Assessing Depth of Anesthesia

More than 150 years after the first public display of surgical anesthesia, most anesthesiologists still infer your depth of anesthesia from your physical signs. Anesthesiologists are still taught in their training that physical signs like heart rate, blood pressure, sweating, tearing, and gross movement are indirect, or surrogate, indicators of the patient's depth of anesthesia. Yet no research has ever shown that these physical signs correlate in any predictable or measurable way with the patient's level of consciousness. In fact, the clinical signs that are widely used by anesthesiologists to assess depth of anesthesia are notoriously inaccurate and unreliable for predicting depth of anesthesia. In a study of patients who had awareness during general anesthesia, only a small number manifested any physical signs of light anesthesia.1 The anesthesiologist should be monitoring these physical parameters for many reasons, but they are not a reliable indicator of depth of anesthesia.

Most anesthesiologists would consider responses to surgical stimulation like increased heart rate, increased blood pressure, and movement as signs of light anesthesia and would deepen your anesthesia. However, there is no scientific way to correlate these physical signs with your depth of anesthesia. Using these physical signs as an indicator often results in excessive amounts of anesthesia administered to abolish heart-rate and blood-pressure response to surgical stimulation even though you are deeply unconscious. Conversely, a patient undergoing heart surgery may have low blood pressure and a slow heart rate in response to anesthesia even though she is awake and paralyzed during surgery.

Movement is another physical sign commonly used to gauge depth of anesthesia. When a patient moves, she is probably inadequately anesthetized, but if the patient is anesthetized to the point where she does not move, is the patient barely unconscious or in a deep coma? We don't know. Movement doesn't correlate in any scientific way with depth of anesthesia. To abolish all movement in all patients using anesthesia means that many patients will be given excessive amounts of anesthesia.

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