Factors influencing conversion of chronic atrial fibrillation with special reference to serum quinidine concentration.

Opinion differs greatly about the value and risk of quinidine. The present communication attempts to demonstrate that more rational use of quinidine is possible by emphasis on serum quinidine concentration rather than on dose alone. Conversion of chronic atrial fibrillation and the appearance of myocardial toxicity are related more closely to the concentration of quinidine in the blood than to the dose. The serum concentration achieved with equal doses of the drug varied strikingly in different individuals; myocardial toxicity was infrequent at levels less than 6.0 ,g./ml. and increased proportionately as this level was exceeded. These levels could not always be predicted from the dose given.