Elsevier

Injury

Volume 37, Issue 1, January 2006, Pages 41-45
Injury

The effect of IV cannula length on the rate of infusion

https://doi.org/10.1016/j.injury.2005.09.001Get rights and content

Summary

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 13 mm. 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.

Introduction

The importance of rapid fluid replenishment in shock due to either trauma or surgery cannot be overstated. Various methods have been shown to increase infusion rates, which include using a wide bore cannula; applying pressure to the fluid bag9; shortening the infusion tubing2 and using special infusion systems.7

In an infusion system, the individual components are connected in series. Hence, the narrowest part offers maximum resistance to the flow in vitro and is the rate-determining factor for a specific fluid or giving set. The IV cannula is the narrowest part of an infusion system and hence is the rate-limiting step to the flow.4 Hagen–Poiseuille's law governs flow through a tubular structure. According to this law flow rate is proportional to fourth power of the radius of the tube and inversely proportional to its length. When Hagen–Poiseuille's law is applied to an IV cannula, an increase in the flow rate is expected when the diameter of the IV cannula is increased or its length is decreased. It is well recognised that higher flow rates can be obtained with wider cannulae. However, little is known about the effect of the length of the cannula on the flow rates.

Table 1 shows the dimensions of commonly used IV cannulae in UK (Venflon). It is evident that a 20 G cannula is 32 mm long whereas wider cannulae are 45 mm long. Despite a diligent search, we could not identify any scientific or practical reason for different lengths of the cannulae (personal communication with R&D department of Becton Dickinson, Helsingborg, Sweden). To the best of our knowledge, no study has been published to date on this subject. The current in vitro study was designed to investigate the effect of shortening IV cannulae of various diameters on flow rates.

Section snippets

Methods

The equipment consisted of a sterile blood infusion set RMC2071B (Baxter S.A., B-7860 Lessiness) and IV cannulae of sizes 14 G, 16 G, and 18 G (‘Venflon’ Becton Dickinson, Helsingborg, Sweden). ‘Stitch cutter blades’ (Swann Morton Ltd., Sheffield, UK), which have a curved cutting edge were used to shorten the cannulae. The cannulae were marked at 32 mm (length of a 20 G venflon) and were cut in a single sweeping motion with the stilette inside the cannula. A fresh blade was used to cut each cannula.

Mathematical model

According to Hagen–Poiseuille's law1 Q=ΔPr4π/8η˙l, for laminar flows, where Q is rate of flow, ΔP the pressure gradient, r the radius of the tube, η˙ viscosity of the fluid, l length of the tube. If the pressure gradient and the viscosity of the fluid are kept constant, the Hagen–Poiseuille's law can be simplified to Q = Kr4/l or Q = Kd4/l, where K and K′ are constants and ‘d’ is the diameter of the tube (d = 2r). Therefore, the rate of flow of a given fluid through a tubular structure is directly

Results

The maximum rates of infusion of IV cannulae (‘Venflon’ Becton Dickinson, Helsingborg, Sweden) as stated by the manufacturers are shown in column 2 of Table 2. Flow rates of 18 G, 16 G and 14 G cannulae after shortening to 32 mm were mathematically calculated from Hagen–Poiseuille's law and are shown in column 3 of Table 2. Forty percent increase in the flow rates was predicted if the cannulae were shorter by 13 mm (column 4 of Table 2). Further, this would mean that the time to infuse 1 L of fluid

Discussion

This study demonstrates that the flow rates increase and the average time required to infuse 1 L of normal saline decrease on shortening 18 G, 16 G and 14 G IV cannulae by 13 mm to the length of a 20 G cannula.

The flow rates of the regular (45 mm long) IV cannulae observed in this study are similar to the flow rates claimed by the manufacturer and rates obtained in previous studies4, 9, thus validating the accuracy of methods used. Cutting the IV cannula in order to shorten it, may alter its

Acknowledgements

We would like to thank Dr. Gemmell, Consultant & Clinical Director of Departments of Anaesthesia and Critical Care, Wrexham Maelor Hospital for support and supply of the materials required for the study. We also like to thank Barbara King, Urology Research nurse at Countess of Chester Hospital for her help with uroflowmeter and Phil McShane, University of Oxford for statistics.

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