EDITOR—We would like to clarify some of the statements that Weaver made about our double blind randomised trial of hyperbaric compared with normobaric oxygen treatment of carbon monoxide poisoning. 1 2 Weaver expresses concern that concomitant depression and use of psychoactive drugs might have influenced the results, given the large percentage of suicide attempts in our cohort of patients. Depression and the use of drugs may indeed have resulted in a higher incidence of poor outcome overall, but this would not have biased the comparison between normobaric and hyperbaric groups: patients were specifically stratified for attempted suicide before randomisation to treatment.

Weaver seems to be concerned about the delay in receiving hyperbaric oxygen treatment. Although the geometric mean treatment delay was 7.1 hours, we performed subgroup analysis of patients treated within four hours (all patients, and severely poisoned patients alone). We also analysed outcome in four groups according to time to treatment (<3, 3-60, 6-12, >12 hours) and found no difference in outcome between hyperbaric and normobaric oxygen groups. Further multivariable analysis did not identify delay in treatment as a predictor of poor outcome. Thus there was no evidence that delay to treatment might have explained the lack of benefit of hyperbaric oxygen.

Weaver also questions our use of cluster randomisation. With this we allocated more than one person simultaneously to the same treatment on 22 occasions (two on 12 occasions, three on …