Plasma volume changes during and after acute variations of body hydration level in humans

This study examined plasma volume changes (Delta PV) in humans during perio ds with or without changes in body hydration: exercise-induced dehydration, heat-induced dehydration and glycerol hyperhydration. Repeated measurement s of plasma volume were made after two injections of Evans blue. Results we re compared to Delta PV calculated from haematocrit (Hct) and blood haemogl obin concentration ([Hb]). Eight well-trained men completed four trials in randomized order: euhydration (control test C), 2.8% dehydration of body ma ss by passive controlled hyperthermia (D) and by treadmill exercise (60% of their maximal oxygen uptake, (V) over dot O-2max) (E), and hyperhydration (H) by glycerol ingestion. The Hct, [Hb], plasma protein concentrations and plasma osmolality were measured before, during and after the changes in bo dy hydration. Different Hct and [Hb] reference values were obtained to allo w for posture-induced variations between and during trials. The Delta PV va lues calculated after two Evans blue injections were in good agreement with Delta PV calculated from Hct and [Hb]. Compared to the control test, mean plasma volume declined markedly during heat-induced dehydration [-11.4 (SEM 1.7)%] and slightly during exercise-induced dehydration [-4.2 (SEM 0.9)%] (P < 0.001 compared to D), although hyperosmolality was similar in these tw o trials. Conversely, glycerol hyperhydration induced an increase in plasma volume [+7.5 (SEM 1.0)%]. These results would indicate that, for a given l evel of dehydration, plasma volume is dramatically decreased during and aft er heat exposure, while it is better maintained during and after exercise.