Comparison of intravenous and oral verapamil dosing

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Abstract
To compare the effects of i.v. and oral verapamil, the prolongation of the PR interval in 11 patients was examined after a single 10 mg i.v. bolus given over 2 min, a single oral dose of 120 mg and a sustained concentration-maintaining infusion. Maximal PR interval prolongation was delayed relative to peak plasma verapamil concentration in all patients after the bolus and in 7 of 11 patients after oral dosing. In all 11 patients oral verapamil was less potent than a single i.v. bolus of verapamil; the plasma verapamil concentration corresponding to a 10% prolongation of the PR was 39.5 .+-. 21.7 ng/ml after the bolus and 146.3 .+-. 75.1 ng/ml after the oral dose (P = 0.001). There was no such difference between oral verapamil and an infusion in 6 patients. The plasma verapamil concentration corresponding to a 10% PR prolongation was 35.7 .+-. 24 ng/ml after the bolus, 132.5 .+-. 80.8 ng/ml after the oral dose and 85.2 .+-. 29.9 ng/ml after the infusion. Maximum PR prolongation (drug efficacy) was comparable for the 3 methods of administration. There was no evidence of tachyphylaxis during prolonged infusions. Both oral doses and infusions of verapamil are evidently less potent than bolus doses, but drug efficacy at the concentrations reached is equivalent for the 3. Plasma verapamil concentrations determined after bolus doses appear to underestimate effective plasma concentration when the drug is given by the oral or infusion methods.