The bispectral index (BIS) is one of several systems used in anesthesiology as of 2003 to measure the effects of specific anesthetic drugs on the brain and to track changes in the patient’s level of sedation or hypnosis. In technical terms, the bispectral index itself is a complex mathematical algorithm that allows a computer inside an anesthesia monitor to analyze data from a patient’s electroencephalogram (EEG) during surgery. BIS, which has been in use since 1997, is a type of automated direct measurement of the patient’s condition, in comparison to the Glasgow Coma Scale and similar scoring systems, which are indirect assessments of sedation.
A brief discussion of anesthetic depth may be helpful in understanding people’s interest in monitoring the brain’s responses to anesthesia. Ever since the first modern anesthetics (ether, chloroform, and nitrous oxide) were used in the 1840s, doctors have been searching for a reliable method of measuring the depth of the patient’s unconsciousness in order to guarantee the safety as well as the painlessness of surgery. Anesthetic drugs, whether inhaled or given intravenously, are toxic in high doses; too high a dose can stop the patient’s breathing. On the other hand, too small a does can result in the patient’s coming to various degrees of awareness during surgery. Events of this type occur frequently enough to be publicized in general medical news sources as well as the professional literature. One Australian medical journal reports that postoperative recall of operations, including the patient’s overhearing conversations among members of the surgical team as well as feeling helpless and experiencing physical pain, occurs in one of every 1,000 patients undergoing non-cardiac surgery and three of every 1,000 cardiac patients. An Israeliresearcher gives the rate of accidental awareness during surgery as between 0.2% and 1.2% of patients. According to an American news account, “An estimated 40,000 to 200,000 mid-operative awakenings may occur each year in the United States alone.” Research has indicated that patients’ attitudes toward undergoing surgery are affected by the possibility of awakening during the procedure. A group of Australian researchers found that 56% of a group of 200 patients awaiting surgery had heard about awareness during operations, mostly from the mass media; 42.5% of the group expressed anxiety about it. Postraumatic stress disorder (PTSD) is a common result of awareness episodes; a 2001 study done at Boston University reported that 56.3% of a group of patients who had awakened during surgery met the diagnostic criteria for PTSD—as late as 17 years after their opera
There are several reasons for anesthesiologists’ difficulty in evaluating dosages of anesthetic agents:
- The lack of a universally accepted definition of “consciousness.” There are a number of scientific periodicals devoted solely to the study of human consciousness, as it concerns philosophers, psychologists, psychiatrists, and lawyers, as well as doctors involved in anesthesiology and critical care medicine. Some researchers emphasize the emotional or psychological dimensions of consciousness while others focus on physiological definitions—for example, the response of skin or muscle tissue to painful stimuli.
- The complex effects of anesthesia on the human organism. Scholarly debates about the nature of human consciousness are reflected in the variety of different goals that surgical anesthesia is expected to achieve. These goals are usually listed as blocking the nervous system’s responses to pain (analgesia), inducing muscular relaxation and blocking reflexes (areflexia), keeping the patient asleep during the procedure (hypnosis), and preventing conscious recall of the procedure afterwards (amnesia). It is not always possible, however, to meet all four goals with the same degree of accuracy, since some patients suffer from health conditions that require the anesthesiologist to keep them under lighter sedation in order to lower the risk of heart or circulation problems.
- The increased use of combinations of anesthetic agents rather than single drugs. At present, anesthesiologists rarely use inhaled anesthetics by themselves; most prefer what is known as balanced anesthesia, which combines inhaled and intravenous anesthetics. When different agents are used together, however, they are often synergistic, which means that they intensify each other’s effects. This characteristic makes it more difficult for the anesthesiologist to predict how much of each drug will be needed during the operation.
- Changes in the patient’s response to anesthesia over the course of the operation.
- Age- and sex-related differences in responsiveness to specific anesthetics. Anesthesiologists have become increasingly aware of the special needs of elderly patients, for example; they are more likely than younger patients to develop cardiovascular complications under anesthesia. With regard to sex, several studies have reported that women appear to emerge from anesthesia more rapidly than men after standardized anesthetic administration with the same agents.
- Large differences among individuals apart from age or sex groupings in regard to sensitivity to anesthesia.
Indirect measurements of consciousness
Indirect methods that allow an observer to assess a person’s level of awareness have been used since the early 1970s. The earliest and most widely used instrument for evaluating impaired consciousness is the Glasgow Coma Scale (GCS), first published in the Lancet in 1974. The GCS evaluates the patient’s responsiveness under three headings: eye response (four levels of responsiveness), verbal response (five levels), and motor (movement) response (six levels). A normally conscious individual would score 15, the maximum score. In practice, however, the total score on the GCS is usually broken down into three sub-scores for the three types of response measured; thus E2V2M3 would represent a total GCS score of 7. Total scores on the Glasgow Coma Scale are interpreted as follows: 13–14 indicates mild impairment of consciousness; 9–12 indicates moderate impairment; 8 or lower indicates coma.
