977-74 Long Electrical Silence Following Unsuccessful Internal Defibrillation Shocks of Atrial Fibrillation in Sheep

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We characterized the atrial activation patterns following unsuccessful atrial defibrillation shocks by performing epicardial mapping studies on 3 anesthetized adult sheep (heart weight 330 ± 25 gms) with atrial fibrillation (AFI) induced by burst pacing. 138 silver chloride electrodes were positioned to cover the epicardium of both atria. Transvenous defibrillation coiled electrodes were positioned in the right atrial appendage (RAap), low right atrium (LRA), distal coronary sinus (CS), and left pulmonary artery (LPA) behind the left atrium. Atrial defibrillation thresholds (ADFT) were determined using 3/3 ms biphasic waveform with a step up protocol (10 volt resolution). ADFT was determined for 7 different electrode configurations: RAap→CS, RAap→LRA, LRA→LPA, RAap→LPA, CS→LRA, CS→LPA, and RAap→CS + LPA. After ADFT was determined, 15 shocks were given starting at the ADFT for each electrode configuration in each sheep. Shock strengths were decreased by 10 volts after a successful shock and increased by 10 volts after an unsuccessful shock. Pre and post-shock shock activations were recorded from all 138 electrodes using a computer assisted mapping system. For each animal 3 to 4 unsuccessful shocks just prior to or after a successful shock were analyzed for each electrode configuration. The earliest site of activation and the time from the shock to the first post-shock activation were determined. The table shows the mean ± standard deviation for the ADFT in terms of total energy (joules) and the first post-shock activation times (ms) for each electrode configuration. RAap→CS RAap→LRA LRA→LPA RAap→LPA CS→LRA CS→LPA RAap→CS + LPA 0.5 ± 0.2 5.5 ± 2.4 2.0 ± 0.7 1.9 ± 1.2 23 ± 2.2 7.2 ± 4.0 1.2 ± 0.9 101.7 98.3 119.0 110.4 99.3 90.0 127.7 ± 40.7 ± 33.0 ± 20.9 ± 30.4 ± 20.2 ± 20.5 ± 49.1 There was no difference (p ≤ 0.05) in first activation times between electrode configurations. A relatively long period of electrical silence occurred following unsuccessful atrial defibrillation shocks. This appeared to be independent of electrode configuration. These findings suggest that a period of time may exist after an unsuccessful atrial defibrillation shock for further intervention.