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Professor Tasker queries whether patients undergoing brainstem death testing could bias the association between exposure to hyperoxemia and mortality in our recent study (1). In our database, these patients represent a tiny fraction of patients evaluated. For the Days 1, 3, 5, and 7 cohorts, there were only 33 (0.1%), 14 (0.1%), 9 (0.1%), and 6 (0.1%) patients, respectively, in whom there was semantic labeling for death confirmed using neurological criteria. Owing to these low numbers, we did not attempt to stratify by this variable. This is likely to be a small underestimate of patients exposed to apnea testing, because this label refers only to patients who met full criteria rather than to patients who underwent brainstem death testing itself, though it is standard practice in the United Kingdom not to proceed with testing unless there is good evidence that the result is likely to be positive. A sensitivity analysis, however …