Failure of long-term digitalization to prevent rapid ventricular response in patients with paroxysmal atrial fibrillation. Digitalis is frequently prescribed to patients with paroxysmal atrial fibrillation to reduce the ventricular rate during subsequent paroxysms. To verify the validity of this assumption, we determined the ventricular rate during paroxysmal atrial fibrillation in 13 patients receiving long-term digoxin therapy (mean plasma digoxin level + 1.28 +/- 0.4 ng/ml) and compared it with that of a group of 14 patients who had not taken digoxin or beta-adrenergic and calcium-blocking agents before the attack. The treated and the untreated groups were similar statistically. The mean ventricular rate of the digitalized patients was 121 +/- 15 beats per minute, while that of the patients in the control group was 118 +/- 16 beats per minute. It is concluded that long-term digoxin therapy is not effective in reducing the ventricular response in patients with paroxysmal atrial fibrillation despite adequate therapeutic levels. 