MAC Awake Definition
MAC awake is the minimum alveolar concentration of an inhaled anesthetic at which 50% of patients appropriately respond to verbal command (such as "open your eyes"), typically occurring at approximately 0.3-0.5 times the MAC value depending on the specific anesthetic agent.
What MAC Awake Represents
MAC awake serves as a critical threshold measurement that indicates when patients transition from an anesthetized state to consciousness during emergence from general anesthesia. This value provides three clinically important measures:
- Awakening kinetics: The ratio of MAC awake to MAC determines the time required for emergence from anesthesia
- Amnestic potency: MAC awake correlates with the anesthetic concentration sufficient to prevent memory formation
- Hypnotic potency: It serves as a traditional index of an anesthetic's ability to suppress consciousness
Agent-Specific MAC Awake Values
The MAC awake varies significantly between different volatile anesthetics and is not uniform across agents 1, 2:
Volatile Anesthetics (as fraction of MAC)
- Isoflurane: 0.25-0.40 MAC (approximately 0.14-0.40% end-tidal concentration) 1, 3
- Sevoflurane: 0.35 MAC (approximately 0.16-0.17% end-tidal concentration) 4, 3
- Enflurane: 0.27 MAC 1
- Halothane: 0.50-0.59 MAC (significantly higher than other agents) 1
- Desflurane: 0.36 MAC (2.60%) 5
- Xenon: 0.46 MAC (32.6%) 4
- Nitrous oxide: 0.61 MAC (63.3%) 4
Intravenous Anesthetics
- Propofol: Cp50-awake is 2.69 μg/mL, which represents only 0.18 of its Cp50 for anesthesia—meaning propofol requires a much larger concentration decrease for awakening compared to volatile agents 5
Clinical Monitoring Implications
The 2021 Association of Anaesthetists guidelines emphasize that maintaining end-tidal anesthetic concentration (ETAC) > 0.7 age-adjusted MAC during maintenance reduces the risk of accidental awareness during general anesthesia 6. An audible alarm should be enabled to alert when ETAC falls below 0.7 MAC 6.
Important Clinical Caveats
Age significantly affects MAC awake values—older patients have lower MAC awake concentrations than younger patients 3. However, MAC awake is not influenced by:
- The concentration of anesthetic administered during maintenance
- Duration of anesthetic exposure
- Patient obesity 3
Washout kinetics matter: MAC awake values differ depending on whether measured during slow equilibrated washout (higher values) versus rapid discontinuation (lower values) 1. For example, isoflurane MAC awake is 0.25 MAC with slow washout but only 0.19 MAC with fast washout.
Drug interactions: When combining anesthetics, xenon demonstrates additive effects with isoflurane and sevoflurane on MAC awake, while nitrous oxide shows supra-additive effects 4.
Practical Application
During emergence, anesthesiologists can use MAC awake values to predict when patients will respond to verbal commands. The recent eMAC fraction concept suggests that during emergence, patients typically awaken at an eMAC fraction of approximately 0.30 7, which aligns with traditional MAC awake measurements and provides a framework for titrating multiple anesthetics simultaneously.