MATTHEW J. WATT, A. P. GARNHAM, M. A. FEBBRAIO, and M. HARGREAVES. Effect of acute plasma volume expansion on thermoregulation and exercise performance in the heat. Med. Sci. Sports Exerc., Vol. 32, No. 5, pp. 958–962, 2000.
Purpose: We investigated the effects of acute plasma volume expansion on exercise performance in the heat.
Methods: Six moderately trained men cycled for 40 min at 64 ± 2% peak pulmonary oxygen uptake (V̇O2peak) followed by an individual performance time trial, where subjects completed a set amount of work (267 ± 15 kJ) in as little time as possible. Exercise trials were performed at 35°C with a relative humidity of 40%. Subjects performed two exercise trials: one after 13.1 ± 1% acute plasma volume expansion (PVE), which was achieved by the intravenous infusion of 8 mL·kg−1 body weight of Hemaccel® (35 g·L−1 polygeline, 145 mmol·L−1 Na+, and 145 mmol·L−1 Cl−) and the other without prior treatment (CON).
Results: Core temperature, skin blood flow, and heart rate progressively increased (P < 0.05) during exercise, but no differences were observed between trials. Plasma glucose and lactate were similar at rest and during exercise, as was V̇O2 during exercise. Exercise performance was not influenced by plasma volume expansion (CON 17.5 ± 0.4 min and PVE 17.1 ± 0.2 min).
Conclusion: These data suggest that, in moderately trained men, plasma volume expansion alone does not enhance thermoregulatory function and exercise performance during moderate intensity exercise in the heat.
Hyperthermia and dehydration impair exercise performance, and further decrements in performance occur when heat stress is imposed on the exercising individual (9,11,17,23). After heat acclimatization, sweat rate (12,18,19) and skin blood flow (21) are higher and core temperature is lower during exercise at the same absolute intensity (19). It has been suggested that plasma volume expansion is an important mechanism promoting enhanced cardiovascular and thermoregulatory stability after heat acclimatization (1,3), which may contribute to improved exercise performance.
The effects of acute plasma volume expansion on cardiovascular function and thermoregulation have provided conflicting results. Fortney et al. (6,8) have demonstrated a reduction in the cardiovascular drift during exercise in the heat after dextran-induced plasma volume expansion. In support of these findings, heart rate was significantly lower (∼13 bpm) during the early stages of low-intensity exercise at high temperatures (22). Such responses have also been observed during moderate exercise in temperate conditions (10,13,15).
Studies that have examined thermoregulatory responses with and without plasma volume expansion have demonstrated no differences in sweat rates (7,22), skin blood flow (6,7,22), and skin temperature (6,22). However, the data pertaining to core temperature are somewhat equivocal. Some investigations have reported unchanged core temperatures with plasma volume expansion (6,22), whereas others have reported progressive reductions (0.10–0.25°C) during exercise in the heat (7). Furthermore, despite working at a higher average power output, core temperature was similar in subjects who had their plasma volume expanded by dextran (16). Taken together, the data suggest that plasma volume expansion may influence thermoregulation during moderate-intensity exercise in the heat.
Limited data are available regarding the effects of plasma volume expansion on exercise performance. Luetkemeier and Thomas (16) reported an increase in average power output during simulated performance time trials at 21.5°C with plasma volume expansion. In contrast, a study conducted on military personnel at low exercise intensities (45% V̇O2max) and high ambient temperatures (45°C) demonstrated no change in walking performance (22). To our knowledge, no study has investigated the effects of plasma volume expansion on more intense exercise performance with moderate heat stress. Thus, the purpose of this investigation was to compare thermoregulatory function and cycling time trial performance in trained, unacclimatized cyclists during exercise in the heat after plasma volume expansion to a level similar to that observed after heat acclimatization. It was hypothesized that acute plasma volume expansion would improve exercise performance in the heat.