Definition of Minimum Alveolar Concentration (MAC)
Minimum Alveolar Concentration (MAC) is defined as the end-tidal concentration of an inhaled anesthetic agent at which 50% of patients do not move in response to a noxious stimulus (typically surgical incision). 1, 2, 3
Core Concept and Clinical Significance
MAC serves as the standard measure of potency for volatile anesthetic agents and was introduced in 1965 as a quantitative metric for comparing inhaled anesthetics 3
The MAC value represents a population-based concentration-response curve with a characteristically steep slope, meaning small changes in anesthetic concentration produce large changes in patient response 4
MAC is primarily mediated through action on the spinal cord to inhibit movement, while the amnesic and hypnotic effects occur at lower concentrations through brain-mediated mechanisms 3
Specific MAC Values for Common Agents
Sevoflurane MAC in 100% oxygen for a 40-year-old adult is 2.1% 1
Isoflurane MAC values vary by age: 1.28% (age 26±4 years), 1.15% (age 44±7 years), and 1.05% (age 64±5 years) in 100% oxygen 2
Desflurane MAC decreases with increasing patient age, ranging from approximately 9-10% in infants to 5.2% in 70-year-olds when administered in 100% oxygen 5
Important Clinical Modifiers
Nitrous oxide significantly reduces MAC requirements: for example, 60% N₂O reduces sevoflurane MAC from 2.1% to approximately 1.4%, and reduces isoflurane MAC by approximately 50% 1, 2
Opioids substantially decrease MAC: fentanyl at 0.5 ng/mL or alfentanil at 28.8 ng/mL produces a 50% reduction in isoflurane MAC 6
Low doses (0.25 to 0.5 MAC) of volatile anesthetics like sevoflurane and isoflurane have been demonstrated to provide cardioprotection through preconditioning and postconditioning mechanisms 7
MAC Derivatives and Clinical Applications
The concentration needed to prevent explicit memory formation and produce unconsciousness is substantially lower than the MAC required to prevent movement—typically around 0.3-0.5 MAC 3
Protocols that alert clinicians when MAC falls below 0.5 or 0.7 have potential to decrease intraoperative awareness with explicit recall 3
The eMAC fraction concept extends MAC to quantify combined potency of multiple coadministered inhalation and intravenous anesthetics, with maintenance values typically ranging from 1.3 to 2.6 during surgery 8
Common Pitfalls
MAC only predicts movement response to noxious stimuli—it does not guarantee amnesia, unconsciousness, or hemodynamic stability, which occur at different anesthetic depths 3
The functional unit for comparing anesthetics in environmental or pharmacoeconomic studies should be "MAC-hours" rather than volume or weight, as this represents clinically equivalent dosing 7
MAC values represent population averages; individual patient requirements vary based on age, comorbidities, and concurrent medications 5, 2