To the Editor We read the study by Brown et al1 with great interest. They report a 45% reduction in the odds of delirium in patients undergoing cardiac surgery by a maintaining mean arterial pressure greater than a computationally identified cerebral autoregulatory threshold. These results suggest that relative hypotension could represent an etiology for this common form of acute temporary brain injury. Their results are impressive; however, the effect size seems biologically and clinically implausible. Moreover, delirium is usually regarded as multifactorial, with the existence of patient-related predisposing factors as well as pharmaceutical and behavioral/environmental factors.