There are about a dozen other scales that have been devised to measure consciousness in addition to the GCS; the two that are the most important in this context are the Ramsay Sedation Score, first published in 1974 as a measurement of sedation in patients receiving intravenous sedatives prior to surgery; and the Observer’s Assessment of Alertness/Sedation Scale (OAA/SS), first used in 1990 for the same purpose as the Ramsay. These two instruments are significant because they are commonly used in research evaluations of the bispectral index and similar monitoring systems. The Ramsay Score is a six-point scale ranging from one (“patient agitated or restless”) through six (“patient asleep; has no response to firm nailbed pressure or other noxious stimuli”). A score of 1 indicates inadequate sedation; 2–4, an acceptable level of sedation; and 5–6, oversedation.
The OAA/SS resembles the Glasgow Coma Scale in that it evaluates different categories of responsiveness, although the categories are different. The OAA/ SS measures the patient’s responsiveness to his or her name, quality of speech, degree of facial relaxation, and ability to focus the eyes.
Algorithm— A procedure or formula for solving a problem. It is often used to refer to a sequence of steps used to program a computer to solve a specific problem.
Analgesia— Absence of the ability to feel pain. The term is also sometimes used to refer to pain relief without loss of consciousness. An analgesic is a drug that is given to relieve pain.
Anesthesia— Loss of the ability to feel pain, brought about by administration of a drug or such other medical interventions as hypnosis or acupuncture.
Anesthesiology— The branch of medicine that specializes in the study of anesthetic agents, their effects on patients, and their proper use and administration.
Areflexia— A condition in which the body’s normal reflexes are absent. It is one of the objectives of general anesthesia.
Balanced anesthesia— The use of a combination of inhaled and intravenous drugs in anesthetizing patients.
Coma— A state of unconsciousness from which a person cannot be aroused, even by strong or painful stimuli.
Electroencephalogram (EEG)— A recording of the electrical activity of the nerve cells in the brain. The first such recording was made in 1929 by Hans Berger, an Austrian psychiatrist.
Hemodynamics— Measurement of the movements involved in the circulation of the blood; it usually includes blood pressure and heart rate.
Hypnosis— The term is used to refer to a specific verbal technique for refocusing a person’s attention in order to change their perceptions, judgment, control of movements, and memory. A hypnotic medication is one that induces sleep.
Proprietary— Referring to a drug, device, or formula that is secret or sold only by the holder of the patent, trademark, or copyright. The algorithm used in BIS systems is proprietary information.
Sedation— A condition of calm or relaxation, brought about by the use of a drug or medication.
Sequela (plural, sequelae)— An abnormal condition or event resulting from a previous disease or disorder.
Synergistic— Enhancing the effects of another drug. Anesthetics given in combination are often synergistic.
Direct measurements of consciousness
A variety of different physiological responses have been used in attempts to measure the depth of a patient’s unconsciousness under anesthesia. Most anesthesiologists use hemodynamic responses—the patient’s blood pressure and heart rate—as basic guidelines for adjusting the amount of anesthetic delivered to the patient during surgery. Other direct measurements have been based on movements of the patient’s body during surgery, hormonal responses, sweating, eye movement, and the reactivity of the eyes to light. One difficulty that has emerged from these attempts at direct measurement is that that they are not good predictors of the likelihood of awareness during surgery or recall of the procedure after surgery.
Another measurement that researchers have explored in their attempts to measure depth of anesthesia directly is the electroencephalogram, or EEG. The EEG is a complex recording of the electrical activity of the nerve cells in the brain. The first published paper on the EEG was written in 1929 by Dr. Hans Berger, an Austrian psychiatrist, on 73 recordings of brain waves using his son Klaus as the subject. Berger was the first to distinguish between alpha and beta brain waves, and to use the term EEG to describe the technique of electroencephalography . In 1931 Berger discovered that brain waves change in amplitude and frequency when a person is asleep or anesthetized; they slow down, shift to lower frequencies, and become more closely synchronized with one another. He also noted that such diseases as multiple sclerosis and Alzheimer disease affect a person’s EEG.
