The effect of IV cannula length on the rate of infusion.

According to the Hagen-Poiseuille's law, rate of laminar flow through a tubular structure varies directly with fourth power of its radius and inversely with its length. Although it is well recognised that faster infusion rates can be achieved with wider-bore IV cannulae, the effect of length on flow rates is less well known. In the current in vitro study, we assessed the effect of length of an IV cannula on the rate of flow of infusion. Mathematical calculations performed using Hagen-Poiseuille's law predicted an increase of 40% in flow rates when the IV cannulae were shortened by 13mm. However, when the flow rates of the shortened cannulae were measured in vitro an increase of only 4-18% was observed. Although the increase in flow rates was statistically significant, it may not be sufficient to be significant in clinical practice. Turbulence resulting from design characteristics of the infusion system is responsible for the measured flow rates to be lower than that predicted by mathematical calculations.