Background: The effect of bystander interventions has been extensively evaluated by cerebral function after 1 month post-resuscitation. However, patients who received bystander cardiopulmonary resuscitation (BCPR) and achieved the return of spontaneous circulation (ROSC) before the arrival of the emergency medical system (EMS) are routinely defined with an unknown electrocardiogram (ECG) and are usually excluded before analysis. The aim is to determine the influence of excluding patients with unknown first monitored rhythm, which includes cases of bystander ROSC, from the out-of-hospital cardiac arrest (OHCA) database.
Methods: This nationwide population-based observational study was conducted in Japan using Utstein data from 2011 to 2014. In total, 91,995 patients with bystander-witnessed cardiogenic OHCA received resuscitation attempts in the pre-hospital setting. These patients were divided into three groups by the first monitored rhythm upon EMS arrival. We analysed the differences of datasets that included and excluded the unknown group and determined the effect on outcomes by multivariate logistic regression and odds ratios (ORs) with 95% confidence intervals (95% CIs).
Results: When the unknown group was excluded from the data, the adjusted odds ratio (AOR) of cardiopulmonary resuscitation (CPR) to favourable cerebral performance category (CPC) 1 or 2 was decreased (conventional CPR: AOR, 1.90 to 1.58; chest-compression-only CPR: AOR, 2.08 to 1.69) compared to the unknown group's inclusion. Conversely, the AOR of public-access defibrillation (PAD) was increased (AOR, 4.51 to 6.13).
Conclusions: The exclusion of unknown ECGs from a dataset may lose ROSC patients by bystander CPR, causing selection bias to affect outcomes.
Keywords: Bystander intervention; Cardiopulmonary resuscitation; First monitored rhythm; Out-of-hospital cardiac arrest; Public-access defibrillation.