Residency · Residency · Anesthesiology

MAC: Concept, Variants, and Clinical Modifiers

Definition of MAC

Minimum Alveolar Concentration (MAC) is the alveolar concentration of an inhaled anesthetic at 1 atmosphere at which 50% of patients do not move in response to a standardized surgical stimulus — specifically, skin incision. The concept was introduced by Eger, Saidman, and Brandstater in 1965 and uses end-tidal concentration as a surrogate for brain partial pressure, assuming equilibrium has been reached. Because MAC is a population median (an ED50), it means that 50% of patients will still move at 1 MAC. An important practical point is that MAC values are additive across agents: 0.5 MAC of sevoflurane combined with 0.5 MAC of nitrous oxide yields a total of 1.0 MAC.

MAC Values for Common Agents (in 100% O2, 40-year-old adult)

MAC Variants

MAC-Awake (MAC-awareness)

MAC-awake is the alveolar concentration at which 50% of patients lose the response to verbal commands, representing the transition from consciousness to unconsciousness. It falls at approximately 0.3-0.5 MAC (roughly one-third of MAC) and is clinically relevant for predicting awareness risk and the timing of recovery and emergence.

MAC-BAR (Block Adrenergic Response)

MAC-BAR is the concentration that blocks the sympathetic (adrenergic) response to surgical stimulation in 50% of patients, sitting at approximately 1.5-1.7 MAC. This is the concentration needed to prevent tachycardia and hypertension. It is not routinely targeted with volatile agent alone; instead, it is achieved clinically by adding adjunctive opioids or other agents.

MAC-Intubation

MAC-intubation is the concentration required to prevent coughing and movement during endotracheal intubation in 50% of patients, approximately 1.3 MAC. It is higher than standard MAC because laryngoscopy and intubation represent a supramaximal stimulus.

MAC-Extubation

MAC-extubation is the concentration allowing smooth extubation without coughing in 50% of patients. It is similar to or slightly higher than MAC-awake.

ED95

The ED95 is the concentration at which 95% of patients do not move — approximately 1.3 MAC for skin incision. This value is more clinically relevant than MAC itself for ensuring immobility during surgery.

Factors That Decrease MAC

Physiologic Factors

Advanced age is one of the most important modifiers: MAC decreases approximately 6% per decade after age 40, and neonates also have lower MAC than infants. Hypothermia reduces MAC by approximately 5% per degree Celsius below 37 degrees. Hypotension (MAP below 40 mmHg), hypoxemia (PaO2 below 40 mmHg), severe anemia (hemoglobin below 5 g/dL), metabolic acidosis, and hyponatremia all decrease MAC. Pregnancy reduces MAC by approximately 25-40% through a progesterone-mediated mechanism that begins in the first trimester.

Pharmacologic Factors

Opioids are the most potent pharmacologic MAC reducers, with fentanyl and remifentanil infusions commonly producing 60% reduction, although a ceiling effect limits further benefit. Benzodiazepines provide moderate MAC reduction (up to 30%). Alpha-2 agonists such as dexmedetomidine and clonidine can produce significant MAC reduction, up to 90% with high-dose dexmedetomidine. Other agents that decrease MAC include ketamine, IV lidocaine, magnesium, lithium, acute alcohol intoxication, other inhaled anesthetics (through their additive effect), barbiturates, and propofol.

Pathologic States

Hypothyroidism and central neurotransmitter depletion (from chronic amphetamine use or reserpine) both decrease MAC.

Factors That Increase MAC

Physiologic Factors

MAC peaks in infants at approximately 6 months of age, the highest MAC of any age group. Hyperthermia increases MAC up to approximately 42 degrees Celsius, above which MAC actually decreases due to neuronal damage. Hypernatremia increases MAC. Red hair is associated with a modest increase of approximately 20%, possibly related to melanocortin-1 receptor polymorphism.

Pharmacologic Factors

Chronic alcohol use increases MAC through tolerance and enzyme induction. Chronic opioid use raises MAC through tolerance. Acute amphetamine or cocaine use increases MAC via catecholamine release. MAO inhibitors increase MAC through elevated CNS catecholamines. Ephedrine and aminophylline also increase MAC.

