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1397 HEMODYNAMIC RESPONSE TO SINGLE AND REPEATED EPISODES OF APNEA IN NEWBORN (NB) PIGLETS

This study investigates the ability of hemodynamic mechanisms to respond to multiple episodes of apnea in 5 NB piglets. Following catheterization and tracheostomy, piglets were paralyzed and mechanically ventilated (PO2 = 60-70 torr, pH = 735-7.45, PCO2 = 30-35). After baseline measurements of blood gases, pH, Hct. BP, HR, and organ blood flow by microspheres, apnea to the point of bracycardia (HR < 80) was induced by disconnecting the ventilator; 30-40 sec later, mechanical ventilation was resumed until HR and BP returned to baseline. A total of 7 apneas were induced over 1½ hrs. Microspheres were injected during the first apnea (PO2 = 16±13, pH = 7.34±.06, PCO2 = 42±5), recovery from first apnea (Rec 1), and ½ hr after the 7th apnea (Rec 2). Tissue blood flows (ml/min/100g, mean ±S.E. and % change):The rapidity of hemodynamic response to apnea, redistribution of blood flow occurring 130-180 sec after cessation of ventilation, 30-40 sec after onset of bradycardia indicates oxygen sensitivity of newborn vasculature. Kidney and skeletal muscle flows return to near baseline during Rec 1 and Rec 2, while heart and brain flows remain elevated; lack of significant difference between Rec 1 and Rec 2 despite 6 intervening apneas suggests no cumulative effect in the newborn piglet of brief, repeated hypoxemic insults.