Verapamil decreases MAC for halothane in dogs.

Verapamil hydrochloride is a calcium entry blocking drug that is being prescribed with increasing frequency for cardiovascular disorders in the perioperative setting. Verapamil's calcium channel blocking effect is not selective, because it also exerts activity on the sodium channel. Because of the well-described effects of sodium channel blockers on anesthetic requirements, the authors studied the MAC for halothane in dogs before and after a therapeutic dose of verapamil 0.5 mg . kg-1. There was a 25% reduction in halothane MAC from 0.97-0.72% (P less than 0.01) when a therapeutic plasma level of verapamil (64 ng . ml-1) was present. Anesthetic requirements for halothane are reduced by dl-verapamil possibly on the basis of its local anesthetic-like sodium channel blocking properties. Adjustments in anesthetic dosage may be necessary in patients receiving verapamil.