ABSTRACT. Myocardial oxygen demand may be intration. FFA and ketoacid uptake are insufficient to fuel creased in chronically hypoxemic lambs because of their LV oxidative metabolism. (Pediatr Res 34: 471-477, 1993) increased heart rate. Therefore, we determined whether left ventricular (LV) oxygen supply, oxygen uptake and Abbreviations oxygen demand were proportionally increased in 6-wk-old lambs, after 4 wk of hypoxemia (n = IS), as compared LVAVD, left ventricular arteriovenous concentration difwith control lambs (n = 14). In addition, we determined ference whether LV glucose, pyruvate, lactate, FFA and ketoacids OER, oxygen extraction ratio uptake were altered in hypoxemic lambs, because of alterations in arterial glucose, pyruvate and lactate concentrations, that may occur in hypoxemia. Hypoxemia was induced by the combination of an atrial septa1 defect and pulmonary stenosis. Arterial oxygen saturation was deLeft ventricular oxygen supply is closely matched to left vencreased in hypoxemic lambs (67 8 versus 91 2 3%, < tricular oxygen uptake under normal conditions, because the 0.001), ~b concentration was increased, so that arterial oxygen extraction reserve of the heart is limited. Adjustments in oxygen concentration was similar in both groups of lambs. left ventricular blood flow are important to maintain oxygen Myocardial mass (61 +. 13 versus 44 + 9 g, < O.OO1) and supply during acute alterations in arterial oxygen saturation or