A 70-year-old patient on long-term phenytoin therapy requires induction of anaesthesia. The dose of thiopentone required is likely to be:

• A Decreased, due to enhanced GABA potentiation
• B Unchanged, as phenytoin does not affect barbiturate metabolism
• C Decreased, due to reduced hepatic blood flow in the elderly
• D Increased, due to hepatic enzyme induction increasing thiopentone metabolism ✓

Explanation

Phenytoin is a potent inducer of hepatic cytochrome P450 enzymes (particularly CYP2C9 and CYP3A4). Thiopentone undergoes hepatic redistribution and oxidative metabolism; chronic enzyme induction by phenytoin increases the rate of thiopentone breakdown, resulting in reduced plasma levels and shorter duration of action. A higher dose may therefore be required. Additionally, cross-tolerance between phenytoin and barbiturates at GABA receptors contributes to increased anaesthetic requirements.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.