THE EFFECT OF A RISE IN BODY-TEMPERATURE ON THE CENTRAL-CHEMOREFLEX VENTILATORY RESPONSE TO CARBON-DIOXIDE

We assessed the central-chemoreflex ventilatory responses to carbon di oxide in six male volunteers using a hyperoxic rebreathing technique. Hyperventilation prior to rebreathing allowed both the threshold and t he sensitivity of the response to be measured. We used immersion in wa ter to control the body temperature (tympanic). The water temperature was adjusted to be either thermo-neutral or hot so that body temperatu re either remained normal [I + 0.2 (0.04) degrees C, mean (SEM)] or wa s elevated by 1.5 (0.08) degrees C. The sensitivities of the central-c hemoreflex ventilatory responses to carbon dioxide were increased at e levated body temperatures, changing from a mean of 1.8 (0.2) 1 . min(- 1). Torr(-1) to 2.7 (0.1) 1 . min(-1). Torr(-1). However, the threshol ds did not change with temperature, and the mean threshold was 48(1) T orr at both normal and elevated temperatures. For all of the volunteer s, ventilation was increased at elevated body temperatures for all lev els of carbon dioxide, mainly by changes in respiratory frequency due to reductions in expiratory times. At subthreshold levels of carbon di oxide, mean ventilation changed significantly from 6.3 (1.1) 1 . min(- 1) at normal temperatures to 10.8 (1.9) 1 . min(-1) at elevated temper atures. Heart rates also increased significantly with temperature: cha nging from a mean of 66 (4) beats .. min(-1) to 102(3) beats . min(-1) at threshold levels of carbon dioxide. The mean rates of rise of carb on dioxide partial pressure during rebreathing were significantly incr eased with temperature as well, changing from 0.075 (0.008) Torr . min (-1) to 0.089 (0.004)Torr . min(-1). We concluded that elevating the b ody temperatures of our subjects not only increased their ventilation, heart rates and metabolic rates at all levels of carbon dioxide, but it also increased the sensitivity of their central chemoreflex ventila tory responses to carbon dioxide. Despite these increases. the thresho lds of the central-chemoreflex ventilatory responses to carbon dioxide remained unchanged.