319 CHRONIC HYPOXEMIA IN THE NEWBORN LAMB

Abstract

The adaptive mechanisms by which oxygen delivery is maintained during chronic hypoxemia, and the costs of such adaptations, are not well defined. In the newborn lamb, we have induced chronic hypoxemia by placing an inflatable balloon around the pulmonary artery after performing an atrial septostomy. Three days after surgery, the balloon was gradually inflated in 11 lambs to create a right-to-left atrial shunt such that arterial oxygen saturation was between 60 and 75%. Twice weekly studies were performed on these lambs and weekly studies on 12 normoxemic lambs. Growth decreased sharply (47 ± 123 vs 221 ± 82 g/day) at the onset of hypoxemia and remained low, although oxygen consumption followed the normal gradual decline. Systemic blood flow decreased at balloon inflation but quickly returned to normal. Mixed venous and coronary sinus saturations were low, but could decrease further with shivering. Systemic oxygen delivery returned to normal as hemoglobin concentration rose (from 9.4 ± 1.5 to 12.5 ± 2.2 g/dl). P50 did not change. Four of the 11 hypoxemic lambs died during the 2 week study period. These data show that systemic oxygen delivery is maintained primarily by rising hemoglobin in the chronically hypoxemic newborn. There is a signal to decrease growth apparently to maintain some reserve. The limited reserve is shown by a high mortality rate despite normal oxygen delivery at rest.