Porth's Essentials of Pathophysiology, 4e

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Circulatory Function

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higher than normal, and thus the size of the left-to-right shunt may initially be limited. As the pulmonary vascu- lar resistance falls during the first few weeks after birth, the size of the left-to right shunt increases. Eventually, a large left-to-right shunt develops, and clinical symptoms (e.g., tachypnea; diaphoresis, especially with feeding; and failure to thrive) become apparent. In most cases pulmonary vascular pressure is only slightly elevated during infancy, and the major contributor to pulmonary hypertension is an increase in pulmonary blood flow. However, in some infants with a large septal defect, pulmonary arteriolar thickness never decreases. With continued exposure to high pulmonary blood flow, pul- monary vascular disease develops. In untreated patients, the pulmonary vascular resistance can eventually exceed the systemic resistance. In this case, a reversal of shunt flow occurs and the child demonstrates progressive cyanosis as deoxygenated blood moves from the right to the left side of the heart. These symptoms, coupled with irreversible changes in the pulmonary vasculature, represent an end-stage form of congenital heart disease called Eisenmenger syndrome , a condition that is quite rare today due to advances in diagnosis and treatment. The treatment of a ventricular septal defect depends on the size of the defect, accompanying hemodynamic derangements, and symptomatology. Children with small or medium-sized defects may be followed without intervention if they remain free from signs of congestive heart failure or pulmonary hypertension. Ventricular defects do not increase in size, and some spontaneously close over time. 22 Prophylactic antibiotic therapy is given during periods of increased risk for bacteremia. The man- agement of infants with large ventricular defects aims to control heart failure and prevent the development of pulmonary vascular disease. Symptomatic infants may require feeding supplements or tube feeding to promote growth and development. In the symptomatic infant in whom complete repair cannot be achieved because of size or other complicating lesions, a palliative procedure may be performed to reduce symptoms. Placement of a synthetic band around the main pulmonary artery (pul- monary artery banding) can reduce pulmonary blood flow until complete repair can be accomplished. Surgical closure of the defect is completed by placement of a syn- thetic or autologous patch effectively to close the shunt across the ventricular septum. These procedures are typically done electively in the infant or young child and are associated with low morbidity and mortality rates. Endocardial Cushion Defects. The endocardial cush- ions form the AV canals, the upper part of the ventricu- lar septum, and the lower part of the atrial septum. The endocardial cushions surround this canal and contribute tissue to the lower part of the atrial septum, the upper part of the ventricular septum, the septal leaflet of the tri- cuspid valve, and the anterior leaflet of the mitral valve. 83 Any flaw in the development of these tissues results in an endocardial cushion defect. Endocardial cushion defects are responsible for approximately 2% of all congenital heart defects. As many as 30% of children with Down syndrome have endocardial cushion defects. 66

Because the endocardial cushions contribute to mul- tiple aspects of heart development, several variations of endocardial cushion defects are possible. The defect may be described as partial or complete. The anatomy of the AV valve determines the classification. In partial AV canal defects, the two AV valve rings are complete and separate. The most common type of partial AV canal defect is an ostium primum defect, often associ- ated with a cleft in the mitral valve. In a complete canal defect, there is a common AV valve orifice along with defects in both the atrial and ventricular septal tissue (see Fig. 19-21E). Other cardiac defects may be associ- ated with endocardial cushion defects and most com- monly include cardiac malposition defects and tetralogy of Fallot. 83 Physiologically, endocardial cushion defects result in abnormalities similar to those described for atrial or ventricular septal defects. The direction and magni- tude of a shunt in a child with an endocardial cushion defect are determined by the combination of defects and the child’s pulmonary and systemic vascular resistance. The hemodynamic effects of an isolated ostium primum defect are those of the previously described atrial septal defect. These children are largely asymptomatic during childhood. With a complete AV canal defect, pulmonary blood flow is increased after the pulmonary vascular resistance falls because of left-to-right shunting across both the ventricular and atrial septal defects. Children with complete defects often have effort intolerance, easy fatigability, failure to thrive, recurrent infections, and other signs of congestive heart failure, particularly when the shunt is large. Pulmonary hypertension and increased pulmonary vascular resistance result if the lesion is left untreated. The timing of treatment for endocardial cushion defects is determined by the severity of the defect and symptoms. With an ostium primum defect, surgical repair usually is planned on an elective basis before the child reaches school age. Corrective surgery is required for all complete AV canal defects. Infants with severe symptoms may require a palliative procedure in which the main pulmonary artery is banded to reduce pulmo- nary blood flow. This typically improves the infant’s ability to grow and develop until a complete repair can be performed. 83 Pulmonary Stenosis. Obstruction of blood flow from the right ventricle to the pulmonary circulation is termed pulmonary stenosis. The obstruction can occur as an isolated valvular lesion, within the right ventricular chamber, in the pulmonary arteries, or as a combination of stenoses in multiple areas. It is a relatively common defect, estimated to account for approximately 10% of all congenital cardiac disease, and is often associated with other abnormalities. 84 Pulmonary valvular defect, the most common type of disorder, usually produces some impairment of pulmo- nary blood flow and increases the workload imposed on the right side of the heart (see Fig. 19-21D). Most children with pulmonic valve stenosis have mild ste- nosis that does not increase in severity. These children