The MAC of isoflurane in a 70-year-old patient at sea level (1 atm) is approximately 1.15%. In a patient with chronic alcoholism on long-term ethanol intake (not acutely intoxicated), the expected MAC would be:

• A Decreased due to cross-tolerance with GABA-enhancing drugs
• B Unchanged as MAC is not affected by alcohol tolerance
• C Decreased due to hepatic enzyme induction reducing circulating agent
• D Increased due to cross-tolerance with CNS depressants ✓

Explanation

Chronic alcohol use induces cross-tolerance to CNS depressants including volatile anaesthetics, resulting in an elevated MAC requirement. This is distinct from acute alcohol ingestion, which decreases MAC by additive CNS depression. Age reduces MAC (approximately 6% per decade after 40 years), and the example already accounts for the patient's age. Hepatic metabolism plays a negligible role in MAC determination since inhalational agents are eliminated via the lungs.

Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.

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