Wisconsin Anesthesia Professionals

What is MAC Fraction?

The potency of inhaled anesthetics is known as the Minimum Alveolar Concentration (MAC). MAC is defined as the concentration of an anesthetic in the alveoli needed to prevent movement in 50% of patients in response to a surgical incision. The MAC fraction refers to the amount of anesthetic administered as a proportion of the MAC value and reflects how strong anesthesia is for a given patient.

MAC is expressed as a percentage of one atmosphere of pressure, indicating the proportion of the gas that is present in the inspired air. For instance, a MAC value of 1.2%, in the case of isoflurane, indicates that 1.2% of the gas mixture being inhaled by the patient is isoflurane, while the remaining 98.8% is typically a mixture of oxygen and other gases. Additionally, a lower MAC value indicates a more potent anesthetic while a higher MAC value indicates lower potency. For example, isoflurane has a MAC value of ~1.2% and is more potent than nitrous oxide, which has a MAC value of ~104% (1) (2) (3).

When discussing MAC fraction, a value of 0.5 is half the concentration needed to achieve the full MAC, while a value of 1.0 means the patient is receiving an anesthetic dose equal to the standard MAC value. These values help standardize doses across different anesthetics and ensure proper anesthetic depth, which can vary depending on the procedure (e.g., lighter anesthesia for minor procedures and higher for more invasive surgeries) (1).

Several factors, such as age, body temperature, the presence of other medications, and the type of procedure occurring, influence the MAC fraction a patient receives. For example, how much anesthetic is needed typically decreases with age, meaning elderly patients require lower concentrations of anesthetic to achieve the same effect as younger adults (1).

Different types of MAC values are used to describe various levels of anesthetic depth, including: MACawake, the anesthetic concentration needed to suppress a voluntary response to verbal command in 50% of patients; MACunconciousness, the concentration of an anesthetic in the alveoli at which 50% of patients will become unconscious; MACimmobility, the concentration of an anesthetic in the alveoli needed to prevent movement in 50% of patients in response to a surgical incision; and MACBAR: the concentration of an anesthetic in the alveoli required to block autonomic responses (e.g., changes in heart rate, blood pressure) to painful stimuli in 50% of patients. As an example, while isoflurane has a MAC value of ~1.2%, its MACawake value is at a lower percentage of ~0.4% (4). These variations of MAC values help anesthesiologists tailor anesthesia plans based on the desired clinical outcome, ensuring both patient safety and comfort (1) (5).

In clinical practice, MAC fraction helps anesthesiologists maintain appropriate anesthetic levels during surgery. Modern anesthesia machines display the end-tidal concentration of anesthetics as a fraction of MAC, facilitating real-time adjustments. The end-tidal concentration, measured at the end of the exhalation phase, provides a stable and accurate reflection of the gas concentrations in the alveoli. For example, an end-tidal concentration of 1 MAC indicates that the patient is at a level where 50% of patients would not move in response to a surgical stimulus. This measure helps maintain anesthetic depth within a safe and effective range, reducing the risk of under-dosing or overdosing (6).

Understanding and utilizing MAC fraction has advanced anesthetic management. This approach not only standardizes the administration of anesthetics but also allows for fine-tuning based on patient-specific factors and surgical requirements. The ability to combine different anesthetic agents using their respective MAC fraction values enables anesthesiologists to achieve desired effects while minimizing side effects. This has become particularly important in complex surgeries where maintaining hemodynamic stability and quick recovery times are critical and has overall enhanced the general quality of care in surgical settings.


  1. Evan D. Kharasch; Anesthetic MAC: Origin, Utility, and Nomenclature Revisited. Anesthesiology 2022; 136:885–887 doi: https://doi.org/10.1097/ALN.0000000000004217
  2. Zafirova Z, Sheehan C, Hosseinian L. Update on nitrous oxide and its use in anesthesia practice. Best Pract Res Clin Anaesthesiol. 2018;32(2):113-123. doi:10.1016/j.bpa.2018.06.003
  3. R. W. D. Nickalls, W. W. Mapleson, Age‐related iso‐MAC charts for isoflurane, sevoflurane and desflurane in man, BJA: British Journal of Anaesthesia, Volume 91, Issue 2, August 2003, Pages 170–174, https://doi.org/10.1093/bja/aeg132
  4. Aranake, A., Mashour, G.A. and Avidan, M.S. (2013), Minimum alveolar concentration: ongoing relevance and clinical utility. Anaesthesia, 68: 512-522. https://doi.org/10.1111/anae.12168
  5. David White, Uses of MAC, BJA: British Journal of Anaesthesia, Volume 91, Issue 2, August 2003, Pages 167–169, https://doi.org/10.1093/bja/aeg160
  6. Jan F. A. Hendrickx, Andre M. De Wolf; End-tidal Anesthetic Concentration: Monitoring, Interpretation, and Clinical Application. Anesthesiology 2022; 136:985–996 doi: https://doi.org/10.1097/ALN.0000000000004218