MAC Modifying Factors — FRCA Primary MCQ
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Correct answer: E — Hypothermia
Hypothermia decreases MAC: as body and central nervous system temperature fall, volatile anaesthetic potency increases, so a lower alveolar partial pressure is required to prevent movement in 50% of patients. Experimental data demonstrate a progressive reduction in isoflurane MAC during cooling. Hyperthermia has the opposite effect and increases MAC. Chronic alcohol dependence produces pharmacodynamic tolerance and generally increases anaesthetic requirements, unlike acute alcohol intoxication, which decreases them. MAC varies non-linearly with age: it is relatively high in infants and reaches a maximum at approximately 6–12 months before declining progressively with increasing age. Hyperthyroidism is not a reliable cause of reduced MAC. Therefore, hypothermia is the single best answer.
Reference: Antognini JF. Hypothermia eliminates isoflurane requirements at 20 degrees C. Anesthesiology. 1993;78:1152–1156. https://pubmed.ncbi.nlm.nih.gov/8512109/