Body cooling attenuates the decrease in maximal oxygen uptake associated with cardiovascular drift during heat stress

Abstract

Previous research suggests cardiovascular drift (CV drift) is associated with decreased maximal oxygen uptake $\left( \dot{\hbox{V}}\hbox{O}_{\rm 2max} \right)$ during heat stress, but more research manipulating CV drift with subsequent measurement of $\dot{\hbox{V}}\hbox{O}_{\rm 2max}$ is needed to assess whether this relationship is causal. To assess causation, $\dot{\hbox{V}}\hbox{O}_{\rm 2max}$ was measured during the same time interval that CV drift occurred (between 15 and 45 min of submaximal exercise under different conditions of body cooling intended to manipulate CV drift). Ten men completed a control graded exercise test (GXT) in 22°C to measure $\dot{\hbox{V}}\hbox{O}_{\rm 2max},$ then on separate occasions they cycled in 35°C at 60% $\dot{\hbox{V}}\hbox{O}_{\rm 2max}$ for 15 min (15max), 45 min with no cooling (NC), and 45 min with fan airflow (FAN) beginning at ∼18 min into exercise, and each bout was immediately followed by a GXT to measure $\dot{\hbox{V}}\hbox{O}_{\rm 2max}.$ In NC, $\dot{\hbox{V}}\hbox{O}_{\rm 2max}$ decreased 18%, heart rate (HR) increased 16%, and stroke volume (SV) fell 12% (P < 0.05) from min 15 to min 45. In FAN, $\dot{\hbox{V}}\hbox{O}_{\rm 2max}$ fell less (5.7%, P < 0.05) , HR rose less (4%, P < 0.05) and SV decreased less (3%, P < 0.05) from 15 to 45 min. The fall in $\dot{\hbox{V}}\hbox{O}_{\rm 2max}$ associated with CV drift during exercise in a hot environment is attenuated with body cooling via fan airflow. The findings support the notion that a causal link exists between CV drift that occurs during prolonged exercise in a hot environment and a decrease in $\dot{\hbox{V}}\hbox{O}_{\rm 2max}.$