Factors That Do Not Affect MAC

Gender, duration of anesthesia, PaCO2 within the physiologic range (20-80 mmHg), metabolic alkalosis, hyper- or hypokalemia, blood pressure above 40 mmHg, the type of surgical stimulus (MAC is defined for skin incision), and thyroid status within the normal range all have no effect on MAC.

Age-Related Changes in Detail

Premature neonates have lower MAC than term neonates due to neuronal immaturity. Term neonates have lower MAC than infants. MAC peaks at 1-6 months of age, then decreases gradually through childhood and continues to decline by approximately 6% per decade from age 40 in adults. By age 80 and beyond, MAC may be only 50-60% of the value seen in a 40-year-old.

MAC and Opioid Interactions

All opioids reduce MAC in a dose-dependent manner, but the relationship follows a ceiling effect: beyond a certain opioid concentration, further increases provide diminishing MAC reduction. Remifentanil at effect-site concentrations of 1-3 ng/mL reduces MAC by 50-70%. This ceiling phenomenon is the basis for balanced anesthesia — combining lower concentrations of volatile agent with opioid to achieve immobility and hemodynamic stability without pushing either drug class to extremes.

<image>A graph showing the relationship between age (x-axis, from preterm neonate to 90 years) and MAC value in percent (y-axis). The curve rises from a moderate level in neonates, peaks at approximately 6 months of age, then gradually declines in a near-linear fashion through adulthood and old age. Key data points labeled for sevoflurane MAC at each age milestone.</image>

<image>A dose-response curve showing MAC reduction (y-axis, percentage of baseline MAC) as a function of opioid plasma concentration (x-axis). Multiple curves for fentanyl, sufentanil, alfentanil, and remifentanil. All curves show a steep initial decline followed by a plateau (ceiling effect) at approximately 60-70% MAC reduction. An annotation highlights the ceiling effect and explains that further opioid doses do not proportionally decrease MAC.</image>

<image>A summary infographic divided into two columns: left column titled "Factors That Decrease MAC" listing physiologic factors (age, hypothermia, pregnancy, hypotension, hypoxemia) and pharmacologic factors (opioids, benzodiazepines, alpha-2 agonists, acute alcohol) with downward arrows; right column titled "Factors That Increase MAC" listing physiologic factors (infancy, hyperthermia, red hair) and pharmacologic factors (chronic alcohol, acute stimulants, MAO inhibitors) with upward arrows. A central column lists "No Effect on MAC" items (gender, PaCO2 20-80, duration, K+).</image>

Clinical Pearls

MAC is an ED50, which means that at 1 MAC half of patients will still move. Targeting 1.2-1.3 MAC (approximately the ED95) provides reliable immobility when no opioids are used. The dramatic effect of age on MAC means elderly patients require substantially less volatile agent, and failure to adjust risks hemodynamic instability from relative overdose. Pregnancy reduces MAC by up to 40%, a fact relevant for any parturient requiring general anesthesia. End-tidal concentration is a reliable surrogate for brain partial pressure only after equilibration, which takes approximately 15 minutes of stable end-tidal values. MAC is additive across agents, so adding N2O allows reduction of the volatile agent concentration while maintaining the same total MAC. The MAC-BAR concept explains why patients can be hemodynamically reactive even at adequate surgical MAC — opioids are far more efficient at blunting autonomic responses than simply increasing the volatile concentration.

References

• Eger EI II, Saidman LJ, Brandstater B. Minimum alveolar anesthetic concentration: a standard of anesthetic potency. Anesthesiology. 1965;26(6):756-763.
• Nickalls RW, Mapleson WW. Age-related iso-MAC charts for isoflurane, sevoflurane and desflurane in man. Br J Anaesth. 2003;91(2):170-174.
• Daniel M, Eger EI II. What determines the minimum alveolar concentration (MAC)? Key Papers in Anaesthesia, 1996.
• Sebel PS, et al. Opioid reduction of MAC. Anesthesiology. 1992.

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