1567 RESPONSE OF THE NEWBORN TO INDUCED HYPOTENSION

Abstract

The response of vasoactive mediators in the neonate to a 20% fall in mean arterial blood pressure (MBP) was studied in six chronically catheterized lambs 4-7 days old, following infusion of sodium nitroprusside (SNP). No significant change in pHa, PaCO2, plasma sodium or osmolality was observed following the infusion of SNP. However, the fall in MBP from 65.3±3.90 to 53.8±2.82 mmHg (Mean ±S.E.) at the end of sixty minutes infusion was associated with the following changes in vasopressin (VP), renin activity (PRA) and catecholamine concentrations (CA) (*p<0.01): These levels remained significantly elevated for at least thirty minutes after the end of infusion while MBP rose above control values and was still elevated (72.8±2.95 mmHg) after sixty minutes. These experiments demonstrate that as in the adult, the newborn sheep responds promptly to a hypotensive episode by secreting vasoactive mediators which may play an important role in blood pressure homeostasis. The prolonged hypertension that followed suggests that as in the fetus, blood pressure is not well regulated due to either slower metabolism or immaturity of feedback mechanisms controlling the release of vasoactive mediators.