Oral amiodarone loading therapy. I. The effect on serial signal-averaged electrocardiographic recordings and the QTc in patients with ventricular tachyarrhythmias.

The effect of amiodarone loading (1400 mg/day for 7 days) and maintenance therapy (400 mg/day) on the signal-averaged electrocardiogram (SAECG) and the QTc were evaluated in 14 patients. Daily recordings were taken for the first 7 days of loading and on the ninth, twelfth, fourteenth, and twenty-first day of maintenance therapy. All patients had inducible sustained ventricular tachycardia (VT), 10 of them had late potential (LP) positive recording, and four were LP negative during control measurements. Amiodarone loading significantly changed all SAECG indices and prolonged the QTc. Changes induced by amiodarone were present within 24 hours of administration. The time to reach peak effect varied from 6 to 9 days. The maximum increase in late potential duration (LPD) was threefold greater than the increase in filtered QRS duration or QTc (61% vs 18% vs 14%, respectively), suggesting a more pronounced effect on the reentrant pathway than on the remaining myocardium. Of the four LP negative patients, three became LP positive while receiving amiodarone. In conclusion, amiodarone affects all SAECG indices. However, the magnitude of the changes varies, being most pronounced on the LPD. Amiodarone affects the SAECG and the QTc within the first 24 hours. The time of onset of the peak effect varies from 6 to 9 days. LP negative patients with inducible VT frequently became LP positive during amiodarone therapy.