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during exercise were about 2.9 g/100 liter of air (body temperature pressure saturated or BTPS2).

Another factor to consider with the increases in ventilation associated with altitude is increased respiratory heat loss. Given the same ventilatory rate at sea level and 4,572 m (15,000 ft) (i.e., a lower workload at altitude) and no decrease in expired air temperature, it is estimated that at the same ventilatory rate of 100 liter/min, the respiratory heat loss would be 1.6 times greater (Gonzalez et al., 1985).

It is generally believed that an altitude-induced diuresis normally occurs during the initial phases of altitude acclimatization when acute mountain sickness, high-altitude pulmonary edema, and high-altitude cerebral edema are not present. When significant symptomology of altitude illness is present, there is evidence of fluid retention and weight gain (Bärtsch et al., 1991; Hackett et al., 1981, 1982; Singh et al., 1969).

Exercise

Exercise is an inescapable consequence of traveling to high mountainous areas. The two factors associated with exercise that relate to weight loss are the increased water loss caused by high ventilatory rates and the increased energy output. The influence of exercise and energy expenditure on nutrition is covered by Schoene and by Hoyt and Honig (see Chapters 17 and 20 in this volume).

Sleep

The quality and quantity of sleep at altitude are important contributory factors that determine whether individuals acclimatize well or slowly deteriorate. Disturbances in sleep are related to the terrestrial elevation and the length of exposure. Many studies at very high and extreme altitudes confirm that sleep arousals of up to 30 to 40 per night can occur. These arousals are probably caused by increased episodes of periodic breathing. There is also a concurrent 50 percent reduction in total sleep time and a fivefold reduction in the REM (rapid eye movement) sleep stage. Less-disturbed sleep is observed at lower altitudes, but the phenomena may still last for weeks. Individuals commonly report "being awake half the night," "not being able to sleep," and having "frequent disturbing dreams." Problems of sleeping can contribute to mood changes during the day, loss of appetite, and daytime somnolence

2  

BTPS is one of the sets of standard conditions under which pulmonary gas volume can be defined; the volume that is actually exhaled.



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