Mechanism of the suppression of repetitive atrial firing by isoproterenol — comparison with disopyramide

Abstract

To investigate whether isoproterenol (Iso) could suppress the initiation of repetitive atrial firing (RAF), we investigated its effect on RAF in comparison with that of disopyramide (Diso). Extrastimuli at a basic cycle length of 500 ms were delivered from the high right atrium in 49 patients who received an intravenous infusion of Iso (0.01 μg/kg per min) and in 39 patients given intravenous Diso (2 mg/kg per 10 min). Induction of RAF, the atrial effective refractory period (A-ERP), and the maximum conduction delay (MCD) were measured. Iso abolished the induction of RAF in 13 19 (68%) patients, while Diso did so in 13 22 (59%) patients. Thirty-four of the 41 patients with RAF in the base-line study had an A-ERP <250 ms and an MCD >40 ms. Iso significantly decreased the A-ERP from 205 ± 26 to 194 ± 23 ms ( P < 0.01) and significantly decreased the MCD from 67 ± 24 to 39 ± 16 ms ( P < 0.0001) in 19 patients with RAF. On the other hand, Diso significantly increased the A-ERP from 203 ± 31 to 235 ± 36 ms ( P < 0.0001), and significantly diminished the MCD from 68 ± 31 to 55 ± 30 ms ( P < 0.01) in 22 patients with RAF. In patients with new RAF ( n = 7) or re-induced RAF ( n = 14) during Iso or after Diso, the MCD was more than 40 ms. Our results suggest that there are two different modes of RAF suppression, i.e. shortening or lengthening of the A-ERP. Thus, suppression of RAF is probably due to a reduction of conduction delay, with Iso accelerating conduction and Diso diminishing conduction delay as a result of prolongation of the A-ERP.