Defibrillation probability and impedance change between shocks during resuscitation from out-of-hospital cardiac arrest

Objective

Technical data now gathered by automated external defibrillators (AEDs) allows closer evaluation of the behavior of defibrillation shocks administered during out-of-hospital cardiac arrest. We analyzed technical data from a large case series to evaluate the change in transthoracic impedance between shocks, and to assess the heterogeneity of the probability of successful defibrillation across the population.

Methods

We analyzed a series of consecutive cases where AEDs delivered shocks to treat ventricular fibrillation (VF) during out-of-hospital cardiac arrest. Impedance measurements and VF termination efficacy were extracted from electronic records downloaded from biphasic AEDs deployed in three EMS systems. All patients received 200 J first shocks; second shocks were 200 J or 300 J, depending on local protocols. Results presented are median (25th, 75th percentiles).

Results

Of 863 cases with defibrillation shocks, 467 contained multiple shocks because the first shock failed to terminate VF (n = 61) or VF recurred (n = 406). Defibrillation efficacy of subsequent shocks was significantly lower in patients that failed to defibrillate on first shock than in patients that did defibrillate on first shock (162/234 = 69% vs. 955/1027 = 93%; p < 0.0001). The failed VF terminations were distributed heterogeneously across the population; 5% of patients accounted for 71% of failed shocks. Shock impedance decreased by 1% [0%, 4%] and peak current increased by 1% [0%, 4%] between 200 J first and 200 J second shocks. Shock impedance decreased 4% [2%, 6%] and current increased 27% [25%, 29%] between 200 J first and 300 J second shocks. In all 499 pairs of same-energy consecutive shocks, impedance changed by less than 1% in 226 (45%), increased >1% in 124 (25%) and decreased >1% in 149 (30%).

Conclusions

Impedance change between consecutive shocks is minimal and inconsistent. Therefore, to increase current of a subsequent shock requires an increase of the energy setting. Distribution of failed shocks is far from random. First shock defibrillation failure is often predictive of low efficacy for subsequent shocks.