Mac concept and factors affecting mac

7 min read Last updated April 8, 2026

Core definitions and key numbers

  • MAC = alveolar concentration of an inhalational anaesthetic at 1 atmosphere that prevents movement in response to a standard surgical stimulus (classically skin incision) in 50% of subjects.
    • It is a population median (ED50) for immobility; not a measure of unconsciousness or analgesia.
    • Standard conditions: healthy adults, normothermia, no other CNS-active drugs, steady state between alveolar and brain partial pressure.
  • MAC is best thought of as a partial pressure (or end-tidal %) required to achieve a defined effect; comparisons between agents are valid because effect relates to partial pressure in CNS.
  • MAC-awake: end-tidal concentration at which 50% of patients respond to verbal command (loss of consciousness surrogate). Typically ~0.3–0.5 MAC (agent dependent).
  • MAC-BAR (block adrenergic response): concentration preventing autonomic response (HR/BP rise) to incision in 50%. Typically ~1.5 MAC (range ~1.3–1.7).
  • MAC95: concentration preventing movement in 95% of subjects. Roughly ~1.2–1.3 × MAC (assuming a typical population dose–response slope).
  • Additivity: MAC fractions of different volatile agents are approximately additive (e.g., 0.5 MAC sevo + 0.5 MAC des ≈ 1 MAC total).

How MAC is measured (exam-friendly outline)

  • Method: apply a standard noxious stimulus (e.g., skin incision) at steady end-tidal concentration; record movement/no movement; adjust concentration in subsequent subjects to bracket the 50% point.
  • Use end-tidal concentration as a surrogate for brain partial pressure (requires equilibration time; influenced by solubility and ventilation/perfusion).
  • Movement endpoint is mediated predominantly at the spinal cord rather than cortex.

Clinical use of MAC

  • Potency comparison between agents: lower MAC = more potent (higher lipid solubility broadly correlates with potency: Meyer–Overton).
  • Guiding dosing: typical maintenance for surgery often ~0.8–1.2 age-adjusted MAC with adjuncts; awareness risk relates more to hypnotic effect (MAC-awake) than immobility.
  • Interpretation caveat: MAC is a population metric; individual requirement varies; MAC does not directly quantify depth of anaesthesia, analgesia, or autonomic suppression.

Factors that decrease MAC (need less volatile)

  • Ageing: MAC decreases with increasing age (approximately 6% per decade after ~40 years; highest MAC in infants ~6 months).
    • Neonates: MAC lower than infants; peaks around 6 months then declines through childhood to adult values.
  • Hypothermia: reduces MAC (roughly 5% per °C decrease in core temperature).
  • Hypotension: MAP < ~50 mmHg reduces MAC (reduced CNS perfusion/anaesthetic delivery and altered neuronal function).
  • Hypoxaemia: significant hypoxia reduces MAC (classically PaO2 < ~5.3 kPa / 40 mmHg).
  • Hypercarbia: marked hypercapnia reduces MAC (mild changes may have little effect).
  • CNS depressant drugs: opioids, benzodiazepines, propofol, barbiturates, etomidate, alpha-2 agonists (clonidine/dexmedetomidine) all reduce MAC (often dose-dependent).
    • Opioids: large MAC reduction at low–moderate doses; ceiling effect at higher doses (immobility becomes less opioid-responsive).
    • Nitrous oxide: contributes MAC fraction additively (e.g., 50% N2O ≈ 0.5 MAC of N2O), reducing required volatile concentration.
  • Acute alcohol intoxication reduces MAC.
  • Pregnancy: reduces MAC (commonly quoted ~25–40% reduction, especially 1st/2nd trimester; multifactorial including progesterone/endorphins).
  • Severe anaemia reduces MAC (reduced oxygen content and altered CNS physiology).

Factors that increase MAC (need more volatile)

  • Young age: infants (peak around 6 months) have the highest MAC; children generally require higher MAC than adults.
  • Hyperthermia increases MAC (opposite of hypothermia).
  • Chronic alcohol use increases MAC (tolerance).
  • Chronic enzyme-inducing/centrally stimulating drugs may increase MAC (e.g., some patients on long-term anticonvulsants may have increased requirements).
  • Acute CNS stimulants: amphetamines/cocaine may increase MAC; effect can be variable in severe toxicity (where physiology may dominate).

Factors with minimal/no clinically important effect on MAC (commonly examined)

  • Sex: minimal effect.
  • Duration of anaesthesia: MAC is relatively constant over time (no significant tachyphylaxis for volatiles).
  • Thyroid function: hyper/hypothyroidism has little direct effect on MAC (though may alter haemodynamics and drug handling).
  • Mild–moderate hypoxaemia/hypercarbia: often little effect until severe derangement.

