Issn Print: 0265-0215

Publication Date: 2007/06/01


Print

Flow through intravenous cannulae: neither easily predictable nor laminar: 3AP8-8

D. McPherson; O. Adekanye; A. R. Wilkes; J. E. Hall

    loading  Checking for direct PDF access through Ovid

Excerpt

Background and Goal of Study: Predicting flow through an intravenous cannula (IVC) is useful to clinicians if changes in flow are required and to guide selection of IVC. We set out to find the usefulness of manufacturers' quoted flows according to standard tests (1) in predicting actual flow and to characterise that flow.
Materials and Methods: We built a model of a vein and inserted IVCs of varying gauges into it. The height of the fluid bag above the vein was varied to set the filling pressure and the flow through the IVC was measured.
Results and Discussions: The graph shows measured (•) and theoretical (▴) flow at a pressure of 10 cmH2O and the quoted (▪) flow. The flow through an IVC is not a consistent ratio of the quoted maximum flow: at 100cmH20 it is 0.83[0.06] (mean[sd]) for 0.75 mm ID IVCs compared with 0.72[0.04] for 1.55 mm. Flow at the manufacturer's quoted rate is turbulent (best curve fit flow = 113.3[1.41]*ID2.21[005] (mean [standard error]), as is the flow at rates above 12ml.min−1 (flow = 2.87[0.13]*pressure0.62[0.01] in 20G IVCs). This is despite Reynolds' number being as low as 328.
Conclusion(s): Flow through IVCs is turbulent at the upper range of clinically used flows, therefore Poiseuille's law is not useful in predicting flow and the effect of changing radius is less than commonly believed. Neither are the quoted maximum flows easily useful. There are many determinants of laminar flow apart from Reynolds' number. Further work would determine useful correlates of flow and find if laminar flow ever occurs in clinical situations.
    loading  Loading Related Articles