At what minimum alveolar concentration (MAC) does stage 2 (excitation or delirium phase) typically occur?

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Last updated: June 8, 2026 • View editorial policy

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Stage 2 of Anesthesia Occurs Below 1 MAC

Stage 2 (the excitation or delirium phase) typically occurs at sub-anesthetic concentrations, generally between 0.4-0.6 MAC, as the patient transitions from consciousness to surgical anesthesia.

Understanding the Stages and MAC Relationship

The classical stages of anesthesia describe a progression from awake to surgical anesthesia:

  • Stage 1 (Analgesia): From awake to loss of consciousness
  • Stage 2 (Excitation/Delirium): From loss of consciousness to onset of automatic breathing
  • Stage 3 (Surgical Anesthesia): Adequate depth for surgery (occurs at ~1.0 MAC)
  • Stage 4 (Overdose): Medullary depression

The Critical MAC-Awake Concept

Research demonstrates that MAC-awake (the concentration at which patients respond to verbal commands) ranges from 0.3-0.5 MAC depending on the agent 1, 2. For sevoflurane specifically, MAC-awake is approximately 0.35 MAC (0.59%) 1, while for xenon it is 0.46 MAC 1.

Stage 2 occurs in the narrow window between MAC-awake and deeper anesthetic levels, roughly 0.4-0.6 MAC. This is the dangerous zone where:

  • Patients have lost consciousness but lack adequate CNS depression
  • Disinhibition leads to involuntary movement, breath-holding, laryngospasm
  • Sympathetic hyperactivity causes tachycardia and hypertension

Clinical Implications and Pitfalls

The key to avoiding prolonged Stage 2 is rapid transition through this concentration range. Modern anesthetic practice minimizes time in Stage 2 by:

  • Using IV induction agents (propofol, etomidate) that bypass Stage 2 entirely
  • When using inhalational induction, rapidly increasing concentration to >1.0 MAC
  • Never maintaining patients at 0.4-0.6 MAC intentionally

Common pitfall: Inadequate anesthetic depth during maintenance (MAC <0.7) increases risk of awareness 3. Guidelines recommend maintaining end-tidal anesthetic concentration >0.7 age-adjusted MAC during maintenance to prevent awareness with recall 3.

Age Considerations

MAC values decrease with age—elderly patients (age 76) have approximately 44% lower MAC compared to young adults (age 25) 4. This means Stage 2 occurs at proportionally lower absolute concentrations in elderly patients, though still at similar MAC fractions (0.4-0.6 MAC).

References

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