Anaesthesiology
General Anaesthesia
General anaesthesia for MBBS and NEET-PG: propofol, ketamine, inhalational agents, MAC, muscle relaxants and reversal, rapid sequence induction and malignant hyperthermia, mapped to NMC codes AS4.
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General Anaesthesia
General anaesthesia for MBBS and NEET-PG: propofol, ketamine, inhalational agents, MAC, muscle relaxants and reversal, rapid sequence induction and malignant hyperthermia, mapped to NMC codes AS4.
This chapter covers the pharmacology and conduct of general anaesthesia, from intravenous and inhalational agents to muscle relaxants and their reversal. It explains minimum alveolar concentration, induction and maintenance techniques, rapid sequence induction and the recognition of malignant hyperthermia.
High-yield: General Anaesthesia
- The minimum alveolar concentration is the alveolar concentration of an inhaled agent that prevents movement in 50% of patients on skin incision, and it measures anaesthetic potency.
- A low minimum alveolar concentration means a more potent agent; the value falls with age, hypothermia, opioids and pregnancy.
- Propofol is the commonest intravenous induction agent, gives a smooth induction and rapid recovery, and can cause pain on injection and hypotension.
- Thiopentone is an ultra-short-acting barbiturate that is contraindicated in porphyria and can cause severe tissue injury if injected outside the vein.
- Ketamine gives dissociative anaesthesia with preserved airway reflexes and bronchodilation, but raises blood pressure and can cause emergence hallucinations.
- Etomidate is haemodynamically stable and useful in shock, but suppresses adrenocortical steroid synthesis.
- Sevoflurane is the agent of choice for inhalational (gas) induction, especially in children, because it is non-pungent and pleasant to breathe.
- Succinylcholine is a depolarising muscle relaxant with rapid onset and short duration, used for rapid sequence induction, and can cause hyperkalaemia and malignant hyperthermia.
- Non-depolarising relaxants such as vecuronium, rocuronium and atracurium are competitive antagonists at the neuromuscular junction and are reversed by neostigmine.
- Neostigmine reverses non-depolarising block but not a deep block, and is given with an antimuscarinic such as glycopyrrolate to prevent bradycardia.
- Sugammadex rapidly reverses rocuronium and vecuronium by encapsulation, even from deep block, where neostigmine is ineffective.
- Rapid sequence induction uses preoxygenation, a predetermined induction dose and succinylcholine, with cricoid pressure, to secure the airway quickly in the aspiration-prone patient.
- The stages of ether anaesthesia (Guedel) are analgesia, excitement, surgical anaesthesia and medullary paralysis, and remain a classic teaching framework.
- Malignant hyperthermia is triggered by volatile agents and succinylcholine, presents with a rising end-tidal carbon dioxide and rigidity, and is treated with dantrolene.
Key induction agents
- **Propofol:** Commonest agent. Smooth induction, rapid recovery, antiemetic. Causes pain on injection and hypotension.
- **Ketamine:** Dissociative, preserves airway reflexes, bronchodilates. Raises BP, emergence phenomena.
- **Etomidate:** Haemodynamically stable, useful in shock. Suppresses adrenal steroid synthesis.
- **Thiopentone:** Ultra-short barbiturate. Contraindicated in porphyria; tissue necrosis if extravasated.
NMC competencies in this chapter
- **AS4.1:** Pharmacology of intravenous induction agents
- **AS4.2:** Inhalational anaesthetic agents and minimum alveolar concentration
- **AS4.3:** Muscle relaxants and their reversal
- **AS4.4:** Airway anatomy and its anaesthetic implications
- **AS4.5:** Induction and maintenance of general anaesthesia
- **AS4.6:** Rapid sequence induction and airway management
- **AS4.7:** Complications of general anaesthesia
Frequently Asked Questions
What is minimum alveolar concentration?
It is the alveolar concentration of an inhaled agent that stops movement in half of patients at skin incision. A lower value means a more potent agent, and the value falls with age, opioids and pregnancy.
Why is succinylcholine used for rapid sequence induction?
It is a depolarising relaxant with the fastest onset and shortest duration, so it secures the airway quickly in a patient at risk of aspiration. Its side effects include hyperkalaemia and malignant hyperthermia.
How is non-depolarising block reversed?
Neostigmine reverses a partial non-depolarising block and is combined with an antimuscarinic such as glycopyrrolate. Sugammadex reverses rocuronium and vecuronium rapidly, even from deep block.
How is malignant hyperthermia recognised and treated?
It is triggered by volatile agents and succinylcholine, with a rising end-tidal carbon dioxide, tachycardia, rigidity and fever. Treatment is to stop the trigger, cool the patient and give dantrolene.
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