Electrophysiology in assessing supraventricular arrhythmias: value of programmed stimulation in predicting and understanding efficacy of encainide.

The effects of intravenous (1.5 mg/kg body weight given over 15 minutes) and oral (mean dose 164.8 +/- 56.2 mg/day) encainide were studied in 11 patients with recurrent, difficult to treat, supraventricular tachycardia. In 2 patients the arrhythmia was atrial in origin. In 9 patients a concealed (slow type in 3, fast type in 6) accessory atrioventricular pathway was used for retrograde conduction during a circus movement tachycardia. Intravenous encainide terminated tachycardia in 8 of the 11 patients. Oral encainide prevented clinical recurrence of tachycardia in 7 patients during a 5- to 20-month follow-up (mean 11). Programmed stimulation of the heart after intravenous encainide administration revealed significant (p less than 0.05) prolongation of HV interval, QRS width and refractory period of the right ventricle. Apart from a slower tachycardia rate no changes in electrophysiologic parameters were observed in 5 patients restudied while receiving oral encainide medication. Although encainide was an effective antiarrhythmic agent, programmed stimulation of the heart was not very helpful in providing insight into its mode of action, nor did programmed stimulation predict clinical efficacy. Prevention of the tachycardia-initiating premature beat seems to be the most likely mechanism for the clinical efficacy of oral encainide.