General Anaesthesia
Background Information
Types of Anaesthesia
There are 3 main types of anaesthesia with varying definitions and features:
| Type | Definition | Key Features |
|---|---|---|
| General anaesthesia (GA) | A reversible, controlled state of unconsciousness affecting the entire body | Complete unconsciousness, loss of reflexes, requires airway management. |
| Regional anaesthesia | Involves blocking sensation in a large region of the body by targeting specific nerves or the spinal cord area. | Patient is usually awake or lightly sedated, with loss of sensation +/- motor function in the targeted area only. |
| Local anaesthesia (LA) | Involves blocking sensation in a small, specific area of tissue by direct injection or topical application. | Patient is fully awake, used for minor procedures. |
GA vs sedation:
- GA induces a total loss of consciousness, requiring airway and breathing support
- Sedation involves a depressed level of consciousness, where the patient breathes spontaneously. Sedation is commonly used for endoscopies.
To put it in context, if a patient is under sedation, and a strong stimulus like a sternal rub is applied, the patient will wake up or at least respond. In contrast, patients under GA are in a deeper, unarousable state of unconsciousness. A strong stimulus like a sternal rub typically will NOT wake the patient up or produce a purposeful response.
Inducing General Anaesthesia (GA)
4 essential components are required to achieve safe and effective GA:
- Unconsciousness – loss of awareness and unresponsiveness
- Amnesia – loss of memory to prevent recall of the surgical event
- Analgesia – to ensure the patient does not feel surgical pain
- Paralysis – suppression of voluntary movement
Building on the above, the following are administered to induce GA:
- Anaesthetic (hypnotic) agent (IV / inhaled) – to induce unconsciousness + amnesia +/- analgesia (ketamine is the only agent with intrinsic analgesic property)
- Analgesia (apart from ketamine, most anaesthetic agents do NOT provide analgesia; additional analgesics should be given)
- Paralytic agent – to induce paralysis (required for intubation and certain surgeries that require the patient to be relaxed / stay still- e.g. abdominal surgeries, neurosurgery)
The choice of agents varies a lot, depending on the indication, the surgery, patient factors, and local availability.
The most common GA induction regimen for most surgeries is: IV propofol (the anaesthetic agent) + IV fentanyl (the analgesic agent) + IV rocuronium (the paralytic agent)
Inhaled anaesthetic agents are more commonly used in the paediatric population to avoid cannulation while the patient is awake.
Maintaining General Anaesthesia (GA)
Definition: the process of sustaining an adequate level of anaesthesia throughout the procedure
Most commonly used agents of maintenance:
- Inhaled anaesthetics (e.g. sevoflurane, desflurane, isoflurane), or
- IV propofol infusion
Paralytics (Neuromuscular Blocking Agents)
Paralytic Agents
It is important to be aware that paralytics provide muscle relaxation but have NO hypnotic, analgesic, or amnestic properties.
Do not only give a paralytic agent for induction:
- The patient is still fully conscious, aware, and able to feel pain, but unable to move or communicate
- This situation would be traumatic because the patient would experience pain and remember the procedure despite being “frozen” physically
There are 2 main classes of paralytic agents with varying MoA:
| Class | MoA | Drug example | Other notes |
|---|---|---|---|
| Depolarising agent | Drug action: acetylcholine receptor agonist
Following administration of suxamethonium, visible muscle fasciculations occur as a result of the initial depolarisation phase, prior to the onset of paralysis. |
Suxamethonium (succinylcholine) | Suxamethonium has a fast onset (<1 min) and is short-acting (lasts ~10 min), therefore favoured in rapid sequence induction
Due to the depolarising mechanism, suxamethonium carries a risk of hyperkalaemia |
| Non-depolarising agent | Drug action: acetylcholine receptor antagonist
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Rocuronium, vecuronium | Non-depolarising agents have a slower onset but are long-acting, therefore preferred to maintain paralysis in surgery (e.g. neurosurgery) |
Non-depolarising agents are usually preferred over depolarising agents due to a rare but serious complication associated with suxamethonium: suxamethonium apnoea
- Inheritance: autosomal recessive
- Cause: mutation in BCHE (butyrylcholinesterase) gene → deficiency of pseudocholinesterase (the enzyme that metabolises suxamethonium → prolonged effect of suxamethonium)
- Presentation: prolonged inability to breathe spontaneously after extubation (normally, suxamethonium’s effect wears off after 5-10 min)
- Management: continue airway and ventilatory support until the body clears the drug
This is why it’s important to inquire about the personal and family history regarding issues with anaesthetics.
Neostigmine is a reversible acetylcholinesterase inhibitor; it is commonly used to reverse the effects of non-depolarising agents at the end of surgery.
Anaesthetic Agents
Intravenous (IV) Anaesthetic Agents
| Drug | MoA | Main indication | Important adverse effects / complications | Additional notes |
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| Propofol |
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| Etomidate |
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| Ketamine |
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| Thiopental |
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Signs of awareness under paralysis during surgery are challenging to detect because the patient cannot move or provide typical behavioural cues.
However, intraoperative awareness under paralysis can sometimes be indicated by:
- Physiological signs: Sudden increases in heart rate (often the most useful sign) or blood pressure
- Tearing or sweating: autonomic nervous system responses may still occur.
- Eye movements or eyelid fluttering: rare but possible even under muscle relaxation.
Volatile Inhalational Anaesthetic Agents
There are 2 main classes:
| Class | MoA | Examples | Important adverse effects / complications |
|---|---|---|---|
| Halogenated hydrocarbons |
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Sevoflurane is generally considered the most side-effect neutral and safest overall |
| Nitrous oxide |
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Halothane is rarely used today primarily because of its risk of serious adverse effects, especially:
- Hepatotoxicity
- Arrhythmias
- Malignant hyperthermia
Malignant hyperthermia is a rare but important complication associated with all halogenated hydrocarbons, and suxamethonium (to a lesser extent):
- Inheritance: autosomal dominant
- Cause: mutation in the RYR1 gene → hyperactive calcium channel in the sarcoplasmic reticulum → excessive and uncontrolled release of calcium into the muscle cytoplasm → sustained muscle contraction and hypermetabolism
- Typically presents during or shortly after administration of the triggering agent:
- Early signs: ↑ ETCO2, tachycardia, tachypnoea, muscle rigidity (masseter spasm)
- Late signs: hyperthermia, metabolic acidosis, arrhythmia, rhabdomyolysis
- Management:
- Immediate discontinuation of the triggering agent
- Dantrolene sodium – the only antidote (reduces calcium release from the sarcoplasmic reticulum)
- Supportive measures (e.g. hyperventilation with 100% oxygen, active cooling, correcting acidosis and electrolyte disturbances)