BJA

Correspondence

Declaration of interest None declared. R. K. Malhotra* C. Johnstone A. Banerjee Liverpool, UK * E-mail: [email protected]

Lack of reduction in overall stroke rate Editor—I read with interest the article about intraoperative epiaortic scanning (EAS) for stroke prevention in off-pump coronary artery bypass grafting (CABG).1 Reducing the incidence of stroke is an important issue in cardiac surgery. However, I believe that several other factors need to be considered as causes for the lack of significant reduction in overall stroke rate. Interpretation of subgroup results is known to be potentially misleading, even when defined a priori.2 This is especially true with such low numbers of both patients and incidence of stroke. I believe that the required sample size to indicate statistical significance and a power of 80% in a randomized controlled study with a two-tailed P-value of ,0.05 is about 5000 patients. Displacement of artherosclerotic plaques resulting in embolic strokes can be caused by carotid puncture during insertion of central lines.3 The authors do mention that Swan –Ganz catheters were inserted, but do not include their incidence of carotid punctures, or record the relationship between carotid puncture and stroke. This is especially important as carotid disease was identified as significantly higher in the EAS group, which could account for the non-significance of the overall stroke rate. Another issue was the potential conversion from off-pump to on-pump CABG. This is not mentioned as an exclusion criterion and no information regarding any such cases were included in the results. In the literature, the conversion rate is cited between 1% and 16%.4 If the type of surgery is of importance in stroke rate, then the number of off-pump to on-pump conversions here would also have had an impact on the overall significance. In order to justify the costs of new equipment (and the support training required), I believe these results require confirmation, preferably in a multicentre study.

1 Abdallah F, Brull R. Facilitatory effects of perineural dexmedetomidine on neuraxial and peripheral nerve blocks: a systematic review and meta-analysis. Br J Anaesth 2013; 110: 915–25 2 Brummett C, Padda A, Amodeo F, Welch K, Lydic R. Perineural dexmedetomidine added to ropivacaine causes a dose-dependent increase in the duration of thermal antinociception in sciatic nerve block in rat. Anesthesiology 2009; 111: 1111– 9 3 Eid H, Shafie M, Youssef H. Dose-related prolongation of hyperbaric bupivacaine spinal anesthesia by dexmedetomidine. Ain Shams J Anesthesiol 2011; 4: 83 –95 4 Mantouvalou M, Ralli S, Arnaoutoglou H, et al. Spinal anesthesia: comparison of plain ropivacaine, bupivacaine and levobupivacaine for lower abdominal surgery. Acta Anaesth Belg 2008; 59: 65 – 71 5 Suzuki S, Gerner P, Colvin A, et al. C-fiber-selective peripheral nerve blockade. Open Pain J 2009; 2: 24–9 6 Hocking G, Wildsmith J. Intrathecal drug spread. Br J Anaesth 2004; 93: 568– 78 7 Inberg P, Annila I, Annila P. Double-injection method using peripheral nerve stimulator is superior to single injection in axillary plexus block. Reg Anesth Pain Med 1999; 24: 509– 13 8 Al-Kaisy A, McGuire G, Chan V, et al. Analgesic effect of interscalene block using low dose bupivacaine for outpatient arthroscopic shoulder surgery. Reg Anesth Pain Med 1998; 23: 469– 73

1 Joo HC, Youn YN, Kwak YL, Yi GJ, Yoo KJ. Intraoperative epiaortic scanning for preventing early stroke after off-pump coronary artery bypass. Br J Anaesth 2013; 111: 374– 81 2 ISIS-2 (Second International Study of Infarct Survival) Collaborative Group. Randomised trial of intravenous streptokinase, oral aspirin, both, or neither among 17 187 cases of suspected acute myocardial infarction: ISIS-2. Lancet 1988; ii: 349– 60 3 Reuber M, Dunkley LA, Turton EPL, Bell MDD, Bamford JM. Stroke after internal jugular venous cannulation. Acta Neurol Scand 2002; 105: 235–9 4 Edgerton JR, Dewey TM, Magee MJ, et al. Conversion in off-pump coronary artery bypass grafting: an analysis of predictors and outcomes. Ann Thorac Surg 2003; 4: 1138– 43

doi:10.1093/bja/aet568

doi:10.1093/bja/aet569

Declaration of interest None declared. J. Scheffczik London, UK E-mail: [email protected]

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First, we note that the doses of dexmedetomidine in the included trials vary from 3 to 100 mg. It has been demonstrated in both animal2 and human studies3 that the duration of sensory block is related to the dose of dexmedetomidine used. A meta-regression would have been useful to assess the influence of dexmedetomidine dosage on the reported findings. Secondly, there was no consistency in the local anaesthetic agent used in the trials. Mantouvalou and colleagues4 showed that the use of intrathecal ropivacaine resulted in a reduced duration of sensory and motor block compared with bupivacaine and levobupivacaine. Subgroup analysis based on local anaesthetic type would have been useful to assess the impact of this factor upon the differences in duration of nerve block. Thirdly, the data presented in the meta-analysis combine that obtained after both peripheral nerve and neuraxial block. There are differences in the mechanism of action of local anaesthetics using these two techniques.5 6 The volumes of local anaesthetic used are also different, which may have an impact upon the duration of block.5 – 8 Hence comparison between the two groups, as demonstrated in Figure 2,1 is of limited benefit due to the influence of several confounding variables.

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