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Letters to the Editor: Letters & Announcements

Observational Studies Identify Associations, Not Causality

Berry, Arnold J. MD, MPH

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doi: 10.1213/01.ANE.0000173754.86959.9D
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To the Editor:

In their recent publication, Monk et al. assessed the effect of anesthetic management on long-term outcomes in a prospective observational study of patients undergoing major noncardiac surgery with general anesthesia (1). Using a multivariate model, they detected an association between cumulative deep hypnotic time (the time that the patient’s Bispectral Index was <45) and 1-yr postoperative mortality.

One of the limitations of an observational study is that statistical analyses can only identify associations between independent variables and the outcome of interest. The authors provide several possible biologic mechanisms to explain the association between accumulative deep hypnotic time and postoperative mortality, but these explanations are speculative and lacking proof. Unfortunately, causality cannot be established by this observational study, and other work must be done to confirm a cause-and-effect relationship between accumulative deep hypnotic time as measured by Bispectral Index <45 and 1-yr postoperative mortality.

Because the finding in the current study is an association, there is another interpretation of the data, i.e., the patients who were more susceptible to general anesthesia, as measured by a lower Bispectral Index, were more likely to die within the year after surgery. If this were the case, the low Bispectral Index values, in response to doses of anesthesia that clinicians felt were appropriate, may be a marker for increased risk of death within 1 yr of surgery.

The association between accumulative deep hypnotic time and 1-yr postoperative mortality is interesting, but additional research is required to understand the significance of this finding and whether a cause-and-effect relationship truly exists.

Arnold J. Berry, MD, MPH

Department of Anesthesiology

Emory University School of Medicine

Atlanta, GA

[email protected]


1. Monk TG, Saini V, Weldon BC, Sigl JC. Anesthetic management and one-year mortality after noncardiac surgery. Anesth Analg 2005;100:4–10.
© 2005 International Anesthesia Research Society