Effects of Encainide on the Function of Implanted Pacemakers

Abstract

The effect of encainide on chronic pacing thresholds was evaluated in 10 patients, age 64-89, who were exposed to weekly increased encainide dosing (25 mg TID, 50 mg TID, 75 mg TID). Median pacing threshold (mujoules) increased modestly at each period compared to placebo and returned rapidly to baseline after discontinuation. (table; see text) No patient experienced a pacing-related clinical event. One patient had a large threshold increase (566%), but no failure to capture on 24-hour ECG, and one patient whose threshold increased minimally had clinically insignificant capture failure, the longest event being 3 beats. No other failure to capture was noted. Thus, encainide, like a number of other antiarrhythmic drugs, appears to affect pacing thresholds. At the highest dose of 225 mg/day (75 mg TID, which exceeds the generally recommended dose of 50 mg TID), but not at lower doses, some patients may experience loss of capture that does not appear related to the change in threshold energy required. Increases in the duration of the paced QRS induced by encainide did not predict increases in threshold. Therefore, when higher doses of encainide are required in patients with pacemakers, clinical observation and ambulatory electrocardiographic monitoring should be carried out.

A Case of Sick Sinus Syndrome that Developed Torsades de Pointes, Pacing Failure and Sensing Failure During Administration of Bepridil

“…Several antiarrhythmic agents have been found to change the thresholds of pacemakers but no such effect has been documented for Bpd. 6,7) In these reports changes in the pacing threshold have been mentioned, yet the sensing thresholds are much less commonly recognized as being affected by cardioactive drugs, and significant clinical sensing problems have not been recognized with any of the drugs discussed in relation to pacing thresholds. The sensing capacity of a pacemaker depends on depolarization of the myocardium at the site of the pacing lead.…”

Atrial Selective Effect of Amiodarone to Increase Threshold of Excitation

“…Changes in pacing thresholds have been described in relation to different antiarrhythmic drugs, especially with the 1C antiarrhythmic agents group, and though changes in defibrillation thresholds with amiodarone have been well described, documentation of changes in the stimulation threshold of pacemakers induced by amiodarone do not exist, as far as we know. Our case presents in a clear form how amiodarone can cause severe elevation of the thresholds of stimulation to the point of producing no capture, even at maximum output voltage.…”

“…They have been related to chronic inflammatory processes at the electrode tip resulting in an output block, in conjunction with metabolic abnormalities such as hyperkalemia, acidosis or alkalosis, hypothyroidism, or by the effect of different antiarrhythmic drugs. Threshold variations induced by flecainide, propafenone, encainide, or moricine are well documented. Defibrillation threshold variations induced by amiodarone are well documented too, but the effects that this antiarrhythmic drug may cause on the excitation threshold of the pacemaker are not.…”

Pacemaker Troubleshooting and Follow-up

“…A rise in threshold more than 6 weeks after implantation is usually considered to be in the chronic phase of the lead maturation process. In addition, some medications may affect capture thresholds, resulting in significant changes from the patient's baseline [189][190][191][192][193][194] (Box 29-11). [153][154][155][156][157][158][159] Older lead models and some epicardial leads are more likely to develop elevated capture threshold levels.…”