Shock Success and Its Determinants in Refractory Ventricular Fibrillation
During Out-of-Hospital Cardiac Arrest

J Nas, JL Bonnes, J Thannhauser, R Starreveld, PM van Grunsven, G Meinsma, N van Royen, JLRM Smeets, MJ de Boer, MA Brouwer

Purpose of the study: Optimising timing of shocks using predictors of shock success may contribute to a more personalised resuscitation strategy, and is currently under investigation for initial ventricular fibrillation (VF). However, the majority of patients requires additional shocks during the course of the resuscitation. More information is needed on shock success and its determinants in this particular subset of patients with refractory VF. 

Methods and materials: Per-shock analysis on cardiac arrest patients (Nijmegen, the Netherlands, 2008-2011) receiving >1 shock on VF (107 patients; 342 shocks). 
Following current guidelines, shocks were divided into shocks on recurrent (previous shock terminated VF) or on persisting VF (previous shock did not terminate VF), figure 1. Initial shocks were excluded. Predictors included defibrillator-derived parameters: amplitude spectrum area (AMSA, a VF-waveform measure), VF time intervals (e.g. pre-shock VF duration) and the number of preceding successful shocks. Shock success was defined as return of organized rhythm.

Results: Shock success was markedly higher for shocks on recurrent VF (n=214) vs. persisting VF (n=128): 65% vs. 32%, p<0.001. For recurrent VF, higher AMSA and increasing numbers of preceding successful shocks were independently associated with shock success (adjusted odds ratio [aOR] 1.15 per mV•Hz increase, p=0.001; aOR 1.43 per additional preceding shock, p=0.03). C-statistic for AMSA=0.63. For persisting VF, we found that longer pre-shock VF duration was significantly associated with reduced shock success (OR 0.96 per 30s increase, p=0.04, C-statistic=0.72)

Conclusions: This study underscores the importance of distinguishing different types of refractory VF, given the considerable differences in shock success and its determinants. Defibrillation failure for a shock on persisting is twice as likely as on recurrent VF. For persisting VF, shocks following longer periods of pre-shock VF duration had lower defibrillation success, which warrants future studies on the impact of a strategy with earlier and/or repetitive defibrillation attempts.

6 - Shock Success and Its Determinants in Refractory Ventricular Fibrillation During Out-of-Hospital Cardiac Arrest