Introduction: The likelihood of survival from ventricular fibrillation (VF) declines 7%-10% per minute until successful defibrillation. When VF duration is prolonged, immediate defibrillation of the ischemic myocardium is less likely to result in ROSC, and repeated unsuccessful defibrillations are associated with post-resuscitation myocardial dysfunction. Thus, the timing of defibrillation should be based upon the probability of shock success-a function of VF duration. Unfortunately, VF duration is often unknown in out-of-hospital cardiac arrest (OHCA) and a better predictor of shock success is needed.
Objective: To assess the ability of end-tidal carbon dioxide (EtCO2) to predict successful defibrillation in OHCA.
Methods: This retrospective study included adult patients among four EMS systems who experienced non-traumatic OHCA from August, 2015-July, 2017 and received one or more defibrillations. First and succedent shocks were analyzed separately. First shocks represented EMS-attempted defibrillation of patients who had not received a prior AED shock, whereas succedent shocks included all shocks subsequent to the first. Logistic regression provided odds ratios (OR) for first shocks resulting in ROSC, while a generalized estimating equation was used to analyze succedent shocks.
Results: Among 324 patients, 869 shocks were delivered by EMS (153 first and 716 succedent shocks). Layperson CPR was performed in 48.1% of cases and 21.6% received an AED shock before EMS arrival. First defibrillation ROSC was more likely with layperson CPR (OR = 4.41;p = 0.01) and increasing EtCO2 (OR = 1.03/mmHg;p = 0.01). No other variables were statistically significant. Notably, only one patient with EtCO2 < 20 mmHg was successfully defibrillated on the first shock. The probability of ROSC was higher with increasing values of EtCO2 when layperson CPR was provided, yet remained relatively unchanged across all values of EtCO2 ≥ 20 mmHg without layperson CPR. The optimal threshold first shock EtCO2 was 27 and 32 mmHg for those with/without layperson CPR, respectively. EtCO2 was not a predictor of ROSC for succedent shocks.
Conclusions: An optimal defibrillation threshold EtCO2 of 27 and 32 mmHg was observed for patients with and without layperson CPR, respectively. Further studies are warranted to verify these results and to evaluate the clinical effect of delaying defibrillation in favor of chest compressions until these values are attained.
Keywords: capnography; cardiac arrest; cardiopulmonary resuscitation; defibrillation; ventricular fibrillation.