MAC & Intraoperative Awareness

Minimum Alveolar Concentration = the alveolar concentration of a gas at 1 atm at steady state at which 50% of subjects do not move in response to surgical incision.

• MAC = ED50. ED95 is roughly 20% higher → 1.2 MAC prevents movement in 95% of patients.
• MAC is a population average, not a predictor of individual response.
• MAC values are additive (0.5 MAC sevoflurane + 0.5 MAC N₂O ≈ 1 MAC).
• MAC is inversely related to potency. Potency tracks oil:gas partition coefficient (Meyer-Overton), NOT blood:gas.
• Blood:gas coefficient determines speed of induction/emergence, NOT potency.

• MAC-awake — concentration that prevents response to verbal/tactile stimulation. Volatiles: ~0.34 MAC. N₂O: ~0.6 MAC.
• MAC-movement — 1.0 MAC (the classic definition).
• MAC-EI (Endotracheal Intubation) — concentration that blunts laryngeal response. ~1.3 MAC (ED95).
• MAC-BAR (Blunt Autonomic Response) — prevents adrenergic response to noxious stimulus. ~1.6 MAC. Opioids and N₂O reduce this requirement.

• Medications: opioids, benzodiazepines, barbiturates, propofol, ketamine, α2-agonists, local anesthetics, verapamil, chronic methamphetamine
• Acute ethanol intoxication
• Age: highest at 6 months; ↓ ~6% per decade after age 40
• Pregnancy (down ~30–40%)
• Hypothermia, hypoxia, hypercarbia
• Severe anemia (Hb < 5)
• Hyponatremia
• Sepsis

• Catecholamine reuptake inhibition: amphetamines, ephedrine, L-dopa, TCAs
• Chronic ethanol abuse (cross-tolerance)
• First months of life (peak MAC at 6 months)
• Hyperthermia, hypernatremia
• Genotype related to red hair (MC1R mutations)

• Estimated 1–2 per 1000 GA cases. Pediatric incidence up to 2.7% in kids > 6 yo (psychological sequelae less common).
• 2× more likely with neuromuscular blockade.
• Higher risk: chronic alcohol/opioid/meth/cocaine use, high-risk surgery (cardiac 1–1.5%, trauma 11–43%, C-section 0.4%).
• Most common sensation: hearing voices.
• Mostly occurs during induction or emergence.
• Dreaming ≠ awareness; not related to anesthetic depth.

Prevention: - Consider amnestic premedication (midazolam) in high-risk cases - Avoid or minimize NMBA when feasible - Use potent end-tidal monitoring; consider BIS/processed EEG in TIVA or high-risk patients - Premedicate with scopolamine in trauma / CV cases when anesthesia must be light

After an episode: acknowledge, apologize, document, refer for early psychological counseling (40–60% benefit).