General anaesthetics are structurally diverse compounds that induce a reversible state of unconsciousness, amnesia, immobility, and analgesia. They are distinct from sedatives and do not act as analgesics.
Intravenous (IV) anaesthetics are preferred for inducing unconsciousness due to their rapid onset, reliability, and generally less painful administration compared with intramuscular or subcutaneous routes. Their clinical action is governed by rapid distribution to highly vascularized, lipophilic tissues such as the brain and spinal cord.
Widely Used IV Anaesthetic Agents
| Agent | Clinical Details & Mechanism |
|---|---|
| Propofol | A potent GABAA receptor agonist used for the induction and maintenance of unconsciousness. |
| Etomidate | Functions as a GABAA receptor agonist to induce sedation and unconsciousness. |
| Barbiturates (Methohexital, Thiopental) |
GABAA receptor agonists are utilized for the rapid induction of an anaesthetic state. |
| Benzodiazepines (Midazolam) |
GABAA receptor agonists are primarily used for their sedative properties in combination with other agents. |
| Ketamine | An NMDA receptor antagonist that induces “dissociative anaesthesia”. Often causes auditory/visual hallucinations and dissociates pain perception from noxious stimuli. |
Pharmacokinetic Principles
- Induction: Highly lipophilic molecules allow for rapid preferential distribution into the brain and spinal cord.
- Termination of Effect: Following a single bolus injection, termination of anaesthesia is determined solely by redistribution kinetics as the drug diffuses into muscle, viscera, and adipose tissue.
- Prolonged Infusion: Half-lives become “context-dependent”. When fat stores are saturated with the drug, redistribution is slowed, prolonging CNS effects and delaying the termination of anaesthesia.
Key Pharmacological Targets
| Target/Mechanism | High-Yield Agents |
|---|---|
| GABAA Receptor Agonists (Inhibitory) | Propofol, Etomidate, Benzodiazepines (e.g., Midazolam), Barbiturates (e.g., Thiopental), Isoflurane. |
| NMDA Receptor Antagonists | Ketamine (causes dissociative anaesthesia). |
| K2P Channel Activation | Halogenated inhalational agents (e.g., TASK-1, TASK-3, TREK-1). |
Stages of Anaesthesia (Guedel’s Signs)
- Stage I (Analgesia): Analgesia followed by amnesia and confusion.
- Stage II (Excitement): Delirium, confusion, vomiting, and irregular respiration.
- Stage III (Surgical): Regular breathing resumes; loss of eyelash reflex.
- Stage IV (Medullary Depression): Respiratory and circulatory failure; requires life support.
Physiological & Pharmacokinetic Pearls
- Respiration: Most inhalational agents reduce tidal volume and decrease respiration. Protective reflexes like gag/cough are dampened, necessitating endotracheal intubation.
- Thermoregulation: General anaesthesia lowers the threshold for cold-induced thermoregulation, increasing the risk of hypothermia.
- Elimination: Termination of effect after a single bolus is dependent on redistribution kinetics. Prolonged infusions have “context-dependent” half-lives due to accumulation in fat stores.
Clinical High-Yield
Inhalational agents like Desflurane, Isoflurane, and Sevoflurane are widely used today. Intravenous induction agents (Propofol, Etomidate, Barbiturates) are preferred for speed and reliability.
Ketamine is notably used for “field anaesthesia” in the UK for on-scene operations due to its dissociative properties. Always remember that the drug approach for IV GA is combinatorial, dependent on the patient’s needs and the specific procedure.
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