Operator Functional State Assessment (l'evaluation de i'aptitude operationnelle de i'operateur humain)

Adequate hydration is essential for maintaining fighting effectiveness, and several common operational stresses can result in relatively large alterations in TBW content and distribution. During most "normal" conditions, humans have little trouble maintaining optimal fluid balance. However, many factors such as sickness, physical exercise, climatic exposure (heat, cold, and altitude), and psychological strain can lead to significant disturbances in water balance (Sawka, 1988). Perhaps the best example involves heat stress and physical activity. For sedentary persons in temperate conditions, water requirements usually range from 2 to 4 L per day and the kidneys primarily regulate fluid balance. For physically active persons who are exposed to heat stress, water requirements can often double (Sawka Montain, & Latzka 2001). Water is the largest single constituent of the body (50- 70% of body weight) and is essential for supporting the cardiovascular and then thermoregulatory systems and cellular homeostasis. TBW is distributed into intracellular fluid (ICF) and extracellular fluid (ECF) compartments. Exercise-heat stress not only stimulates fluid loss (primarily through sweating) but also induces electrolyte imbalances and renal function changes%. As a result, fluid losses and gains with and without proportionate solute changes can occur. In addition, exercise-heat stress will alter transcompartmental and transcapillary% forces that redistribute fluids between various compartments, organs, and tissues (Sawka et al., 2001). For these reasons, the accuracy of most methods used to assess hydration status is highly limited by the circumstances in which the measurements are made and the purposes for which they are intended. TBW is the "gold standard" measurement to assess hydration status (Aloia, Vaswani, Flaster, & Ma, 1998; Lesser & Markofsky, 1979). TBW can be directly measured with doubly labeled water (DLW) and other dilution techniques.