A large ASD gives a common filling pressure for both ventricles (Table 4.14). As the right ventricular wall thins post-birth, a left to right shunt develops, producing dilatation of the right atrium, right ventricle, and pulmonary vessels. The ASD itself is silent; what is heard is the increased flow through the tricuspid orifice producing a mid-diastolic rumble and a systolic ejection murmur due to increased flow through the pulmonic valve.
ASD's can be detected by echocardiography, by ECG, and by the dilatation of the right atrium, right ventricle, and pulmonary vessels on X-ray. An O2 step-up occurs at the level of the right atrium.
Note that neither ASD's nor VSD's can be heard immediately post-birth due to the nearly equal right and left heart pressures. That is why the infant must return to the pediatrician several weeks after birth so heart sounds can be taken. Long continued high pressure and flow applied to the pulmonary vasculature causes development of pulmonary vascular obstructive disease. Beyond a certain stage (approximately 18-24 mos) the condition becomes irreversible and in most cases, eventually lethal (Figure 4.04).
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