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*Pain Management Center; Tennessee Valley Healthcare System; Department of Veterans Affairs,; Nashville, TN; sukdeb{at}hotmail.com
Department of Anesthesiology; University of Cincinnati College of Medicine; Cincinnati, OH
Department of Anesthesiology; Tennessee Valley Healthcare System; Department of Veterans Affairs; Department of Anesthesiology; Vanderbilt University Medical School; Nashville, TN
To the Editor:
Ahmed et al. (1) postulate that by reducing norepinephrine available in central structures, mainly the locus ceruleus, clonidine might decrease the seizure threshold. Previously, Nishikawa et al. (2) reported that clonidine added to lidocaine tended to increase the plasma lidocaine concentrations. However, Yokomaya et al.’s study (3) on the effect of clonidine on IV lidocaine induced hemodynamic changes and convulsions in awake rats reported that pretreatment with 1 µg/kg or 10 µg/kg of IV clonidine had neither anticonvulsant nor proconvulsant effect on lidocaine induced convulsions in rats. They concluded that clinical doses of clonidine would not affect the lidocaine seizure threshold in humans. Doses of 1–3 µg/kg IV are used in clinical practice. Ahmed et al. (1) used 30 µg, which is even smaller. This low dose of clonidine used was probably less than the threshold to produce seizures.
The authors used 20 mL of solution containing 150 mg (1.5%) of lidocaine for the Bier block. This translates to 10 mL of lidocaine. What was the composition of the other 10 mL of solution?
The pharmacokinetics of lidocaine has been well studied. Simon et al. (4) found that plasma concentration of lidocaine was zero before tourniquet release and rose immediately to maximal concentrations within 1 min. Prieto-Alvarez et al. (5) demonstrated that plasma concentrations of lidocaine decreased significantly between 5 and 60 min after the tourniquet deflation. Why then did the seizure occur 10 min after the tourniquet release?
The duration of tourniquet inflation has varied from 20 to 30 min (6,7), so the tourniquet time of 60 min appears unjustified. This longer duration of tourniquet inflation would certainly lead to increased tissue absorption and consequently lesser peak plasma concentration immediately after release of the tourniquet. Thus, it would actually lead to minimal plasma concentrations of clonidine and lidocaine, certainly below the seizure threshold.
Although it is possible that the seizure resulted from a combination of causes, highlighting clonidine as the main culprit is probably not justified. Determining whether plasma or the tissue levels of the two drugs in combination were responsible for the seizure is an important point.
References
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