Porth's Essentials of Pathophysiology, 4e

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Circulatory Function

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the capillaries (equal to 5 g of deoxygenated hemo- globin). Defects that result in a right-to-left shunting or obstruction of pulmonary blood flow are catego- rized as cyanotic disorders and those involving left- to-right shunting are usually categorized as acyanotic disorders. Of the congenital defects discussed in this chapter, patent ductus arteriosus, atrial and ventricu- lar septal defects, endocardial cushion defects, pulmo- nary valve stenosis, and coarctation of the aorta are considered acyanotic; tetralogy of Fallot, transposition of the great vessels, and single-ventricle anatomy are considered cyanotic defects. A right-to-left shunt results in deoxygenated blood moving from the right side of the heart to the left side and then being ejected into the systemic circulation. With a left-to-right shunt, oxygenated blood intended for ejection into the systemic circulation is recirculated through the right side of the heart and back through the lungs. This increased volume distends the right side of the heart and pulmonary circulation and increases the workload placed on the right ventricle. 68 Alterations in Pulmonary Blood Flow Many of the complications of congenital heart disor- ders result from a decrease or an increase in pulmonary blood flow. Defects that reduce pulmonary blood flow (e.g., pulmonary stenosis) typically cause symptoms of fatigue, dyspnea, and failure to thrive. In contrast to the arterioles in the systemic circulation, the arterioles in the pulmonary circulation are normally thin-walled vessels that can accommodate the various levels of stroke vol- ume that are ejected from the right heart. The thinning of the pulmonary vessels occurs during the 1st weeks after birth, during which the vessel media thin and pul- monary vascular resistance decreases. In a term infant who has a congenital heart defect that produces mark- edly increased pulmonary blood flow (e.g., ventricular septal defect), the increased flow stimulates pulmonary vasoconstriction and delays or reduces the normal invo- lutional thinning of the small pulmonary arterioles. In most cases pulmonary vascular resistance is only slightly elevated during early infancy, and the major contribution to pulmonary hypertension is the increased blood flow. However, in some infants with a large right-to-left shunt, the pulmonary vascular resistance never decreases. Congenital heart defects that persistently increase pulmonary blood flow or pulmonary vascular resistance have the potential of causing pulmonary hypertension and producing irreversible pathologic changes in the pulmonary vasculature. When shunting of systemic blood flow into the pulmonary circulation threatens per- manent injury to the pulmonary vessels, a surgical pro- cedure should be done to reduce the flow temporarily or permanently. Pulmonary artery banding consists of placing a constrictive band around the main pulmonary artery, thereby increasing resistance to outflow from the right ventricle. The banding technique is a temporary measure to alleviate symptoms and protect the pulmo- nary vasculature in anticipation of later surgical repair of the defect.

adulthood and consider having children of their own. Recent evidence suggests that the genetic contribution to congenital heart disease has been underestimated in the past. 65,66 Some heart defects, such as aortic stenosis, atrial septal defect of the secundum type, pulmonary valve stenosis, tetralogy of Fallot, and certain ventricu- lar septal defects, have a stronger familial predisposition than others. Chromosomal abnormalities are also asso- ciated with congenital heart defects, as evidenced by the observation that as many as 30% of children with con- genital heart disease have an associated chromosomal abnormality. Heart disease is found in nearly 100% of children with trisomy 18, 50% of those with trisomy 21, and 35% of those with Turner syndrome. 66 Congenital heart diseases are commonly classified according to their anatomic site (atrial septal or ventric- ular septal defects), the hemodynamic alterations caused by the anatomic defects (left-to-right or right-to-left shunts), and their effect on pulmonary blood flow and tissue oxygenation (cyanotic or noncyanotic defects). Shunting Shunting of blood refers to the diversion of blood flow from one system to the other—from the arterial to the venous system (i.e., left-to-right shunt) or from the venous to the arterial system (i.e., right-to-left shunt). 64 The shunting of blood in congenital heart defects is determined by the presence of an abnormal opening between the right and left circulations and the degree of resistance to flow through the opening. The shunting of blood can affect both the oxygen content of the blood and the volume of blood being delivered to the vessels in the pulmonary circulation. The direction of shunting (right-to left or left-to- right) is largely determined by the vascular resistance of the systemic and pulmonary circulations. Due to the high pulmonary vascular resistance in the neonate, atrial and ventricular septal defects usually do not produce a significant shunt during the 1st weeks of life. As the pul- monary vascular smooth muscle regresses in the neo- nate, the resistance in the pulmonary circulation falls below that of the systemic circulation, causing a left-to- right shunt in uncomplicated atrial or ventricular septal defects. In more complicated ventricular septal defects, increased resistance to outflow may affect the pattern of shunting. For example, defects that increase resistance to aortic outflow (e.g., aortic valve stenosis, coarctation of the aorta, hypoplastic left heart syndrome) increase left-to-right shunting, whereas defects that obstruct pul- monary outflow (e.g., pulmonary valve stenosis, tetral- ogy of Fallot) increase right-to-left shunting. Crying, defecating, or even the stress of feeding may increase pulmonary vascular resistance and cause an increase in right-to-left shunting in infants with septal defects. Cyanosis, a bluish color of the skin most notable in the nail beds and mucous membranes, develops when sufficient deoxygenated blood from the right side of the heart mixes with oxygenated blood in the left side of the heart. 67 Abnormal color becomes obvi- ous when the oxygen saturation falls below 80% in