B18 Ultrasound is superior than injection pressure monitoring detecting the low-volume intraneural injection

Abstract

<h3>Background and Aims</h3> Inadvertent intraneural injection is not infrequent during peripheral nerve blocks. To this end, injection pressure monitoring is suggested as a safeguard method that warns of a potentially hazardous needle tip location. However, doubts remain if this method is superior to the sonographic nerve swelling in terms of earlier detection of the intraneural injection. <h3>Methods</h3> A cadaveric study was designed to assess injection pressures during an ultrasound-guided intraneural injection of the median nerve. We hypothesized that the sonographic swelling occurred first than elevated injection pressures (&gt;15 pound per square inch) using an in-line monitor. 33 injections of 11 median nerves from unembalmed human cadavers were performed at proximal, mid and distal locations. 1 ml of a mixture of local anesthetic and methylene blue was injected at 10 ml/min. Afterwards, dissection was performed to assess spread location. Videos of the procedures including ultrasound images were blindly analyzed to evaluate nerve swelling and injection pressures. <h3>Results</h3> 31 injections were analyzed (2 were excluded due to uncertain needle tip location). &gt;15 pound per square inch was attained in 6 injections (19%) following a mean volume of 0.7 ml. Nerve swelling was evident in all 31 injections (100%) with a mean volume of 0.4 ml. Upon dissection, spread was confirmed intraneural in all injections, with a proximal-distal longitudinal diffusion of an average 6 cm per injection. <h3>Conclusions</h3> Ultrasound is a more sensitive and earlier indicator of the intraneural injection than injection pressure monitoring. Further research is required to consolidate the role of pressure monitors in the clinical setting.