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LETTER TO THE EDITOR

Is Depth of Anesthesia, as Assessed by the Bispectral Index, Related to Postoperative Cognitive Dysfunction and Recovery?

Department of General Anesthesiology, Cleveland Clinic, Cleveland, OH, schubea{at}ccf.org

In Response:

We appreciate Dr. Royse’s (1) interest in our work (2) and agree with some of his points (as indicated in our paper’s Discussion section) although we would like to clarify others. Certainly one of the limitations of our data is that the observed difference in cognitive ability occurred in only one of three assessments. Three were chosen on practical grounds because the time needed to complete them was the maximally tolerable for our patients. To perform 10 assessments, as suggested, would have been impossible for most of our patients. Reviewing two consensus statements (one not cited by Dr. Royse) for cardiac surgical patients (a patient population which was not the subject of our report), we are unable to find statements recommending 10 tests (3,4). Furthermore, one of the papers cited by Dr. Royse likewise used a limited set of cognitive function assessments (essentially the set used for our patients plus the grooved pegboard test for manual dexterity) in cardiac surgical patients (5). The authors of this paper also accepted cognitive dysfunction to mean impairment in only one of the four domains they assessed.

In a randomized study (6) where the groups have the same mean pretest score, a comparison of groups on mean change from baseline and an analysis of covariance comparing groups on the posttest score adjusting for pretest score (our primary analysis) will give very similar, if not identical, results. However, in cases where the randomized groups are not balanced on the pretest score, the analysis of covariance is the preferred analysis, even though one degree of freedom is spent to estimate the slope for posttest regressed on pretest. In the analysis of mean change, this slope is arbitrarily assumed to be 1.0, which of course may not be the case. We are therefore confident that we have chosen the better analysis.

The groups did not differ on baseline means, as Dr. Royse questions; however, if they did, the analysis of covariance would most assuredly be the better analysis because it compares groups at a common baseline! Nevertheless, it may satisfy the reader to know that we did reach the same conclusions using a mean change analysis (results not published). We also used a reliable change method that adjusts the posttest scores for the estimated practice effect as well as the pretest score, and obtained the same conclusions.

The fact that we had power to detect a difference of 15 but found significance with a smaller observed difference is not counterintuitive, but is rather a common phenomenon in clinical trials. We clearly qualified our conclusions of this significant outcome in the Discussion, and agree that the evidence even for this outcome is not entirely convincing. We did not use a binary outcome (e.g., a 20% change in one or more outcomes) in this trial because of practical sample size considerations. Further studies, perhaps with larger sample size and larger separation between groups on the bispectral index level, may give a more definitive answer to this research question and are certainly needed.

REFERENCES

1. Royse C. Is depth of anesthesia, as assessed by the bispectral index, related to postoperative cognitive dysfunction and recovery? Anesth Analg 2007;104:1297.[Free Full Text]
2. Farag E, Chelune GJ, Schubert A, Mascha EJ. is depth of anesthesia, as assessed by the Bispectral Index, related to postoperative cognitive dysfunction and recovery? Anesth Analg 2006:103:633–40.[Abstract/Free Full Text]
3. Murkin JM, Stump DA, Blumenthal JA, McKhann G. Defining dysfunction: group means versus incidence analysis—a statement of consensus. Ann Thorac Surg 1997;64:904–5.[Web of Science][Medline]
4. Murkin JM, Newman SP, Stump DA, Blumenthal JA. Statement of consensus on assessment of neurobehavioral outcomes after cardiac surgery. Ann Thorac Surg 1995;59:1289–95.[Free Full Text]
5. Murkin JM, Baird DL, Martzke JS, Yee R. Cognitive dysfunction after ventricular fibrillation during implantable cardiovertor/ defibrillator procedures is related to duration of the reperfusion interval. Anesth Analg 1997;84:1186–92.[Abstract]
6. Maxwell S.E, Delaney HD. Designing experiments and analyzing data: a model comparison perspective. Pacific Grove, CA: Brooks/Cole, 1990.

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