What Is Minimum Alveolar Concentration?

Minimum alveolar concentration (MAC) is a test for determining the strength of an anesthetic compound during surgery. It is specifically reliant on how a given substance keeps at least half of the patients tested immobile. Created in the 1960s, the test can be conducted while people are awake, by asking questions or looking for a response to stimuli. The MAC can also be determined by whether someone responds to a surgical incision. Lower values trend towards anesthetic drugs that have a higher potency, and minimum alveolar concentration tends to decrease the older a person gets, meaning less anesthetic is needed to stop muscle movement and the sensation of pain.

The typical measurement process lasts for 15 minutes. Once the anesthetic gases are mixed equally in the lungs, blood, and brain, surgeons can be confident that the MAC has been determined. A substance’s MAC is higher the less soluble it is. More soluble types of anesthesia have a lower effectiveness because they are absorbed and broken down easier. This discovery was made at the beginning of the 20th century and is called the Meyer-Overton hypothesis.

Other than age, few physical factors have an effect on MAC. Whether a subject is male or female does not play a role, although pregnant women have a lower tolerance to anesthesia. Height and weight are not factors either. It has also been observed that conditions such as hypothermia, hypothyroidism and low blood pressure can strengthen the effects of an anesthetic substance, so less of it is required.

The goal of minimum alveolar concentration is to establish a baseline of the effect of different types of anesthesia. Depending on the drug used, different parts of the nervous system are affected. Most anesthetics work within the spinal cord, so numbing the connection with the brain won’t stop reflexive movements. Surgical patients sometimes say they were aware of what was happening during surgery, but awareness is influenced by differing processes than most types of anesthesia control.

Patients can unconsciously form memories while under anesthesia. Such drugs block memories when someone awakens, so sounds are not typically recalled even if they are heard during an operation. People do sometimes remember what certain things felt like, but once the minimum alveolar concentration has been reached, pain does not get processed. It is difficult to determine exactly how much information is retained in memory during an anesthetized state, so surgeons must always have MAC in mind for each individual patient.