Atrial Fibrillation: The Science behind Its Defiance
Ibutilide can prolong refractory period and terminate reentry. Whether ibutilide has the same effects on pulmonary vein (PV) focal discharge (FD) is unclear. We induced sustained atrial fibrillation (AF) in seven dogs by rapid left atrial (LA) pacing for 74 +/- 46 days. Ibutilide was repeatedly infused until it terminated AF (0.02 +/- 0.01 mg/kg) or when a cumulative dose was reached (0.04 mg/kg). High-resolution computerized epicardial mapping was performed. We found intermittent FD at the PVs and reentry at the PV-LA junction during AF. Ibutilide increased the cycle length of consecutive reentry from 97 +/- 13 to 112 +/- 18 ms and increased FD from 96 +/- 7 to 113 +/- 9 ms. In four dogs with both FD and reentry at the PVs, the incidence of reentry decreased from 3.5 +/- 1.9/s at baseline to 2.2 +/- 1.8/s after ibutilide administration. However, the incidence of FD remained unchanged. The conducted wave fronts between PV and LA were significantly reduced by ibutilide (10.4 +/- 2.0/s vs. 8.0 +/- 1.6/s). The ibutilide dose needed to terminate AF correlated negatively with the baseline effective refractory period of PV and LA. We conclude that ibutilide reduces reentrant wave fronts but not PV FD in a canine model of pacing-induced sustained AF. These findings suggest that the PV FD during AF is due to nonreentrant mechanisms. High doses of ibutilide may completely terminate all reentrant activity, converting AF to PV tachycardia before the resumption of sinus rhythm.