Verapamil



The calcium antagonists have been discussed in Chapter 7. Verapamil has the most potent an­tiarrhythmic actions. It does not affect cells with normal fast response characteristics (atrial and ventricular muscle, His-Purkinje system), but in fast channel-dependent cells rendered abnormal by disease, verapamil may suppress electrical ac­tivity. Slow channel-dependent tissue (sinus and AV nodes) exhibits an increase in conduction time and refractoriness after verapamil adminis­tration. Therefore, verapamil prolongs the AH in­terval without affecting His-Purkinje conduction or the QRS interval. Sinus rate may decrease, but in intact animals it often does not’change signif­icantly because of counteraction by sympathetic reflexes activated by peripheral vasodilation. Ver­apamil does not affect directly refractoriness of atrial or ventricular muscle or the accessory path­way. Combined therapy with propranolol and ver­apamil can be attempted in patients with normal cardiac contractility, but the patient should be ob­served for the development of heart failure and/ or symptomatic bradycardias because the com­pensatory sympathetic response to slow channel blockade is blocked. Calcium infusion or isopro­terenol may counteract some of the adverse effects of verapamil until temporary pacing can be ini­tiated.

Intravenous verapamil is the drug of choice for terminating sustained paroxysmal supra­ventricular tachycardias that are not terminated by vagal maneuvers, such as those reciprocating tachycardias employing the AV node or SA node in the tachycardia circuit. Verapamil can decrease the ventricular response in patients with atrial fi­brillation or flutter but converts only a small num­ber of these rhythms to sinus rhythm. Verapamil may be used in patients with congestive heart fail­ure and supraventricular tachycardia if it is thought that termination of the arrhythmia will relieve the heart failure. Verapamil may increase the ventricular response in patients with atrial fi­brillation and Wolff-Parkinson-White syndrome, and the drug is relatively contraindicated in that situation. Verapamil is usually not effective in pa­tients with recurrent ventricular tachyarrhyth­mias.