Digitalis Intoxication following Conversion to Sinus Rhythm.

IN 1963 a study was published from our laboratory concerning cardiac performance during exercise before and after conversion from atrial fibrillation to sinus rhythm.' All subjects were receiving digitalis at the time of conversion. One of the findings of that study was a markedly low heart rate at exercise in sinus rhythm, much lower than expected for a group of subjects with a comparable degree of cardiac disability. Furthermore, electrocardiographic abnormalities suggesting digitalis intoxication often occurred after conversion, even in subjects who showed no evidence of digitalis intoxication prior to conversion. These results led to the speculation that digitalis requirements might be less in sinus rhythm than in atrial fibrillation. The conversions in the above-mentioned study were accomplished with quinidine, usually in a dose range of 1 to 3 Gm. over a period of 8 hours. The role of these relatively large doses of quinidine in either potentiating or suppressing this phenomenon of digitalis intoxication could not be assessed. The recent introduction of direct-current countershock for the treatment of cardiac arrhythmias2 has made it possible to circumvent this difficulty. The present report is a study of the results of conversion to sinus rhythm with direct-current countershock with special emphasis on the appearance of digitalis intoxication. The subjects received either no quinidine or much smaller doses than those previously used for conversion.