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

Summary Animal studies have shown that injection pressures > 75 kPa 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 20 ml.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 (2.7–92 kPa). Flow rates > 17 ml.min −1 and needle sizes 22 G and smaller produced mean plateau pressures > 75 kPa. 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 ≤ 15 ml.min −1 (0.25 ml.s −1 ) to reduce the effect of factors upstream from the needle tip as a cause of high pressure readings.