61 - 65 Pharmacology SBAs for the Primary FRCA
Question 61
You are deciding between ketamine and propofol for your RSI for the next case, when your wisened consultant suggests etomidate.
Which of the following is not a feature of an ideal induction agent?
- Stable in storage and transport at room temperature
- Minimal metabolism
- Mainly ionised at physiological pH
- Highly lipid soluble
- Highly water soluble
Answer
- Mainly ionised at physiological pH
A drug that is in its ionised state is by definition charged and will therefore struggle to cross fatty cell membranes, and thus will have a much slower onset of action.
A drug that is mainly unionised at physiological pH will have a faster onset of action.
This question is an old favourite as it tests your understanding of the flaws of the induction agents we have available to us.
You can break the properties down into separate categories for a more structured response:
Physicochemical
- Cheap and easy to produce
- Stable in storage and transport at room temperature
- Long shelf life
- Non-flammable, non-explosive
- Inert in standard syringes and giving sets
- Compatible with all other drugs encountered in anaesthetic practice
- Water soluble
Pharmacological
- Hypoallergenic and painless on injection - no histamine release
- Safe if given intra-arterially by accident
- Potent - High lipid solubility
- Rapid onset and offset - pKa that means mainly unionised at physiological pH
- Minimal cardiorespiratory depression
- Blunts hypertensive response to laryngoscopy
- Minimal metabolism, or rapid metabolism with minimal accumulation
- Anti-emetic and analgesic effect
- Smooth and comfortable emergence without hangover or excitation
- Predictable recovery