We are pleased that neither Drs. Spackman and Abel nor Dr. Gage disagree with the fundamental conclusions of our article—that the more rare awareness is, the more difficult it is to prove that the BIS®monitor (Aspect Medical Systems, Natick, MA)—or any other depth of anesthesia monitor—prevents it, and the more expensive it is to use for this purpose. Dr. Gage's reference to the study that failed to demonstrate a benefit to the use of pulse oximetry is further evidence of how difficult conducting such studies in the clinical setting can be.
We are wary of the pulse oximetry analogy for a variety of reasons, not the least of which is that reasoning by analogy can be treacherous. The value of analogy depends critically on underlying similarity, and analogies may mislead if the similarity is not present. Pulse oximeters measure a well-defined physiologic variable; their performance can be calibrated with other instruments available to practitioners. No such calibration exists for the BIS®monitor. Pulse oximeters are used in a fashion different from the BIS®monitor. The majority of the use of pulse oximetry in the operating room is to detect hypoxia; only rarely is it used to titrate other therapies (such as fraction of inspired oxygen, positive end-expiratory pressure, and others). The BIS®monitor, when used as advocated, is used to titrate the anesthetic itself, trading between deeper levels of anesthesia and lower drug acquisition costs, improved recovery, and fewer side effects. To realize these benefits, practitioners deliberately conduct an anesthetic that provides a state that is closer to awake than to asleep, and that may paradoxically increase the risk of awareness in the patient. Of course, it is difficult to prove or disprove this conjecture for the reasons we explored in our article.