Influence of verapamil on the inotropism and pharmacokinetics of digoxin

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Abstract
Verapamil has been demonstrated to inhibit the elimination of digoxin and to increase its steady state plasma level by 60–80%. Animal studies suggest that verapamil abolishes the intropic action of other drugs such as ouabain and dopamine. The clinical consequences of this drug interaction were investigated by examining the inotropic activity of single doses of digoxin (assessed from systolic time intervals), with and without coadministration of verapamil. Verapamil decreased total-body clearance of digoxin from 4.68±0.41 to 3.29±0.26 ml/min/kg (pp<0.01). Verapamil had no influence on the base-line values of the systolic time intervals. Both in the absence and presence of verapamil, digoxin caused significant shortening of the total electromechanical systole and the left ventricular ejection time. However, compared to control conditions, the decay of these changes was slower in the presence of verapamil, in parallel with the prolongation of the plasma half-life of digoxin. A linear relationship was established between reductions in the systolic time intervals and the computer-derived concentration of digoxin in the deep compartment. These regression lines, which represent the concentration-effect relationships of the inotropism of digoxin, were not affected by verapamil. Thus, verapamil per se had no measurable effect either on base-line contractile function of the heart or on digoxin-induced inotropism. The elevated plasma digoxin concentration induced by verapamil appears cardioactive in terms of inotropism.