Fig. 11.3 ). The ventricular septum normally is formed by a muscular ridge that grows upward from the heart apex fusing with a thinner membranous partition that grows downward from the endocardial cushions. The basal (membranous) region is the last part of the septum to develop and is the site of approximately 90% of VSDs. Most VSDs close spontaneously in childhood, but only 20% to 30% of VSDs occur in isolation; the remainder are associated with other cardiac malformations.
Fig.11.3 ),rangingfromminutedefectsinthemembranousseptumtolargedefectsinvolvingvirtuallytheentireinterventricularwall.Indefectsassociatedwithasignificantleft-to-rightshunt,therightventricleishypertrophiedandoftendilated.Thediameterofthepulmonaryarteryisincreased,owingtotheincreasedrightventricularoutputandhigherrightsidedpressures.Vascularchangestypicalofpulmonaryhypertensionarecommon(Chapter13).
Fig. 11.3 Ventricularseptaldefectofthemembranoustype(arrow).(Cour-tesy of William D. Edwards, MD, Mayo Clinic, Rochester, Minnesota.) http://ebooksmedicine.net
Small VSDs may be asymptomatic; half of those in the muscular portion of the septum close spontaneously during infancy or childhood. Larger defects, however, result in chronic left-to-right shunting, often complicated by pulmonary hypertension and CHF. Progressive pulmonary hypertension, with resultant reversal of the shunt and cyanosis, occurs earlier and more frequently with VSDs than with ASDs. Early surgical correction is therefore indicated for such lesions. Small-or medium-sized defects that produce jet lesions in the right ventricle cause endothelial damage and increase the risk for infective endocarditis.
The ductus arteriosus arises from the left pulmonary artery and joins the aorta just distal to the origin of the left subclavian artery. During intrauterine life, it permits blood to flow from the pulmonary artery to the aorta, bypassing the unoxygenated lungs. Within 1 to 2 days of birth in healthy term infants, the ductus constricts and closes; these changes occur in response to increased arterial oxygenation, decreased pulmonary vascular resistance, and declining local levels of prostaglandin E2. Complete obliteration occurs within the first few months of extrauterine life, leaving only a strand of residual fibrous tissue known as the ligamentum arteriosum. Ductal closure can be delayed (or even absent) in infants with hypoxia (related to respiratory distress or heart disease). PDAs account for about 7% of congenital heart lesions (see
Table 11.1
Fig. 11.2 ); the great majority of these (90%) are isolated defects.
PDAs are high-pressure left-to-right shunts that produce harsh, “machinery-like” murmurs. A small PDA generally causes no symptoms, although larger defects eventually can lead to Eisenmenger syndrome with cyanosis and congestive heart failure. High-pressure shunts also predispose patients to developing infective endocarditis. While isolated PDAs should be closed as early in life as is feasible, preservation of ductal patency (by administering prostaglandin E) can be lifesaving when a PDA is the only means to sustain systemic or pulmonary blood flow (e.g., in infants with aortic or pulmonic atresia).
Cardiac malformations associated with right-to-left shunts are distinguished by early cyanosis. This occurs because poorly oxygenated blood from the right side of the heart flows directly into the arterial circulation. Two of the most important conditions associated with cyanotic congenital heart disease are tetralogy of Fallot and transposition of the great vessels (