Using Poiseuille's law and standardized gauge sizes, an 18-gauge (g) intravenous catheter (IV) should be 2.5 times faster than a 20-g IV, but this is not borne out by observation, in vitro testing, and manufacturer's data. Our objective was to determine if the infusion rate of a single 18-g IV was equivalent to the infusion rate of two 20-g IVs.

This was a prospective study in healthy adult volunteers. Subjects simultaneously received 500 mL of normal saline via an 18-g IV in one arm and 500 mL of normal saline via two 20-g IVs in the other arm. We measured the rates of fluid administration. Paired Student's t test was used for comparison of the 2 arms of the study. We estimated that 18 trials were needed in sample size analysis.

Eighteen trials were completed. The mean infusion rate for a single 18-g 500-mL IV administration was 35.6 mL/min (95% confidence interval [CI], 30.3-40.8), with manufacturer's rating being 105 mL/min. The mean infusion rate for two 20-g IVs was 41.3 mL/min (95% CI, 36.1-46.4), with manufacturer's rating being 120 mL/min. The rate of infusion via two 20-g IVs were statistically significantly faster than the single 18-g IV, with a mean difference in flow rate of 5.7 mL/min (95% CI, 1.3-10; P = .026).

In healthy volunteers, administration of intravenous fluids through two 20-g IVs is faster than a single 18-g IV, although both approaches are markedly slower than the manufacturer's estimates.