To the Editor:—
I read with interest the recent report by Pollard et al. 1on the incidence of awareness in a regional medical system in the United States. The incidence of awareness in their study was comparatively low at approximately 1 in 15,000. This is an order of magnitude less than numerous other large studies conducted in the United States and around the world (incidence around 1 in 1,000).2–5The authors attribute this difference to “the anesthesia providers involved, the types of anesthetics chosen, and/or investigational bias.” I believe that their result may have been due, in part, to investigational biases of their own.
The authors did not define awareness in their report (awareness is often defined as “postoperative recall of intraoperative events”4). They imply that the definition was “recall or possible recall” but later mention that cases of dreaming may have been missed by their data collection method. Dreaming during anesthesia is a remarkably common phenomenon that is rarely indicative of awareness and should not be confused with it.6In studies of incidence, the primary endpoint needs to be prospectively and precisely defined.
In addition, the authors did not ask their patients a direct question about awareness. They omitted the question used by researchers in all of the large studies with which they wish to compare their work2–5: “Can you remember anything in between these two periods (that is, between going to sleep and waking up)?” Would a study on postoperative vomiting be valid if the patients were not asked “Did you vomit?” Moerman et al. 7documented that patients are reluctant to report awareness without being questioned directly. In addition, in my routine use of this question in research and clinical practice, I have not found that it alarms patients.6,8The omission of this question is a potent reason for the lower incidence of awareness reported in this study.
The authors comment on possible investigational bias in previous studies, without discussing it any further or addressing the potential for investigational bias of their own. One could make the case that the aim of previous studies was to detect every case of awareness that had occurred. One could also make the case that an internal quality assurance program such as the one described in this study may be designed to minimize the incidence of adverse events such as awareness. The authors did not describe the procedures for adjudication of awareness reports, impanel an independent endpoint adjudication committee (with members from outside their practice group), or provide a description or count of awareness reports that were rejected.8Readers therefore cannot interpret for themselves the subjective reports of awareness. These factors together throw doubt on the accuracy of their awareness incidence.
A lower incidence of awareness may be expected in a community-based setting with relatively few patients at high risk of awareness, relatively few anesthesiologists or nurse-anesthetists in training, a relative abundance of patients who were not given neuromuscular blockers, and a protocolized anesthesia care plan that may have promoted more-than-adequate general anesthesia. Unfortunately, Pollard et al. did not provide data about any of these potential confounding factors, and their methods cast doubt on their conclusion that the incidence of awareness is 1 in 15,000 in their practice.
Royal Melbourne Hospital and University of Melbourne, Melbourne, Australia. kate.leslie@mh.org.au
