Carotid baroreflex control of heart rate is enhanced, while control of mean arterial pressure is preserved during whole body heat stress in young healthy men

Abstract

Whole-body heat stress (WBH) results in numerous cardiovascular alterations that ultimately reduce orthostatic tolerance. While impaired carotid baroreflex (CBR) function during WBH has been reported as a potential reason for this decrement, study design considerations may limit interpretation of previous findings. We sought to test the hypothesis that CBR function is unaltered during WBH. CBR function was assessed in ten healthy male subjects (age, 26 ± 3; height, 185 ± 7 cm; weight, 82 ± 10 kg; BMI, 24 ± 3 kg/m2; mean ± sd) using 5 s trials of neck pressure (+45, +30 and +15 Torr) and neck suction (-20, -40, -60 and -80 Torr) during normothermia (NT) and passive WBH (∆ core temp ~1 °C). Analyses of stimulus response curves (four parameter logistic model) for CBR control of heart rate (CBR-HR) and mean arterial pressure (CBR-MAP), as well as separate 2-way ANOVA of the hypoand hypertensive stimuli (factor 1: thermal condition, factor 2: chamber pressure) were performed. For CBR-HR, maximal gain was increased during WBH (-0.73±0.11) compared to NT (-0.39±0.04, mean±SE, p=0.03). In addition, the CBR-HR responding range was increased during WBH (33±5) compared to NT (19±2 bpm, p=0.03). Separate analysis of hypertensive stimulation revealed enhanced HR responses during WBH at -40, -60 and -80 Torr (condition*chamber pressure interaction, p=0.049) compared to NT. For CBR-MAP, both logistic analysis and separate 2-way ANOVA revealed no differences during WBH. Therefore, in response to passive WBH, CBR control of heart rate (enhanced) and arterial pressure (no change) is well-preserved.