Effects of marked hyperthermia with and without dehydration on VO(2) kinetics during intense exercise

This study determined whether marked hyperthermia alone or in combination with dehydration reduces the initial rate of rise in O(2) consumption (VO(2) on-kinetics) and the maximal rate of O(2) uptake (VO(2 max)) during intense cycling exercise. Six endurance-trained male cyclists completed four maximal cycle ergometer exercise tests (402 +/- 4 W) when euhydrated or dehydrated (4% body wt) with normal (starting esophageal temperature, 37.5 +/- 0.2 degrees C; mean skin temperature, approximately 31 degrees C) or elevated (+1 and +6 degrees C, respectively) thermal strain. In the euhydrated and normal condition, subjects reached VO(2 max) (4.7 +/- 0.2 l/min) in 228 +/- 34 s, with a mean response time of 42 +/- 2 s, and fatigued after 353 +/- 39 s. Hyperthermia alone or in combination with dehydration reduced mean response time (17-23%), VO(2 max) (16%), and performance time (51-53%) (all P < 0.01) but did not alter the absolute response time (i.e., the time to reach 63% response in the control trial, 3.2 +/- 0.1 l/min, 42 s). Reduction in VO(2 max) was accompanied by proportional decline in O(2) pulse and significantly elevated maximal heart rate (195 vs. 190 beats/min for hyperthermia vs. normal). Preventing hyperthermia in dehydrated subjects restored VO(2 max) and performance time by 65 and 50%, respectively. These results demonstrate that impaired high-intensity exercise performance with marked skin and internal body hyperthermia alone or in combination with dehydration is not associated with a diminished rate of rise in VO(2) but decreased VO(2 max).