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.
Highlights
Inadvertent injections of local anesthetic into nerve may occur during peripheral nerve blocks, even with ultrasound guidance [1,2], and if the needle is placed within a nerve fascicle, some experimental data suggest that high injection pressure and axonal injury may result [3,4]
Nerve swelling was evident during cervical root injections, for all peripheral nerve injections as shown in Figure 1, except for the two femoral nerve injections, which remained essentially unchanged in size (both nerves were clearly stained internally with ink, Peak Pressure of Injection (PSI)
Intrafascicle Group (n=8) Extrafascicle Group (n=15) P value. In this non-preserved cadaver model of neural injection, we determined that mean pressures were highest with USguided injection into the ample fascicular tissue that comprises the majority of the cervical nerve root; lowest for injections placed in a perineural location, and intermediate for injections placed within peripheral nerves
Summary
Inadvertent injections of local anesthetic into nerve may occur during peripheral nerve blocks, even with ultrasound guidance [1,2], and if the needle is placed within a nerve fascicle, some experimental data suggest that high injection pressure and axonal injury may result [3,4]. While it has been suggested to use pressure monitoring during injection to help detect intrafascicle needle placement in the clinical realm [9], the relationship between pressures of injection for intrafascicle injections and subepineurial (but not intrafascicle) injections in humans has not been well studied. In this investigation we performed deliberate nerve injections, guided by ultrasound, into the fascicular tissue of the cervical nerve roots of a cadaver specimen, and into peripheral nerves of the extremities, as well as into perineural soft tissues, to evaluate the relationship between pressures generated in these locations. We determined needle tip position for the peripheral nerve injections, based on histologic analysis of dye deposition, for categorization into intra-fascicle versus extra-fascicle injection, and re-analyzed pressures generated at these sites
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