Plasma levels of local anaesthetic following supraclavicular block

Editor, Plasma levels of local anaesthetic following peripheral nerve blockade are not well studied and as discussed by Rosenberg et al.1 a decade ago, there is no scientific basis for a universal maximum mg or mg kg−1 dose. Since the advent of widespread use of ultrasound for peripheral nerve blockade, our institution now uses supraclavicular brachial plexus block as the primary brachial plexus block for hand and arm surgery. The brachial plexus at the supraclavicular level is almost always accessible regardless of body habitus and there is relatively rapid onset of the block. Despite these advantages, it does require a large dose of local anaesthetic to achieve surgical anaesthesia. Previous studies suggest up to 32 ml for a 90% success rate2. Not infrequently, this puts the dose into what would normally be considered the toxic range if considered in a mg kg−1 dose. To assess our safety margin with regards to systemic toxicity, we assessed the plasma levels of local anaesthetic in 21 patients who presented for hand surgery under supraclavicular block. After ethical approval for this study, which was provided by the Health Research Ethics Board of the University of Alberta, Edmonton, Alberta, Canada (Dr Shane Kimber) on 16 November 2010, reference number Pro00017658 and written consent from the patients, blood samples were obtained at 10, 15, 20 and 25 min to assess for lignocaine and bupivacaine levels. Blood samples were centrifuged, after which the plasma component was removed and frozen at −70°C until analysis. Plasma lidocaine and bupivacaine concentrations using 100 ul of human plasma were measured by HPLC. All blocks were carried out by an anaesthetist experienced with ultrasound-guided supraclavicular blocks. As the block was intended for surgical anaesthesia, our institution's ‘surgical mix’ was used. This consists of 15 ml of 2% lignocaine and 5 ml of 0.5% bupivacaine drawn up into a 30 ml syringe. The absolute volume used was at the discretion of the anaesthetist, but most commonly 30 ml (or one and half syringes of this mixture) was used. Any local anaesthetic used in the skin was also included when total dose was calculated for each patient. The patient demographics and local anaesthetic doses are shown in Table 1, and the plasma concentrations are shown in Table 2. The bupivacaine concentrations in all patients were below the measureable threshold at all time points.Table 1: Patient age and weight and local anaesthetic doseTable 2: Mean (± SD) lignocaine plasma levels and range at each time pointFour of the patients had minor symptoms of local anaesthetic toxicity (nystagmus, circumoral tingling and tinnitus). There were no episodes of serious local anaesthetic toxicity. Interestingly, the patients with mild symptoms of local anaesthetic toxicity did not have the highest plasma lignocaine levels. The patient with the highest levels did not have any signs of local anaesthetic toxicity. Using the Kendall correlation coefficient, we were unable to show any relationship between either the total dose, or the mg kg−1 dose, and the plasma concentration. Using a log normal distribution to model the data (Fig. 1), the probability of the lignocaine level being above 6 μg ml−1 following administration of the block was 25%. The chance of a patient having a plasma lignocaine level of above 12 μg ml−1 was 6%.Fig. 1: Histogram of plasma levels with a log normal distribution curve.In this preliminary study, we have shown that high plasma levels of local anaesthetic can occur after a supraclavicular brachial plexus block. This suggests that the primary reason for a reduction of local anaesthetic toxicity that is seen with the use of ultrasound by Barrington et al.3 is related to the avoidance of intravascular injection. Although there is a reduction in the local anaesthetic volume required for a successful block when using ultrasound, this study suggests that it not reduced sufficiently to ensure that high plasma levels of local anaesthetic are reliably avoided. However, the high plasma concentrations of local anaesthetic that occur following supraclavicular block seem to be well tolerated in a young healthy population. Symptoms of mild local anaesthetic toxicity are common, and in our experience of over 1500 supraclavicular blocks in the past decade there has not been an episode of serious local anaesthetic toxicity associated with this block. The relationship between plasma levels, toxicity and dose of local anaesthetic is unclear, and it is likely that patient factors play a significant role in the likelihood of local anaesthetic systemic toxicity. Further research is required in this area. Acknowledgements relating to this article Assistance with the study: none. Financial support and sponsorship: BCT is supported in part by a Clinical Scholar award from the Alberta Heritage Foundation for Medical Research, Edmonton, Alberta, Canada and a CAS–Abbott Laboratories Ltd Career Scientist Award in Anesthesia from the Canadian Anesthesiologists’ Society. Conflicts of interest: none.

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