Effect of injection speed on anaesthetic spread during axillary block using the orthogonal two-needle technique

One hundred patients undergoing scheduled upper limb orthopaedic surgery were studied to determine if the speed of injection of the local anaesthetic solution could modify the extent and depth of anaesthesia with the axillary approach to the brachial plexus. The blocks were performed using the 'orthogonal two-needle technique' in which two needles are positioned on the upper and on the lower aspect of the axillary artery, orthogonal to the neurovascular bundle and directed towards the fascial compartment containing the radial nerve. The patients were randomly assigned to one of three groups to receive one of three injection rates (10, 20 and 30 s) (n = 30). In consequence to the results obtained in these patients, a further group of 10 patients was a posteriori added for whom the injection time for each single shot of anaesthetic solution was increased to 1 min. In each group the same mixture and volume of anaesthetic solution (a 30 mL mixture of equal parts of 0.5% bupivacaine with adrenaline 1:200,000 and 2% lignocaine) was injected in two separate boluses of 15 mL each through the two positioned needles. A significant association was found between the injection rate and the anaesthetic spread for all tested areas with the exception of the regions supplied by the median nerve. A greater speed of injection was associated with less anaesthetic spread and more frequent block failure. A clear association between the anaesthetic spread to all branches of the brachial plexus and a slower injection rate of the local anaesthetic was found.