Variability in Orthostatic Tolerance during Heat Stress: Role of Reductions in Cerebral Perfusion

Abstract

The mechanism(s) responsible for heat stress induced reductions in orthostatic tolerance are unclear. Although a high degree of variability exists in the level of reduction in tolerance amongst individuals, syncope will always occur when cerebral perfusion is inadequate. Thus, heat stress mediated reductions in cerebral perfusion may explain the variation in tolerance in this thermal condition. This study tested the hypothesis that individuals with the greatest heat‐stress induced reduction in orthostatic tolerance would have the largest reduction in cerebral perfusion in this thermal condition. On different days, lower body negative pressure (LBNP) was imposed on 15 individuals during normothermia, and after a 1.5 °C rise in core temperatures (Tc). Tolerance was quantified as cumulative stress index (CSI) and the difference in CSI (CSIdiff) between thermal conditions was used to categorize individuals most (HighDif) and least (LowDif) affected. Heat stress reduced cerebral vascular conductance (CVC) relative to normothermia in both groups (P<0.05 for each); however, the magnitude of reduction in CVC was similar between groups (HighDif: 31.3±24.9 cm·s−1·mmHg−1; LowDif: 11.4±19.2 cm·s−1·mmHg−1, P=0.18). Our data indicate that variability in heat stress induced reductions in orthostatic tolerance is not related to reductions in cerebral perfusion in this thermal condition.