Oxygen transport in conscious newborn dogs during hypoxic hypometabolism.

We questioned whether the decrease in O2 consumption (VO2) during hypoxia in newborns is a regulated response or reflects a limitation in O2 availability. Experiments were conducted on previously instrumented conscious newborn dogs. VO2 was measured at a warm ambient temperature (30 degrees C, n = 7) or in the cold (20 degrees C, n = 6), while the animals breathed air or were sequentially exposed to 15 min of fractional inspired O2 (FIO2): 21, 18, 15, 12, 10, 8, and 6%. In normoxia, VO2 averaged 15 +/- 1 (SE) and 25 +/- 1 ml . kg-1 . min-1 in warm and cold conditions, respectively. In the warm condition, hypometabolism (i.e., hypoxic VO2 < normoxic VO2) occurred at FIO2 </=10%, whereas in the cold condition, hypometabolism occurred at FIO2 </=12%. The same results were obtained in a separate group (n = 14) of noninstrumented puppies. For all levels of FIO2 with hypometabolism, the relationships between measures of O2 availability (arterial O2 saturation or content, venous PO2 or saturation, x-axis) vs. VO2 (y-axis) had lower slopes in warm than in cold conditions. Hence, VO2 during hypometabolism in the warm condition was not the maximal attainable for the level of oxygenation. The results do not support the possibility that the hypoxic drop in VO2 in the newborn reflects a limitation in O2 availability. The results are compatible with the idea that the phenomenon is one of "regulated conformism" to hypoxia.