127 CHRONIC HYPOXEMIA IN THE 5-WEEK-OLD LAMB

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The mechanisms involved in the adaptation to chronic hypoxemia, so as to maintain normal systemic oxygen delivery (SOD), are not quite clear. Therefore we studied these mechanisms by inducing chronic hypoxemia in 6 newborn lambs. We placed an inflatable balloon around the pulmonary artery after atrial septostomy, and inserted catheters. On days 3-4 after surgery the balloon was gradually inflated. In another 6 lambs only catheters were placed. At 5 weeks of age the lambs were studied. The hypoxemic lambs had a lower aortic and mixed venous oxygen saturation (60±10 (SD) vs 93±2, and 34±7 vs 58±2%, resp., p<0.001), and higher hemoglobin concentration (132±15 vs 101±10 g·1−1, p<0.002). Their P50 as well as their systemic blood flow were not significantly different. The heart rate of the hypoxemic lambs, however, was higher (205±22 vs 153±42, p<0.05). Although SOD in the hypoxemic lambs was lower than in the control lambs, this difference was not statistically significant (17.0±3.4 vs 21.2±5.9 ml.min−1kg−1). The mean weight gain of the hypoxemic lambs was lower (90±39 vs 153±34 g·day−1, p<0.02), while their oxygen consumption was the same as in the control lambs (7.0±2.2 vs 7.2±1.5 ml·min−1kg−1). These data show that to adapt to chronic hypoxemia the lamb increases its hemoglobin concentration to improve arterial oxygen content, and its heart rate to maintain cardiac output. We speculate that the costs of chronic hypoxemia, such as increased cardiac work, are probably effected at the expense of growth.