Effects of local anaesthetic dilution on the characteristics of ultrasound guided axillary brachial plexus block: a randomised controlled study

Aims: Ultrasound guidance has led to marked improvement in the success rate and characteristics of peripheral nerve blocks. However, effects of varying the volume or concentration of a fixed local anaesthetic dose on nerve block remains unclear. The purpose of our study was to evaluate whether at a fixed dose of lidocaine, altering the volume and concentration will have any effect on the onset time of ultrasound-guided axillary brachial plexus block. Material and methods: Twenty patients were randomised to receive an ultrasound-guided axillary brachial plexus block with either lidocaine 2% with epinephrine (20 ml, Group 2%) or lidocaine 1% with epinephrine (40 ml, Group 1%). The primary endpoint was block onset time. Secondary outcomes included duration of the block, performance time, number of needle passes, incidence of paraesthesia and vascular puncture. Results: The median [IQR] onset time of surgical anaesthesia was shorter in Group 1% when compared to Group 2% (6.25 [5-7.5] min vs 8.75 [7.5-10] min; p=0.03). The mean (SD) overall duration of surgical anaesthesia was significantly shorter in Group 1% compared to Group 2% (150.9 +/- 17.2 min vs 165.1 +/- 5.9 min; p=0.02). Group 1% had a shorter performance time with fewer needle passes. The incidence of vascular puncture and paraesthesia was similar in the two groups. Conclu- sion: Ultrasound-guided axillary brachial plexus blocks performed using a higher volume of lower concentration lidocaine was associated with shorter onset time and duration of surgical anaesthesia.