A bispectral index (BIS) monitor is commonly utilized to assess depth of sedation when administering sedative, hypnotic, or anesthetic agents during surgical and medical procedures. The depth of sedation is calculated by measuring cerebral electric activity via an electroencephalogram (EEG). The BIS algorithm processes the frontal EEG and converts the signal to a waveform on the BIS monitor. Let's take a closer look.
Nuts and bolts
The BIS device consists of a transducer that's connected to a monitor on one end and the patient's forehead via an adhesive electrode sensor strip on the other. The two to four sensors on the strip each have numbers that indicate where on the forehead they should be placed for optimal EEG signal analysis. The adhesive electrode sensor strip should be replaced every 24 hours to ensure it remains firmly attached to the skin and minimize the risk of infection.
The monitor calculates the data received by the two to four sensors and displays this information as a numeric value from 0 to 100 with a 10- to 30-second delay. Each numerical range correlates to a degree of sedation:
* 100 to 90-awake and responding appropriately to verbal stimulation
* 80 to 70-responsive to loud commands or mild shaking
* 70 to 60-intense tactile stimulation is needed for a response
* 60 to 40-unresponsive to verbal stimulus; general anesthesia obtained with a low chance for explicit recall
* less than 40-deep hypnotic state; possible protective responses still intact
* less than 20-burst suppression (EEG pattern characterized by cycles of high-voltage electrical movement alternating with cycles of no activity in the brain); respiratory drive is limited, but possible protective responses still intact
* 0-totally suppressed EEG (flat line).
If a patient reaches a deeper level of sedation than desired, medications can be decreased. Continued monitoring is required for the ventilated patient. Numerical values below 70 may require assisted mechanical ventilation. Ensure that you provide your patients and their family with education on the goal of BIS monitoring and the meaning of the numerical values.
Because the probes need to make good contact with the forehead, BIS monitoring isn't recommended for patients with extensive facial trauma. It's also contraindicated in patients who are extremely agitated because the increased muscular activity will interfere with the EEG reading.
Certain anesthetic medications don't perform as we would expect with BIS monitoring. Nitrous oxide or ketamine can leave the BIS value unchanged or increased. This may be because BIS values are known to correlate well with the cerebral metabolic rate (CMR) and both of these drugs, in contrast to other anesthetic agents, are known to increase CMR.
Benefits of BIS
In the past, vital signs and subjective sedation scales were used to measure depth of sedation. This inaccurate method often resulted in either over- or undersedation. Oversedation can prolong the postanesthesia care unit stay, delay extubation and wound healing, decrease gastrointestinal motility, and increase costs due to unnecessary increased usage of medications. Oversedation is also directly linked to postintensive care syndrome-a disorder that results in cognitive and physical problems for patients and their families after extensive ICU stays, occurring in 50% of patients who are mechanically ventilated for over 5 days. We can help prevent this disorder by using BIS monitoring to ensure that the level of sedation is adequate and meets the needs of the patient during his or her ICU stay. Undersedation may cause the patient to have awareness during the procedure, which can lead to increased pain and anxiety.
In a study conducted with patients older than age 60 in Berlin, delirium incidence was lower in patients with BIS monitoring. Post-op delirium was detected in 95 patients (16.7%) in the intervention group compared with 124 patients (21.4%) in the control group. Another study analyzing the economic benefits and clinical effectiveness of BIS monitoring found a reduction in general anesthesia consumption and recovery times when using BIS monitoring. Research is also proving an association between low intraoperative BIS readings and decreased intermediate-term survival in noncardiac and cardiac surgical patients. Responding to these numbers during surgery allows the healthcare provider to decrease the level of sedation, thus impacting the survival rate of patients during the recovery phase.
Intraoperative awareness
Current evidence-based practice research reveals that intraoperative awareness occurs in 1 to 2 patients per 1,000 receiving general anesthesia. This awareness can result in the patient feeling severe pain and experiencing long-term issues such as posttraumatic stress disorder. Intraoperative awareness is considered a sentinel event by The Joint Commission. BIS monitoring can prevent this occurrence by adding another clinical test to determine the depth of anesthesia.
Symptoms of intraoperative awareness include:
* responding to verbal commands
* eyelash reflex still functioning
* 20% elevation in heart rate or BP from previous baseline
* muscle tensing or movement
* pupillary response.
Conflicting views
A 2008 research study titled "Anesthesia Awareness and the Bispectral Index" reported awareness of patients during surgical procedures. The study compared a group of patients who didn't receive BIS monitoring with another group who did. The conclusion of the study was that a structured BIS monitoring protocol didn't result in a lower rate of intraoperative awareness or decreased volatile anesthetic gas usage compared with a structured end-tidal anesthetic gas measurement.
Should this dissuade us from using BIS monitoring? The authors themselves acknowledge the limitations of the study, such as the repeated questioning of the patients may have evoked memories that didn't exist or may have been confused with other memories that patients had from the ICU.
A well-stocked toolbox
The advantages of using BIS monitoring are numerous, including a decrease in time to extubation, use of hypnotic anesthetic drugs, incidence of nausea and vomiting, and occurrence of intraoperative awareness. As nurses, our responsibilities are extensive, yet using all of the available tools at our disposal is one way to increase positive outcomes in many areas of our patients' lives, including the physical and mental. The BIS monitor can offer the opportunity to impact many facets of our patients' outcome.
consider this
During a routine assessment of one of my patients, he mentioned that during his last surgery, he had total awareness and this was causing him extreme stress. He talked with me in detail about how he remembered the room and what the people were saying, and he could feel the pressure and tugging that was occurring in his midsection. This patient repeatedly pleaded with the healthcare team that he didn't want this to reoccur. He was placed on BIS monitoring and, after the surgery, had no recall of the surgical events. We interviewed the patient the day after the surgery and he still had no surgical recall. He stated that he was overjoyed with not being able to remember what for him was a traumatic event.
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