Influence of heat stress on arterial baroreflex control of heart rate in the baboon.

Abstract

SUMMARY. The influence of environmental heat stress on the arterial baroreflex control of heart rate (HR) was studied in eight conscious, chronically instrumented baboons. Inflations of balloon occluders around the inferior vena cava (IVC) and thoracic descending aorta (DA) were used to produce acute, graded changes in mean arterial blood pressure (MABP) in 5 mm Hg intervals ranging from ±5 to ±25 mm Hg. After determination of the HR responses to changes in MABP in the normothermic baboon (blood temperature <37.6°C), the animal was subjected to environmental heating to produce hyperthermia. When blood temperature reached approximately 39.5°C, HR responses to graded DA and IVC occlusions were again determined. During hyperthermia, the HR sensitivity (AHR/AMABP) to MABP changes was markedly diminished for reductions in MABP and significantly enhanced for increases in MABP. To determine whether these alterations in the HR response to changes in MABP were due to an alteration of the baroreflex control of HR, full, sigmoid-shaped HR-MABP curves for both the normothermic and hyperthermic states were constructed and characterized by total HR range, estimated slope of the steep portion of the curve, and MABP at the midpoint of the HR range (BPM). During hyperthermia (1) the whole HR-MABP curve shifted significantly upward by 35-40 beats/min, (2) total HR range, the estimated slope, and BP50 did not change, and (3) the control point (pre-occlusion HR-MABP value) shifted upward along the steep portion of the HR-MABP curve. In six of the eight baboons, full HR-MABP curves were also constructed during either /?-adrenergic blockade or cholinergic (Ch)-receptor blockade in the normothermic and hyperthermic state. Similar to that seen for the unblocked heart, the whole HR-MABP curves were also shifted upward during hyperthermia in this group of baboons with no alteration in the total HR range, the estimated slope, or BP r , 0 . The upward shift in the HR-MABP curve during Ch-receptor blockade, unlike during /S-receptor blockade, was much greater than that which could be attributed only to the local effect of blood temperature. Although the control point was also shifted upward along the steep portion of the curve during /?-or Ch-receptor blockade, the upward shift observed during /S-adrenergic blockade was similar to that observed in the unblocked state. Thus, a heat stress-induced hyperthermia produces a rise in HR without significantly altering the characteristics of the reflex control of HR by arterial baroreceptors. To rely solely on changes in HR sensitivity may lead to erroneous conclusions as to the effect of a particular stress on the baroreceptor reflex control of HR. Further, these results indicate that: (1) the upward shift in the HR-MABP curve is mediated by both the local effect of blood temperature on HR and cardiac sympathetic efferent neurons which are independent of the baroreceptor reflex, and (2) the upward shift in the control point is mediated predominantly by vagal withdrawal, probably as part of the comp...

Cardiovascular and thermoregulatory dysregulation over 24 h following acute heat stress in rats

“…The hemodynamic and thermoregulatory responses noted in the present study are consistent with, and extend, the findings of Gorman and Proppe (16). If we make the assumption that the heated animals are attempting to regulate MAP at a similar set point, it would follow that Moderate animals have a higher HR than Mild or Severe, as follows.…”

“…Similarly, the hemodynamic pattern during the first 24 h of recovery would fluctuate predictably with changes in T c . However, data from rats and primates indicate that ϳ60% of hyperthermia-related tachycardia is autonomic in origin, and only 40% is the direct result of temperature on the SA node (16). Given the strong autonomic influence on hemodynamic responses to heat stress, it is likely that changes in HR during periods of changing T c (e.g., during heat exposure and cooling) include a central role for autonomic mechanisms.…”

Effects of whole-body and local thermal stress on hydrostatic volume changes in the human calf

“…It is thought that the rapid shift of blood to the lower extremities induces a transient reduction of blood pressure in the early phase of orthostatic stress. This may be applicable especially during hyperthermia, where there is a reduction in the tachycardia response to sudden hypotensive stimulation (Gorman and Proppe 1982;Yamazaki et al 1997). Thus, while working in a hot environment, a sudden change of posture to a standing position should be avoided.…”

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

“…Previously, studies investigated the effects of heat stress on baroreflex function in rats (18,22) and baboons (10). With the use of different techniques, these investigators concluded that hyperthermic exposure either increased (22), decreased (18), or did not change (10) the gain expressing baroreflex control of heart rate.…”