Spontaneous arterial baroreflex control of the heart rate during head-down tilt in heat-stressed humans

Abstract

The purpose of this study was to elucidate the effect of raised body temperature per se during acute heat stress on the spontaneous arterial baroreflex control of heart rate (fc) in humans. To investigate whether unloading of cardiopulmonary baroreceptors during whole-body heating would alter the arterial baroreflex control of fc, we controlled loading of the cardiopulmonary baroreceptors by head-down tilt (HDT) at angles of 5 degrees, 10 degrees, 15 degrees, and 30 degrees during heat stress produced by hot-water-perfused suits. The sensitivity of the arterial baroreceptor-cardiac reflex was calculated from the spontaneous changes in beat-to-beat arterial pressure and fc. As an index of cardiopulmonary baroreceptor loading, the left atrial diameter (LAD) was measured by echocardiography. During whole-body heating, the LAD decreased with the rising body core temperature and increased with the HDT. The decreased LAD during heating almost recovered to the normothermic control level by 10 degrees HDT. In the supine position, cardiac baroreflex sensitivity remained unchanged during heating. Arterial pressure, fc and cardiac baroreflex sensitivity were not changed by HDT ranging from 5 degrees to 30 degrees during heating. These results suggest that cardiac baroreflex sensitivity remain unchanged during graded loading of the cardiopulmonary baroreceptors in heat-stressed humans. Also, we conclude that the sensitivity of the spontaneous arterial baroreflex controlling the fc is not influenced by raised body temperature per se during acute heat stress.

Human Cardiovascular Responses to Passive Heat Stress

“…Similar to that observed with carotid-cardiac baroreflex responsiveness, the outcome of this assessment depends on whether the evaluated variable was heart rate or R-R interval. When heart rate was analyzed, heat stress did not change baroreflex responsiveness (128,275), however when R-R interval was analyzed, heat stress decreased baroreflex responsiveness (128,272,276). A primary limitation of the sequencing technique is that the range of changes in arterial blood pressure can be relatively small, which can affect the calculation of baroreflex responsiveness and the range to which these results can be applied.…”

“…These observations raise the possibility that integrated baroreflex responsiveness may be impaired by heat stress. A number of other techniques have been used to evaluate integrated baroreflex responsiveness (44–46,48,272,275–277), including pharmacological manipulations of blood pressure (e.g., modified oxford technique), hypotensive challenges induced by simulated hemorrhage (e.g., LBNP and head-up tilt), and mathematical evaluations of heart rate and neural responses to spontaneous changes blood pressure (i.e., both the transfer function gain analysis and the sequencing technique). Results from these differing techniques sometimes contradict each other which complicate interpretation, though an alternate interpretation is that each of these different techniques may highlight different aspects of the reflex arc.…”

“…A number of investigators have assessed the effects of heat stress on baroreflex control of heart rate via an approach that has been termed the sequence technique (88,128,272,275,276). This method evaluates the slope between spontaneous changes in arterial blood pressure and corresponding changes in heart rate or R-R interval.…”

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

“…With regard to the observed hysteresis in the immediate recovery period, several studies have demonstrated the preservation of baroreflex control of HR during heat stress (8,11,38,40,41). Interestingly, although vascular sympathetic nerve activity is increased during heat stress (12), heat-induced attenuation of postsynaptic vasoconstrictor responsiveness results in progressive and profound reductions in MAP (14) in rats (20,26,28) and humans (11,14,39,42).…”

Whole-Body Heating Decreases Skin Vascular Response to Low Orthostatic Stress in the Lower Extremities

“…In protocol 1, the left atrial diameter (LAD) (in mm), as an index of cardiopulmonary baroreceptor loading [22][23][24], was measured by two-dimensional and M-mode echocardiography (Logiq 400 pro, GE Yokogawa Medical Systems, Tokyo) with a 3.5 MHz transducer. The images were obtained from the fourth left intercostal space in the parasternal short axis view at the level of the aortic valve at the end of the ventricular systole.…”