Injection pressure monitoring during peripheral nerve blocks: from bench to operating theatre

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

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.

Combination of real-time needle-tip pressure sensing and minimal intensity stimulation limits unintentional intraneural injection during an ultrasound-guided peripheral nerve block procedure: A randomized, parallel group, controlled trial

STUDY OBJECTIVE

Ultrasound guidance does not eliminate the risk of intraneural injection, which must be avoided during PNB. Combining ultrasound guidance (USG), nerve stimulation (NS), and injection pressure monitoring is advocated to prevent nerve injury during PNB. We hypothesized that combining patient-tailored dynamic NS and real-time pressure sensing (RTPS) could reduce the incidence of intraneural injection and nerve puncture during USG PNB compared with a traditional fixed thresholds (Control) procedure.

DESIGN

Randomized, prospective study.

SETTING

Operating room.

PATIENTS

One hundred ASA physical status I to III patients undergoing orthopedic surgery.

INTERVENTIONS

Patient anesthetized using axillary, sciatic or femoral USG PNB were randomized to the PresStim group (Dynamic RTPS and NS set at 1.5 mA then decreased; n = 50) or Control group (fixed thresholds for in-line pressure mechanical manometer and NS at 0.2 mA; n = 50).

MEASUREMENTS

Procedural ultrasound images and videos were recorded, stored and reviewed in random order by two experts in ultrasound-guided PNB blinded to the group. They noted the needle-to-nerve relationship and intraneural injection for all blocked nerves.

MAIN RESULTS

One hundred and twenty-three USG PNBs were performed (56 axillary brachial plexus blocks, 40 femoral nerve blocks and 27 sciatic popliteal nerve blocks); 235 blocked nerves and videos were recorded and analyzed (PresStim, 118; Control, 117). Less paresthesia was noted in the PresStim group (12.7%) compared with the Control group (18.8%). The risk of intraneural injection was significantly higher in the Control group (odds ratio [OR], 17.1; 95% confidence interval [CI], 2.2-135, P = 0.007). The risk of nerve puncture (OR, 22.7; 95% CI, 2.9-175, p = 0.003) and needle-nerve contact (OR, 4.7; 95% CI, 2.4-9.5, p < 0.001) was significantly higher in the Control group than the PresStim group.

CONCLUSIONS

Under the conditions of the study, dynamic triple monitoring combining RTPS, NS and USG decreases intraneural injection and unintentional needle-nerve contact and puncture during a PNB procedure.