Heat Exhaustion with Dehydration
A Michigan National Guard infantry unit was sent at the end of May to Louisiana for a field training exercise. Spring in Michigan was cool, but during the exercise in Louisiana, the temperature reached at least 30°C (86°F) every afternoon. At 3:30 pm on the second day of the exercise, a 70-kg infantryman became unsteady and, after a few more steps, sat on the ground. He told his comrades that he was dizzy and had a headache. When they urged him to drink from his canteen, he took a few swallows and said that he was sick in his stomach.
At the field aid station, he is observed to be sweating, his rectal temperature is 38.5°C, and his pulse is rapid. He appears dazed, and his answers to questions are coherent but slow. He cannot produce a urine sample. Blood samples are drawn, and an intravenous drip is started. The laboratory report shows serum [Na+] of 156 mmol/L (normal range,135 to 145 mmol/L). Two liters of normal saline (0.9% NaCl) are infused over 45 minutes. Well before the end of the infusion, the patient is alert, his nausea disappears, and he asks for, and is given, water to drink. After the end of the infusion he is sent back to his unit with instructions to consume salt with dinner, drink at least three quarts of fluid before going to bed, and to return for follow-up in the morning. Questions
1. What is the likely basis of the patient's nausea, which also contributes to his inability to produce a urine specimen?
2. If we assume that the patient's total body water was 36 L when he came for treatment, it can be shown that giving the patient 3 L of water without salt (by mouth and/or as an intravenous infusion of glucose in water) would reduce serum [Na+] to 144 mmol/L. Such treatment would improve the patient's condition considerably. How might the medical officer argue the case for giving 2 L of normal saline?
3. What other (and relatively unusual) condition could produce the patient's symptoms? Did the medical officer rule this possibility out by appropriate means?
1. The patient's nausea is probably a result of constriction of the splanchnic vascular beds, which is part of the homeo-static cardiovascular response that helps maintain cardiac output and blood pressure when central blood volume is reduced. Central blood volume, in turn, was reduced by the loss of body water and pooling of blood in the peripheral vascular beds. This homeostatic response also includes constriction of the renal vascular beds, which, in turn, contributes (along with the release of vasopressin and activation of the renin-angiotensin system) to scanty urine production.
2. Because the weather was cool back home, the patient probably was probably not acclimatized to heat and was not conserving salt in his sweat. He was probably secreting large amounts of sweat, and losing correspondingly large amounts of salt because of the weather and the activity involved in the exercise. If the patient returns to training the next morning without correcting the salt deficit, he is likely to have further difficulties in the heat. Even if the medical officer has guessed incorrectly about the patient's salt balance, a patient with normal renal function and adequate fluid intake should be able to excrete any excess salt resulting from the treatment.
3. Hyponatremia can produce symptoms similar to the patient's symptoms. However, the medical officer was able to exclude hyponatremia (although not necessarily some degree of salt deficit) on the basis of elevated serum [Na+]. Giving a hyponatremic patient large volumes of fluid without an equivalent of salt (which would have been a reasonable alternative treatment for the patient in this example) would worsen the hyponatremia, perhaps to a dangerous degree.
Knochel JP. Clinical complications of body fluid and electrolyte balance. In: Buskirk ER, Puhl SM, eds. Body Fluid Balance: Exercise and Sport. Boca Raton, FL: CRC Press,
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.