Heat Injury Prevention

Introduction

This section is intended to be a practical guide to preventing heat casualties in military training and operations. The first part introduces the three interacting factors that influence the risk of heat casualties: the soldier, the environment and the mission. The second part discusses preventing heat casualties among recruits in basic training and introduces the Appendices in the back of this handbook that can be used to estimate safe limits for heat stress exposure. The third part discusses operations planning to minimize heat casualties. The fourth section discusses medical planning and readiness to manage heat casualties should they occur.

Heat casualties are the result of the interaction of three factors: the condition of the soldier, the external heat stress from the environment and the internal heat stress required by the mission. Medical officers should assess each component in their preparation of plans for primary prevention of heat casualties.

The Soldier

A soldier is optimally capable to manage heat stress when he is fully hydrated, physically fit, acclimatized, well nourished and well rested.

Hydration is the most important element in a plan to prevent heat casualties. Full hydration is critical to the prevention of heat casualties because it is essential to maintain both blood volume for thermoregulatory blood flow and sweating. Both are reduced by dehydration. Consequently, the dehydrated soldier has less ability to maintain body temperature in the heat.

Water requirements are not reduced by any form of training or acclimatization. Exercises to teach soldiers to work or fight with less water are fruitless and dangerous.

!REMEMBER!

Soldiers cannot reduce thermoregulatory water requirements by water deprication diring training. Acclimatization does not reduce water requirements. Commanders must understand this principle and recognize its logistic and operational implications.

Avoiding heat casualties requires that soldiers drink enough water to replace what they lose. In hot environments, soldiers do not drink enough water to voluntarily to maintain hydration. This phenomenon has been called "voluntary dehydration", although there is nothing willful about it. In hot environments, thirst is not stimulated until plasma osmolarity rises 1-2% above the level customarily found in temperate climates. Consequently, if thirst is used as the guide to drinking, soldiers will maintain themselves at a level that is 1-2% dehydrated relative to their usual state. If soldiers are to fully replace the water they lose in their daily activities and eliminate voluntary dehydration, they must understand the need to drink even though they are not thirsty and leaders must enforce water drinking discipline.

Even in the face of a clear understanding of the importance of water and hydration, soldiers may decide that water drinking creates problems that outweigh its importance. For example, soldiers may not drink before going to sleep to avoid having to wake up and dress to urinate or they may not drink before convoys if no rest stops are planned.

Units which have soldiers who do not drink because they do not have opportunities to urinate have a leadership problem. Unit leaders must reinforce of hydration by planning for all aspects of adequate hydration: elimination as well as consumption.

The medical officer must be aware that soldiers may not follow drinking discipline. Be sure operations are planned so that drinking does not become a problem. Be aware of the soldiers hydration status. Urine color, body weight change and orthostatic blood pressure change can all be used as guides to hydration.

CONSEQUENCES OF DEHYDRATION

Acutely, mild dehydration (2-3% of body weight) reduces physical capacity and heat tolerance. As dehydration progresses, cognitive function deteriorates and both thermoregulation and physical capacity become seriously compromised. 5-6% dehydration is incompatible with further functioning.

Chronic mild dehydration is associated with renal stones and urinary infection, severe constipation, rectal afflictions and cutaneous drying.

In hot environments, water losses can reach 15 liters per day per soldier. Complete replacement requires realistic estimates of potable water requirements, an adequate water logistic system and soldiers who understand and act on their water requirement. Water for hygiene will be needed in addition to water for drinking.

There is no advantage to carbohydrate/electrolyte beverages beyond their palatability which may encourage drinking. They should not be the sole source of water as they can be mildly hypertonic.

Aerobic fitness provides the cardiovascular reserve to maintain the extra cardiac output required to sustain thermoregulation, muscular work and vital organs in the face of heat stress. In addition, regular strenuous aerobic physical training will provide a small degree of heat acclimatization.

Regardless of their physical condition, however, soldiers who are required to deploy on short notice to hot environments, will arrive incompletely acclimatized. Adequate acclimatization will require several days to achieve. During this initial acclimatization period, soldiers must be provided copious quantities of water and carefully supervised to prevent excessive heat exposure. If possible, work tasks should be regulated using work-rest cycles tailored to the soldiers' physical capacity by direct medical oversight.

In the first few days of acclimatizing, sweat salt conservation will not be fully developed. Salt depletion is a risk if soldiers are exposed during this time to sufficient heat or work stress to induce high sweating rates (>several liters per day), particularly if ration consumption is reduced. Salt depletion can be avoided by providing a salt supplement in the form of salted water (0.05 to 0.1%). A 0.1% salt solution can be prepared using the directions outlined in Appendix A to this handbook. Acclimatization should eventually eliminate the need for salt supplementation.

The requirements of military operations frequently mean lack of sleep and missed meals. All these factors reduce thermoregulatory capacity and increase the risk of heat injury. Recommendations to planning staffs should emphasize the importance of adequate sleep and food to reduce the likelihood of heat casualties.

Coincidental illnesses increase heat casualty risk through fever and dehydration. The consequences of dehydration are discussed above. Fever, whether due to immunization or illness, reduces thermoregulatory capacity by resetting the hypothalamus toward heat conservation