195 MYOCARDIAL BLOOD FLOW (Q) AND O₂ CONSUMPTION (VO₂) DURING ACUTE HYPOXEMIA IN NEWBORN LAMBS

Abstract

We previously showed that myocardial Q and left ventricular (LV) VO₂ at rest are higher in neonatal lambs than in fetal or adult sheep. To determine whether the neonate has similar reserve to further increase myocardial Q and O₂ supply during hypoxemia, we measured myocardial Q and LV VO₂ in 5 lambs (4-5 days) three days after thoracotomy, before and during hypoxia (FIO₂=0.09). Myocardial Q was measured by the microsphere method; LV VO₂ was calculated from Q and the O₂ content difference between aortic and coronary sinus (CS) blood (LV(A-V)). LV work was estimated from the rate-pressure product. Hypoxia produced a 51% decrease in arterial O₂ content, a 16% increase in cardiac output, and a 46% increase in LV work. Although CS O₂ content fell and myocardial O₂ extraction coefficient increased (.62 vs .66), LV(A-V) decreased 47%. LV VO₂ increased 27% because of a 139% increase in LV Q (183 vs 438 ml/100g). Right ventricular Q (126 vs 451 ml/100g) increased even more, probably because of the increase in pulmonary arterial systolic pressure (24 to 41 mmHg). Despite the increase in heart rate there was only a small decrease in inner:outer flow ratio (1.14 to 0.97 in LV, and 1.22 to 1.1 in right ventricle). Our studies show that despite high resting values there is considerable reserve for increasing flow to the neonatal myocardium. This reserve allows the LV to increase its O₂ supply and VO₂ during severe hypoxemia and therefore to increase its work and output without subendocardial ischemia and probably without anaerobic metabolism.

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