The effects of hyperbaric oxygen therapy (HBOT) on mortality or morbidity in patients with carbon monoxide (CO) poisoning remain unknown. We examined the effects of HBOT on CO poisoning and further strived to delineate its inherent effects on specific subgroups of patients using a nationwide inpatient database.

We identified adult patients with CO poisoning who were registered in the Japanese Diagnosis Procedure Combination inpatient database from 2010 to 2016. Propensity score-matching was performed to compare patients who received HBOT within 1 day of admission (HBOT group) with those who did not receive HBOT (control group). The primary outcome was in-hospital mortality. The secondary outcomes were a depressed mental status and reduced activities of daily living (ADL) at discharge. We also performed subgroup analyses divided according to severity of CO poisoning.

Eligible patients were categorized into the HBOT group (n = 2034) or the control group (n = 4701). One-to-one propensity score-matching created 2034 pairs. In-hospital mortality was not significantly different between the HBOT and control groups (0.8% vs. 1.2%, risk difference: −0.4%, 95% confidence interval: −1.0 to 0.2). Patients in the HBOT group had significantly lower proportions of a depressed mental status and reduced ADL at discharge than did those in the control group. Similar associations were shown in the non-severe poisoning subgroup.

Although HBOT was not significantly associated with reduced mortality, it was significantly associated with a favorable consciousness level and ADL in patients with CO poisoning. HBOT may be beneficial even for patients with non-severe CO poisoning.