Fluid balance measurements in disclosing fluid deposition during prolonged hypokinesia in healthy subjects.

Measuring fluid absorption and fluid homeostasis, the aim of this study was to establish if hypokinesia (HK) could depress fluid deposition and thus contribute to the development of fluid depletion. Studies were performed during 30 days pre-HK period and during 364 days HK period. Twenty healthy male individuals 24.0 +/- 6.6 years of age were chosen as subjects. They were equally divided into two groups: active control subjects (ACS) and hypokinetic subjects (HKS). All HKS were walking average distances of 0.7 +/- 0.2 km x day(-1) for 364 days, while all ACS were running average distances of 8.5 +/- 1.2 km x day(-1) for 364 days. Water imbalance, whole blood hemoglobin (Hb) and hematocrit (Hct), plasma protein, plasma osmolality, urinary and plasma sodium (Na+) and potassium (K+) levels and fluid loss increased significantly (p < 0.05), while fluid absorption, fluid consumption, glomerular filtration rate (GFR), and renal blood flow (RBF) reduced significantly (p < 0.05) in HKS compared with their pre-HK values and their respective active controls (ACS). Conversely, water balance, whole blood Hb and Hct, plasma protein, plasma osmolality, fluid absorption, GFR, RBF, urinary and plasma Na+ and K+ levels, fluid consumption and fluid loss did not change in ACS compared with their pre-HK control values. Significant increase of fluid loss with fluid imbalance may demonstrate decreased fluid deposition. Dissociation between fluid loss and fluid imbalance may demonstrate decreased fluid deposition as the mechanism of development of fluid depletion. It was concluded that fluid imbalance and the significant increase of Hb, Hct, plasma protein, plasma osmolality, urinary and plasma Na+ and K+ levels may demonstrate the presence of fluid depletion during prolonged HK in humans.