Acclimization in a hot, humid environment: cardiovascular adjustments

Area of Science:

• Exercise Physiology
• Environmental Physiology
• Human Adaptation

Background:

• Understanding central circulatory responses is crucial for heat acclimatization.
• Previous research indicates complex physiological adjustments during heat exposure.

Purpose of the Study:

• To investigate the distinct phases of central circulatory adaptation during prolonged heat exposure.
• To examine the relationship between circulatory changes and thermoregulation during acclimatization.

Main Methods:

• Four trained men exercised at 40-50% max aerobic capacity for 13 days under varying heat conditions (25°C and 45°C).
• Central circulatory parameters (stroke volume, heart rate, cardiac output) and body temperatures were monitored throughout the study.

Main Results:

• Heat acclimatization exhibited four distinct central circulatory phases over 13 days.
• Phase I (Day 1): Decreased stroke volume, maintained cardiac output via elevated heart rate.
• Phase II (Days 2-3): Increased stroke volume, decreased heart rate.
• Phase III (Days 4-8): Increased cardiac output due to enhanced stroke volume.
• Phase IV (Days 6-8): Decreased rectal and skin temperatures, with declining stroke volume and heart rate, resulting in near-control cardiac output levels.

Conclusions:

• Central circulatory and temperature-regulating adaptations during heat acclimatization occur in distinct phases.
• Circulatory and thermoregulatory events are not directly causally linked during the acclimatization process.