Preexisting abnormalities in SA & AV node activity, congenital or documented acquired QT prolongation, poor cardiac reserve, unstable angina pectoris, recent MI. Concomitant use w/ medications that prolong the QT interval eg, class I & III antiarrhythmics, TCAs, some antipsychotics, antibiotics (eg, erythromycin) & some antihistamines (eg, terfenadine). Hepatic impairment. Pregnancy & lactation. Childn.