Anesthesia and Awareness

Anesthetic Technique

The first successful use of ether by William Morton on October 16, 1846, is still celebrated as “Ether Day” by anesthesiologists worldwide. Chloroform was introduced in 1847 and gained popularity when used by Queen Victoria during the birth of her eighth child. Modern anesthetic practice, known as balanced anesthesia, employs a “cocktail” of different drugs to achieve three different end points: sedation, loss of consciousness, and muscle relaxation. Commonly employed sedatives include diazepam and midazolam. Typical anesthetic agents include inhalants such as nitrous oxide and oxygen, isoflurane, or sevoflurane, and intravenous drugs such as sufentanil or propofol. Muscle relaxation is typically achieved with a drug such as tubocurarine. Because the neuromuscular blockade induces a total paralysis of the skeletal musculature, the patient must be artificially respirated during the procedure, until the blockade is reversed by a drug such as neostigmine. For the same reason, the patient is unable to communicate with the surgical team or respond behaviorally to requests; this situation raises the question of how anyone knows that the patient is really unconscious.

Monitoring Anesthetic Depth

Clinically, the success of general anesthesia is shown by the patient's lack of response to verbal commands or to what is euphemistically referred to as “surgical stimulation.” Moreover, postoperatively, the patient will report no sensation of pain during the operation; nor will the patient recall any events that took place during the operation. By these standards, far less than 1% of surgical patients report any surgical awareness. Still, a postoperative interview is a little late to determine that a patient was not adequately anesthetized, and so considerable effort has been made to develop means of monitoring the patient's state of consciousness, as well as vital functions, during anesthesia. One “on line” measure, based on presumed autonomic signs of conscious awareness, is the PRST score, which takes into account the patient's blood pressure, heart rate, sweating, and secretion of tears. Another standard simply relies on biochemical measures of anesthetic concentration in the lungs or bloodstream. A common measure is MAC, which is the minimum alveolar concentration of inhalant anesthetic, measured through the respirator, which eliminates motor response to surgical stimulation in 50% of patients. A weaker concentration, MAC-aware, also known as MAC-awake, typically about 0.3 to 0.5 MAC, is sufficient to eliminate awareness (measured by response to a verbal request), without necessarily eliminating all reflexive motor responses. A stronger concentration, MAC-BAR (1.7–2.0 MAC) is required to block autonomic as well as skeletal reflexes. Including muscle relaxants in balanced anesthesia, then, allows administration of lower doses of anesthetic agents. Similar standards, based on blood plasma levels, have been determined for intravenous anesthetic agents. Because the operational definition of MAC means that 50% of patients will respond to surgical events, [Page 31]anesthesiologists generally administer a dose equivalent to approximately 1.3 MAC to ensure adequate anesthesia.

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