A shunt is an abnormal communication between chambers or blood vessels. Depending on pressure relationships, shunts permit the flow of blood from the left to the right side of the heart (or vice versa).
With right-to-left shunt, a dusky blueness of the skin (cyanosis) results because the pulmonary circulation is bypassed and poorly oxygenated blood collected from the venous system enters the systemic arterial circulation.
By contrast, left-to-right shunts increase blood flow into the pulmonary circulation and are not associated (at least initially) with cyanosis. However, they expose the low-pressure, low-resistance pulmonary circulation to high pressures and increased volumes; these alterations lead to adaptive changes that increase lung vascular resistance to protect the pulmonary bed, resulting in right ventricular hypertrophy and—eventually rightsided—failure. With time, increased pulmonary resistance also can cause shunt reversal (right to left) and late-onset cyanosis.
• Some congenital anomalies obstruct vascular flow by narrowing the chambers, valves, or major blood vessels. A malformation characterized by complete obstruction is called an atresia. In some disorders (e.g., tetralogy of Fallot), an obstruction (pulmonary stenosis) also is associated with a shunt (right-to-left, through a VSD).
The altered hemodynamics of congenital heart disease usually lead to chamber dilation or wall hypertrophy. However, some defects result in a reduced muscle mass or chamber size; this is called hypoplasia if it occurs before birth and atrophy if it develops postnatally.
Disorders associated with Left-to-right shunts are the most common types of congenital cardiac malformations. They include atrial septal defects (ASDs), ventricular septal defects (VSDs), and patent ductus arteriosus (PDA) (
Fig.
11.2 ). ASDs typically increase only right ventricular and pulmonary outflow volumes, while VSDs and PDAs cause both increased pulmonary blood flow and pressure. Manifestations of these shunts range from completely asymptomatic to fulminant heart failure.
Cyanosis is not an early feature of these defects. However, as discussed earlier, prolonged left-to-right shunting may eventually give rise to pulmonary hypertension and right-to-left shunting of unoxygenated blood into the systemic circulation, a change marked by the appearance of cyanosis (Eisenmenger syndrome). Once significant pulmonary hypertension develops, the structural defects of congenital heart disease are considered irreversible. This is the rationale for early intervention, in most cases surgically.
Fig. 11.2 Commoncongenitalcausesofleft-to-rightshunts(arrows indicatedirectionofbloodflow).(A)Atrialseptaldefect(ASD).(B)Ventricularseptaldefect(VSD).(C)Patentductusarteriosus(PDA).Ao, Aorta;LA, leftatrium;LV, leftventricle;PT, pulmonarytrunk;RA, rightatrium;RV, rightventricle.
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