Awareness during anesthesia is a problem receiving increased attention by patients,
clinicians, and the general public. The incidence of intraoperative awareness has been
reported to be between 1-2/1000 cases, but recent data suggest that this may be an
overestimate. The Bispectral Index (BIS) Monitor is an electroencephalographic method of
assessing depth of anesthesia that has been shown in one study to reduce the incidence of
awareness during anesthesia in the high-risk population (Myles et al, 2004). In the study of
Myles et al, the number needed to treat (NNT) in order to prevent one case of awareness in
the high-risk population was 138, with an associated cost of approximately US$2200. Since the
NNT and the associated cost of treatment would be much higher in the general population, the
efficacy of the BIS monitor in preventing awareness in all anesthetized patients needs to be
clearly established. Furthermore, recent data suggest that the BIS may not be useful in the
high-risk population. The investigators propose a prospective, randomized, controlled trial
comparing the BIS monitor to electronic alerts based on non-electroencephalographic gauges of
Electronic alerts have been developed and employed at our institution and have been shown to
increase compliance with both clinical and administrative tasks (O'Reilly et al, 2006;
Kheterpal et al, 2007). The investigators have developed electronic alerts for the purpose of
informing the clinician of potentially insufficient anesthesia based on minimum alveolar
concentration (MAC). The algorithm is as follows:
- Every 5 minutes the alerting system checks every active case in our operating rooms. It
takes approximately 1 second for this scan of all active cases to occur.
- Conditions for an "active case" are:
1. data capture is possible (i.e., not a paper record)
2. data capture is active (i.e., "patient in room" has been electronically entered and
end-tidal [Et] CO2 is detected)
3. case has been identified as a general anesthetic
4. "anesthesia induction end" has already been documented
5. request for recovery room bed or transport to an intensive care unit has not been
6. surgical dressing completion has not been documented
- The alerting system checks the most recent value (within a specified time period) of:
1. Et Sevoflurane (MAC=2.0)
2. Et Isoflurane (MAC=1.2)
3. Et Desflurane (MAC=6)
4. Et Nitrous Oxide (MAC=105) and compares it to the MAC of each agent. It adds the
resulting MAC values together for "current total MAC."
- The system then checks for a charted propofol infusion in mcg/kg/min and divides by 150,
assuming that 150 mcg/kg/min is "1.0 MAC" for propofol. The analogous concept of MAC for
propofol is "Cp50"- the plasma or blood concentrations at which 50% of patients do not
move in response to a noxious stimulus. Since the investigators do not have the
technology at our institution to calculate Cp50 or Cp50-awake, the investigators have
chosen the above propofol dose as an initial value based on clinical experience. The
resultant MAC equivalent is added to current total MAC.
- The system next checks for a dexmedetomidine infusion with a rate of 0.2 mcg/kg/hour or
greater. If present, it multiplies the current total inhalational MAC by 2, as
dexmedetomidine can reduce MAC by 50%.
- At this point, the "current total MAC" is defined as: Et Sevo /2 + Et Iso /1.2 + Et
Des/6 + Et Nitrous /105 + propofol rate (in mcg/kg/min)/150. If dexmedetomidine is >0.2
mcg/kg/hour, inhalational MAC is multiplied by 2.
- If this total mac < 0.50, it checks to see if a bolus of propofol, midazolam, etomidate,
or thiopental has been given in the preceding 10 minutes.
- It alerts the clinician signed into the case within 30-60 seconds if total age-adjusted
MAC < 0.50 AND no bolus has been documented in the last 10 minutes.
This will be the protocol in the MAC-guided group. In the BIS-guided group an electronic
alert will be sent if the BIS value is >60. If an alert is triggered, the clinician
electronically signed into the case receives an alphanumeric page stating "Potentially
insufficient anesthesia, please check vaporizers and intravenous lines."
The investigators are collaborating with Washington University, the University of Chicago,
and the University of Mannitoba, who will also be testing a MAC-based protocol in comparison
to the BIS.
- At least 18 years of age, English-speaking, available for follow-up interview at one
- Pre-existing neurologic or neuropsychiatric condition, surgical manipulation around
forehead, not available for follow-up interview.