Components of general anaesthesia
General anaesthesia is a reversible, drug-induced state: the patient is unconscious, has no memory (amnesia), feels no pain (analgesia) and does not move in response to painful stimuli (immobility).
Remember
Five separate effects make up general anaesthesia: analgesia (absence of pain), amnesia or hypnosis (loss of consciousness and of recall), immobility (muscle relaxation), autonomic stability (blunting the heart rate, blood pressure and breathing responses to surgery) and antiemesis (preventing post-operative nausea and vomiting). No single drug provides all of these reliably.
Drugs produce this state in a controlled way, and it is reversible: the effects wear off and the patient wakes up. The defining feature is hypnosis, meaning reversible loss of consciousness, so the patient is asleep and unaware. The other effects are added to it. The classical triad of balanced anaesthesia is hypnosis, analgesia and muscle relaxation, and modern practice adds autonomic stability and antiemesis, which gives the five components.
- Analgesia is absence of pain. An opioid (a drug given for pain) such as fentanyl mainly supplies it.
- Immobility, or muscle relaxation, keeps the patient still. It gives the surgeon access and makes tracheal intubation (passing a breathing tube into the windpipe) easier. A neuromuscular blocking drug such as rocuronium, which relaxes the muscle, usually produces it.
- Autonomic stability blunts the responses of the autonomic nervous system (the involuntary system that controls heart rate, blood pressure and breathing) to surgery.
No single drug provides all of these effects reliably, so anaesthetists give a balanced combination: propofol brings unconsciousness, an opioid deals with pain, a muscle relaxant gives stillness and a volatile agent (an anaesthetic gas that is breathed in) such as sevoflurane maintains the anaesthetic. The relaxant is added to the other drugs and does not replace them.
Conscious sedation gives enough drug to allow a procedure without full loss of consciousness, and the aim is to preserve airway reflexes (the cough and gag that normally stop saliva or stomach contents entering the windpipe), although sedation can deepen unintentionally. On its own, a regional nerve block numbs one area while the patient stays awake. Sedation and regional anaesthesia are not general anaesthesia.
Key facts
- Defining feature
- hypnosis, meaning reversible loss of consciousness, with the other effects added to it.
- Classical set
- hypnosis, analgesia and muscle relaxation make up the classical set, and modern practice adds autonomic stability and antiemesis.
- Immobility
- gives the surgeon access and makes tracheal intubation easier, usually through a neuromuscular blocking drug such as rocuronium.
- Autonomic stability
- blunts the rise in heart rate and blood pressure, and the breathing response, provoked by surgery.
- Balanced technique
- no single drug provides all of these reliably, so different drugs are combined.
- Lighter or narrower techniques
- sedation leaves the patient responsive, and regional anaesthesia on its own leaves the patient awake with one area numb.
Exam traps
- Analgesia means absence of pain, so it does not stop skeletal muscle moving: that needs a neuromuscular blocking drug.
- Immobility or muscle relaxation does not damp the rise in heart rate and blood pressure, because relaxing skeletal muscle is a different job.
- Anxiolysis (relieving anxiety) is not one of the five components, because it is the aim of premedication, a preparatory step given before the anaesthetic starts.
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