Oxygen transport in conscious newborn dogs during hypoxic hypometabolism

We questioned whether the decrease in O2consumption (V˙o2) during hypoxia in newborns is a regulated response or reflects a limitation in O2availability. Experiments were conducted on previously instrumented conscious newborn dogs.V˙o2was measured at a warm ambient temperature (30°C, n= 7) or in the cold (20°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,V˙o2averaged 15 ± 1 (SE) and 25 ± 1 ml ⋅ kg−1⋅ min−1in warm and cold conditions, respectively. In the warm condition, hypometabolism (i.e., hypoxicV˙o2< normoxicV˙o2) occurred atFIO2≤10%, whereas in the cold condition, hypometabolism occurred atFIO2≤12%. The same results were obtained in a separate group (n= 14) of noninstrumented puppies. For all levels of FIO2with hypometabolism, the relationships between measures of O2availability (arterial O2saturation or content, venousPO2or saturation,x-axis) vs.V˙o2(y-axis) had lower slopes in warm than in cold conditions. Hence, V˙o2during 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 inV˙o2in the newborn reflects a limitation in O2availability. The results are compatible with the idea that the phenomenon is one of “regulated conformism” to hypoxia.