Depth of anesthesia can be defined as the degree to which a general anesthetic agent depresses the central nervous system. This in turn depends on the potency and the concentration of the agent used.

Learning Objectives:

1. Definition and understanding of the concept of depth of anesthesia.
2. Clinical importance and use of depth of anesthesia.
3. Unconsciousness and awareness during anesthesia
4. Long term effects of general anesthesia
5. Monitors available for measuring the depth of anesthesia - their advantages and limitations

Ever since the day of first public demonstration of anesthesia with ether, people have attempted to define, measure or monitor the depth of anesthesia.  John Snow described “degrees of narcotism” for ether, and then Gudel in 1937 published the classical signs of ether anesthesia and defined four stages of anesthesia. In the 20th century, pioneers like Prys-Roberts and Kissin changed the concept of depth of anesthesia more in tune with the modern anesthetic agents and practice.

Modern anesthetic practice offers various monitors to measure the depth of anesthesia. These monitors differ in how and what they measure to reflect the depth of anesthesia.  Some of the monitors commercially available include BIS monitor, PSA analyser, Cerebral state monitor, Entropy module and Auditory evoked potential monitor. In order to analyze and compare these monitors we need to understand the basic principles of their functioning including the data collected, the method of analysis and the algorithm used by these monitors.

Adequate depth of anesthesia is required not only to provide excellent operating conditions for the surgeons but also to avoid unpleasant experience for the patients (avoid awareness during surgery). On the other hand, avoiding too deep an anesthetic provides advantages such as quicker wake-up (recovery), shorter PACU stay, and quicker discharge from hospital. Some of the recent studies even imply that too deep an anesthetic may have adverse outcomes such as increased mortality.

The consensus is that general anesthesia consists of several components representing separate pharmacological actions, even if it is produced by one drug. Therefore, a monitor of depth of anesthesia may be measuring only one of the several components of general anesthesia. Some of the common components used to measure the depth of anesthesia are loss of consciousness, lack of motor response to painful stimulus and lack of hemodynamic response to painful stimulus.

As our understanding of the mechanism of anesthesia has improved it has only increased the questions about the depth of anesthesia.  The evidence available is not compelling enough to make the depth of anesthesia monitoring a requirement or a standard yet. We have a choice of monitors to help assess the depth of anesthesia but the search for a better monitor continues.