Effect of lowered temperature on respiratory rhythm generation in rabbit

It was tried in this study to determine the effects of temperature and carbon dioxide on the respiratory drive under experimental hypothermia in rabbits under urethane-chloralose anaesthesia after muscle-relaxant administration, after bilateral vagotomy and during artificial ventilation with a biologically-controlled respirator. Hypercapnia was produced in the animals during normothermia (37.3 +/- 0.7 degrees C) and hypothermia (30.0 +/- 1.5 degrees C). The basic physiological parameters and efferent activity of the phrenic nerve were recorded, and arterial blood gasometric parameters were determined. The electrophysiological equivalent of minute ventilation (Veq) decreased during hypothermia by 33% on the average while the PaCO2 value was unchanged. The hypercapnic stimulus applied during hypothermia failed also to raise the Veq value to its initial level. A 9% fall of blood flow was observed in the common carotid artery when the animals received a hypercapnic gas mixture for breathing during hypothermia. The results obtained in this study and earlier observations confirm unequivocally the hypothesis of a direct influence of temperature lowering on respiratory rhythm generation and regulation of arterial blood flow to the brain.