Effect of application of force to self-adhesive defibrillator pads on transthoracic electrical impedance and countershock success.

STUDY OBJECTIVE: To examine the effect of the application of force to self-adhesive defibrillator pads on transthoracic electrical impedance and countershock success. METHODS: A prospective, randomized, controlled pilot study was carried out in an experimental animal laboratory, involving 32 mixed-breed swine weighing 36.5 to 55.7 kg each. Ventricular fibrillation (VF) was induced, and the animals were randomly allocated to 1 of 4 groups, with 8 animals per group. Animals in groups I and II remained in VF for 30 seconds; those in groups III and IV remained in VF for 5 minutes. At the end of the VF period, up to 3 countershocks were given. In groups I and III, countershocks were delivered through the self-adhesive defibrillator pads alone; in groups II and IV, they were delivered through the defibrillator pads with 25 lb of applied force. Any animal without return of spontaneous circulation after 3 countershocks was given epinephrine after 1 minute of CPR, followed by 1 additional minute of CPR and 1 additional countershock if required. RESULTS: The main measurements included baseline and countershock transthoracic impedance, cumulative countershock success rate, and 30-minute survival rate. Application of 25 lb of force to the defibrillator pads significantly decreased transthoracic impedance, compared with baseline values (group II, 15. 1% decrease; group IV, 16.1% decrease). The first-shock success rate was higher when force was applied during the countershock (87.5% versus 50% for groups II and I, respectively; 62.5% and 37.5% for groups IV and III, respectively). In the animals who experienced 5 minutes of VF, there were greater rates of success after the second, third, and fourth countershocks when force was applied (group IV) than when no force was applied (group III). Groups I and II (VF for 30 seconds) demonstrated 100% survival at 30 minutes. Group IV had a higher 30-minute survival rate (3/8 animals) than did group III (1/8). However, the rates of countershock success and 30-minute survival were not statistically different among the groups. CONCLUSION: Application of force to self-adhesive defibrillator pads decreases transthoracic impedance during countershock. This effect may contribute to improving the countershock success rate.