Oxygen transport in conscious newborn dogs during hypoxic hypometabolism

We questioned whether the decrease inO₂ consumption(O₂) during hypoxia innewborns is a regulated response or reflects a limitation inO₂ availability. Experiments wereconducted on previously instrumented conscious newborn dogs.O₂ was measured at a warmambient temperature (30°C, n = 7)or in the cold (20°C, n = 6),while the animals breathed air or were sequentially exposed to 15 minof fractional inspired O₂(FI_O2): 21, 18, 15, 12, 10, 8, and 6%. In normoxia,O₂ averaged 15 ± 1 (SE)and 25 ± 1 ml · kg¹ · min¹in warm and cold conditions, respectively. In the warmcondition, hypometabolism (i.e., hypoxicO₂ < normoxicO₂) occurred at FI_O2 10%, whereas in thecold condition, hypometabolism occurred atFI_O2 12%. The sameresults were obtained in a separate group(n = 14) of noninstrumented puppies.For all levels of FI_O2 withhypometabolism, the relationships between measures ofO₂ availability (arterialO₂ saturation or content, venousPO₂ or saturation,x-axis) vs.O₂(y-axis) had lower slopes in warm than in coldconditions. Hence, O₂ during hypometabolism in the warm condition was not the maximal attainable for the level of oxygenation. The results do not support thepossibility that the hypoxic drop inO₂ in the newborn reflects a limitation in O₂availability. The results are compatible with the ideathat the phenomenon is one of "regulated conformism" tohypoxia.