Effect of Fluid Intake on Acute Changes in Plasma Volume: A Randomized Controlled Crossover Pilot Trial.

Plasma volume (PV) undergoes constant and dynamic changes, leading to a large intra-day variability in healthy individuals. Hydration is known to induce PV changes; however, the response to the intake of osmotically different fluids is still not fully understood. In a randomized controlled crossover trial, 18 healthy individuals (10 females) orally received an individual amount of an isotonic sodium-chloride (ISO), Ringer (RIN), or glucose (GLU) solution. Hemoglobin mass (Hbmass) was determined with the optimized carbon monoxide re-breathing method. Fluid-induced changes in PV were subsequently calculated based on capillary hemoglobin concentration ([Hb]) and hematocrit (Hct) before and then every 10 minutes until 120 min (t) after the fluid intake and compared to a control trial arm (CON), where no fluid was administered. Within GLU and CON trial arms, no statistically significant differences from baseline until t were found ( > 0.05). In the ISO trial arm, PV was significantly increased at t (+138 mL, = 0.01), t (+191 mL, < 0.01), and t (+182 mL, = 0.01) when compared to t. Moreover, PV in the ISO trial arm was significantly higher at t ( = 0.02), t ( = 0.04), and t ( = 0.01) when compared to the same time points in the CON trial arm. Within the RIN trial arm, PV was significantly higher between t and t (+183 mL, = 0.01) and between t (+194 mL, = 0.03) and t (+186 mL, < 0.01) when compared to t. These results demonstrated that fluids with a higher content of osmotically active particles lead to acute hemodilution, which is associated with a decrease in [Hb] and Hct. These findings underpin the importance of the hydration state on PV and especially on PV constituent levels in healthy individuals.