The effects of digoxin and β-methyldigoxin on the heart rate of decompensated patients with atrial fibrillation

Eighteen patients with atrial fibrillation were given digoxin 0.13 mg twice daily for 3 weeks and beta-methyldigoxin 0.10 mg twice daily for another 3 weeks. At the end of each 3 week period an exercise test was performed and the effects on the heart rate of the two drugs were compared. No difference in heart rate was obtained at rest, whereas the heart rate after 6 min of exercise was higher during treatment with digoxin (131 beats/min) than when the patients were taking beta-methyldigoxin (124 beats/min). There were no significant differences between digoxin and beta-methyldigoxin in their effects on the ECG (R-R intervals, T-wave, Q-T duration). The plasma concentrations of the two glycosides were determined by radioimmunoassay and by 86Rb-uptake inhibition assay. Comparable plasma concentration values (1.0 ng/ml for digoxin, 1.1 ng/ml for beta-methyldigoxin, mean values) were obtained by radioimmunoassay, but the 86Rb-technique gave significantly higher values (mean 1.5 ng/ml) for beta-methyldigoxin. It is concluded that beta-methyldigoxin is equal to digoxin for producing slowing of the heart rate in patients with atrial fibrillation.