Injection pressure mapping of intraneural vs. perineural injections: further lessons from cadaveric studies.

BACKGROUND: The aim of the study was to investigate the difference between intraneural and perineural injection pressures in human cadavers. Targeted nerves included the cervical roots, the supraclavicular and infraclavicular brachial plexus, the sciatic-subgluteal nerve and the common peroneal and tibial nerves.

METHODS: Ten readings were obtained for each nerve location. Over ten seconds, 1 mL of 0.9% NaCl was injected - deliberately slower than in clinical practice to eliminate the risk of aberrant readings relating to the speed of injection. Perineural injections occurred at least 1 mm outside the epineurium. After pressure recordings were completed 0.1mL of dye was injected, and dissection performed to confirm needle placement. Ultrasound and dissection images were matched with light microscopy pictures for all locations.

RESULTS: The average pressure for intraneural injections was 24.1±5.7 psi and 6.1±2.1 psi for perinereural. The average injection pressure generated for the cervical trunk, supraclavicular, infraclavicular, sciatic subgluteal, peroneal and tibial nerves respectively were 31.2±6.0 psi, 24±15.0 psi, 23.4±9.5 psi, 22.6±8.8 psi 19.7±6 psi, 17±7.3 psi intraneurally and 6.1±2.0 psi, 9.1±5.5 psi, 10±4.9 psi, 6±2.4 psi, 6±2.4 psi and 7±2.5 psi perineurally. For intraneural injections statistically significant differences were demonstrated between the peroneal and tibial nerves compared to cervical roots/trunks/division/cords of brachial plexus.

CONCLUSIONS: The study has consistently demonstrated statistically significant differences between intraneural and perineural injection pressures. It effectively created a "map" of intraneural injection pressures for the most common peripheral nerves blocks and demonstrated a pattern between proximal and distal locations. The study also revealed limitations of either techniques, ultrasound and injection pressure monitoring reinforcing the concept of their simultaneous application.