Raphael 1989.

Methods	Prospective, randomised, unblinded trial. Randomization stratified according to history of loss of consciousness (LOC). Allocation by sealed opaque envelopes, not sequentially numbered. Only those with no history of LOC randomised to HBO vs. NBO; more severe patients randomised to different regimens of HBO. Only the HBO vs NBO group included in this review.
Participants	629 adults admitted within 12 hours of termination of CO exposure. Inclusion: age >15 years, admitted within 12 hours, COHb >10% (smoker) or 5% (nonsmoker). Exclusion: other intoxication, pregnancy, cardiovascular collapse, pulmonary edema, non‐feasible HBO (technical problems etc.), difficulty in stratifying into groups A or B (by LOC), refusal by patient. Of enrolled patients, 343 were randomised to receive either HBO or NBO.
Interventions	Only those without history of loss of consciousness randomised to HBO [HBO for 2h followed by 100% oxygen by mask for 4h (where HBO regimen included 30 minutes compression & decompression flanking 60 minutes at 2.0 ATA.)] vs. NBO [100% oxygen by mask for 6h]. Other patients randomised to HBO x 1 vs. HBO x 2 not included in this review's analysis.
Outcomes	Intention to treat analysis. Outcome measures included self‐assessment questionnaire and physical examination by neurologist (unblinded) at one month, with no difference in outcome (symptoms present in 50 of 158 patients (32%) treated with NBO vs. 51 of 159 patients (32%) treated with HBO at one month).
Notes	The second part of this study involving patients who had initial impairment of consciousness did not meet inclusion criteria for this review. In this group of patients, the proportion with residual symptoms 1 month after poisoning was greater among patients who received two HBO sessions (48%; group B2) than in those treated with one HBO session (46%; group B1). This difference was not statistically significant and the trend did not provide any support for a dose‐response relationship for HBO.
Risk of bias
Bias	Authors' judgement	Support for judgement
Adequate sequence generation?	Unclear risk	Quote: "Treatment was randomly selected by means of sealed envelopes and randomisation was stratified according to patient group."
Allocation concealment?	Unclear risk	No mention of concealment strategies.
Blinding? All outcomes	High risk	Quote (from discussion): "A double‐blind trial with sham HBO therapy might have been the ideal solution but both feasibility and security problems would have made it impossible to conduct such a study on a large scale." No record of attempts to blind participants or assessment. The descriptions of the outcome measures employed were vague and therefore potentially open to interpretation by the unblinded assessors.
Incomplete outcome data addressed? All outcomes	Low risk	Outcome data reported on 317 of the 343 patients randomised. Quote "The differences remained non significant even when all patients lost to follow‐up were assumed to have recovered or when all were assumed to have been left with sequelae."
Free of selective reporting?	Low risk	Intent to treat analysis: Quote: "Patients were not informed of the randomisation procedure because informed consent would have been difficult to obtain in an emergency setting and was not required by French regulations at the time. However, the treatment to be given was explained in detail to the patient (or family if the patient was comatose). If patients refused the allocated treatment after randomisation they were still retained in the study and analysed according to the treatment intended" Quote "Most patients received the treatment allocated. 9 patients (3 Al, 2 Bl, 4 B2) refused HBO, 2 group AO received one HBO session by mistake".