Respiration during hypothermia: effect of rewarming intermediate areas of ventral medulla

We studied respiration (phrenic nerve activity) during progressive hypothermia to as low as 30.5 degrees C in five anesthetized, paralyzed, glomectomized, and vagotomized cats. PCO2 was maintained at a constant level throughout the experiments. We confirmed the results of a previous study (J. P. Kiley, F. L. Eldridge, and D. E. Millhorn, J. Appl. Physiol. 58: 295-312, 1985) in which respiratory minute output decreased progressively with cooling and respiratory frequency decreased markedly. In addition we show that focal rewarming to normal temperature (37.5 degrees C) of the structures in the intermediate areas on the ventral surface of the medulla resulted in a significant reversal of the depressed respiratory minute activity observed with hypothermia. Respiratory frequency, however, was unaffected by intermediate area rewarming. We conclude that the decreased respiratory activity during hypothermia is due to a generalized interference with neural function. A major portion of these effects is due to cooling of the intermediate areas, but the slowing of respiratory frequency appears to be an independent effect.