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FLUID REPLACEMENT AND HEAT STRESS
Potassium deficiency under the conditions described above could also occur as a result of diffuse skeletal muscle injury secondary to intense exertion, especially when conducted during hot weather. Thus, realizing that muscle cell injury or rhabdomyolysis, as reflected by elevated muscle enzyme activity in the blood, invariably occurs when an untrained individual is subjected to severe muscular exercise in the heat (Demos et al., 1974), it would seem logical to assume that injured muscle cells could not retain sufficient ion transport activity or membrane integrity to maintain the normal distribution of sodium and potassium ions between the muscle cell and the plasma (Bilbrey et al., 1973). If this were so, potassium would leak from muscle cells and be excreted into the urine.
A study was designed to examine these possibilities (Knochel et al., 1972). Healthy young Army recruits who were in good physical condition but untrained and poorly acclimatized to heat were studied during the summer and another group was studied during the winter. The two groups were studied while they were undergoing basic training conducted at Fort Sam Houston, Texas. Training activities were identical to those performed by recruits in basic training at other basic training facilities, with the exception that weapons training was replaced by field training for medical corpsmen activities. Training activities on many of these days were of 12 to 14 hours in duration. The caloric expenditure under such conditions probably exceeds that associated with weapons training. Each day the men consumed a constant diet containing 4,135 kcal that included 100 mEq of potassium, 149 g of protein, 158 g of fat, and 556 g of carbohydrate. Sodium chloride intake was 150 mEq/day in one group and 350 mEq/day in another. The men consumed their diets under the direct observation of a trained dietitian each day throughout the study. Total body potassium content was estimated by weekly determination of exchangeable 42K, which was then indexed as a function of lean body mass. Lean body mass was estimated from body density and from total body water. Each Thursday morning of the study body density was estimated by weighing the men underwater and measuring the total body volume after subtracting measured lung capacity. On the same morning, total body water was estimated by tritium dilution. Tritiated water was given by mouth. Tritiated aldosterone and NaS35O4, which were used to measure aldosterone secretory and excretory rates and extracellular fluid volume, were administered intravenously. Sampling for these determinations was conducted at appropriate intervals after measurement of total body volume. On the same day, a 24-h urine collection was obtained for measurements of creatine, creatinine, urea, calcium, phosphorus, electrolytes, and osmolality. Blood was obtained for measurements of the same biochemical parameters, and in addition, creatine kinase activity was measured as an index of muscle