Statistical guidelines for contributors to medical journals

SIR,-In reply to Dr Wildsmith's letter we would like to reiterate the main purpose of our short paper-namely, to highlight the erratic way in which lignocaine is commonly used and to offer a simple guide to safer practice. We accept that the correlation between body weight, total dose injected, and plasma concentration produced is not always high; in our study, however, most doctors did not use any objective physiological variables to guide their use of local anaesthetics. To suggest that they calculate a safe maximum dose according to factors other than body weight is, we feel, unrealistic. Our chart covers body weights up to 70 kg only, and we would not recommend extrapolation to include obese patients. We accept that the caption may be misinterpreted in the manner described, though in Britain intravenous regional analgesia should be practised only by specialists, whom we hope would not consider that possibility. Although we agree with Dr Dennison that the site of injection and type of procedure are important factors in the production of toxic plasma concentrations of lignocaine, we would like to point out that our article was aimed at a general medical readership, and 95O, of junior hospital doctors questioned were using lignocaine for local infiltration or topical analgesia. Few of our respondents were anaesthetists, and we hope that doctors attempting the specialist procedures he mentions would be aware of the many other factors involved in drug toxicity. We do not suggest that the use of 3 mg,/kg of lignocaine produces predictable plasma concentrations, but we do believe that if the total dose is kept at or below those derived from the chart then toxic plasma concentrations are unlikely to be achieved. This is borne out by Dr Dennison's statement that even in the vascular sites used in intercostal, interscalene brachial plexus, and epidural blocks, toxic concentrations (of greater than 5 mg,l) are not found in total doses of less than 200 mg in a man weighing 70 kg.