Several attempts were made in the late 1980s and early 1990s to make use of what is known about changes in the EEG in order to monitor anesthetic depth. One attempt is known as spectral edge frequency, or SEF. SEF is the frequency just above 95% of the total power spectrum of electrical energy recorded on an EEG. It was thought that the spectral edge frequency would be useful in guiding adjustments of anesthetics administered during surgery. Unfortunately, SEF is difficult to use with balanced anesthesia; it is also difficult to correlate with such other measures of anesthetic depth as movement or memory of the procedure. Another method that has been tried is median frequency, which is based on the median frequency of the complex EEG electrical signal at any given moment. This method proved to have the same drawbacks as spectral edge frequency. The bispectral index can be understood historically as a slightly later and more sophisticated attempt to use EEG signals to monitor patients’ responses to anesthesia.
Development of the bispectral index
The bispectral index was first developed in the early 1990s by applying bispectral analysis to EEG recordings. Bispectral analysis is a method of analyzing the mathematical relationships among the various components of an EEG signal (phase couplings) as well as measuring amplitudes and frequencies. To compile a database for the index, researchers recorded EEGs from several thousand patients and volunteers anesthetized with a range of commonly used anesthetics and anesthetic combinations. Each subject’s depth of unconsciousness was evaluated on the basis of a modified version of the OAA/SS described earlier (in the volunteers) or the amount of drug concentration in blood serum (in the patients). Segments of the recorded EEGs were used to draw up a set of EEG features that were then tested for their ability to distinguish between different levels of sedation or unconsciousness. The index that resulted from this process was then tested on different EEG recordings from the researchers’ larger database. It is scaled from 100 to 0 so that the BIS value decreases linearly with increasing doses of anesthesia.
It should be noted that the bispectral index is a work in progress. As new anesthetic agents are developed and used, the BIS algorithm is continually retested and refined. In addition, the algorithm is proprietary information, which means that it is kept secret by the company that developed it.
When a patient is brought into the operating room, special BIS sensors are applied to his or her forehead. No additional gels or electrodes are required. The anesthesiologist can attach the sensors to the patient in less than 30 seconds, since preparing the patient’s skin requires no more than an alcohol wipe to provide good electrical contact. The BIS system itself is integrated into patient monitoring devices produced by a number of different manufacturers that use enhanced EEG monitors. The BIS system displays both raw data from the EEG and a single number between 100 (indicating an awake patient) and 0 (indicating the absence of brain activity) that represents the patient’s degree of sedation. The target number for most anesthetized patients is between 50 and 60.
Current applications of BIS
BIS is presently used in intensive care units (ICUs) and some emergency departments as well as in operating rooms. According to company information, the bispectral index is used in about 26% of all hospital operating rooms in the United States as of late 2002. It is claimed that BIS reduces the risk of patient awareness during surgery as well as lowering hospital costs by speeding patient recovery and reducing the overuse of anesthetic agents.
Limitations of BIS
As of 2003, published studies of the bispectral index and other anesthetic monitoring systems presently available indicate that none of them can be considered a “gold standard” for preventing instances of patient awareness under anesthesia or for predicting the depth of anesthesia in a specific patient. Researchers have noted several specific limitations of the BIS system:
- BIS values are affected by the choice of anesthetic agent. This finding means that a patient with a BIS score of 60 anesthetized with one combination of agents may be more deeply sedated than another patient with the same score but anesthetized with a different combination of drugs. In addition, the BIS monitor appears unable to accurately track changes in consciousness produced by certain anesthetics, specifically ketamine and nitrous oxide.
- The changes in the BIS algorithm resulting from updating and refinement of the producer’s database make it difficult to compare results obtained by different investigators using different versions of the BIS monitor. This fact also leaves hospital-based anesthesiologists uncertain as to whether findings based on earlier versions of the BIS system are still valid.
- BIS values are difficult to correlate with other measurements of anesthetic depth or altered consciousness. One group of Norwegian researchers found that BIS values had little relationship to serum blood concentrations of anesthetic agents. Other researchers in the United States have found that BIS scores showed wide variability when compared with Glasgow Coma Scores for emergency room patients.
- Standard BIS scores are not useful in monitoring special patient populations, particularly critically ill patients with unstable body temperatures and patients with dementia.
Other anesthesia monitoring systems that are in use as of 2003 include the Patient State Analyzer, or PSA 4000, which is also based on EEG data; and the A-Line (R) monitor, which processes signals derived from auditory stimuli. Current opinion among anesthesiologists appears to be that none of the present monitoring systems are sufficiently sensitive to guarantee that patients will not awaken during surgery while simultaneously preventing undesirable cardiovascular reactions, other stress responses, or overuse of anesthetic agents.
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Rebecca Frey, PhD