Plasma volume and blood viscosity during 4 h sitting in a dry environment: effect of prehydration.

Background: Deep vein thrombosis and pulmonary thromboembolism are potential problems for travelers, including those who fly. We hypothesized that prehydration with an electrolyte-glucose beverage (EGB) would be better than water for maintaining body fluid balance and preventing increased blood viscosity in immobilized men. Methods: There were 12 healthy men (24-38 yr) who participated in crossover trials of prehydration using EGB and H 2 O as well as a control condition (Con) with no prehydration. Fluid intake was set at 6 ml kg - 1 body weight (mean 418 ml). For each trial, subjects sat for 4 h at a dry-bulb temperature of 23.0-23.5°C and a relative humidity of 18-36%. Plasma volume (PV) and whole blood viscosity (Bvis) were determined every hour; routine laboratory hematological tests, urine volume, and body weight were recorded at 2 h and 4 h. Results: For Con, subjects lost approximately 110 ml h - 1 ; at 2 h, PV had decreased significantly by 3.4%, and Bvis had increased significantly by 9.3%, with no further change at 4 h. For prehydration, retention of the consumed fluid at 2 h was significantly higher for EGB (57%) than for H 2 O (38%), while both drinks prevented significant change in PV and Bvis. There were no significant differences between trials in coagulation variables, but Bvis measured at higher shear rates for EGB were significantly attenuated compared with Con. Conclusion: EGB and water prevented the increase of blood viscosity that occurred without prehydration. EGB was better than water for maintaining body fluid balance and preventing hypovolemia.