Introduction Previous studies have demonstrated that synchronized shocks reduce intracardial defibrillation threshold. In the present study, we investigated the effect of synchronized shocks on external defibrillation success.
Methods The experimental data were collected from a male domestic pig (38 kg). The right ventricular electrogram (EGM) was obtained from a unipolar pacing lead placed in the right ventricular apex. The defibrillation pads were placed in lateral-to-lateral position. The peak detection threshold was set manually to 80% of the mean peak value of EGM. VF was induced electrically and untreated for 10 seconds. A synchronized shock of 100 Joule from an external defibrillator was delivered with a time delay after a peak was detected. Four different time delays (25, 50, 75 and 0 ms) were introduced aiming the shock at four phases of EGM waveform: descending limb (P1), trough (P2), ascending limb (P3) and peak (P0). The animal received at least four minutes for recovery before the study sequence was repeated. The targeted shocking phases were corrected with the phases the energy was actually delivered before data were analyzed. The descending phase was defined as P1+P2 and the ascending phase was P3+P0.
Results A total of 43 shocks were included for analysis, in which 30 shocks were delivered correctly in targeted phases while other 13 shocks were not. The delivered defibrillation current, energy and transthoracic impedance had no significant difference between ascending and descending phases. The logistic regression, adjusted for delivered current, energy and impedance, demonstrated that odds of successful defibrillation on the descending phase was 5.7 times larger than that on the ascending phase (p=0.023). 
Conclusions This study result suggested that the descending phase of a right ventricular endocardial electrogram may be a vulnerable phase for external defibrillation.

7 - Improved External Defibrillation Success with Shocks Timed to Descending Phase of a Right Ventricular Endocardial Electrogram