Influence of temperature on metabolism and breathing during mammalian ontogenesis

A literature survey of the ventilatory responses to changes in ambient temperature (T) in neonatal mammals reveals that, as in adults, the metabolic response to T is the major determining factor. In fact, the newborn's metabolic response to changes in T determines not only the pulmonary ventilation and the breathing pattern, but also the magnitude of the ventilatory responses to chemical stimuli and the intensity of the pulmonary reflexes at different T. The important difference from the adult is that in many neonatal mammals the control of body temperature (Tb) is poorly developed. Hence, the metabolic response can be more similar to that of an ectothermic, rather than endothermic, animal, and Tb can vary substantially with T. When hypoxia occurs in cold, Tb can decrease greatly, because of the hypoxic drop in the thermoregulatory set-point, and this lowers pulmonary ventilation. Hence, in addition to the metabolic response, also the changes in Tb are a factor modulating the ventilatory responses to T. Artificial warming of the newborn during hypoxia causes heat-dissipation responses that can be counterproductive. During ontogenesis, with prolonged cold conditions, the sustained alterations in metabolic rate and body growth do not modify the postnatal development of the respiratory control mechanisms. Presumably, this indicates that respiratory regulation develops independently from the individual's metabolic history.