In Wolff-Parkinson-White (
WPW) syndrome, the two most commonly occurring arrhythmias are circus movement
tachycardia (CMT) and
atrial fibrillation (AF). In 70% of patients with clinically documented CMT in whom the
arrhythmia could be initiated by programmed electrical stimulation of the heart, the same CMT could still be initiated after long-term oral
amiodarone administration. Spontaneous clinical recurrence of the
arrhythmia was, however, observed in only 10% of patients. This finding suggests that the beneficial effect of
amiodarone on CMT is primarily based on the prevention of the CMT-initiating
premature beat. This may also apply to
atrioventricular nodal reentrant tachycardia, in which
amiodarone is also extremely effective in preventing relapses. The role of
amiodarone in other forms of reentrant, or ectopic,
supraventricular tachycardias is less well defined. During AF in
WPW syndrome, the ventricular rate is related to the duration of the anterograde refractory period of the accessory pathway.
Amiodarone prolongs this value, resulting in the reduction of ventricular rate during AF. Unfortunately, in the presence of a short anterograde refractory period of the accessory pathway,
amiodarone results in only a small amount of lengthening of this value. In these patients the beneficial effect of
amiodarone may primarily be related to the prevention of episodes of AF. We also found that the effect of oral
amiodarone on the duration of the anterograde refractory period of the accessory pathway can (1) be abolished by sympathetic stimulation with
isoproterenol and (2) be predicted from the effect of
ajmaline or
procainamide given intravenously. These observations clearly have practical clinical implications.