1. Cardiac output in lambs less than 3 days old was 50-100% higher during summit metabolism than during exposure to thermoneutral conditions.2. The Fick method, based on oxygen consumption, and the dye dilution method for determination of cardiac output, gave qualitatively similar results, but there were considerable quantitative discrepancies.3. The thermogenic tissues, muscle and brown fat, extracted some 80% of the oxygen from the blood perfusing them during summit metabolism.4. Significant right to left shunts through the foramen ovale were detected in about half of the lambs during the first few days of life; the size and incidence of shunts decreased with advancing age but were apparently independent of environmental conditions.5. During exposure to extreme cold, left to right shunts through the ductus arteriosus were detected in a high proportion of lambs, < 12 hr old, with detectable foramen ovale shunts. Pressure measurements in pulmonary and systemic arteries in very young lambs indicated that right to left shunting via the ductus was unlikely to occur; none was detected.6. Summit metabolism was poorly related to oxygen carrying capacity of blood, but significant correlations of summit metabolism with cardiac output (positive) and with oxygen saturation of mixed venous blood (negative) were observed, though not consistently.7. It is concluded that the great variability of summit metabolism between lambs older than 6 hr is not readily explained in terms of cardiovascular phenomena, but that the quantity and efficiency of thermogenic tissues probably play an important role in limiting summit metabolism.

Adipose Tissue, Brown↗

PubMed1970

Summit metabolism and cardiovascular function in young lambs during hyperoxia and hypoxia.

1. Exposure of lambs at sea level to air containing more than 33% oxygen resulted in a small increase in summit metabolism (maximum oxygen consumption on severe cold exposure) in some lambs, and in an improvement in the ability to maintain summit metabolism in others; there was a concomitant improvement in the maintenance of cardiac output, but no change in the degree of right to left shunting.2. Hypoxia (12-13% oxygen in inspired air) reduced summit metabolism of new-born lambs by 25%; but there was no effect in lambs 6-11 days old. Hypoxia neither reduced cardiac output, nor affected the degree of right to left shunting.3. The results suggest that in very young lambs there is some limitation of summit metabolism imposed by the supply of either oxygen or substrate to thermogenic tissues.

Animals↗

PubMed1970