Fluid volume measurements in normal subjects to disclose body hydration during acute bed rest.

Measuring intercompartmental, blood and urinary biochemical parameters during acute bed rest (ABR) and rigorous bed rest (RBR) the aim of this work was to disclose if ABR or RBR could influence significantly more and significantly faster the body hydration level in normal subjects. Studies conducted during pre-bed rest (BR) period of 3-days and during 7-days period of ABR and RBR. Thirty normal male individuals 24.6 +/- 5.7 years of age were chosen as subjects. They were divided into three groups: unrestricted active 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 subjected to RBR. RBRS were submitted to RBR in a predetermined date and time known to them right from the start of the study. UACS were not subjected to any form of BR. Fluid loss, urinary and plasma sodium (Na+) and potassium (K+), plasma osmolality, whole blood hematocrit (Hct) and hemoglobin (Hb), and total plasma protein level increased significantly (p < 0.05), while urinary osmolality, extracellular volume (ECV), plasma volume (PV), red cell volume (RCV), blood volume (BV), interstitial volume (IV) and fluid consumption decreased significantly (p < 0.05) in ABRS and RBRS compared with their pre-BR values and their control (UACS). However, the measured variables changed significantly (p < 0.05) more and significantly faster in ABRS than RBRS. Conversely, whole blood Hct and Hb levels, fluid consumption and fluid loss, urinary and plasma osmolality, urinary and plasma electrolytes, plasma protein, ECV, PV, RCV, BV and IV levels were not change in UACS compared with their pre-BR values. Significantly greater and significantly faster intercompartmental, blood and urinary biochemical changes were observed in ABRS than RBRS. Body hydration was affected significantly more and significantly faster in ABR than RBR. It was concluded, the more abruptly normal activity is restricted the smaller the body hydration in bed rested subject is, and probably in patients who are abruptly confined to RBR.