Effects of hypothermia and rewarming on the neonatal circulation.

Thirteen unanesthetized lambs, aged 1-3 days, were cooled from 40 degrees to 30 degrees C and then rewarmed. During cooling, heart rate dropped from 196 +/- 47 to 140 +/- 42 beats per minute, a 29% change; cardiac output, femoral arterial and pulse pressures did not change significantly; but inferior vena cava pressure (IVC) increased from 5 to 9 mm of mercury, and left ventricular end diastolic pressure (LVEDP) increased from 8 to 25 mm/Hg. Mean pulmonary arterial pressure increased from 29 to 40 mm/Hg. Metabolic acidosis and hypoxia developed during the cooling. There were marked changes in the distribution of cardiac output; the proportion of cardiac output increased by 450% to brown fat, 66% to the skeletal muscles, and 30% to the right ventricular myocardium and atria. No significant change in the distribution to the left ventricule and septum was observed. Distribution to the other organs decreased markedly. After rewarming, all the previous parameters returned to normal except CVP and LVEDP, which remained elevated.