Table 4.18. Characteristics in large VSD with pulmonary vascular obstructive disease.

Hypertrophy of the smooth muscle lining of the pulmonary arterioles produces increase in PVR; hence increase RV afterload progressively reduces L ---> R shunt, eventually reversing it to R --->L; mild cyanosis.
Systolic murmur disappears and S₂ becomes loud and narrowly split.
Left heart size diminishes, leaving isolated RV enlargement.
X-ray shows diminished peripheral lung vascularity.
Pulmonary hypertension seen by catheterization, permitting calculation of blood flows and resistance.
In the premature baby the muscularis of the pulmonary arterioles is not completely developed, so a patent ductus or VSD produces excessive pulmonary flow and pulmonary congestion.
In the full term infant mature muscularis provides high Pulmonary Vascular Resistance at birth, limiting lung flow, but also obscuring murmurs due to septa defects. Regression of the muscularis at 3-8 weeks gives higher lung flow and reveals signs of L ---> R shunting for the first time.