Maturational changes in body temperature and ventilation during hypoxia in kittens

Developmental changes in body temperature (Tb) and minute ventilation (V) were studied in unanesthetized kittens of 1, 2, 4, 6 and 8 weeks of postnatal age while breathing either air, 11% O2, or 11% O2 + 2.5% CO2. Experiments were mainly carried out at the ambient temperature (Ta) of 26-28 degrees C at which Tb was similar to that measured in the nesting box with the mother and littermates. In air, Tb progressively rose with increasing age to approach adult values by the age of 8 weeks. In hypoxia, Tb significantly decreased relative to room air values at all ages. However, the greatest decline occurred during the first month after birth, whereas by 6 and 8 weeks Tb was only slightly affected by hypoxia. The hypoxic-induced fall in Tb was partially prevented by the addition of 2.5% CO2 to the hypoxic mixture at 1, 2 and 4 weeks and slightly enhanced in the oldest kittens. On the other hand, V was consistently depressed by hypoxia relative to room air in the 1- and 2-week-old kittens. After 4 weeks of age, hypoxia induced a marked and sustained increase in V as in adult cats. Therefore, the depressant effect of hypoxia on body temperature which becomes progressively less effective during the first two months after birth appears to follow a similar postnatal evolution as the ventilatory response to hypoxia. It is suggested that maturity may be a contributing factor to the hypoxic-induced fall in Tb and that the maturational changes in V and Tb in response to hypoxia may be under the influence of the same central mechanisms.