Amplitude spectrum area to predict true shock-refractory ventricular fibrillation during basic life support-treated out-of-hospital cardiac arrest

Aim: Refractory ventricular fibrillation (VF) is associated with decreased survival. Guideline definition of refractory VF, i.e. a VF persisting after 3 shocks, includes cases of true shock-refractory VF or recurrent VF with transient defibrillation response. Predicting true shock-refractory VF before repeated shock failure could prioritize targeted interventions to improve out-of-hospital cardiac arrest (OHCA) outcomes. We hypothesized that amplitude spectrum area (AMSA) may predict cases of true refractory VF. Methods: ECGs recorded by automated external defibrillators were obtained from OHCAs in 8 cities in Italy. A 2-second VF window before each defibrillation was analyzed to calculate Amplitude Spectrum Area (AMSA). Defibrillation success was defined by occurrence of a perfusing rhythm, while refractory VF was defined as: “pragmatic-refractory” (based on guideline definition); or within this category, as “True shock-refractory”, if VF continuously persisted over the period needed to deliver the first 3 shocks; or “refractory-recurrent”, if VF recurred after any of the first three shocks transiently terminated VF. Results: 1646 OHCAs with shockable presenting rhythm were included, 360 (22 %) of whom met the definition of pragmatic-refractory VF. Among the 360 cases of pragmatic-refractory VF, 18 % were true shock-refractory and 82 % were refractory-recurrent VF. AMSA was significantly lower in true shock-refractory VF than in refractory-recurrent VF. A lower first AMSA was associated with occurrence of true shock-refractory VF (aOR:0.81; 95 %CI: 0.73–0.88; p < 0.0001). Conclusions: Lower AMSA is associated with true shock-refractory VF, a subtype associated with persistent defibrillation failure and worse long-term survival. Identifying different VF subtypes early may help guide advanced resuscitation strategies in OHCA.