We thank Dr. Litvan for his thoughtful comments about our recent editorial on the issue of “depth of anesthesia” monitors. 1We agree that at best these devices should be considered “hypnosis” monitors, because by definition “adequacy of anesthesia” also implies adequate analgesia. The issue of prediction of movement has received considerable interest since Rampil et al . showed that an intact brain is not a prerequisite for determination of minimal alveolar concentration (MAC) in rats. 2However, we feel that the old misconception that blurs these two aspects of anesthesia has now been replaced by a new one: namely that analgesia and hypnosis are entirely separate entities without any interrelation. Several observations suggest that this may not be the case. Patients who remain conversant after a low dose of benzodiazepine do fall asleep when given large doses of opioids; similarly, when opioid levels are moderately high, patients will not respond to verbal command even when propofol or sevoflurane concentrations have decreased to levels that, in isolation, would yield only minimal sedation. We are just scratching the surface of the pharmacodynamic interaction between hypnotics and opioids. Fortunately the issue is now receiving considerable attention in well-designed clinical studies. As Dr. Litvan points out, even the “gold standard” assessment of sedation/hypnosis by verbal or tactile stimuli (Observer's Assessment of Alertness/Sedation) is an example of a measurement instrument affecting the measured variable.
Dr. Litvan seems not to be concerned about whether we should measure spontaneous or evoked cortical electrical activity, as long as the electrophysiological variable that is chosen tracks hypnotic level or anesthetic concentration. Recent data from the UK 3and Belgium 4suggest that the MLAER reacts in a different way to a changing anesthetic concentration than does the electroencephalogram. This apparent difference in the slope of the concentration-effect curve may have implications for the usefulness of the monitor at hand. Whereas the BIS® variable appears to begin to increase almost immediately after discontinuation of the hypnotic, the MLAER is likely to remain unchanged until just before the transition from unconsciousness to the awake state. Both response patterns may contain information useful to the clinician.
Finally, we agree that the information derived from a dedicated electroencephalogram or MLAER monitor may represent a step forward in helping the anesthesiologist to titrate anesthetics to the desired level. We also agree that if BIS® or AAI guided titration can serve to decrease unnecessary overdosing then both physiologic stresses and wasteful anesthetic consumption might be avoided. However, we remain unconvinced that there is a magic number that clinicians can “ride” that will simultaneously allow a reduction in anesthetic consumption while at the same time reducing the incidence of intraoperative awareness with explicit recall. In fact, we harbor the concern that the practice of “riding the numbers” might actually have the potential to increase the incidence of awareness, if some very specific threshold, e.g. , a BIS® of 60, were applied too aggressively.