Mechanism of action and efficacy of verapamil and beta-blockers in exercise-induced ventricular tachycardia

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Abstract
To determine the antiarrhythmic efficacy of beta-blockers (β-B) and verapamil (V) in exercise-induced ventricular tachycardia (Ex-VT), nine patients with reproducible Ex-VT (in two consecutive exercise tests) were studied by means of electrophysiologic study (EPS) in basal conditions and serial exercise testing after β-B (metoprolol 25 mg tid to 100 mg qid; oxprenolol 40 mg tid) and/or V (80–160 mg tid). Ejection fraction was normal in four cases and depressed in five. Of these nine patients, four developed Ex-VT during chronic amiodarone treatment, which was continued. During EPS, VT was induced at a critical atrialpacing rate in one case, and with the extrastimulus technique in four. Ventricular tachycardia was not inducible with either technique in four patients. Five of the six patients on β-B and none of the seven on V developed Ex- VT, although they achieved the same or higher work-loads as compared to the basal exercise tests. In the case with rate-dependent VT, β-B and V prevented VT at work-loads, sinus rates and double products significantly higher than those obtained in basal conditions. In the others, maximal heart rate and double product were lower on β-B and showed a wide variability on V. V and β-B appeared to be highly effective in preventing Ex-VT, in patients with normal heart as well as in those with greatly depressed ejection fraction. Both of the drugs appeared to suppress re-entry or triggered activity in the patient with rate-dependent Ex-VT; in the others the arrhythmia was prevented by β-B probably by decreasing the adrenergic tone and myocardial oxygen uptake, and by V probably by depressing slow-response calcium-dependent fibres.

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