Ketamine as an induction agent in shocked trauma patients
Ketamine is an induction agent (the drug that starts anaesthesia) that tends to raise heart rate and blood pressure, which suits trauma patients whose circulation is failing.
Remember
For a shocked trauma patient, ketamine is the induction agent to choose: it gives pain relief, tends to maintain breathing and the protective reflexes that keep the airway clear, and tends to raise rather than lower heart rate and blood pressure.
Memory aid
ketamine keeps the pulse and pressure up, propofol and thiopental let them fall, and etomidate leaves them roughly where they were
Use it to sort the four induction agents by what they do to heart rate and blood pressure.
Ketamine is an NMDA receptor antagonist (it blocks a receptor on nerve cells). It also has sympathomimetic effects, meaning it mimics the sympathetic nervous system, the body's fight or flight system. Ketamine tends to cause tachycardia (a fast heart rate) and hypertension, the opposite of most anaesthetics. Ketamine has a moderate depressant effect on heart muscle in laboratory studies, but in patients the sympathetic stimulation is usually the dominant effect.
Ketamine gives analgesia (pain relief) and amnesia (loss of memory of the event) without switching off the patient's own breathing. It maintains airway reflexes (the protective reflexes, such as coughing and swallowing, that keep the airway clear) and respiratory drive (the brain's automatic signal to keep breathing). It also produces dissociative anaesthesia, a state in which the patient appears awake but is disconnected.
This pattern is why ketamine is particularly useful in haemodynamically unstable trauma patients (those whose circulation is failing). Propofol, the standard induction agent, causes vasodilation and dose-dependent hypotension. Thiopental, an ultra-short-acting barbiturate (a sedative drug class), tends to cause hypotension. Etomidate leaves the circulation roughly unchanged, but it gives no pain relief and causes adrenal suppression (a fall in the stress hormone cortisol for up to 24 hours).
The classic downside is emergence phenomena: vivid dreams or hallucinations on waking, particularly in adults. A co-administered benzodiazepine (a type of sedative) can reduce them.
Key facts
- Mechanism
- ketamine is an NMDA receptor antagonist (it blocks a receptor on nerve cells).
- Circulation
- it is sympathomimetic, so tachycardia and hypertension tend to follow rather than hypotension.
- Airway and breathing
- airway reflexes and respiratory drive are maintained.
- Pain relief
- ketamine gives analgesia, which etomidate does not, and it also gives amnesia.
- Particular use
- it is particularly useful in haemodynamically unstable trauma patients.
- Downside
- emergence phenomena (vivid dreams or hallucinations on waking), particularly in adults, reduced by a co-administered benzodiazepine.
Exam traps
- Etomidate leaves the circulation roughly unchanged, which makes it a recognised choice in unstable patients, but it gives no pain relief and causes adrenal suppression (a fall in the stress hormone cortisol for up to 24 hours), so it misses part of what ketamine offers.
- Propofol is the standard induction agent, but it causes vasodilation and dose-dependent hypotension, so in a shocked trauma patient it would lower an already low blood pressure further.
- Barbiturates tend to cause hypotension, and thiopental carries a risk of laryngospasm (sudden closure of the airway by the voice-box muscles), so it is a poor choice when blood pressure is low.
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