Heat-stress-induced changes in central venous pressure do not explain interindividual differences in orthostatic tolerance during heat stress

Abstract

The extent to which heat stress compromises blood pressure control is variable among individuals, with some individuals becoming very intolerant to a hypotensive challenge, such as lower body negative pressure (LBNP) while heat stressed, while others are relatively tolerant. Heat stress itself reduces indexes of ventricular filling pressure, including central venous pressure, which may be reflective of reductions in tolerance in this thermal condition. This study tested the hypothesis that the magnitude of the reduction in central venous pressure in response to heat stress alone is related to the subsequent decrement in LBNP tolerance. In 19 subjects, central hypovolemia was imposed via LBNP to presyncope in both normothermic and heat-stress conditions. Tolerance to LBNP was quantified using a cumulative stress index (CSI), and the difference between normothermic CSI and heat-stress CSI was calculated for each individual. The eight individuals with the greatest CSI difference between normothermic and heat-stress tolerances (LargeDif), and the eight individuals with the smallest CSI difference (SmallDif), were grouped together. By design, the difference in CSI between thermal conditions was greater in the LargeDif group (969 vs. 382 mmHg × min; P < 0.001). Despite this profound difference in the effect of heat stress in decreasing LBNP tolerance between groups, coupled with no difference in the rise in core body temperatures to the heat stress (LargeDif, 1.4 ± 0.1°C vs. SmallDif, 1.4 ± 0.1°C; interaction P = 0.89), the reduction in central venous pressure during heat stress alone was similar between groups (LargeDif: 5.7 ± 1.9 mmHg vs. SmallDif: 5.2 ± 2.0 mmHg; interaction P = 0.85). Contrary to the proposed hypothesis, differences in blood pressure control during LBNP are not related to differences in the magnitude of the heat-stress-induced reductions in central venous pressure.

Fig. 1.

Effects of heat stress on the difference in lower body negative pressure (LBNP) tolerance between those with the smallest (SmallDif) and largest reduction in LBNP tolerance between normothermic and heat-stressed conditions (LargeDif; A), and the reduction (Δ) in central venous pressure (CVP) for the respective groups in response to the heat stress alone (B). Despite large differences in the reduction in LBNP tolerance between groups, indexed by the cumulative stress index (CSI; A), the reduction in CVP during heat stress alone was similar (P = 0.56; B). These findings strongly suggest that the reduction in CVP that accompanies heat stress is unlikely to be responsible for intersubject variability in heat-stress-induced reductions in LBNP tolerance. Values are means ± SD.

Fig. 2.

Relationship between the reduction in CVP to heat stress alone and the reduction in LBNP tolerance (defined as a CSI) from normothermic to heat-stressed conditions. Shaded circles represent responses for the SmallDif group, open circles represent the responses from the middle group, and the solid circles represent responses for the LargeDif group. There was no relationship between heat-stress-induced reductions in CVP and the reduction in LBNP tolerance between normothermic and heat-stress conditions (r = 0.27, P = 0.26).