## Minimum Alveolar Concentration (MAC) — Definition and Clinical Relevance **Key Point:** MAC is the alveolar concentration of an inhalational anesthetic at 1 atmosphere pressure at which 50% of patients do not move in response to a standard surgical stimulus (skin incision). ### MAC Values of Common Inhalational Agents | Agent | MAC (%) | Potency | Clinical Use | | --- | --- | --- | --- | | Methoxyflurane | 0.16 | Highest (most potent) | Rarely used; nephrotoxicity | | Isoflurane | 1.15 | High | Common; rapid emergence | | Sevoflurane | 2.0 | Moderate | Common; smooth induction | | Desflurane | 6.0 | Low | Airway irritant; rapid emergence | | Nitrous oxide | 104 | Very low | Adjuvant only | **High-Yield:** Lower MAC = greater potency. Methoxyflurane has the lowest MAC (0.16%), making it the most potent volatile anesthetic, though it is rarely used clinically due to dose-dependent nephrotoxicity and hepatotoxicity. ### Inverse Relationship: MAC and Lipid Solubility MAC is inversely related to blood:gas partition coefficient (lipid solubility). Agents with higher lipid solubility (e.g., methoxyflurane) have lower MAC values because they dissolve more readily in the CNS lipid bilayer, requiring less alveolar concentration to achieve anesthesia. **Clinical Pearl:** Although methoxyflurane is the most potent (lowest MAC), it is contraindicated for prolonged use because metabolic degradation produces fluoride ions and oxalic acid, leading to high-output renal failure and hepatic injury. ### Why Other Agents Have Higher MAC - **Isoflurane (MAC 1.15%):** Moderate potency; commonly used in India for maintenance anesthesia. - **Sevoflurane (MAC 2.0%):** Lower potency than isoflurane; preferred for smooth inhalational induction in pediatrics. - **Desflurane (MAC 6.0%):** Lowest potency among agents in current use; airway irritant; rapid emergence makes it useful for short procedures. **Mnemonic: "MAC Potency Ladder" (from highest to lowest potency):** **M**ethoxyflurane > **I**soflurane > **S**evoflurane > **D**esflurane (MISD = inverse order of MAC values).
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