An animal model of "syringe feel" during peripheral nerve block

The perception of high resistance during injection of a local anesthetic during regional anesthesia may indicate intraneural injection. Anesthetists' ability to detect high resistance by "syringe feel" has been questioned in the past. The aim of our study was to investigate the anesthetist's ability to detect abnormal resistance to injection using an animal model.

We created a model using nerve, muscle, bone, and tendon tissue dissected from a sheep. Regional anesthesia needles of 21-gauge and 100 mm were placed into each of these tissues under direct vision, and 40 anesthetists were then asked to inject normal saline from a 20-mL syringe. They were unable to see the needle position. Once injected into all 4 tissues, they were asked to state which tissue they thought each was.

Of the 40 anesthetists, 12 (30%) correctly identified the nerve. This was no better than chance (25%) and shows that the tested anesthetists were unable to correctly identify beyond chance what tissues they were injecting into. When those who did not practice regional anesthesia regularly (n = 7) were excluded, similar results were obtained. Ten (30%) of the 33 self-identified experienced regional anesthetists correctly identified the nerve. A score that measured the number of correctly identified tissues (score 1) was used to compare anesthetists on grade and level of experience. This showed that the more experienced anesthetists did better than the less experienced ones.

Under the conditions of this study model, anesthetists were unable to correctly identify intraneural injection by syringe feel during simulated regional anesthesia.