Response to Low Cardiac Output: Developmental Differences in Metabolism during Oxygen Deficit and Recovery in Lambs

Abstract

ABSTRACT: When cardiac output is critically lowered, whole body O₂ consumption decreases and an O₂ deficit accumulates. With restoration of cardiac output, an excess O₂ consumption is expected in order to “repay” some or all of the O₂ deficit. We tested the hypothesis that young lambs, with higher resting O₂ consumption, cardiac output and growth rates than older lambs, would repay less of their O₂ deficit because they have a higher proportion of nonessential metabolism. We reduced cardiac output acutely by inflating a Foley catheter in the right atrium of spontaneously breathing, sedated lambs at ages 2 (n = 5) and 8 (n = 5) wk. Each lamb was studied with low cardiac output periods of 30 and 60 min on different days. Aortic and pulmonary artery blood pressures, gases, and O₂ saturations, venous Hb, and arterial lactate concentrations were measured every 10 min during baseline, low output, and 60 min of recovery. Oxygen consumption was monitored continuously. Oxygen deficit and excess O₂ of recovery were calculated from the time integral of the difference between O₂ consumption at baseline and during the respective study period. The average percent decreases in cardiac output (66.4 and 64.6%) and O₂ consumption (38.1 and 35.1%) were similar in the 2- and 8-wk groups, respectively. There was no consistent relation between O₂ deficit and O₂ repayment in either age group. However, on average the 2-wk group repaid a significantly lower percentage (3 ± 16%, mean ± SEM) than 8-wk-olds (76 ± 29%). Moreover, lactate increased linearly with time and O₂ deficit in both age groups, but lactate accumulated at a significantly lower rate in the 2-wk as compared to the 8-wk group for comparable deficits. We speculate that, during periods of very low cardiac output, 2-wk-old lambs not only decrease O₂ consumption, but also decrease metabolic demands, possibly by suppressing nonessential metabolism. This results in fewer energy stores that need to be replenished during recovery. This is a potential mechanism for the greater tolerance of young animals to hypoxic stresses.

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