The minimum alveolar concentration (MAC) of inhalational anaesthetics is reduced in pregnancy. What is the approximate percentage reduction in MAC during term pregnancy and what mediates this change?
- A 10% reduction; caused by increased plasma volume diluting the anaesthetic
- B 50% reduction; caused by decreased hepatic metabolism due to reduced hepatic blood flow
- C 25–40% reduction; mediated by elevated progesterone and beta-endorphin levels acting on CNS ✓
- D 15% reduction; due to metabolic alkalosis from hyperventilation of pregnancy
Correct answer: C. 25–40% reduction; mediated by elevated progesterone and beta-endorphin levels acting on CNS
Explanation
MAC is reduced by approximately 25–40% during term pregnancy. The mechanism is multifactorial but elevated progesterone (which has sedative/anaesthetic-potentiating properties) and elevated beta-endorphin levels are the primary mediators. This means pregnant women achieve the same anaesthetic depth at lower inhaled concentrations, reducing the dose needed for Caesarean section. It also means they are at greater risk of awareness if standard non-obstetric concentrations are not maintained, and it highlights the importance of adequate agent delivery confirmation.
Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.