The problems of digitalis therapy from the viewpoint of serum concentration with special reference to the sampling time, to the overlapping range of serum concentration where intoxicated and non-intoxicated patients are located, and to atrial fibrillation.

Clinical studies were carried out during digoxin maintenance therapy to clarify three questions concerning digitalis therapy: optimal time for blood sample collection for serum digoxin concentration (SDC), the overlapping range of SDC levels in which some patients may be intoxicated while others are not, and both resistance and sensitivity of atrial fibrillation (AF) to digitalis. The SDC curve after a single dose of digoxin or beta-methyldigoxin shows the appropriate sampling time to be at least 12 hours after the administration. The optimal time is 24 hours. The overlapping SDC range was 1.7-2.7 ng/ml. There were significant differences in CTR and ventricular rates in AF between intoxicated and non-intoxicated groups. This suggests that susceptibility to digitalis increases with the severity of underlying heart disease. Precipitating factors such as CTR should be taken into consideration, if the SDC is in the overlapping level. Digitalis resistance occurs in 6.7% of 105 patients with AF, and more frequently in hypertensive heart disease than others. Digitalis sensitivity occurs more often in the elderly and in patients with dilated cardiomyopathy. But it is not necessary to attain higher therapeutic levels for AF than for sinus rhythm. Clearly optimal digitalis therapy for AF can be best accomplished when precise clinical findings, SDC and EKG recordings are carefully monitored and used to correct treatment.