Like ASD's, VSD's (Figure 4.05) may be of several different types (Table 4.15): Most common is the variety due to incomplete closure of the membranous part of the interventricular septum. VSD's may occur lower in the ventricle, in the muscular part of the septum, e.g. the "Swiss cheese" defect. Finally, the septum may completely fail to develop, resulting in a 3-chambered heart, or cortriloculare biatrium.
VSD's may be small and restrictive or as large as the mitral valve annulus, thus unrestrictive (Table 4.16, Table 4.17, Figure 4.06). The volume of blood shunted across a VSD is proportional to defect size. VSD's are noisy and can be easily heard with the stethoscope, the pansystolic murmur resulting from continuous, turbulent blood flow from left to right. This is evidenced by an increased right ventricle pressure and elevated blood O2 content ("O2 step-up"). Pulmonary blood flow is increased and the increase in right ventricle size and lung vascularity can be seen on X-ray.
The left atrium and left ventricle are usually dilated due to the increased flow. ECG, echo, and X-ray are used to determine the degree of enlargement. VSD's are usually initially acyanotic, but if they are large and unrestrictive, flow may eventually reverse due to right ventricle hypertrophy, producing cyanosis.
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