Characterization Of Regional Cerebral Perfusion During Pre-syncope In Heat-stressed Individuals

Orthostatic tolerance is reduced in heat-stressed individuals, likely resulting from inadequate cerebral perfusion. However, the precise mechanism(s) responsible for orthostatic intolerance in this thermal state remain unclear. Herein we characterize regional cerebral perfusion during an orthostatic challenge in heat-stressed individuals. PURPOSE: To characterize anterior (internal carotid artery; ICA) and posterior (vertebral artery; VA) cerebral blood flow responses during pre-syncope in heat-stressed individuals. METHODS: Ten normotensive subjects (5 males; 5 females)performed a progressive lower body negative pressure (LBNP) challenge until pre-syncope while heat-stressed with a water perfused suit. VA and ICA blood flows were measured via vascular ultrasound. RESULTS: Internal temperature was increased by 1.4 ± 0.1 °C, resulting in ICA blood flow decreasing by ~12 % from the pre-heat baseline (from 378 ± 72 to 331 ± 80 ml/min, P = 0.03), without any changes in VA blood flow (from 146 ± 53 to 146 ± 60 ml/min) prior to LBNP. Just before the observation of any pre-syncope symptoms, LBNP had decreased ICA and VA blood flows to 76 ± 20 % and 81 ± 22 % of the pre-heat baseline values, respectively. At pre-syncope, the VA blood flow had further decreased by ~5% (to 75 ± 22 %, P = 0.045) without a change in ICA blood flow (75 ± 19 %, P = 0.474). Interestingly, at pre-syncope we observed consistent retrograde flow from the VA, contributing to the observed further reduction in VA blood flow, while ICA blood flow remained antegrade. CONCLUSIONS: Thus, a greater reduction in VA blood flow at the onset of pre-syncope due to the retrograde blood flow perhaps causes insufficient delivery of blood to cardiovascular control centers, which may induce pre-syncope during an orthostatic challenge while heat stressed. Supported by: 18H03166