EFFECTS OF ACUTE HYPERTHERMIA ON THE CAROTID BAROREFLEX CONTROL OF HEART-RATE IN HUMANS

The purpose of this study was to examine the effect of hyperthermia on the carotid baroreceptor-cardiac reflexes in humans. Nine healthy mal es underwent acute hyperthermia (esophageal temperature similar to 38. 0 degrees C) produced by hot water-perfused suits. Beat-to-beat heart rate (HR) responses were determined during positive and negative R-wav e-triggered neck pressure steps from +40 to -65 mm Hg during normother mia and hyperthermia. The carotid baroreceptor-cardiac reflex sensitiv ity was evaluated from the maximum slope of the HR response to changes in carotid distending pressure. Buffering capacity of the HR response to carotid distending pressure was evaluated in % from a reference po int calculated as (HR at 0 mm Hg neck pressure-minimum HR)/HR range x1 00. An upward shift of the curve was evident in hyperthermia because H R increased from 57.7+/-2.4 beats/min in normothermia to 88.7+/-4.1 be ats/min in hyperthermia (P < 0.05) without changes in mean arterial pr essure. The maximum slope of the curve in hyperthermia was similar to that in normothermia. The reference point was increased (P < 0.05) dur ing hyperthermia. These results suggest that the sensitivity of the ca rotid baroreflex of HR remains unchanged in hyperthermia. However, the capacity for tachycardia response to rapid onset of hypotension is re duced and the capacity for bradycardia response to sudden hypertension is increased during acute hyperthermia.