Small Variations in Defibrillation Electrode Placement Affect Shock Success

Background: Defibrillation is essential in treating cardiac arrest patients with ventricular fibrillation (VF). The success of the shock in terminating VF depends on many previously studied factors, including waveform and energy dose. It has been observed clinically that defibrillation electrode placement varies widely. The goal of this study was to assess the effect of small differences in defibrillation electrode placement on first shock defibrillation success rates in swine. Methods: In 10 anesthetized swine weighing 32.7 ± 2.5 kg (mean ± standard deviation), electrode pads were attached at 3 different symmetric positions: the medial edge of each electrode was placed 3% (2.1 ± 0.1 cm), 7% (4.7 ± 0.1 cm), or 11% (7.5 ± 0.2 cm) around the circumference of the chest from mid-sternum (Fig. 1). Biphasic impedance-compensated shocks were delivered through the pads with a 50-ohm resistor added in series to replicate human impedance. The order of the 3 positions was block randomized for each experiment. At each electrode position, 24 episodes were run: VF was electrically induced and up to 2 shocks were delivered. Primary outcome was defined as successful VF termination after the first shock; secondary outcome was the cumulative success of both the first and second shock. Results: Across all experiments, at the 3%, 7%, and 11% positions, first shock success was 38.3% (92/240), 48.3% (116/240), and 36.7% (88/240) (p = 0.02, Chi-square Test); cumulative 2-shock success was 59.6% (143/240), 63.8% (153/240), and 55.4% (133/240) (p = 0.18), respectively. The lowest first shock success was at the 3% position in 6 of 10 animals, at the 11% position in 4 of 10 animals, and never at the 7% position. Conclusions: Small changes in electrode pad placement can significantly affect defibrillation shock success. Although anatomic differences may prevent the existence of one optimal electrode position, one position may be more effective for VF termination than another in a given patient.