Changes in work rate to oxygen consumption ratio during hypoxia and ischemia in immature and mature rabbit hearts

This study was designed to evaluate the relative response of myocardial efficiency to reduced oxygen supply (hypoxia and ischemia) in immature and mature isolated rabbit hearts. Hearts were subjected to either 15 min of hypoxia (60% or 30% O 2) or reductions in coronary flow to 75%, 50%, 25%, and 15% of basal flow followed by 12 min of total global ischemia and 15 min of reperfusion. In order to examine changes in cardiac efficiency, we utilized the ratio of isovolumic contractile function (rate-pressure product) to myocardial oxygen consumption ( RPP MVO 2 ). Under basal conditions, immature hearts displayed lower aortic pressure. RPP, coronary resistance and RPP MVO 2 . Moderate hypoxia (60% O 2) resulted in similar reductions in RPP and MVO 2 in both age groups, with RPP MVO 2 remaining unchanged. During severe hypoxia, RPP MVO 2 increased significantly in mature hearts but not in immature hearts ( P < 0.05). Underperfusion produced greater reductions in RPP and heart rate, whereas reperfusion after ischemia resulted in greater recovery of RPP, dP dt and MVO 2 in immature compared to mature hearts. When oxygen supply was limited by reductions in coronary perfusion. RPP MVO 2 tended to increase in mature hearts, whereas the ratio declined significantly in immature hearts. These data demonstrate that, in this model, a reduction in oxygen supply by hypoxia or hypoperfusion decreases efficiency in immature hearts, but increases efficiency in mature hearts under the same conditions.