Heat acclimation is usually associated with an increase in plasma volume (Sawka et al. 2000), and it is known that expansion of an individual’s plasma volume results in an increase in tolerance to orthostatic stress. The aim of this preliminary study was to determine whether orthostatic tolerance was influenced by heat acclimation, induced by repeated bouts of exercise in a hot environment.
The study, which had been approved by the University of Leeds and Leeds Teaching Hospitals ethics committees, was performed on four male and four female volunteers (aged 19-34 years). Heat acclimation was achieved by exercising at approximately 60 % of VO2,max for two hour-long sessions per week for 4 weeks, at 40°C and 25-35 % humidity. Before, and 2 days after completing the acclimation protocol, the following measurements were made: plasma volume (n = 5, Evans’ blue dye dilution, El-Sayed et al. 1994), orthostatic tolerance (n = 6, El-Bedawi & Hainsworth, 1994) and plasma electrolytes. Heart rates and aural temperatures were recorded during each heat exposure. All data are presented as means ± S.E.M. Statistical significance was assessed using paired t tests. Correlations were assessed using the Spearman ranked correlation coefficient.
Following acclimation, there was a reduction in the end-exercise core temperature (38.0 ± 0.1 to 37.6 ± 0.1 °C, P < 0.005) and in maximal heart rate (168.6 ± 5.5 to 152.4 ± 4.8 b.p.m., P < 0.005). VO2,max did not change (35.1 ± 3.6 to 32.5 ± 3.1 ml kg-1 min-1). There was a significant reduction in plasma Na+ concentration (143.6 ± 0.9 to 140.9 ± 0.6 mmol l-1, P < 0.05), but no change in plasma volume (40.4 ± 3.4 to 41.9 ± 4.2 ml kg-1). Orthostatic tolerance, assessed as time to presyncope, did not change significantly (35 ± 2.4 to 38 ± 2.8 min), but there was a significant negative correlation between orthostatic tolerance and aural temperature during heat exposure, (r = -0.567, P < 0.05).
All subjects were successfully heat acclimated without affecting aerobic fitness. Mean orthostatic tolerance did not change significantly, but was significantly negatively correlated with the increase in body temperature during heat exposure (a measure of the degree of heat acclimation). The failure of plasma volume and orthostatic tolerance to increase significantly following the heat acclimation may be related to sodium depletion.
All procedures accord with current local guidelines.