Minimal Alveolar Concentration

Minimal Alveolar Concentration (MAC) is a numerical scale that was created to correlate a depth of anesthesia with the amount of inhaled anesthetic that a patient is exposed to. The definition of MAC is the alveolar concentration of an inhalation anesthetic agent at 1 atmosphere (atm) and at stead-state concentration that is necessary to suppress a gross purposeful movement in 50% of patients in response to a surgical skin incision. Another way to look at MAC is it is similar to the effective dose (ED50) for medications.

The potency of each anesthetic gas is inversely related to their MAC. The most potent gas, isoflurane, has the lowest MAC at 1.3%. Potency is related to the anesthetics lipid solubility (or oil:gas partition coefficient). The percentage of alveolar concentration is measured as the end-tidal concentration of gas. This is measured by the gas analyzer built into the expiratory limb of the ventilator. At a stead-state, the end-tidal concentration of the gas equals the alveolar concentration which also equals the central nervous system (CNS) concentration.

The percentage dial on the variable bypass vaporizer correlates to the partial pressure at a given altitude. For example, at sea level where the atmosphere pressure is 760 mmHg, a vaporizer dials to 1% will deliver 7.6 mmHg of gas. Vaporizer are designed to adjust to various altitude and deliver the same partial pressure of gas in order to achieve the same anesthetic effect. For example, at 0.5 atmosphere or 380 mmHg, a vaporizer with a dial set to 1% would actually deliver 2% gas which would equal 7.6 mmHg of gas. This does not apply to the desflurane vaporizer.

For the most part, MAC is quite similar across all people. There are a few factors that affect MAC. First of all, MAC is effected by age. It is greatest in infants aged 3 to 6 months. After a baby turns 1 year old, their MAC will decrease by 6-7% for each decade of life. Temperature also has a linear affect on MAC with each decrease in 1 degree Celsius of body temperature resulting in a 4-5% reduction in MAC.

Other things that decrease MAC include:

• opioids

• Benzodiazepines

• Barbiturates

• Propofol

• Ketamine

• Alpha 2 agonists

• Intravenous local anesthetic

• Acute ethanol ingestion

• Pregnancy

• Severe hypotension (MAP < 50 mmHg)

• Severe hypoxemia (PaO2 < 50 mmHg

• Severe hypercarbia (PaCO2 > 40 mmHg)

• Severe anemia (Hematocrit < 10%)

• Acute metabolic acidosis

• Sepsis

Things that increase MAC include:

• inhibition of catecholamine reuptake (amphetamines, cocaine)

• Chronic ethanol abuse

• Hyperthermia

• Hyperthyroidism

• Increased extra cellular Na+ in CNS

• Red hair

Besides the definition of 1 MAC in regards to a surgical incision, there have been other definitions of MAC for various depths of anesthesia. For example, a MAC of 0.4 has been said to reduce awareness and response to a verbal or tactile stimulation. 1.2 MAC is the level at which 95% of patients will not response to a surgical incision. 1.3 MAC is the level that is require for no laryngeal response to endotracheal intubation. And at 1.6 MAC, this is the level to blunt autonomic response (BAR) to a surgical stimulus.

Awareness under general anesthesia (GA) happens in about 1 to 2 cases per 1000 cases. The most common reported feature is hearing voices and occurs most frequently during induction or emergence. There is a higher risk any time paralysis is used.

There are particular surgeries and groups of patients that are at higher risk for awareness:

• patients chronically using alcohol, opiates, meth, cocaine

• Trauma (11-43%)

• Cardiac (1-1.5%)

• Cesarean section under GA (0.4%)

If you suspect your patient had an episode of awareness, the best thing to do is early counseling. There is patient education available on the ASA HQ website.