Effects of acute hyperthermia on the carotid baroreflex control of heart rate in humans

Abstract

The purpose of this study was to examine the effect of hyperthermia on the carotid baroreceptor-cardiac reflexes in humans. Nine healthy males underwent acute hyperthermia (esophageal temperature -38.0 degrees C) produced by hot water-perfused suits. Beat-to-beat heart rate (HR) responses were determined during positive and negative R-were-triggered neck pressure steps from +40 to -65 mm Hg during normothermia and hyperthermia. The carotid baroreceptor-cardiac reflex sensitivity 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 point calculated as (HR at 0 mm Hg neck pressure-minimum HR)/HR range x 100. An upward shift of the curve was evident in hyperthermia because HR increased from 57.7 +/- 2.4 beats/min in normothermia to 88.7 +/- 4.1 beats/min in hyperthermia (P < 0.05) without changes in mean arterial pressure. The maximum slope of the curve in hyperthermia was similar to that in normothermia. The reference point was increased (P < 0.05) during hyperthermia. These results suggest that the sensitivity of the carotid baroreflex of HR remains unchanged in hyperthermia. However, the capacity for tachycardia response to rapid onset of hypotension is reduced and the capacity for bradycardia response to sudden hypertension is increased during acute hyperthermia.

Baroreflex modulation of sympathetic nerve activity to muscle in heat-stressed humans

“…A lack of change in baroreflex gain is consistent with prior findings (2,26,27), but it is in contrast to one study (5). Conflicting findings between these studies are likely due to the methodology of assessing baroreflex regulation.…”

“…In humans, we (2) and others (26) reported that the gain of the carotid-cardiac baroreflex was not significantly affected by whole body heating, whereas the gain of the carotid-vasomotor baroreflex was significantly attenuated in this environment (2). Furthermore, we reported that the transfer function gain of the relationship between spontaneous fluctuations in blood pressure and heart rate was significantly attenuated within the high-frequency range (5).…”

“…Moreover, in hyperthermic rats, the gain expressing the relationship between splanchnic sympathetic nerve activity and blood pressure was unaltered relative to normothermia (18). In humans, we and others have shown that whole body heating does not alter the maximal gain of baroreflex control of heart rate during carotid baroreceptor perturbations (2,26), it decreases the maximal gain of carotid-vasomotor responses (2), whereas the gain expressing changes in heart rate relative to spontaneous changes in blood pressure was either unchanged (27) or was reduced by heat stress (5).…”

Phenylephrine-induced elevations in arterial blood pressure are attenuated in heat-stressed humans

“…However, we and others have shown that whole body heating does not alter the maximal gain of baroreflex control of heart rate during carotid baroreceptor perturbations (4,35) or during acute changes in arterial blood pressure (7,36). Moreover, the gain expressing the relationship between blood pressure and muscle sympathetic nerve activity (MSNA) was unaffected by whole body heating (7).…”

Heat stress and orthostatic tolerance

“…Compared with normothermic control, whole-body heating rapidly increases calf volume during the early phase of HUT, whereas whole-body cooling decreases the magnitude and the speed of change in calf volume 48) . Local heating to the legs in normothermia increases the blood flow in the warmed skin area and increases the speed of the change in calf volume the same nique, the effect of heat stress on the relationship between carotid distending pressure and HR has been examined in the supine position [50][51][52] . Prior reports showed that wholebody heating did not alter the sensitivity of the carotid baroreflex for controlling HR, as defined by the maximal slope of the linear portion of the baroreflex curve around the operating point, irrespective of heating levels [50][51][52] (Fig.…”