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Intravenous induction agents
Every induction agent produces anaesthesia; what differs is what it does to blood pressure, breathing and the rest of the body — so it is chosen for the patient: propofol for most, ketamine in shock and asthma, etomidate for a weak heart.
Why it matters in the exam
Exam questions present a particular patient — in haemorrhagic shock, with asthma, with porphyria, with tamponade or with severe nausea after previous anaesthetics — and ask which drug is suitable and which is forbidden. They also ask about signature side effects: adrenal suppression with etomidate, emergence reactions with ketamine, propofol infusion syndrome and necrosis after intra-arterial thiopental.
Most agents act on the GABA-A receptor (propofol, etomidate, barbiturates, benzodiazepines), whereas ketamine blocks the NMDA receptor and dexmedetomidine stimulates alpha-2 receptors. The effect starts within one arm-brain circulation time, and waking after a single dose comes mainly from redistribution from the brain to muscle and fat, not from metabolism.
In exam questions look for the word that decides the choice: "low blood pressure", "wheeze", "porphyria", "severe nausea in the past", "sepsis" or "prolonged intensive care". Once it is found, usually only one suitable drug remains. Remember too that all induction agents reduce vascular tone to some degree, so in a dehydrated or bleeding patient the dose is reduced and a vasopressor is prepared in advance. In a frail older patient the required dose is much smaller than in a healthy young adult, and the fall in blood pressure is sharper. Even after induction the question remains how to continue: with a halogenated agent, with a propofol infusion, or combined with a short-acting opioid, and each such choice affects the speed of waking and the nausea afterwards.
Key points
- Propofol — the usual choice; lowers blood pressure, antiemetic.
- Ketamine — shock, asthma, tamponade; preserves breathing and reflexes.
- Etomidate — cardiac stability at the price of adrenal suppression.
- Barbiturates are forbidden in porphyria.
- Waking from a single dose is due to redistribution.
Summary table
| Drug | Induction dose | Main advantage | Main drawback |
|---|---|---|---|
| Propofol | 1.5–2.5 mg/kg | Clear-headed waking, antiemetic, lowers ICP | Hypotension and apnoea, pain on injection |
| Ketamine | 1–2 mg/kg | Preserves blood pressure and breathing, bronchodilator, analgesic | Emergence reactions, secretions, raised heart rate and blood pressure |
| Etomidate | 0.2–0.3 mg/kg | Haemodynamic stability | Adrenal suppression (11-beta-hydroxylase), myoclonus, nausea |
| Thiopental | 3–5 mg/kg | Anticonvulsant, lowers ICP | Forbidden in porphyria; necrosis after intra-arterial injection |
| Midazolam | 0.1–0.3 mg/kg | Anxiolysis and amnesia | Slow onset; reversed by flumazenil |
A free exam question
איזו מהתרופות הבאות לא עוברת REVERSE על ידי FLUMAZENIL ? (the question as examined, in Hebrew)
- א. Chloral Hydrate
- ב. Methohexital
- ג. Cannabis
- ד. Carbamazepine
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Sources
- Miller's Anesthesia, 9th edition (2020) — Intravenous Anesthetics
- Morgan & Mikhail's Clinical Anesthesiology, 7th edition (2022) — Intravenous Anesthetics
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-02. The text may contain errors; when in doubt, check the source.