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_0–120) 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 t₁₂₀ were found (p > 0.05). In the ISO trial arm, PV was significantly increased at t₇₀ (+138 mL, p = 0.01), t₈₀ (+191 mL, p < 0.01), and t₁₁₀ (+182 mL, p = 0.01) when compared to t₀. Moreover, PV in the ISO trial arm was significantly higher at t₇₀ (p = 0.02), t₁₁₀ (p = 0.04), and t₁₂₀ (p = 0.01) when compared to the same time points in the CON trial arm. Within the RIN trial arm, PV was significantly higher between t₇₀ and t₉₀ (+183 mL, p = 0.01) and between t₁₁₀ (+194 mL, p = 0.03) and t₁₂₀ (+186 mL, p < 0.01) when compared to t₀. 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.