Fluid volume compartments and biochemical measurements for disclosing fluid depletion during acute and rigorous bed rest in normal subjects.

Rigorous bed rest (RBR) induces fluid depletion, however, little is known about the mechanisms of development of fluid depletion during acute bed rest (ABR). Measuring fluid retention and body hydration parameters, the aim of this study was to establish whether ABR than RBR could depress significantly more fluid retention contributing to greater fluid depletion development. Studies were conducted during pre bed rest (BR) period of three days and during seven days period of ABR and RBR. Thirty normal male individuals aged, 24.2 +/- 5.0 years chosen as subjects. They were divided into three groups: unrestricted ambulatory control subjects (UACS), acute bed rested subjects (ABRS) and rigorous bed rested subjects (RBRS). Acute bed rested subjects confined abruptly to RBR, while they did not have any prior knowledge of the exact date and time when they would be asked to stay in bed. Rigorous bed rested subjects submitted to RBR on a predetermined date and time known to them right from the start of the study. Unrestricted ambulatory control subjects were not confined to any type of bed rest. Fluid loss, urinary and plasma sodium (Na+) and potassium (K+), urinary osmolality and plasma osmolality, whole blood hematocrit (Hct), whole blood hemoglobin (Hb), and total plasma protein increased significantly (p < 0.05) with time, while fluid retention, extracellular volume (ECV), plasma volume (PV), red cell volume (RCV), blood volume (BV), interstitial volume (IV) and fluid intakes reduced significantly (p < 0.05) with time in ABRS and RBRS compared with their pre-BR values and the control values (UACS). However, the measured parameters changed significantly (p < 0.05) more with time in ABRS than RBRS. Fluid rention, fluid loss, fluid intake, urinary and plasma Na+ and K+, Hct, Hb, plasma protein, urinary and plasma osmolality, ECV, PV, RCV, BV and IV did not change in UACS compared with their pre-BR values. It was concluded that significant increase of urinary and plasma osmolality, whole blood Hct and Hb, total plasma protein, plasma Na+ and K+, and significant decrease of ECV, PV, RCV, BV, and IV may demonstrate fluid depleteion, while a higher increase of fluid loss in spite of fluid depletion may show decreased fluid retention. Dissociation between fluid losses and fluid depletion may suggest the presence of impair fluid retention as a mechanism of development of fluid depletion.