Pulmonary hypertension is 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 congestion.
In contrast, in the full term infant mature muscularis provides high pulmonary vascular resistance at birth, limiting lung flow, but also obscuring murmurs due to septal defects. Regression of the muscularis at 3-8 weeks of age gives higher lung flow and reveals signs of left to right shunting for the first time.
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