To the Editor:
Brown et al.1 nicely described their work comparing spinal anesthesia with targeted sedation based on Bispectral Index values compared with general anesthesia (masked Bispectral Index) and the outcome of delirium. Intraoperative hypotension has been associated with delirium.2,3 The adjusted hazard ratio associated with a 1-mmHg increase in time-weighted average of mean arterial pressure less than 65 mmHg was 1.11 (95% CI, 1.03 to 1.20).2 The study by Brown et al.1 found that the lowest mean arterial pressure was similar in both groups (general anesthesia, 59 [51 to 64] vs. spinal anesthesia, 60 [52 to 64]); however, the relationship between intraoperative hypotension and the subsequent development of delirium might be more of a cumulative exposure response than a single threshold. It would be of interest to know the hypotension exposure by time under the 65- or 60-mmHg threshold and to consider that in a treatment-by-covariate interactions analysis. In short, we appreciate Brown et al. for their great contributions to this important topic.