Porth's Essentials of Pathophysiology, 4e

477

Disorders of Cardiac Function

C h a p t e r 1 9

of the great arteries). Intravenous infusion of pros- taglandin E 1 (PGE 1 ) has proved extremely effective in maintaining ductal patency or reopening the ductus in newborns. Today, this therapy is routinely administered to newborns with suspected congenital heart defects until they can be transported to a specialized center where a diagnosis can be confirmed. 80 Atrial Septal Defects. In atrial septal defects, an open- ing in the atrial septum persists as a result of improper septal formation 81 (see Fig. 19-21A). Partitioning of the atria takes place during the 4th and 5th weeks of devel- opment and occurs in two stages, beginning with the formation of a thin, crescent-shaped membrane called the septum primum followed by the development of a second membrane called the septum secundum. As the septum secundum develops, it gradually overlaps an opening in the upper part of the septum primum, form- ing an oval opening with a flap-type valve called the foramen ovale (Fig. 19-22). The foramen ovale, which closes shortly after birth, allows blood from the umbili- cal vein to pass directly into the left heart, bypassing the lungs. Atrial septal defects may be single or multiple and vary from a small, asymptomatic opening to a large, symptomatic opening. The type of defect is determined by its position and may include an abnormal opening in the septum primum (ostium primum defects), the sep- tum secundum (ostium secundum defects), or a patent foramen ovale. An ostium secundum atrial septal defect in the region of the foramen ovale is the most common defect. The defect may be single or multiple (fenestrated atrial septum). Most atrial septal defects are small and young children with these defects are often asymptom- atic, with the defects discovered inadvertently during a routine physical examination at a few years of age. 81 In the case of an isolated septal defect large enough to

allow shunting, the flow of blood usually is from the left side to the right side of the heart because of the more compliant right ventricle and because the pulmonary vascular resistance is lower than the systemic vascular resistance. This produces right ventricular volume over- load and increased pulmonary blood flow. In most cases there is a moderate shunt resulting in dilation of the right heart chambers and overperfusion of the pulmo- nary circulation. Children with undiagnosed atrial defects are at risk for pulmonary vascular disease, although this is a rare occurrence before 20 years of age. Rarely, infants with a large shunt may develop congestive heart failure and failure to thrive, prompting early intervention to close the defect. 81 Adolescents and young adults may experi- ence atrial flutter or atrial fibrillation and palpitations because of atrial dilatation. Larger symptomatic defects are usually treated surgically or by transcatheter device closure. Smaller defects may be observed for spontane- ous closure in the young child. Ventricular Septal Defects. A ventricular septal defect is an opening in the ventricular septum that results from an incomplete separation of the ventricles during early fetal development (see Fig. 19-21B). These defects may be single or multiple and may occur in any position along the ventricular septum. Ventricular septal defects are the most common form of congenital heart defect, accounting for 25% to 30% of congenital heart disor- ders. 82 A ventricular septal defect may be the only car- diac defect, or it may occur in association with multiple cardiac anomalies. The ventricular septum originates from two sources: the interventricular groove of the folded tubular heart that gives rise to the muscular part of the septum, and the endocardial cushions that extend to form the mem- branous portion of the septum. The upper membranous portion of the septum is the last area to close, typically by the 7th week of gestation, and it is here that most defects occur. Depending on the size of the opening and the pulmo- nary vascular resistance, the signs and symptoms of a ventricular septal defect may range from an asymptom- atic murmur to congestive heart failure. 82 If the defect is small, it allows a small shunt and small increases in pulmonary blood flow. These defects produce few symp- toms, and approximately one third close spontaneously. With medium-sized defects, a larger shunt occurs, pro- ducing a larger increase in pulmonary blood flow. Most of the children with such defects are asymptomatic and have a low risk for development of pulmonary vascular disease. In children with large nonrestrictive defects, right and left ventricular pressure is equalized and the degree of shunting is determined by the ratio of pulmonary to sys- temic vascular resistance. Pulmonary vascular resistance normally falls rapidly after birth, owing to the onset of ventilation and subsequent release of hypoxic pulmo- nary vasoconstriction. This process is often delayed in infants with large ventricular septal defects. As a result, the pulmonary vascular resistance remains somewhat

Septum secundum

Foramen ovale

Septum primum

Endocardial cushions

Left atrioventricular canal

Right atrioventricular canal

Interventricular septum

FIGURE 19-22. Development of the endocardial cushions, right and left atrioventricular canals, interventricular septum, and septum primum and septum secundum of the foramen ovale. Note that blood from the right atrium flows through the foramen ovale to the left atrium.