Commentary

Although carbon monoxide, the most frequent cause of poisoning, results in both immediate and delayed cognitive deficits, physicians have been justifiably skeptical about treating it with HBO rather than normobaric oxygen. HBO often incurs the cost and risk associated with transferring a potentially unstable patient to a referral centre, and observational studies that initially suggested a benefit were recently contradicted by a well designed but negative randomised trial by Scheinkestel et al.¹

The study by Weaver et al showed a large neuropsychological benefit in the HBO group. This study avoids 2 of the limitations of the study by Scheinkestel et al: loss to follow up and cluster rather than patient level randomisation for group exposures. The Scheinkestel trial also differed in other respects. More patients had attempted suicide (69% compared with 31% in the study by Weaver et al), and depression and other toxins may have influenced outcomes. Patients in the Scheinkestel trial control group received continuous face mask oxygen at 15 l/min for 3 days, whereas patients in the study by Weaver et al received oxygen only when needed to maintain oxygen saturation. Both studies concealed allocation and used sham hyperbaric treatments to enhance blinding, and both used persistent or delayed neurological sequelae as a primary outcome. However, each used a different battery of tests. Similarly, differences in treatment delay, case severity, and HBO protocols might also explain the disparity between the results of these 2 studies.

The study by Weaver et al shifts the balance of evidence to support HBO over normobaric oxygen. Future studies should be designed to determine optimal combinations of the dose and timing of HBO, and planned subgroup analyses should attempt to define which patients could benefit most from hospital transfer. Hopefully, these studies will provide the evidence needed to erase the remaining doubt about the effectiveness of HBO.