DEHYDRATION MARKEDLY IMPAIRS CARDIOVASCULAR FUNCTION IN HYPERTHERMIC ENDURANCE ATHLETES DURING EXERCISE

We identified the cardiovascular stress encountered by superimposing d ehydration on hyperthermia during exercise in the heat and the mechani sms contributing to the dehydration-mediated stroke volume (SV) reduct ion. Fifteen endurance-trained cyclists [maximal Ca consumption (VO2ma x) = 4.5 1/min] exercised in the heat for 100-120 min and either becam e dehydrated by 4% body weight or remained euhydrated by drinking flui ds. Measurements were made after they continued exercise at 71% VO2max for 30 min while 1) euhydrated with an esophageal temperature (T-es) of 38.1-38.3 degrees C (control); 2) euhydrated and hyperthermic (39.3 degrees C); 3) dehydrated and hyperthermic with skin temperature (T-s k) Of 34 degrees C; 4) dehydrated with T-es of 38.1 degrees C and T-sk Of 21 degrees C; and 5) condition 4 followed by restored blood volume . Compared with control, hyperthermia (1 degrees C T-es increase) and dehydration (4% body weight loss) each separately lowered SV 7-8% (11 +/- 3 ml/beat; P < 0.05) and increased heart rate sufficiently to prev ent significant declines in cardiac output. However, when dehydration was superimposed on hyperthermia, the reductions in SV were significan tly (P < 0.05) greater (26 +/- 3 ml/beat), and cardiac output declined 13% (2.8 +/- 0.3 1/min). Furthermore, mean arterial pressure declined 5 +/- 2%, and systemic vascular resistance increased 10 +/- 3% (both P < 0.05). When hyperthermia was prevented, all of the decline in SV w ith dehydration was due to reduced blood volume (similar to 200 mi). T hese results demonstrate that the superimposition of dehydration on hy perthermia during exercise in the heat causes an inability to maintain cardiac output and blood pressure that makes the dehydrated athlete l ess able to cope with hyperthermia.