Mechanistic links worth stating in vivas

  • MAC reflects immobility to noxious stimulus: predominantly spinal cord effect; hypnotic endpoints (awareness/command response) correlate better with cortical effects and MAC-awake.
  • Meyer–Overton: potency correlates with lipid solubility (oil:gas partition coefficient), but exceptions exist (e.g., non-immobilisers) so it is not a complete mechanistic explanation.
  • MAC is agent-specific and varies with age; use age-adjusted MAC when comparing end-tidal targets across patients.
Define MAC and explain what endpoint it measures.

Give the formal definition, then clarify what MAC does and does not represent.

  • MAC is the alveolar concentration at 1 atm that prevents movement in response to a standard surgical stimulus in 50% of subjects.
  • It is an ED50 for immobility (spinal cord mediated) and is not a direct measure of hypnosis, analgesia, or autonomic blockade.
Why is end-tidal concentration used when discussing MAC? What assumptions are being made?

Link partial pressure to effect and mention equilibration.

  • Effect correlates with partial pressure in the CNS; end-tidal concentration approximates alveolar partial pressure.
  • Assumes near steady state: alveolar ↔ arterial ↔ brain partial pressures have equilibrated (time depends on solubility, ventilation, cardiac output).
Describe how MAC is determined experimentally.

Outline the bracketing method and the endpoint.

  • Maintain a set end-tidal concentration, apply a standard noxious stimulus (e.g., incision), observe movement/no movement.
  • Repeat across subjects (or up/down method) to find the concentration at which 50% do not move.
What is MAC-awake? How does it relate to MAC and clinical awareness?

Different endpoint (response to command) and typical fraction of MAC.

  • MAC-awake is the end-tidal concentration at which 50% of patients respond to verbal command.
  • Typically ~0.3–0.5 MAC; it relates more to hypnosis/awareness than immobility.
What is MAC-BAR? Give a typical value relative to MAC and explain its clinical relevance.

Autonomic response endpoint; higher than MAC.

  • MAC-BAR is the concentration preventing adrenergic (autonomic) response to incision in 50% of patients.
  • Typically ~1.5 MAC; explains why 1 MAC may allow tachycardia/hypertension unless analgesic/adjuncts are used.
List factors that decrease MAC and give a brief mechanism for two of them.

Prioritise common FRCA answers and add mechanisms succinctly.

  • Decrease MAC: increasing age, hypothermia, hypotension (MAP <~50), severe hypoxaemia, marked hypercarbia, pregnancy, acute alcohol, CNS depressants (opioids, benzodiazepines, propofol, barbiturates, alpha-2 agonists), nitrous oxide (MAC fraction additivity).
  • Mechanisms (examples): hypothermia reduces neuronal metabolic activity; opioids reduce nociceptive transmission and spinal reflexes (with ceiling effect).
List factors that increase MAC.

Include age-related increase in infants and chronic tolerance states.

  • Increase MAC: infants/children (peak around 6 months), hyperthermia, chronic alcohol use, acute CNS stimulants (e.g., amphetamines/cocaine), possible increased requirement with some chronic anticonvulsants.
Does sex affect MAC? Does duration of anaesthesia affect MAC?
  • Sex: minimal/no clinically important effect.
  • Duration: MAC is relatively constant over time (no significant tachyphylaxis for volatiles).
Explain the concept of MAC additivity and give a clinical example.

MAC fractions add approximately linearly for agents producing immobility.

  • MAC fractions are approximately additive: total MAC ≈ sum of individual MAC fractions.
  • Example: using 50% N2O (≈0.5 MAC of N2O) reduces the required sevoflurane/desflurane end-tidal concentration to achieve 1.0 MAC total.
How does age affect MAC across the lifespan (including paediatrics)?

This is commonly examined: neonate vs infant vs adult vs elderly.

  • MAC is lower in neonates than in infants; it peaks at around 6 months of age, then gradually declines through childhood to adult values, and continues to fall with ageing (≈6% per decade after ~40).
A patient becomes hypothermic (35°C) during surgery. What happens to MAC and what are the practical implications?
  • MAC decreases (roughly ~5% per °C). At 35°C (≈2°C below 37°C), MAC may be ~10% lower than expected.
  • Practical: if you do not reduce volatile, you may deliver a deeper-than-intended anaesthetic (hypotension, delayed emergence).
Why can a patient move at 0.8 MAC but still be unconscious? Relate your answer to sites of action.
  • Immobility (MAC) is predominantly a spinal cord effect; unconsciousness relates more to cortical/subcortical networks and is better reflected by MAC-awake.
  • Therefore, a concentration below 1.0 MAC may still exceed MAC-awake (unconscious) but be insufficient to suppress spinal motor response to incision (movement).

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