Minimum effective volume of 0.5% bupivacaine with epinephrine in ultrasound-guided interscalene brachial plexus block.

Background: The use of ultrasound (US) in regional anaesthesia enables a reduction in the local anaesthetic volume. The present study aimed to determine the minimum effective volume (MEV(90)) of 0.5% bupivacaine with epinephrine for interscalene brachial plexus block (ISBPB).
Methods: The volume of the anaesthetic was determined using a step-up/step-down method and was based on the outcome of the preceding block. A positive or negative block resulted in a 1 ml reduction or increase in volume, respectively. The success of the block was defined as the presence of motor block in three muscle groups and the absence of thermal and pain sensations in three dermatomes within 30 min of the injection. Diaphragmatic paralysis and analgesia were assessed at 30 min, 4, and 6 h.
Results: The MEV(90) for US-guided brachial plexus block under the conditions of the present study was 0.95 ml [R(2): 0.97, 95% confidence interval (CI): 0.6-1.22 ml]. The estimated maximum volume that did not cause diaphragmatic block was 4.29 ml (R(2): 0.84, 95% CI: 3.56-4.98 ml). Effective postoperative analgesia was achieved with 2.34 ml (R(2): 0.87, 95% CI: 0.48-11.47 ml).
Conclusions: The MEV(90) of 0.5% bupivacaine with epinephrine (1:200 000) for US-guided ISBPB was 0.95 ml. Adequate postoperative analgesia and a reduced incidence of diaphragmatic block can be obtained using from 2.34 to 4.29 ml. ClinicalTrials.gov. Registry NCT01244932.