Predefibrillation end-tidal CO2 and defibrillation success in out-of-hospital cardiac arrest: an observational cohort study

Background Predefibrillation end-tidal CO₂ (ETCO₂) may predict defibrillation success and could guide defibrillation timing in ventricular fibrillation (VF) cardiac arrest. This relationship has only been studied using advanced airways. Our aim was to evaluate this relationship using both basic (bag–valve–mask (BVM)) and advanced airways (supraglottic airways and endotracheal tubes).

Methods Prehospital patient records and defibrillator files were abstracted for patients with out-of-hospital cardiac arrest in Ontario, Canada, with initial VF cardiac rhythms between 1 January 2018, and 31 December 2019. Analyses assessed the relationship between each predefibrillation ETCO₂ reading and defibrillation outcomes at the subsequent 2 min pulse check (ie, VF, asystole, pulseless electrical activity (PEA) or return of spontaneous circulation (ROSC)), accounting for airway types used during resuscitation. Multivariable logistic regression evaluated the association between the first documented predefibrillation ETCO₂ and postshock VF termination or ROSC.

Results Of 269 cases abstracted, 153 had predefibrillation ETCO₂ measurements and were included in the study. Among these cases, 904 shocks were delivered and 44.4% (n=401) had predefibrillation ETCO₂ measured. The first ETCO₂ reading was more often from BVM (n=134) than advanced airways (n=19). ETCO₂ readings were lower when measured through BVM versus advanced airways (30.5 mm Hg (4.06 kPa) (±14.4 mm Hg (1.92 kPa)) vs 42.1 mm Hg (5.61 kPa) (±22.5 mm Hg (3.00 kPa)), _adjANOVA p<0.01). Of all shocks with ETCO₂ reading (n=401), no difference in preshock ETCO₂ was found for subsequent shocks that resulted in persistent VF (32.2 mm Hg (4.29 kPa) (±15.8 mm Hg (2.11 kPa))), PEA (32.8 mm Hg (4.37 kPa) (±17.1 mm Hg (2.30 kPa))), asystole (32.4 mm Hg (4.32 kPa) (±20.6 mm Hg (2.75 kPa))) or ROSC (32.5 mm Hg (4.33 kPa) (±15.3 mm Hg (2.04 kPa))), analysis of variance p=0.99. In the multivariate analysis using the initial predefibrillation ETCO₂, there was no association with VF termination on the subsequent shock (adjusted OR (_adjOR) 0.99, 95% CI 0.97 to 1.02, p=0.57) or ROSC (_adjOR 1.00, 95% CI 0.97 to 1.03, p=0.94) when evaluated as a continuous or categorical variable.

Conclusion Predefibrillation ETCO₂ measurement is not associated with VF termination or ROSC when basic and advanced airways are included in the analysis. The role of predefibrillation ETCO₂ requires careful consideration of the type of airway used during resuscitation.