Describe Ketamine in detail.

Pharmacology & ‘dissociative’ Anaesthesia

Ketamine is unique among the IV agents: it acts mainly by antagonising the NMDA (glutamate) receptor and produces ‘dissociative anaesthesia’ — a trance-like, cataleptic state with profound analgesia and amnesia in which the eyes may remain open. It can be given intravenously (1–2 mg/kg) or intramuscularly (useful where IV access is difficult, e.g. Children, field/disaster settings).

Distinctive Effects

Unlike other IV agents, ketamine is sympathomimetic — it tends to maintain or raise blood pressure and heart rate and to bronchodilate — and it relatively preserves airway reflexes and respiration. This makes it valuable in the shocked/hypovolaemic or asthmatic patient, in trauma and burns (dressings), and in paediatric and resource-poor settings. It provides potent analgesia even at sub-anaesthetic doses.

Adverse Effects

The main drawbacks are emergence phenomena — vivid dreams, hallucinations, delirium on waking (reduced by benzodiazepines and a quiet recovery) — increased secretions (an antisialagogue helps), and rises in intracranial and intra-ocular pressure and in heart rate/blood pressure, which limit its use in some patients. It can cause hypertension and tachycardia.

Practical Use & Combinations

Ketamine’s unusual profile makes it useful in situations that defeat other agents: it can be given intramuscularly when no vein is available, it provides analgesia and anaesthesia while largely maintaining ventilation and airway reflexes, and its bronchodilating, blood-pressure-supporting properties suit the asthmatic and the shocked trauma patient. In practice its unwanted effects are managed by co-administering a benzodiazepine to reduce emergence phenomena and an antisialagogue to control the increased secretions, and by recovering the patient in a calm, quiet environment; at low, sub-anaesthetic doses it is increasingly used purely for its analgesic and opioid-sparing effects.

Contraindications & the Balance of Effects

The decision to use ketamine is a balance between its valuable cardiovascular stability and analgesia and its unwanted rises in intracranial pressure, intra-ocular pressure, heart rate and blood pressure. It is therefore used cautiously or avoided in patients with a raised intracranial pressure, a penetrating eye injury, severe systemic or pulmonary hypertension, or significant ischaemic heart disease, in whom the sympathetic stimulation could be harmful, while it is positively indicated in the hypovolaemic, shocked, asthmatic or trauma patient in whom its blood-pressure-supporting, bronchodilating and airway-preserving effects are exactly what is wanted — a striking example of the same drug being contraindicated in one patient and ideal in another.

Preserves airway reflexes and cardiovascular stability — useful in shock.

• Ketamine: NMDA antagonist → ‘dissociative’ anaesthesia with profound analgesia & amnesia; IV or IM.
• Sympathomimetic: maintains BP/HR, bronchodilates, preserves airway/respiration.
• Ideal in shock/hypovolaemia, asthma, trauma/burns, paediatric & field settings.
• Adverse: emergence phenomena (cover with benzodiazepine), secretions, ↑ICP/↑IOP, tachycardia/hypertension.
• Cautious/avoid in raised ICP, penetrating eye injury, severe hypertension/IHD.