Acute volume expansion preserves orthostatic tolerance during whole-body heat stress in humans

Abstract

Whole-body heat stress reduces orthostatic tolerance via a yet to be identified mechanism(s). The reduction in central blood volume that accompanies heat stress may contribute to this phenomenon. The purpose of this study was to test the hypothesis that acute volume expansion prior to the application of an orthostatic challenge attenuates heat stress-induced reductions in orthostatic tolerance. In seven normotensive subjects (age, 40 +/- 10 years: mean +/- S.D.), orthostatic tolerance was assessed using graded lower-body negative pressure (LBNP) until the onset of symptoms associated with ensuing syncope. Orthostatic tolerance (expressed in cumulative stress index units, CSI) was determined on each of 3 days, with each day having a unique experimental condition: normothermia, whole-body heating, and whole-body heating + acute volume expansion. For the whole-body heating + acute volume expansion experimental day, dextran 40 was rapidly infused prior to LBNP sufficient to return central venous pressure to pre-heat stress values. Whole-body heat stress alone reduced orthostatic tolerance by approximately 80% compared to normothermia (938 +/- 152 versus 182 +/- 57 CSI; mean +/- S.E.M., P < 0.001). Acute volume expansion during whole-body heating completely ameliorated the heat stress-induced reduction in orthostatic tolerance (1110 +/- 69 CSI, P < 0.001). Although heat stress results in many cardiovascular and neural responses that directionally challenge blood pressure regulation, reduced central blood volume appears to be an underlying mechanism responsible for impaired orthostatic tolerance in the heat-stressed human.

Figure 1. Group averaged (±s.e.m.) central venous pressure (CVP) during normothermia (Baseline), heat stress and heat stress plus volume expansion (Infusion) prior to lower-body negative pressure on the volume expansion day

*Significantly different from baseline and infusion (P < 0.05).

Figure 2. Responses from three representative subjects on each experimental day

NT, normothermia day; WBH, whole-body heating day; DXT, heat stress plus volume expansion day. The data depict responses from the LBNP stage prior to the final LBNP stage that resulted in syncopal symptoms and subsequent cessation of the LBNP challenge. MAP, mean arterial pressure; HR, heart rate; CVP, central venous pressure. LBNP, lower-body negative pressure.

Figure 3. Cumulative stress index and tolerance probability curves

A, group averaged (±s.e.m.) cumulative stress index (CSI) as a quantitative measure of orthostatic tolerance during graded lower-body negative pressure on each experimental day. NT, normothermia day; WBH, whole-body heating day; DXT, heat stress plus volume expansion day. *Significantly different from NT and DXT days (P < 0.05). B, ‘Tolerance probability’ curves for each experimental day. Tolerance probability for each stage of lower body negative pressure (LBNP) depicted as a ratio of subjects that completed the respective stage of LBNP; *significantly different from NT and DXT days (P < 0.05).