One of the demonstrated advantages of US guidance for peripheral nerve blocks is the ability to administer lower LA volumes for successful block. 1-5 Volumes as low as 5 mL for interscalene, 2 mL for each nerve for the axillary brachial plexus, and 10 mL for sciatic block have been reported. However, when deciding on the value of this low volume approach, one has to be mindful of the benefits vs. risks. By enabling a total LA dose reduction, lower LA volumes should theoretically lower the risk of local and systemic LA toxicity; however, published evidence does not provide evidence to support this argument. 6, 7 Likewise, the risk of blockade of adjacent nerves (e.g. the phrenic nerve during interscalene block) should theoretically be reduced; though, studies have demonstrated conflicting results with some studies demonstrating an advantage, and others no advantage . 8-10 Another unresolved issue is the effect of LA volume on block duration. Recent evidence has demonstrated that a lower volume ankle block results in marginally inferior postoperative analgesia compared to a higher volume technique. 3 It is also likely that LA volume at other block locations correlates with block duration.
Therefore, when considering the potential advantages of administering a low volume block, consideration should be given to the probable shorter block duration, and the inevitably higher block failure risk.
1. Danelli G, Ghisi D, Fanelli A, et al. The effects of ultrasound guidance and neurostimulation on the minimum effective anesthetic volume of mepivacaine 1.5% required to block the sciatic nerve using the subgluteal approach. Anesth Analg 2009;109:1674-8.
2. O'Donnell BD, Iohom G. An estimation of the minimum effective anesthetic volume of 2% lidocaine in ultrasound-guided axillary brachial plexus block. Anesthesiology 2009;111:25-9.
3. Fredrickson MJ, White R, Danesh-Clough TK. Low-volume ultrasound-guided nerve block provides inferior postoperative analgesia compared to a higher-volume landmark technique. Reg Anesth Pain Med 2011;36:393-8.
4. van Geffen GJ, van den Broek E, Braak GJ, et al. A prospective randomised controlled trial of ultrasound guided versus nerve stimulation guided distal sciatic nerve block at the popliteal fossa. Anaesth Intensive Care 2009;37:32-7.
5. McNaught A, Shastri U, Carmichael N, et al. Ultrasound reduces the minimum effective local anaesthetic volume compared with peripheral nerve stimulation for interscalene block. Br J Anaesth 2010;106:124-30.
6. Neal JM, Brull R, Chan VW, et al. The ASRA evidence-based medicine assessment of ultrasound-guided regional anesthesia and pain medicine: Executive summary. Reg Anesth Pain Med 2010;35:S1-9.
7. Neal JM. Ultrasound-guided regional anesthesia and patient safety: An evidence-based analysis. Reg Anesth Pain Med 2010;35:S59-67.
8. Sinha SK, Abrams JH, al Be. Decreasing the Local Anesthetic Volume From 20 to 10 mL for Ultrasound Guided Interscalene Block at the Cricoid Level Does Not Reduce the Incidence of Hemidiaphragmatic Paresis. Reg Anesth Pain Med 2011;36:17-20.
9. Riazi S, Carmichael N, Awad I, et al. Effect of local anaesthetic volume (20 vs 5 ml) on the efficacy and respiratory consequences of ultrasound-guided interscalene brachial plexus block. Br J Anaesth 2008;101:549-56.
10. Renes SH, van Geffen GJ, Rettig HC, et al. Minimum effective volume of local anesthetic for shoulder analgesia by ultrasound-guided block at root C7 with assessment of pulmonary function. Reg Anesth Pain Med 2010;35:529-34.