Can early timed internal atrial defibrillation shocks reduce the atrial defibrillation threshold?

The defibrillation threshold is markedly reduced very early following the i nitiation of ventricular fibrillation. The purpose of this study was to det ermine if the same finding holds true for atrial defibrillation. Sustained, reproducible AF was induced with programmed atrial pacing using acetyl-bet a-methylcholine chloride (40-640 mu L/min) in six adult sheep (heart weight 245-300 g). Seven timing intervals (125 ms, 200 ms, 2 s, 3 s, 20 s, 30 s, and 5 min after AF induction) and two lead configurations: (1) RA as cathod e and CS as anode; and (2) RA as cathode and RV apex as anode were tested. Single capacitor biphasic waveforms (3/1 ms) were delivered and atrial defi brillation thresholds (ADFTs) were determined in random order. No significa nt differences in leading edge voltage and total energy were detected for t he RA-CS configuration for the seven timing intervals. For the RA-RV config uration, a significant difference was detected comparing the voltage for 12 5 ms to the 5-minute timing interval. For all times except 125 ms, the RA-R V threshold was significantly higher than the RA-CS level. in contrast to v entricular defibrillation, the ADFT does not change significantly within th e first 5 minutes after the initiation of AF for the RA-CS configuration, H owever, if the shock is given very early (125 ms after AF induction) with t he RA-RV configuration, the ADFT is lowered almost to the RA-CS level.