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Department of Anaesthesia; Wellington Hospital; Wellington, New Zealand; colin.marsland{at}ccdhb.org.nz
In Response:
We appreciate the reiteration by Woodall et al. of our concerns regarding lidocaine toxicity in endoscopy volunteers in an educational setting. It is, however, incorrect to suggest that our study protocol did not include an upper dose limit. Our topical anesthesia procedure was discontinued if the lidocaine dose approached 15 mg/kg or if local anesthetic effect diminished before adequate topical anesthesia. This upper limit was based on two published studies (1,2) with ranges for total applied lidocaine of 5.5 to 16 mg/kg (average, 9.3 mg/kg) and 5.0 to 15 mg/kg (average, 8.2 mg/kg) in sedated patients. Mean plasma concentrations in these studies were 2.9 µg/mL and 1.29 µg/mL. Plasma concentrations exceeding 5 µg/mL were noted in two patients (1). No signs or symptoms of lidocaine toxicity were reported in either study.
The dose range of applied lidocaine in our study was 7.1 to 14.7 mg/kg with a median dose of 9.6 mg/kg. This is consistent with the previously published work and reflects the dose distribution in a study population of 39 nonsedated subjects. We reported a high incidence of subjective cerebral side effects. Similar symptoms are alluded to in the published description of the training course in local anesthesia of the airway conducted by Woodall et al. themselves (3).
The "acceptable range" for topically applied lidocaine is suggested from studies where the drug has been used successfully and without complications (1,2). This does not mean that the margins of the dose range are completely safe or effective. One-sided 95% upper confidence intervals can be calculated when the occurrence of an event (e.g., clinical lidocaine toxicity) is reported as zero, using the formula 3/n where n is the number of patients (4). In the series of Efthimiou et al. and Langmack et al., the risks of clinical lidocaine toxicity were less than or equal to 7.3% (3 of 41) and 5.9% (3 of 51), respectively. We employed a similar dose range in unsedated subjects and 7.7% (3 of 39) of participants showed objective signs of early lidocaine toxicity. No major adverse events occurred.
The 2001 British Thoracic Society guidelines (5) recommend limiting the topical dose of lidocaine to 8.2 mg/kg. This is based on the mean dose reported in the study of Langmack et al. (2). As many as half of the patients in this study required doses of lidocaine higher than the subsequent recommendations of the British Thoracic Society to achieve adequate topical anesthesia. Similarly, the upper limit used by Woodall et al.’s group of 9 mg/kg (3) is referenced to the work of Efthimiou et al. presumably based on their average dose of 9.3 mg/kg (1).
Although the imperative for successful topical anesthesia in the clinical setting may justify doses of lidocaine in the upper level of the acceptable range, especially when followed by general anesthesia, the same cannot be said when the procedure is performed on volunteers for educational purposes. It is not possible to predict which people in a healthy population will develop toxicity from doses in the upper range.
Successful application of topical anesthetic to the airway is not a safe endpoint (6). Despite the problems associated with nominating a maximum dose of lidocaine, it is nevertheless important if the risk of toxicity is to be minimized. We have subsequently adopted the published average dose (1) of approximately 9 mg/kg as our maximum dose for awake volunteers. This will not guarantee adequate topical anesthesia in all subjects nor will it guarantee plasma concentrations limited to the therapeutic range in all subjects. However, it should eliminate the risk of significant clinical toxicity.
References
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