Heat tolerance following diuretic induced dehydration

In an effort to assess the effects of acute fluid loss on body temperature regulation during exercise, seven subjects (one female and six males) were studied on two occasions during two hours of cycling (35% Vo2max) in a hot environment (39 degrees C = DB, 35% = RH). One trial (D) was conducted following a 3% body weight reduction, achieved with 40-80 mg oral administration of a diuretic "lasix." A control exercise bout was performed with the subject normally hydrated (H). As a result of the diuresis, plasma volume (PV) decreased 15.3% (SE +/- 1.3), while heart rates during exercise increased 20-25 beats/min when compared to the H condition. Although significantly greater increases (P less than 0.05) in rectal and muscle temperatures occurred during exercise in the D trial, mean skin temperature (TS) was lower. Skin blood flow estimated from conductance values averaged 800 ml/min less during the D condition than during exercise in the H trial. Since the preliminary diuresis had no affect on sweat rate during exercise (D = 0.80 vs. H = 0.80 liters/hr), body heat dissipation appeared limited mainly by a reduction in peripheral blood flow. The diuretic induced dehydration was found to increase the sensitivity of some subjects to the exercise-heat stress, producing symptoms of heat exhaustion.