The real-time visualization of a needle and nerve during an ultrasound-guided nerve block can
be challenging. These difficulties may partly explain the systemic complications of local
anesthetics under ultrasound. Injection of small amounts of a solution around the
anesthetized nerve (hydro-dissection) has been proposed to enhance contrast outlining its
borders and also to improve the visualization of the needle tip. The glucose solution 5%
solution is interesting because it allows, unlike saline, to maintain the motor response with
neurostimulation. The hydro-dissection can be particularly useful when one suspect hypoechoic
vessels near the nerve to be anesthetized. Thereby, the nerve well demarcated and separated
from the vessels, injection of local anesthetic is performed in the circumferential diffusion
space (like a small pocket) without redirecting needle.
The influence of this hydro-dissection on the nerve block efficiency is unknown. The nerve
block quality can be improved because the entire anesthetic is injected in contact with the
nerve, but it can also be reduced due to the dilution of the local anesthetic by the glucose
In this randomized study, the investigators test the hypothesis that hydro-dissection does
not alter the nerve block onset time.