Confirmed cardiac output on emergency medical services arrival as confounding by indication: an observational study of prehospital airway management in patients with out-of-hospital cardiac arrest.

OBJECTIVES: Many registry studies on patients with out-of-hospital cardiac arrest (OHCA) have reported that conventional bag-valve-mask (BVM) ventilation is independently associated with favourable outcomes. This study aimed to compare the data of patients with OCHA with confirmed cardiac output on emergency medical services (EMS) arrival and consider the confounding factors in prehospital airway management studies.

METHODS: This was a cohort study using the registry data for survivors after out-of hospital cardiac arrest in the Kanto region at 2012 in Japan (SOS-KANTO 2012). Survivors who received advanced airway management (AAM) group and a BVM group were compared for confirmed cardiac output on EMS arrival and neurolgical outcome at 1 month. Favourable neurological outcome was defined as a score of one or two on the Cerebral Performance Categories Scale. Multivariable logistic regression was used to adjust the neurological outcome by age, gender, cardiac aetiology, witnessed arrest, shockable rhythm, cardiopulmonary resuscitation performed by a bystander, BVM at prehospital ventilation and presence of confirmed cardiac output on EMS arrival.

RESULTS: A total of 16 452 patients were enrolled in the SOS-KANTO 2012 study, and of those data 12 867 were analysed; 5893 patients comprised the AAM group and 6974 comprised the BVM group. Of the study participants, 386 (2.9%) had confirmed cardiac output on EMS arrival; 340 (2.6%) of the entire study group had a favourable neurological outcome. The proportion of patients with confirmed cardiac output on EMS arrival was significantly higher in the BVM group (272: 3.9%) than in the AAM group (114: 1.9%) (95% CI: 1.65 to 2.25). The proportion of patients with favourable neurological outcomes was 30% (117/386) in those with cardiac output on EMS arrival compared with 1.8% (223/12481) in those without. The OR for a good neurological outcome with BVM decreased from 3.24 (2.49 to 4.20) to 2.60 (1.97 to 3.44) when confirmed cardiac output on EMS arrival was added to the multivariable model analysis.

CONCLUSION: Confirmed cardiac output on EMS arrival should be considered as confounding by indication in observational studies of prehospital airway management.