A number of high altitude-related syndromes are associated with abnormal salt and water retention. The first such syndrome was described in cattle grazing at high altitudes. This condition, called brisket disease because of fluid retention in the brisket area, is characterized by severe congestive heart failure due to pulmonary arterial hypertension (Hecht et al., 1959). Some degree of peripheral edema is often seen in temporary visitors to high altitudes. Hackett et al. (1981) noted that fluid retention is common to all forms of acute mountain sickness and postulated a spectrum of derangements in fluid metabolism in these conditions. Fluid retention also occurs in two high altitude-related syndromes recently described by these authors: Infants born at low altitude in mainland China and taken to reside at high altitudes in Tibet develop a fatal condition called subacute infantile mountain sickness (Sui et al., 1988). This condition is characterized by severe hypoxic pulmonary hypertension leading to congestive heart failure. The second condition, adult subacute mountain sickness, occurs in adults exposed to extreme altitude and presents with severe salt and water retention without significant pulmonary hypertension (Anand et al., 1990). Therefore, changes in fluid metabolism at high altitudes appear to have important pathogenetic implications and merit detailed study.
One of the major difficulties in making a meaningful interpretation of fluid metabolism at high altitudes is the lack of reliable data. Because experiments at high altitudes are difficult to organize, various approaches have been used to predict the response at high altitudes. These include acute and chronic experiments in hypobaric hypoxic chambers and use of low fraction of inspired oxygen (FIO2) gas mixtures at sea level. Studies during ascent (trekking, flight, etc.) have been made at different altitudes and after varying periods of time at high altitudes. Therefore, the lack of any uniformity in the degree or type of hypoxia makes the comparison of these studies very difficult. Moreover, serial measurements of body fluid compartments on the same subjects have seldom been made despite the fact that changes in fluid metabolism appear to depend on the duration of stay at high altitudes (Hannon and Rogers, 1975; Jain et al., 1980, 1981). Furthermore, some of the older studies were carried out with inadequate methodology, which makes interpretation of data difficult. Thus it is not uncommon to find the same group of investigators reporting different conclusions in successive studies because of methodological differences.