Introduction: Inadvertent intraneural injection is not infrequent during peripheral nerve blocks. For this reason, injection pressure monitoring has been suggested as a safeguard method that warns the clinician of a potentially hazardous needle tip location. However, doubts remain whether it is superior to the sonographic nerve swelling in terms of earlier detection of the intraneural injection.
Methods: An observational cadaveric study was designed to assess injection pressures during an ultrasound-guided intraneural injection of the median nerve. We hypothesized that the evidence of nerve swelling occurred prior to an elevated injection pressure (>15 pound per square inch) measured with a portable in-line monitor. 33 ultrasound-guided intraneural injections of 11 median nerves from unembalmed human cadavers were performed at proximal, mid and distal forearm. 1 mL of a mixture of local anesthetic and methylene blue was injected intraneurally at a rate of 10 mL/min. Following injections, specimens were dissected to assess spread location. Video recordings of the procedures including ultrasound images were blindly analyzed to evaluate nerve swelling and injection pressures.
Results: 31 injections were considered for analysis (two were excluded due to uncertainty regarding needle tip position). >15 pound per square inch was reached in six injections (19%) following a median injected volume of 0.6 mL. Nerve swelling was evident in all 31 injections (100%) with a median injected volume of 0.4 mL. On dissection, spread location was confirmed intraneural in all injections.
Discussion: Ultrasound is a more sensitive and earlier indicator of the low-volume intraneural injection than injection pressure monitoring.
Keywords: nerve block; peripheral nerve injuries; regional anesthesia; ultrasonography.
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