Study Design and Methodology

The researchers studied two groups of endurance-trained cyclists. One group examined the effects of hyperthermia (elevated core temperature) alone, while the other focused on dehydration and its effects both with and without hyperthermia. In total, 15 male athletes participated in these trials.

• Hyperthermia Study: Seven cyclists exercised in a hot environment (35°C) for 100 minutes, then performed additional exercise while their core temperatures were maintained at around 39.3°C, either with or without dehydration.
• Dehydration Study: Eight cyclists exercised for 120 minutes in the heat to lose approximately 4% of their body weight through dehydration. Additional exercise trials were conducted in a cold environment to prevent hyperthermia and isolate the effects of dehydration. In some cases, intravenous fluid was administered to restore blood volume.

Measurements included heart rate, stroke volume (the amount of blood pumped per heartbeat), cardiac output (the total amount of blood pumped by the heart), and blood pressure. The athletes’ perceived effort and other cardiovascular responses were also recorded.

Key Results

1. Hyperthermia Alone:
   • Stroke volume decreased by 8% (approximately 11 milliliters per beat).
   • Heart rate increased by 5% (around 9 beats per minute).
   • Cardiac output and blood pressure were maintained.
2. Dehydration Alone:
   • Stroke volume dropped by 7% (about 11 milliliters per beat).
   • Heart rate rose by 5% (around 7 beats per minute).
   • Similar to hyperthermia, cardiac output and blood pressure were not significantly affected.
3. Combined Dehydration and Hyperthermia:
   • Stroke volume fell sharply by 20% (26 milliliters per beat).
   • Cardiac output declined by 13% (2.8 liters per minute).
   • Blood pressure decreased by 5% (approximately 5 mmHg).
   • Systemic vascular resistance (the resistance faced by blood in vessels) increased by 10%.
4. Restoring Blood Volume:
   • When blood volume was restored via intravenous fluids in dehydrated but non-hyperthermic athletes, stroke volume returned to normal levels. This suggests that blood volume reduction is a primary cause of cardiovascular strain during dehydration.

Implications and Practical Takeaways

This study highlights the significant cardiovascular strain that occurs when dehydration is combined with elevated body temperatures. The findings suggest:

• Dehydration exacerbates heat stress: Athletes’ ability to maintain cardiac output and blood pressure diminishes when dehydration and hyperthermia occur simultaneously.
• Hydration is critical: Ensuring proper hydration can mitigate some of the cardiovascular strain associated with exercise in the heat.
• Blood volume restoration: Strategies to maintain or restore blood volume may help alleviate the adverse effects of dehydration on heart function.

Conclusion

Dehydration significantly impairs cardiovascular function during exercise, especially when combined with hyperthermia. For athletes and individuals engaging in physical activities in hot conditions, maintaining hydration is crucial to prevent declines in performance and health risks. Future research is needed to explore more strategies to manage dehydration and support cardiovascular health in extreme conditions.