## Minimum Alveolar Concentration (MAC) — Definition and Clinical Significance **Key Point:** MAC is the alveolar concentration of an inhalational anesthetic at 1 atm pressure at which 50% of patients do not move in response to a standard surgical stimulus (skin incision). ### MAC Values of Common Volatile Agents | Agent | MAC (%) | Clinical Implication | |-------|---------|----------------------| | Nitrous oxide | 104 | Potent analgesic; cannot be used alone for general anesthesia | | Isoflurane | 1.15 | Rapid emergence; good muscle relaxation | | Sevoflurane | 2.0 | Smooth induction; non-irritant to airway | | Desflurane | 6.0 | Rapid emergence; airway irritant | **High-Yield:** Nitrous oxide has a MAC of 104%, which means you would need 104% alveolar concentration to achieve anesthesia — **impossible at 1 atm pressure**. This is why nitrous oxide is used as an analgesic adjunct (50–70%) rather than as a sole anesthetic agent. **Clinical Pearl:** The inverse relationship between MAC and potency: agents with **lower MAC values are more potent** (require less concentration). Isoflurane (MAC 1.15) is more potent than sevoflurane (MAC 2.0), which is more potent than desflurane (MAC 6.0). ### Why Nitrous Oxide Cannot Be Used Alone The **"Second Gas Effect"** and **"Concentration Effect"** allow nitrous oxide to enhance the uptake of volatile agents when used as a carrier gas, but its own MAC exceeding 100% precludes its use as a sole anesthetic. It is always combined with volatile agents or IV agents. **Mnemonic:** **"MAC in Order of Potency"** — **I-S-D** (Isoflurane < Sevoflurane < Desflurane in potency, i.e., increasing MAC values: 1.15 → 2.0 → 6.0). Nitrous oxide stands apart with a supramaximal MAC. [cite:Gupta & Rao Textbook of Anesthesiology Ch 8]
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.