General anaesthesia: principles, effects and special patients
What general anaesthesia does to the brain and body, why patients get cold or sick afterwards, and how care changes for children and older people.
General anaesthesia is a reversible, drug-induced state with several components: unconsciousness, amnesia, immobility and blunted responses to pain. Different drugs produce these components by acting on different molecular targets and brain regions. The same drugs also lower blood pressure, depress breathing and disturb temperature control, which shapes care before, during and after surgery.
The anaesthetic state and its components
Clinically, anaesthesia combines unconsciousness, amnesia, immobility in response to surgery and control of the stress response. The concentration needed for each varies by drug: inhaled agents cause amnesia at lower concentrations than those needed to stop movement. That is why anaesthetists combine drugs (a hypnotic, an opioid, sometimes a neuromuscular blocker and regional anaesthesia) instead of relying on one agent at high dose.
Molecular mechanisms
- GABA-A receptors: propofol, etomidate, barbiturates and the halogenated gases bind allosteric sites and make the receptor more sensitive to GABA, increasing inhibition.
- Glycine receptors: inhaled agents enhance them in the spinal cord and brainstem.
- NMDA receptors: ketamine, nitrous oxide and xenon block them and have little effect on GABA-A.
- Two-pore domain potassium channels: activated by halogenated agents and by xenon and nitrous oxide.
- Alpha-2 adrenoceptors: dexmedetomidine sedates through this route, used for sedation rather than full anaesthesia.
Anatomical sites of action
Anaesthetics can act at the peripheral nerves, spinal cord, brainstem and cortex. A consistent finding is suppressed metabolism in the thalamus, a relay for arousal and sensory input. GABA-acting agents also boost sleep-promoting nuclei in the hypothalamus, and depression of the hippocampus probably explains amnesia. Effects on the spinal cord contribute to immobility.
Effects on circulation, breathing and temperature
Induction usually lowers arterial blood pressure through vasodilation and reduced cardiac function. Most agents depress breathing and airway reflexes, so the airway is supported and protected. Patients commonly become hypothermic during surgery; hypothermia increases perioperative complications, so warming is a major goal of care. Shivering after surgery is common and follows core cooling.
After anaesthesia
- Postoperative nausea and vomiting is still frequent; 5-HT3 antagonists such as ondansetron are effective in suppressing it.
- Delirium, temporary confusion and transient neurological signs can appear on emergence.
- Sore throat, drowsiness, dry mouth and feeling cold usually pass within hours.
- Malignant hyperthermia: in susceptible people (for example, with certain ryanodine receptor variants), agents such as sevoflurane can trigger a hypermetabolic muscle crisis with rapidly rising temperature; the trigger is stopped and intravenous dantrolene is given.
Special populations
- Older adults: a smaller initial volume of distribution and, for propofol, slower clearance mean lower doses; other sedatives call for further reduction.
- Children: young children may need relatively larger doses of some intravenous agents. US labels warn that repeated or prolonged anaesthetic or sedation drugs in children under three may harm the developing brain, with a window thought to begin in the third trimester of pregnancy; procedures that cannot be delayed still go ahead, and no agent has been shown to be safer.
- Obesity: body composition changes how lipid-soluble drugs distribute, so dosing and airway management are planned individually.
Test yourself
Think of the answer, then tap the question to see it.
Which general anaesthetics act mainly by blocking NMDA receptors rather than by enhancing GABA-A receptors?
Ketamine, nitrous oxide and xenon.
Why do patients often shiver after surgery?
Core body temperature commonly falls during surgery under anaesthesia, and shivering follows that core hypothermia.
Which brain region is thought to explain the amnesia produced by anaesthetics?
The hippocampus, whose neurotransmission is depressed by both intravenous and inhaled agents.
Why do older adults usually need smaller induction doses?
They have a smaller initial volume of distribution and, for drugs such as propofol, slower clearance.
Example drugs
Connections
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: General anesthesia: MedlinePlus Medical Encyclopedia (MedlinePlus (U.S. National Library of Medicine), 2026); DailyMed - SEVOFLURANE liquid (DailyMed (U.S. National Library of Medicine), 2026).
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Representative model, for educational purposes only.