Fitness Level Does Not Impact Cardiovascular Drift and Decreased Maximal Oxygen Uptake during Heat Stress

Abstract

Cardiovascular drift (CV drift) has been shown to be related to reduced maximal oxygen uptake (V[Combining Dot Above]O2max) during heat stress. At a given relative metabolic intensity (%V[Combining Dot Above]O2max), individuals with higher fitness levels would be expected to experience greater CV drift, and thereby greater decrements in V[Combining Dot Above]O2max, because of a greater metabolic heat production resulting from exercising at a higher absolute intensity. However, this has not been directly investigated. PURPOSE: To test the hypothesis that individuals with a higher initial V[Combining Dot Above]O2max (i.e., fitness level) will experience a greater magnitude of CV drift and accompanying decrement in V[Combining Dot Above]O2max compared to those with a lower initial V[Combining Dot Above]O2max during prolonged, moderate-intensity exercise in the heat. METHODS: Data from 7 studies (n = 62) were used to assess the relationships between fitness level and 1) CV drift (change in heart rate and stroke volume) and 2) V[Combining Dot Above]O2max. CV drift and VO[Combining Dot Above] 2max were assessed between 15 and 45 min or between 15 and 60 min of cycling at 60% V 2max in 35 °C or 30 °C. RESULTS: Initial V[Combining Dot Above]O2max (i.e., fitness level) was not related to change in heart rate (r=0.17, p=0.20), change in stroke volume (r=-0.06, p=0.63), and decrease in V[Combining Dot Above]O2max (r=-0.14, p=0.28). CONCLUSION: Contrary to our hypothesis, fitness level was unrelated to the magnitude of CV drift or decrement in V[Combining Dot Above]O2max during prolonged submaximal exercise in the heat. However, these findings support the results of previous studies in that the greater the magnitude of CV drift—regardless of fitness level—the greater the decrement in V[Combining Dot Above]O2max during constant-rate, moderate-intensity exercise in the heat.