Stages of Anaesthesia (Guedel's Classification)

Stage I - Analgesia

• Begins from the onset of anaesthetic administration until loss of consciousness.
• The patient is conscious and can maintain a conversation.
• Early Stage I: analgesia without amnesia - the patient feels reduced pain but remains aware and can recall events.
• Late Stage I: both analgesia and amnesia are present.
• Reflexes remain intact; airway reflexes are active.
• Respiration and cardiovascular parameters are largely normal.
• Nitrous oxide used for dental procedures and obstetric analgesia produces this stage.

Stage II - Excitement (Delirium)

• Extends from loss of consciousness to the onset of regular, automatic breathing.
• The patient appears delirious, may vocalize (shout, sing, struggle), and may vomit.
• Despite the agitation, the patient is completely amnesic for this stage.
• Physiological signs:
  • Respiration is rapid and irregular
  • Heart rate and blood pressure increase
  • Breath-holding may occur
  • Laryngospasm is a serious risk - any stimulation (secretions, airway manipulation) can trigger it
  • Pupils dilate; eyelid reflex present
• This stage is dangerous and should be passed through as rapidly as possible.
• Clinical strategy: Use rapidly acting induction agents (propofol, thiopentone) or rapidly increase inhaled agent concentration to shorten or eliminate this stage. Avoid stimulation during Stage II.

Stage III - Surgical Anaesthesia

• Begins when breathing becomes regular and automatic, and extends to cessation of spontaneous respiration.
• The stage at which surgery can be safely performed.
• Divided into 4 planes based on eye movements, reflexes, and respiratory pattern:

• Most surgical procedures require Plane 2-3.
• Progression through the planes is tracked by the sequential loss of: eyelid reflex → conjunctival reflex → laryngeal reflex → corneal reflex → all reflexes.

Stage IV - Medullary Depression (Overdose)

• Represents severe, life-threatening CNS depression.
• Both the vasomotor centre (in the medulla) and the respiratory centre (in the brainstem) are suppressed.
• Features:
  • Complete cessation of spontaneous respiration (apnoea)
  • Cardiovascular collapse - profound hypotension
  • Maximally dilated, fixed, unreactive pupils
  • No reflexes
• Without immediate cardiorespiratory support, death rapidly ensues.
• This stage is an anaesthetic overdose - not a therapeutic target.

Summary Table

Clinical Notes

• Modern relevance: With current IV induction agents (propofol), patients transition almost directly from Stage I to Stage III without a prolonged Stage II. Guedel's signs are most relevant during inhalation induction (especially in paediatrics using sevoflurane).
• Monitoring depth: Modern practice uses EEG-derived indices (BIS - Bispectral Index) and clinical signs (movement, haemodynamic response) rather than relying solely on Guedel's stages.
• Laryngospasm risk peaks in Stage II and during light Stage III - the commonest airway emergency during inhalation induction. Ten centimetres of CPAP/PEEP usually resolves it.
• MAC (Minimum Alveolar Concentration): The concept of MAC has largely replaced Guedel staging for titrating volatile agents in modern anaesthesia.