Real time continuous monitoring of injection pressure at the needle tip is better than ultrasound in early detecting intraneural injection.

Introduction: During peripheral nerve blocks, inadvertent intraneural injection is not infrequent. Recently, with sonographic nerve swelling, it has been shown that intraneural injection can be detected as early as 0.4 mL. A new method based on injection pressure monitoring at the needle tip, the real pressure in the tissues, is compared with sonographic performance in early detection of intraneural injection.

Methods: This cadaver study was conducted on the upper limb of fresh cadavers. Senior anesthesiologists performed ultrasound guided intraneural injections of the nerves using a modified Visioplex needle with a fiberoptic sensor embedded. 12 injections were performed, two on each nerve (radial, ulnar, and median) for each anatomical preparation. 3 mL of saline methylene blue mix was injected at 10 mL/min. Recordings of ultrasound screen and pressure curves were blindly analyzed to evaluate nerve swelling and injection pressures.

Results: An immediate increase in injection pressure from time zero was observed in all 12 intraneural injections, with similar trends. After only 0.2 mL, intraneural injection was clearly identified as the pressure curves had already risen to an average of 120 mm Hg (SD 5). Nerve swelling was observed in 10 of 12 injections (83%). This was evident after an average of 1.2 mL (SD 0.5). Following injections, dissection of specimens confirmed intraneural spread in all cases.

Conclusion: Pressure monitoring at the needle tip consistently showed a pressure rise from the onset of injection and proved to be a more sensitive and earlier indicator of intraneural injection than sonographic nerve swelling.