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Department of Anesthesia and Perioperative Care, University of California, San Francisco, CA, egere{at}anesthesia.ucsf.edu
In Response:
Enlund questions our suggestion (1) that recall may be more likely during IV versus inhaled anesthesia. We agree that there is no proper answer because no adequate prospective study has been done. Our argument notes the absence of an online, direct measure of a total IV anesthetic concentration at the effect site. The available indirect measures of suppression of awareness or anesthetic concentration at the effect site (e.g., the Bispectral Index or BIS monitor) do not produce consistent results (2). In contrast, the partial pressure of an inhaled anesthetic at the effect site can be measured, albeit with a modest time delay, as the end-tidal concentration of anesthetic.
Experimental difficulties compound the problem. We suggest that Brice's structured interview (3) does not provide a solution and agree with Dr. Enlund's suggestion "that the diagnostic potential of the interview technique is not optimal." Brice's structured interview poses three questions to patients after their anesthesia: "What was the last thing you remembered happening before you went to sleep?" "What is the first thing you remember happening on waking?" and "Did you dream or have any other experiences while you were asleep?" We are not convinced that such questions would reveal remembrance. The subjects of Brice's study also received auditory input of tape recordings of either choir music or piano music or nothing during their anesthesia and were asked if they heard one or the other. "Twelve of the patients....claimed to be able with absolute or considerable certainty to identify the tape recording which had been played. Four of these chose the correct tape, four recognized the opposite tape to the one which had been played, and four recognized a tape when no tape had been played. There is thus in this series little evidence that the tape recordings played could be recalled."
We continue to believe what we argued in our letter (1): "Awareness during anesthesia is difficult to certify objectively. In our view it requires remembrance of an event by the patient that uniquely connects that remembrance to an event unequivocally occurring during anesthesia/surgery." By such criteria, remembrance of events during adequate levels of anesthesia (e.g., more than 0.5 MAC of a potent inhaled anesthetic) is rare. The reasoning discussed in paragraph 1 of our response suggests that it would be rarer with inhaled anesthetics.
References
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