To the Editor:—
Vuyk et al., in a recent series of case reports, concluded that three volunteers were responsive with bispectral index (BIS) values of 40–50.1The case reports (and corresponding video) raise some interesting questions regarding the methodology utilized as well as the applicability of this information to routine clinical practice.
First, it is our understanding that these patients were volunteers undergoing pharmacokinetic-dynamic study while monitored with BIS® (BIS® monitor, Aspect Medical Systems, Newton, MA). It has been shown previously that healthy asleep volunteers demonstrate BIS levels well into the 40s (and below), independent of pharmacologic intervention.2It is not surprising, therefore, that BIS decreased as it did, nor is it surprising that it rose with prodding. The video clearly and repeatedly shows the rapid increase in BIS after either verbal or physical stimulation despite the inherent lag time of 15 to 30 s for raw data smoothing.
Second, the value of 40 is reemphasized in the report, when in fact the graphs clearly show that values in the 40–50 range were only very briefly obtained. These episodes did correspond to a level of Deep Sedation, as defined by the American Society of Anesthesiologists, as evidenced by the fact that the subjects did not maintain adequate ventilation and were repeatedly told to take deep breaths. It would have been far more valuable for the researchers to report the corresponding data from pulse oximetry without the use of supplemental oxygen. This would have provided meaningful information as to the relationship between BIS and respiratory function.
Third, it would have been interesting to note whether these patients had recall of the prodding. Movement to command does not necessarily translate into conscious decision-making or recall of the event. An interesting follow-up study design would include specific voice and tactile commands with a subsequent assessment of recall. It should be emphasized that the researchers demonstrated responsiveness and not awareness. The clinical applicability is debatable and in our opinion does not warrant the suggested reevaluation of the BIS-XP algorithm.
* University of South Florida, Tampa, Florida. firstname.lastname@example.org