obstruction causes deoxygenated blood from the right ventricle to shunt across the ventricular septal defect and be ejected into the systemic circulation. The degree of obstruc- tion may be dynamic and can increase during periods of stress, causing hypercyanotic attacks (“tet spells”). These spells typically occur in the morning during cry- ing, feeding, or defecating. These activities increase the infant’s oxygen requirements. Crying and defecating may further increase pulmonary vascular resistance, thereby increasing right-to-left shunting and decreasing pulmonary blood flow. With the hypercyanotic spell, the infant becomes acutely cyanotic, hyperpneic, irritable, and diaphoretic. Later in the spell, the infant becomes limp and may lose consciousness. Placing the infant in the knee–chest position increases systemic vascular resistance, which increases pulmonary blood flow and decreases right-to-left shunting. During a hypercyanotic spell, toddlers and older children may spontaneously assume the squatting position, which functions like the knee–chest position to relieve the spell. Total surgical correction is required for all children with tetralogy of Fallot. Early definitive repair in infancy

is currently advocated in most centers experienced in intracardiac surgery in infants. 86 Transposition of the Great Arteries. In complete transposition of the great arteries, the aorta arises from the right ventricle, and the pulmonary artery arises from the left ventricle (see Fig. 19-21F). The defect is more common in infants whose mothers have diabetes and is two to three times more common in boys. 87 Cyanosis is the most common presenting symptom, resulting from an anomaly that allows the systemic venous return to be circulated through the right heart and ejected into the aorta, and the pulmonary venous return to be recirculated to the lungs through the left ventricle and main pulmonary artery. In infants born with this defect, survival depends on communication between the right and left sides of the heart in the form of a patent ductus arteriosus or septal defect. Ventricular septal defects are present in 50% of infants with trans- position of the great arteries at birth and may allow effective mixing of blood. Prostaglandin E 1 should be administered to neonates when this lesion is suspected in an effort to maintain the patency of the ductus arteri- osus. Balloon atrial septostomy may be done to increase the blood flow between the two sides of the heart. In this procedure, a balloon-tipped catheter is inserted into the heart through the vena cava and then passed through the foramen ovale into the left atrium. The balloon is then inflated and pulled back through the foramen ovale, enlarging the opening as it goes. Corrective surgery is essential for long-term survival. An arterial switch procedure, which corrects the relation of the systemic and pulmonary blood flows, is the cur- rent procedure of choice, and has survival rates greater than 90%. 87 This procedure is preferably performed in the first 2 to 3 weeks of life, before the postnatal reduc- tion in pulmonary vascular resistance occurs. The cor- onary arteries are moved to the left-sided great artery and any ventricular septal defects are closed during the same operation. Complications of the arterial switch procedure may include coronary insufficiency, supra- valvar pulmonary stenosis, neoaortic regurgitation, and rhythm abnormalities. 87 Coarctation of the Aorta. Coarctation of the aorta consists of a localized narrowing of the aorta. It can occur proximal to (preductal) (see Fig. 19-21H), distal to (postductal), or opposite of (juxtaductal) the entry of the ductus arteriosus. Approximately 98% of coarcta- tions are juxtaductal. The defect is frequently associated with other congenital cardiac lesions, most commonly bicuspid aortic valve, and occurs in approximately 10% of subjects with Turner syndrome, suggesting a genetic linkage 88 (see Chapter 6). The classic sign of coarctation of the aorta is a dis- parity in pulsations and blood pressures in the arms and legs. The femoral, popliteal, and dorsalis pedis pulsa- tions are weak or delayed compared with the bound- ing pulses of the arms and carotid vessels. Normally, the systolic blood pressure in the legs obtained by the cuff method is 10 to 20 mm Hg higher than in the arms.