Study objective: To determine--through reconstruction of a clinical situation in which a syringe filled with insulin was removed from the syringe pump, placed above the patient, then emptied into the patient--the different physical forces at work, and to examine the height of the syringe (and thus the hydrostatic force) necessary to move the plunger.
Design: Prospective study.
Setting: Research laboratory of a university.
Measurements: The clinical situation was simulated using eight 50-mL and eight 20-mL syringes. A pressure transducer, placed between the syringe and the extension tubing, measured the pressure difference over the extension tubing. The Poiseuille equation of the viscous resistance was used to calculate flow.
Main results: The mean height needed for initiation of flow in the 50-mL syringes was 76 cm (range, 60-90 cm). In the 20-mL syringes, no flow could be generated up to heights of 200 cm. There was a large variability in the height and time required to generate flow in apparently identical syringes, probably due to differences in the static sticking of the plunger to the barrel.
Conclusions: Studies testing the effect of siphoning using one syringe cannot be interpreted reliably. Smaller syringes are safer to avoid siphoning.