The effects of atrial electrical remodeling on atrial defibrillation thresholds

Electrical remodeling of atrial fibrillation may account for the increase i n atrial defibrillation thresholds over time. The aim of this study was to examine the time course of electrical remodeling and the benefit of early d efibrillation on the defibrillation threshold, Twenty-six mongrel dogs weig hing 27.6 +/-3.3 kg were induced into AF by repeated high output burst atri al pacing. Eight dogs were paced for multiple time periods of 5, 20, 40, an d 60 minutes. Five dogs each had burst pacing for 4 hours and 8 hours, and eight dogs were paced at a high rate (640 beats/min) for 48 hours. Biphasic atrial defibrillation shocks with a pulse width of 3/3 ms synchronized to the left apical electrogram were delivered to coil electrode catheters posi tioned in the lateral left and right atria. Defibrillation voltage was incr eased from 50 V in 20- to 30-V steps until defibrillation was successful. A s the pacing period increased, a decrease in atrial fibrillation cycle leng ths and atrial effective refractory period was not observed before 8 hours. Similarly, the defibrillation threshold did not change significantly until the 8-hour pacing period was reached. The defibrillation thresholds were 6 9 +/- 28 Vfor 5 minutes, 64 +/- 20 Vfor 20 minutes, 99 +/- 85 Vfor 40 minut es, 78 +/- 51 Vfor 60 minutes, 78 +/- 38 Vfor 4 hours, 124 +/- 33 Vfor 8 ho urs, and 133 +/- 32 Vfor 48 hours (mean SD) (P < 0.05). Atrial electrical r emodeling in a rapid atrial pacing canine model is not observed until after 4 hours of burst atrail pacing. The atrial defibrillation threshold increa ses with increasing duration of burst atrial pacing, and follows a similar time course to other parameters of electrical remodeling.