Fluid Intake

In addition to improving cooling mechanisms, acclimatization can increase plasma volume up to 30%. An increase in plasma volume gives the body more leeway in terms of maintaining body fluids. No matter what the increase, there will come a time when fluid replacement is needed. How much fluid and what should be in that fluid is a topic of ongoing investigation. While some of the literature is based on good scientific work, some is based on perceived experience including out-dated information. Some is also based on the claims of manufacturers with a product to sell. Caution is advised (17,18).

Increasing plasma volume, either by acclimatization or by increased pre-exposure fluid intake has been proposed as a way of improving performance in the heat. While it is true that plasma volume may expand in acclimatization, studies have shown that plasma volume expansion alone is not adequate to improve tolerance to heat. In fact, over-expansion can result in dilution of vital electrolytes and can worsen the clinical situation (18).

Salt tablets have long been recommended as a means of preventing heat-related illness, but their use is controversial. Problems arise when salt tablets are taken without adequate fluid intake, which is counterproductive in terms of preventing heat-related illness. Far more effective are pre-mix solutions containing both water and electrolytes. Many studies have been done on various fluid-electrolyte combinations. The results appear to depend on what is hypothesized and who is doing the study. A balanced electrolyte solution is likely the best way to replace fluids. As a general guide to fluid replacement, at temperatures less than 82°C, one-half to three-quarters quart water per hour is recommended, depending on workload. As the temperature rises above 90°C, at least a quart of fluids per hour is needed (Figure 33.1) (19-25).

Figure 33.1. Times of exposures to heat in an hour under various conditions of work-rest cycles. (Data from U.S. Army Research Institute of Environmental Medicine (24).

Figure 33.1. Times of exposures to heat in an hour under various conditions of work-rest cycles. (Data from U.S. Army Research Institute of Environmental Medicine (24).

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