Ontario health technology assessment series··
Review of the Scientific Evidence on the Bispectral Index Monitor
Updated
Abstract
The incidence of intraoperative awareness during general anesthesia ranges from 0.1% to 0.2% in the United States.
- Awareness during anesthesia is rare but varies by type of surgery, with trauma patients experiencing the highest rates (11%-43%).
- The BIS monitor uses electrical signals from the brain to quantify the depth of anesthesia, providing a numerical score that correlates with consciousness levels.
- A BIS score below 60 is associated with a low probability of response to commands, indicating deeper anesthesia.
- A large randomized trial found 2 cases of awareness in the BIS group compared to 11 cases in the standard practice group, a statistically significant difference.
- BIS monitoring may reduce the incidence of intraoperative awareness, but its algorithm is not reliable for accurately predicting unconsciousness.
- An unknown percentage of patients may receive unnecessary anesthetic doses due to falsely elevated BIS values, which may lead to overly deep anesthesia.
Simplified
The objective of this analysis was to evaluate the effectiveness and cost-effectiveness of the bispectral index (BIS) monitor, a commercial device to assess the depth of anesthesia. Conventional methods to assess depth of consciousness, such as cardiovascular and pulmonary measures (e.g., heart rate, systolic/diastolic blood pressure, mean arterial pressure, respiratory rate, and level of oxygen in the blood), and clinical signs (e.g., perspiration, shedding of tears, and limb movement) are not reliable methods to evaluate the brain status of anesthetized patients. Recent progress in understanding the electrophysiology of the brain has led to the development of cerebral monitoring devices that identify changes in electrophysiologic brain activity during general anesthesia. The BIS monitor, derived from electroencephalogram (EEG) data, has been used as a statistical predictor of the level of hypnosis and has been proposed as a tool to reduce the risk of intraoperative awareness. Anesthesia that is too light can result in the recall of events or conversations that happen in the operation room. Patients have recalled explicit details of conversations that happened while under anesthesia. This awareness is frightening for patients and can lead to post-traumatic stress disorder. Conversely, anesthesia that is too deep can cause hemodynamic disturbances necessitating the use of vasoconstrictor agents, which constrict blood vessels, to maintain normal blood pressure and cardiac output. Overly deep anesthesia can also result in respiratory depression requiring respiratory assistance postoperatively. Monitoring the depth of anaesthesia should prevent intraoperative awareness and help to ensure that an exact dose of anaesthetic drugs is given to minimize adverse cardiovascular effects caused by overly large doses. Researchers have suggested that cerebral monitoring can be used to assess the depth of anesthesia, prevent awareness, and speed early recovery after general anesthesia by optimizing drug delivery to each patient. Awareness is a rare complication in general anesthesia. The risk of intraoperative awareness varies among countries, depending on their anesthetic practices. In the United States, the incidence of intraoperative awareness is 0.1% to 0.2% of patients undergoing general anesthesia. The incidence of intraoperative awareness depends on the type of surgery. Trauma patients have reported the highest incidence of intraoperative awareness (11%-43%) followed by patients undergoing cardiac surgery (1.14%) and patients undergoing Cesarean section (0.9%). The BIS monitor, licensed by Health Canada, is the first quantitative EEG index used in clinical practice as a monitor to assess the depth of anesthesia. It consists of a sensor, a digital signal converter, and a monitor. The sensor is placed on the patient's forehead to pick up the electrical signals from the cerebral cortex and transfer them to the digital signal converter. A BIS score quantifies changes in the electrophysiologic state of the brain during anesthesia. In patients who are awake, a typical BIS score is 90 to 100. Complete suppression of cortical activity results in a BIS score of 0, known as a flat line. Lower numbers indicate a higher hypnotic effect. Overall, a BIS value below 60 is associated with a low probability of response to commands. There are several alternative technologies to quantify the depth of anesthesia, but only the BIS and SNAP monitors are licensed in Canada. The list price of the BIS monitor is $13,500 (Cdn). The sensors cost $773 (Cdn) for a box of 25. Because intraoperative awareness and recall happen rarely, only 1 randomized controlled trial of all the studies reviewed, was adequately powered to show the impact of BIS monitoring. This was a large prospective, randomized, double-blinded, multicentre study that was designed to investigate if BIS-guided anesthesia reduces the incidence of intraoperative awareness. The study confirmed 2 cases of intraoperative awareness in the BIS group and 11 cases in the standard practice group. This difference was statistically significant (P =.022). There were 36 reports of possible awareness that were not confirmed by the study group (20 patients in the BIS group and 16 in the standard practice group). Additionally, the results of small randomized controlled trials and prospective cohort studies show that, overall, BIS monitoring is relatively good at indicating the state of being alert; however, its algorithm does not accurately predict an unconscious state. BIS monitoring has low sensitivity for the detection of the state of being asleep, and it may show values higher than 60 in those already asleep. Therefore, an unknown percentage of patients will not be identified as being asleep and will receive anesthetics unnecessarily. Based on the literature review, the Medical Advisory Secretariat concludes the following: Prevention of awareness should remain a clinical decision for anesthesiologists to make based on their experience with intraoperative awareness in their practices.Although BIS monitoring may have a positive impact on reducing the incidence of intraoperative awareness in the general population, its negative impact on individual patients may overshadow this positive outcome.BIS monitoring is good at indicating an "alert" state, which is why it can reduce the incidence of intraoperative awareness; however, its algorithm does not accurately predict an "asleep" state. This means an unknown percentage of patients who are already asleep will not be identified because of falsely elevated BIS values. These patients will receive unnecessary dosage of anesthetics resulting in a deep hypnotic state.Adherence to the practice guidelines will reduce the risk of intraoperative awareness.
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