The development of the ventilatory response to cold in very young rats.

To assess the range of functional responses of the ventilatory apparatus of developing rats and the degree to which ventilatory function is developed in advance of other functional characteristics, rat pups at five ages (between 4 and 20 days old) were exposed to temperatures of 28, 32 and 36 degrees C while in a flow through metabolic chamber modified to serve as a whole body plethysmograph. Ventilatory frequency, tidal volume and oxygen extraction 'efficiency' (EO2 = VO2/FEO2 x VI) were measured at each age and temperature. Mean breathing frequency at 4 days old was 2.56 breaths per second, decreasing to 1.99 at 20 days old. There was insignificant modification of breathing frequency with temperature. Four day old rat pups at 28 degrees C had mass specific tidal volumes of 0.017 ml/g, 142% of the value at 36 degrees C (0.012 ml/g). Twenty day old pups at 28 degrees C had mass specific tidal volumes of 0.027 ml/g, also 142% of the thermoneutral value (0.019 ml/g at 32 degrees C). At all ages, increases in tidal volumes were similar and increases in tidal volume were the only response to increased metabolic demand. Oxygen extraction 'efficiency' was about half that previously observed in adult rodents. These observations of ventilation during a cold challenge suggest that although structural development is not complete until much later, functional development is sufficient, either at birth or shortly thereafter.