Resuscitation From Out-of-Hospital Cardiac Arrest When Is EtCO ₂ Reliably Associated With ROSC?

BACKGROUND: Exhaled end-tidal carbon dioxide (EtCO 2 ) trajectory is associated with out-of-hospital cardiac arrest (OHCA) outcomes. However, the minimum EtCO 2 monitoring duration needed to discriminate return of spontaneous circulation (ROSC) from non-ROSC remains unknown. We sought to determine the EtCO 2 trajectory observation time required to differentiate ROSC from non-ROSC patients. METHODS: We performed a secondary analysis of the cluster-randomized Pragmatic Airway Resuscitation Trial (PART), which assessed endotracheal intubation or laryngeal tube strategies in OHCA resuscitation. We summarized mean EtCO 2 in 1-minute epochs over the resuscitation. Cases were stratified a priori by: (1) witnessed versus unwitnessed status, and (2) initial EtCO 2 : low (≤30 mm Hg), moderate (31–49), and high (≥50). Within each stratum, group-based trajectory modeling (GBTM) was used to identify latent EtCO 2 trajectory classes, and patients were categorized into an upward or downward trajectory. To balance trajectory groups on baseline characteristics including age, sex, race, initial rhythm, location, and bystander CPR, we applied inverse probability of treatment weighting. We fit weighted pooled logistic regression models to estimate risk ratios (RRs) for ROSC comparing upward versus downward EtCO 2 trajectories. Within each stratum, we identified the earliest minute when CIs between upward versus downward EtCO 2 trajectories no longer overlapped. RESULTS: EtCO 2 data were available for 1168 patients: 452 (38.6%) witnessed and 716 (61.1%) unwitnessed. Patients were predominantly men (63.5%), with a median age of 65 years (Q1, Q3: 53–75), majority White race (51.3%), and presenting in a nonpublic setting (85.4%). Overall ROSC was 18.2%: 30.5% of witnessed and 10.5% of unwitnessed. Among witnessed arrests, 95% CI for upward versus downward EtCO 2 trajectories no longer overlapped at 8 minutes for low initial EtCO 2 (RR, 3.06; 95% CI, 1.49, 6.71), 12 minutes for moderate EtCO 2 (RR, 1.95; 95% CI, 1.23, 3.48), and 21 minutes for high EtCO 2 (RR, 2.12; 95% CI, 1.30, 3.73). Among unwitnessed arrests, nonoverlapping CIs were first observed at 7 minutes (RR, 3.56; 95% CI, 1.53, 10.37). CONCLUSIONS: Depending on witness status and initial EtCO 2 , between 7 and 21 minutes of monitoring are needed to reliably differentiate upward from downward EtCO 2 trajectories during OHCA resuscitation. Dynamic EtCO 2 trajectory monitoring may provide early prognostic information to guide resuscitation.