Pressures of Injection in a Cadaver Model of Peripheral Nerve Blockade

Abstract

Objective: Pressure monitoring during injection has been suggested to help detect intrafascicle needle placement. We performed injections, guided by ultrasound, into the cervical nerve roots, peripheral nerves of the extremities, and perineural soft tissues of an unpreserved cadaver. We hypothesized that needle tip placement into these three sites would yield significantly different injection pressures, and that histologic analysis would allow comparison of intrafascicle vs. extrafascicle pressures of injection. Methods: Injections of 5 ml ropivacaine 0.5% were conducted at cervical roots (n=4), peripheral nerves of the extremities (n=10) and perineural soft tissues (n=9), at constant rate while monitoring pressure. Dye was instilled at the termination of the injection for histologic determination of needle position. Peak pressures and time to peak pressures, were compared for these three sets of injections. After microscopic examination, all intrafascicular and extrafascicular pressures were grouped together and compared. Results: Mean peak injection pressures for the three groups were found to be significantly different, (p=0.0002). At histological examination, four of 10 peripheral nerve injections resulted in deposition of dye within fascicles, while six of 10 did not. Mean peak intrafascicle injection pressures were significantly higher than those for injections outside of fascicles (p<0.0001). Time to peak injection pressure was not different for these two groups. Discussion: Comparison of intrafascicle versus extrafascicle injections showed a clear delineation of peak pressures into two ranges. This adds to prior evidence, from both human cadavers and live animals, showing that intrafascicle injections generate high pressures, whether conducted in nerve roots or peripherally.