To the Editor:
Common knowledge is that lack of fluid intake can cause a sudden uncontrolled rise in body temperature resulting in cooking of the brain or heat stroke; therefore, recommended is to take in fluids with salt periodically. I was surprised to read the author's conclusion of restricting water to avoid `death by water intoxication' in recruits or when exercising vigorously [Military Medicine, 167:432-434 and 435-437]. Extremely rare is for someone to get water intoxication in otherwise healthy individuals unless its psychogenic polydipsia. Physicians see water intoxication in people with psychological, kidney or neurological problems and those taking certain medications such as those with anticholinergic side effects such as dry mouth. The importance of maintaining adequate hydration is critical, but so is maintaining sodium (salt) balance. When one sweats, one loses water and salt. If one replenishes water without also replenishing the salt, the water dilutes the sodium in the blood. In turn, the dilution causes the brain, and other organs, to take the extra fluid in to try to balance the higher sodium concentration in the cell compared with the low sodium concentration outside the cell. The consequence of trying to balance the sodium causes the brain to swell with water and eventually causes death. The recommendation of the author should have been to replenish salt while replenishing fluids, but not to reduce water intake even if the water intake were truly measured and not speculated as was the case with the recruits. If one takes salt with fluids, the concentration of the sodium inside and outside the cells of the brain remains balanced, in turn keeping the water distribution equal. Drinking too much water did not cause the recruits' death; the deaths occurred because the recruits did not take enough salt when they replenished the water they lost from sweating. The author's own data tell us this.
The author noted low blood sodium levels in all of the cases, which by itself can be misleading causing one to think of water intoxication, or water imbalance, but in reality it is an imbalance in the salt concentration. Implementing the author's suggestion calling for a new guideline that only reduces water intake is contrary to basic physiology.
Richard McDonald, JD., Ph.D. Chief Scientist
Department of Advanced Developments AUI, Inc. Geno Var Diagnostics
To the Editor:
Dr. McDonald raises some important points about the role of fluid intake in prevention of exertional heat illness. As we stated, "The importance of maintaining adequate hydration in exertional heat illness prevention cannot be overemphasized..." Serious problems occur, however, when water intake far exceeds that required to maintain hydration and replace losses due to sweating. Although heavy sweating can occur at rates up to 2-3 L per hour, fluid absorption during heavy exercise appears to be limited to 1-2 L per hour. Thus it becomes impossible to keep up with these losses, and this level of sweating (and corresponding exertion) cannot be sustained due to progressively worsening dehydration. Even in the context of large fluid losses, oral intake beyond 1-2 L per hour may result in severe hyponatremia and its serious consequences, as in the cases we discussed.
We agree that water intoxication in otherwise healthy individuals has been extremely rare, but in the past decade several deaths and cases of serious illness have occurred, which stimulated our reports. In all cases discussed in our papers, the circumstances are analogous to the phenomena seen in psychogenic polydipsia, since excessive water intake is a result of misguided perceptions of the volume of water needed. All of our cases were "associated with more than 5 L (usually 10-20 L) of water intake during a period of a few hours" - far beyond that needed to replenish the water they lost from sweating. Rather than recommending `restricting water' and to `reduce water intake,' we have simply recommended avoiding the extremely excessive water intake that occurred in the cases we discussed. At the same time, we need to emphasize that fluid intake up to 1- 1.5 L per hour may be necessary to avoid dehydration and exertional heat illness when performing heavy exercise.
The `common knowledge' Dr. McDonald refers to that `lack of fluid intake can cause a sudden uncontrolled rise in body temperature resulting in cooking of the brain or heat stroke' is a common myth. We must emphasize that the brain does not `cook,' dehydration does not precipitate a large increase in body temperature, nor does heat stroke represent a phenomenon of thermal tissue damage. Elevated body temperature results from extensive heat production during exercise which exceeds thermal dissipation capacity, and temperatures up to at least 106 OF are extremely well tolerated. Rectal temperatures are commonly seen above 108deg F in exercising individuals without heat stroke, and there is a case report of recovery in a heat stroke survivor with body temperature in excess of 115deg F. The specific pathophysiology of heat stroke is poorly understood, but its serious complications reflect metabolic and circulatory disorders, rather than thermal damage from high tissue temperatures. Dr. McDonald's focus on replenishing salt is important, but salt replacement is adequate if a normal American (high salt) diet is consumed. In addition, once acclimatization is accomplished (within 2 weeks of daily exercise in a warm climate), salt losses in sweat are minimal. The antiquated practice of ingesting salt tablets is dangerous, as it ignores the need for water replacement and salt ingestion increases water requirements. If carried to excess, this antiquated practice may lead to extracellular fluid volume expansion and its consequent adverse effects. In trying to understand the complexities of salt and water balance, one must not only consider the concentration of sodium in the body fluid compartments, but also the volume. A recommendation to replenish salt along with unlimited fluids ignores the physiologic issues of rate limitations of water absorption from the gut and excretion by the kidneys, especially during vigorous exercise. Our guidance is to "Replace water losses hour by hour, and salt losses day by day."
COL John W Gardner, MC, FS, USA Frank D. Gutmann, MD, MPH
Copyright Association of Military Surgeons of the United States Mar 2003
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