A patient anaesthetised with isoflurane shows end-tidal isoflurane of 1.3% but remains haemodynamically unstable with excessive movement to surgical stimulus. The anaesthetist suspects intraoperative awareness. Which of the following physiological states MOST increases MAC for isoflurane?

• A Hypothyroidism with TSH of 45 mIU/L
• B Age 75 years with baseline dementia
• C Severe chronic anaemia (Hb 4 g/dL)
• D Chronic alcohol use disorder ✓

Explanation

Chronic alcohol use upregulates NMDA receptors and increases CNS excitability, raising MAC by up to 40%. Hypothyroidism and advanced age both decrease MAC; elderly patients have significantly reduced MAC (approximately 6% per decade above 40 years), and dementia further reduces it. Severe anaemia decreases MAC because reduced oxygen delivery impairs neuronal metabolism, lowering the anaesthetic requirement. Chronic alcohol abusers therefore require higher inhalational agent concentrations for equivalent depth.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.