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Intraoperative Management/Utility of depth of consciousness monitoring

QUESTION:

According to the ASA official 2003 position on MAC (monitored anesthesia care), the mere loss of consciousness (without regard to whether local or systemic analgesia is administered) transforms a MAC case into a general anesthetic.

Bispectral index or BIS levels compatible with general anesthesia are well published to occur at 45-60. Adding systemic analgesia to BIS 45-60 would be my definition of general anesthesia.

Inasmuch as the ASA has not, to date, embraced the notion of level of consciousness monitoring as worthwhile, it then places the anesthesiologist using BIS monitored propofol ketamine MAC (at BIS 60-75) in the position of having to have an anesthesia machine, scavenging and dantrolene. None of these items improves patient safety, yet enormously increases costs to the office-based OR.

Does SAMBA yet have a position on the utility of level of conscious monitors in general and BIS in specific? If not, why not?

-- From Barry L. Friedberg, M.D., Corona del Mar, CA

REPLY:

You ask a complicated question with many complicated sub-questions. Let me try to address each of them, and hopefully they’ll result in a coherent and satisfactory answer.

1. The definition of loss of consciousness is difficult to pin down. The ASA’s continuum of sedation (1) describes a progression from light to moderate to deep sedation, ultimately culminating in general anesthesia. Furthermore, deep sedation is defined as a state during which ventilation and airway protection may be inadequate, which to my definition makes it a general anesthetic! So when is consciousness lost? Is it during the transition from moderate to deep sedation? From deep sedation to general anesthesia? In reality the continuum isn’t as black and white as described, with many patients requiring vastly different levels of sedation to achieve similar clinical endpoints.
2. Which brings us to consciousness monitoring. Clinically available monitors (eg. BIS, PSA4000, Entropy) do not monitor depth of anesthesia, but rather level of hypnosis. Multiple studies have examined the correlation between propofol concentration and consciousness monitoring, and unfortunately the point at which level of consciousness is actually lost (for instance during induction of general anesthesia) is variable, ranging from 38.2–70.4 for BIS and 42.2–60.4 for State Entropy following initiation of propofol infusion (2). Furthermore, there is no indication that depth of consciousness, as defined by a monitor, predicts the likelihood that a patient is adequately sedated for a surgical stimulus, making the situation much more complicated. We have all had snoring, seemingly deeply sedated patients that wake with a start as soon as an endoscope is passed into the mouth (or elsewhere).
3. Finally, the drug cocktail used also affects the validity and utility of these monitors. You mention a propofol/ketamine technique which I have also successfully used in practice. Ketamine is an excellent analgesic, sedative, and amnestic, but it is also an EEG stimulant. A recent study found that patients anesthetized with sevoflurane had a significant rise in BIS and Entropy (from 33 to 46 for BIS and 30 to 50 for State Entropy) when ketamine was added to that anesthetic (3). So it is unclear what your BIS values of 60-75 actually reflect.
4. So to answer your question, there is no set SAMBA policy on consciousness monitoring during general anesthesia or sedation because the verdict is still out as to how useful these monitors truly are for all, some, or most patients receiving all, some, or most anesthetics. The stance may or may not change as the ASA Task Force on Intraoperative Awareness prepares its findings, but awareness is another complicated (although related) subject altogether. My personal opinion is that it never hurts to be too safe, and consciousness monitors provide an extra level of information about what is actually going on in the brain (which, after all, is the end-organ of consciousness). When using these monitors, as with any other device, it is imperative that the limits of the technology be understood, and that anesthetic technique be tailored to the individual patient to maximize patient safety.
5. Finally, I must object to your comment that dantrolene, anesthesia machines, and scavenging systems do not improve patient safety. It is rather in the context in which they are used. When using non-trigerring agents it is unlikely that dantrolene provides value, but in an environment where general anesthesia is provided in an office setting and multiple providers may use varying drugs to provide anesthesia, dantrolene clearely is important to have immediately available. The same type of argument holds true for the use of anesthesia machines and scavenging systems.

-- Roy G. Soto, M.D., Stony Brook, NY

References: 

1. Definition of General Anesthesia and Levels of Sedation/Analgesia as approved by House of Delegates on October 13, 1999
2. Iannuzzi M, Iannuzzi E, Rossi F, Berrino L, Chiefari M.  Relationship between Bispectral Index, electroencephalographic state entropy and effect-site EC50 for propofol at different clinical endpoints.  Brit J Anest.  2005;
3. Hans P, Dewandre P, Brichant J, Bonhomme V.  Comparative effects of ketamine on Bispectral Index and spectral entropy of the electroencephalogram under sevoflurane anaesthesia.  Brit J Anest.  2005

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