No abstract

The frequency-dependent response of the electrophysiologic effects of amiodarone are similar after 11 days of loading or > or = 1 year of chronic therapy. Amiodarone does not exert frequency-dependent effects on ventricular repolarization; it prolongs refractoriness by both time- and voltage-dependent mechanisms and exerts frequency-dependent effects on ventricular conduction. The absence of amiodarone-induced reverse frequency-dependent effects on repolarization, together with its time-dependent effects on refractoriness may account in part for the high efficacy of the drug and its low propensity to cause torsade de pointes.

BACKGROUND The autonomic nervous system appears to play an important role in the development of clinical ventricular arrhythmias, and beta-adrenergic sympathetic stimulation may be important in modulating the electrophysiologic effects of class III antiarrhythmic agents. This study prospectively determined the effects of isoproterenol on the frequency-dependent actions of sematilide (a pure class III agent that selectively blocks the delayed rectifier potassium current) and amiodarone (a class III agent with a complex pharmacologic profile) on ventricular repolarization, refractoriness, and conduction. METHODS AND RESULTS The frequency-dependent electrophysiologic effects of sematilide (n = 11) and amiodarone (n = 22) were determined at (1) drug-free baseline, (2) during steady-state (> 48 hours) dosing with sematilide (455 +/- 5 mg/d [mean +/- SEM]) or after 10.5 days of amiodarone loading (1618 +/- 32 mg/d), and (3) during isoproterenol administration (35 ng/kg per minute) to patients receiving sematilide or amiodarone. Electrophysiologic determinations were made at paced cycle lengths of 300 to 500 ms. The two groups were similar in all clinical characteristics. The ventricular action potential duration at 90% repolarization (APD90) was significantly prolonged by sematilide (mean increase, 7 +/- 1%, P < .01 by ANOVA) and amiodarone (mean increase, 12 +/- 1%, P < .001). However, while sematilide-induced APD90 prolongation was fully reversed to baseline values during isoproterenol infusion, the APD90 in patients receiving amiodarone remained significantly prolonged by a mean of 6 +/- 1% compared with baseline (P = .005). The reduction in the APD90 was frequency dependent for both agents, with a greater reduction at longer than shorter paced cycle lengths (P < .02). During isoproterenol infusion the right ventricular effective refractory period (RVERP) in patients receiving sematilide was significantly reduced to mean values of 8 +/- 2% below baseline (P < .05), whereas the RVERP in patients receiving amiodarone remained significantly prolonged by a mean of 7 +/- 1% above baseline values (P = .01). Sematilide and sematilide/isoproterenol had no effect on ventricular conduction. Amiodarone increased the QRS duration by 14 +/- 4% (paced cycle length, 500 ms) to 32 +/- 5% (paced cycle length, 300 ms) compared with baseline values. Isoproterenol attenuated amiodarone-induced QRS prolongation by a mean of 5 +/- 1% (P = .005), without frequency-dependent effects, consistent with isoproterenol-induced increases in the sodium current. During isoproterenol infusion there was a trend for the sustained VT cycle length to be reduced below baseline in patients receiving sematilide (275 +/- 16 versus 298 +/- 55 ms, P = .06), whereas it remained significantly prolonged compared with baseline in patients receiving amiodarone (327 +/- 17 versus 257 +/- 12 ms, P < .001). CONCLUSIONS Isoproterenol fully reversed the effects of selective potassium channel block with sematilide on the APD90 and further reduced the RVERP to values significantly below baseline; it partially attenuated but did not fully reverse amiodarone-induced prolongation of the APD90 and RVERP, which remained significantly prolonged beyond baseline values. Isoproterenol exerted frequency-dependent effects in both patient groups on the APD90; it modestly attenuated amiodarone-induced conduction slowing without frequency-dependent actions; and the sustained VT cycle length remained significantly prolonged during isoproterenol administration to patients receiving amiodarone but not in those receiving sematilide. These findings may have important clinical implications regarding protection from arrhythmia development in patients receiving pure class III agents or amiodarone.