Oxygen transport and metabolism in the conscious lamb: the effects of hypoxemia

We studied the response of O2 consumption, systemic O2 transport, arterial blood lactate concentration, base deficit, and respiratory exchange ratio (CO2 production/O2 consumption) to graded alveolar hypoxia (fractional concentration of inspired oxygen 0.21, 0.16, 0.12, and 0.08) in seven intact, conscious chronically catheterized lambs at less than 1 wk after birth, at 25-40 days and at greater than 40 days. To test whether there was an age related difference in the metabolic response to a decline in O2 transport due to hypoxemia, we related the changes in blood lactate and base deficit to the systemic O2 transport and to the fractional change in O2 consumption. Systemic O2 transport and O2 consumption decreased with severe hypoxemia, while lactate concentration and respiratory exchange ratio increased in all age groups. The changes in base deficit were not significant. When the hypoxemia-induced reduction in O2 consumption was less than 15% of the baseline value (at fractional concentration of inspired oxygen 0.21) there was no elevation in lactate concentration; however, when O2 consumption was reduced by more than 15%, lactate concentration consistently increased by more than 1 mmol/liter. There were no apparent differences amongst the age groups in this response. Therefore, the newborn lamb does show metabolic consequences of a fall in O2 consumption with hypoxemia which is similar to the older sheep, however, there is a "buffer zone" in which O2 consumption may decrease before evidence of tissue hypoxia can be found. The mechanism by which O2 consumption can decrease without accumulation of lactate remains speculative.