GLOSSARY

Cardiovascular drift is the gradual rise in heart rate with a fall in stroke volume during prolonged exercise (usually beyond 10–20 minutes) at constant intensity, especially in hot environments, while cardiac output stays roughly constant.

Causes

• Rising body temperature: directly raises heart rate; recent research considers this the main factor.
• Falling stroke volume: a higher heart rate shortens ventricular filling time.
• Shift of blood to the skin: for heat loss.
• Dehydration and reduced plasma volume from sweating.
• Increased sympathetic activity and possibly recruitment of more fibres with fatigue.

Values

Heart rate may rise by 10–20 beats or more during an hour of steady exercise.