Effect of crystalloid infusion on hematocrit and intravascular volume in healthy, nonbleeding subjects

We evaluated the effect of IV crystalloid administration on hematocrit in 28 healthy, nonbleeding volunteers. Normal saline boluses of 10, 20, and 30 mL/kg were delivered at a mean rate of 115 +/- 4 mL/min, followed by a continuous infusion of either 1 or 5 mL/kg/hr. Hematocrit values determined immediately after bolus infusion declined from baseline a mean +/- SEM of 4.5 +/- 0.6 (P less than .001), 6.1 +/- 0.4 (P less than .001) and 6.3 +/- 0.6 points (P less than .001) in the 10, 20, and 30 mL/kg groups, respectively. Twenty minutes into the maintenance infusion, hematocrits had risen 1.5 +/- 0.8 (P = .03), 2.4 +/- 0.4 (P = .004) and 2.3 +/- 0.7 points (P = .005) when compared with post-bolus hematocrits in the 10, 20, and 30 mL/kg groups, respectively. These data show that in healthy, nonbleeding subjects, crystalloid infusion may cause significant changes in hematocrit. Calculations reveal that approximately 60% of infused saline, when delivered as a bolus, will diffuse from the intravascular space within 20 minutes of administration.