The effects of an isoproterenol infusion on the duration of the human right ventricular endocardial monophasic action potential at 90% repolarization were recorded in the absence and in the presence of an antiarrhythmic drag regimen containing class III effects in two similar groups of patients. The drugs used were amiodarone (N = 3, 300 ± 50 mg), sotalol plus quinidine (N = 11, 156 ± 13 mg sotalol, 1688 ± 594 mg quinidine), and sotalol alone (N = 3, 300 ± 20 mg). All patients had underlying coronary disease but no evidence of inducible ischemia. In the absence of antiarrhythmic drug, isoproterenol did not significantly change the relationship of action potential duration at 90% repolarization to cycle length; there was a linear decrease in action potential duration by 19.8% between a paced cycle length of 600 and 300 ms. Isoproterenol did not significantly shorten the action potential duration at any cycle length. However, isoproterenol decreased the ventricular effective refractory period at 400 ms drive from 240 ± 5.0 to 225 ± 6.0 ms (p < 0.05) accompanied by no change in the ratio of refractory period to steady-state action potential duration. In the presence of class III drug effects, the action potential duration was increased by an average of 9.2% at all paced cycle lengths longer than 300 ms (p < 0.05). The ventricular refractory period was increased from 240 ± 5 to 269 ± 9.0 ms (p < 0.05 compared with baseline) with a concomitant increase in the ratio of refractory period to action potential duration from 96 ± 2 to 103 ± 2% (p < 0.05 compared with baseline). With infusion of isoproterenol in the presence of a class III containing regimen, the drug-prolonged action potential duration was shortened (p < 0.05) by an average of 8.1% at all cycle lengths longer than 300 ms. These results suggest that isoproterenol simulation of enhanced sympathetic tone can antagonize drug prolongation of action potential duration and that in the absence of drugs, the effects of isoproterenol on the steady-state action potential duration are modest. The clinical utility of class III agents may be augmented by the addition of concomitant β-blockade.Key words: action potential duration, antiarrhythmic drugs, isoproterenol.
Differences in the Aging-Associated Trends of the Monophasic Action Potential Duration and Effective Refractory Period of the Right and Left Atria of the Rat
