CARDIOVASCULAR AND RENAL EFFECTS OF HEAD-OUT WATER IMMERSION IN MAN - APPLICATION OF MODEL IN ASSESSMENT OF VOLUME HOMEOSTASIS

Studies of the circulatory, renal and endocrine changes induced in man by water immersion demonstrated that immersion in the seated posture results in a redistribution of blood volume with a relative central hypervolemia. Profound alterations in fluid and electrolyte homeostasis ensue including a marked natriuresis, kaliuresis, diuresis and suppression of the renin-aldosterone system and ADH [antidiuretic hormone] release. Characterization of the immersion model and the demonstration that it constitutes a potent central volume stimulus that does not necessitate infusing exogenous volume expanders permitted its successful application in the investigation of abnormal Na and water homeostasis in patients with decompensated cirrhosis and hypertension. The successful application of the immersion model and its future use as an investigative tool to dynamically assess the control mechanisms for the renin aldosterone system in normal man and in a wide array of disease states characterized by a deranged volume homeostatis are discussed. In an edematous or hypertensive subject, water immersion constitutes a less hazardous means to evaluate the effects of volume expansion than administration of a saline load. In contrast to saline administration, water immersion is associated with a decrease in body weight rather than an increase, is associated with a significant decrease in mean arterial blood pressure in contrast to the increase in blood pressure induced by saline administration and the volume stimulus of immersion is promptly reversible after cessation of immersion in contrast to the relatively sustained hypervolemia which follows saline administration.