End‑tidal Alveolar Concentration

End‑tidal alveolar concentration is the measured concentration of an inhaled anesthetic in the exhaled gas at the end of expiration, and it is used as a non‑invasive surrogate for the alveolar (and therefore brain) partial pressure of the anesthetic.

This matters because the alveolar partial pressure determines anesthetic depth, and end‑tidal values closely approximate it at steady state.
Europe PMC

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🧠 Why end‑tidal concentration reflects alveolar concentration

• At the end of expiration, the gas sampled comes from the alveoli, not the conducting airways.
• Because volatile anesthetics equilibrate such that:
  PA (alveolar) ≈ Pa (arterial) ≈ Pbr (brain) at steady state,
  the end‑tidal concentration is a practical clinical indicator of anesthetic effect.
  Part One

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🔗 Relationship to MAC

• MAC is defined using the end‑tidal alveolar concentration of an anesthetic that prevents movement in 50% of patients.
• End‑tidal monitoring therefore provides a real‑time measure of anesthetic potency and depth.
  droracle.ai

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📌 Clinical significance

• Guides titration of volatile anesthetics.
• Helps ensure adequate depth for immobility, hypnosis, and amnesia.
• Allows additive calculations when combining agents (e.g., N₂O + volatile).

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Would you like a deeper dive into:

• How end‑tidal concentration equilibrates with brain partial pressure
• How anesthetic uptake and distribution affect end‑tidal values
• How MAC is measured using end‑tidal concentration