Bispectral Index Health Article

Definition

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.

Anesthetic depth

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 dose 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 Israeli researcher 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. Post-traumatic 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 operation.

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.