The effect of needle dimensions and infusion rates on injection pressures in regional anaesthesia needles: a bench-top study

Animal studies have shown that injection pressures >75kPa indicate probable intrafascicular needle tip position. This study describes the flow/pressure characteristics of seven common needle systems. A syringe pump delivered flow rates of 5, 6.67, 10, 13.3, 15 and 20ml.min(-1) through these needle systems, while keeping the needle tips open to atmosphere. A pressure transducer connected between the syringe and needle provided a real-time graphical display for analysis. Mean plateau pressures increased linearly with flow and with decreasing needle diameter (2.7-92kPa). Flow rates >17ml.min(-1) and needle sizes 22G and smaller produced mean plateau pressures >75kPa. Pressure monitors upstream from the needle may produce false-positive alarms at high flow rates due to needle resistance, and unreliable readings due to non-laminar flow. We recommend injection rates 15ml.min(-1) (0.25ml.s(-1)) to reduce the effect of factors upstream from the needle tip as a cause of high pressure readings.