The Influence of Injected Volume on Discomfort During Administration of Digital Block.

Background: Digital nerve block is associated with pain. In a search for methods to reduce the discomfort, we investigated how the volume of anaesthetic fluid influences pain during subcutaneous digital nerve block, and how it affects the success of the anaesthesia.

Methods: A randomized blinded prospective study was performed on 36 healthy volunteers. The single injection subcutaneous digital block technique was used to anaesthetize the participants´ 4th digit on both hands. The same amount of lidocaine was used, but in two different volumes; 1 ml 2% lidocaine and 2ml 1% lidocaine. After each injection the participant was asked to estimate pain intensity on a visual analogue scale (VAS). The distribution of anaesthesia was then measured by using a Semmes-Weinstein 4.56 monofilament. Finally, participants gave a verbal assessment of which injection was least painful.

Results: In total, 72 blocks were performed. There were no statistically significant differences in pain intensity or preference between the two groups. Furthermore, the 1 ml injection gave poorer anaesthesia and had longer time to onset. Neither injection anaesthetized the dorsal aspect of the proximal phalanx.

Conclusions: The two volumes cause the same degree of discomfort. Greater volume gives a greater area of distribution and more rapid onset of anaesthesia. It seems unreasonable to use a smaller volume of more concentrated anaesthetic when performing the subcutaneous technique.