Antiarrhythmic efficacy of a new class III agent, UK-68,798, during chronic myocardial infarction: evaluation using three-dimensional mapping. UK-68,798 is a potent class III antiarrhythmic agent that selectively lengthens the effective refractory period (ERP) in isolated tissue without affecting conduction velocity. The present study was performed to evaluate the antiarrhythmic efficacy of UK-68,798 (30 micrograms/kg i.v.) in dogs with a previous myocardial infarction. UK-68,798 did not alter the PQ interval or QRS duration of the surface electrocardiogram but did increase the Q-Tc interval significantly. The ventricular ERP was increased significantly (P less than .01) at a basic cycle length of stimulation of either 300 ms (ERP increased 24 +/- 10 ms) or 250 ms (ERP increased 20 +/- 12 msec), indicating that the response was preserved at more rapid rates. UK-68,798 prevented the induction of sustained ventricular tachycardia in six of seven animals (86%, P = .03). However, UK-68,798 failed to prevent the induction of ventricular fibrillation (VF) in dogs where VF was the only arrhythmia induced. To evaluate the mechanisms responsible for the prevention of ventricular tachycardia and lack of efficacy against inducible VF, detailed three-dimensional activation mapping of the heart in vivo was used. Induction of ventricular tachycardia was prevented by UK-68,798 due to a lengthening of the ERP in the epicardial region surrounding the infarct with no effect on conduction velocity even in periinfarct regions bordering the infarct. In contrast, the induction of VF was dependent on a rapid nonreentrant or focal mechanism that was not altered by the lengthening of ERP with UK-68,798. Thus, UK-68,798 is a selective class III antiarrhythmic agent that is likely to be efficacious in preventing arrhythmias due to a reentrant mechanism in patients with a previous myocardial infarction. 