2009A01 Explain the concept of MAC and its clinical utility.
List the patient factors which a) increase MAC b) decrease MAC c) are known to have no effect on MAC.



·      Definition and utility

·      Variants

·      ↑MAC

·      ↓MAC

·      ↔MAC


MAC definition and utility:

“The minimum alveolar concentration of inhaled anaesthetic agent at steady state and at one atmosphere
required to prevent purposeful movement in response to a standard surgical stimulus
in 50% of rested, fasted, healthy 40 year old adult men.”


Short definition

Minimum alveolar concentration of inhaled anaesthetic agent required to prevent movement in response to a painful stimulus in half the population.

Drug conditions

·  Atmospheric pressure

·  Steady state (e.g. 30 mins)

·  No pre-medication

Patient conditions

·  40 year old men

·  Healthy

·  Fasted

·  Rested

Stimulus criteria

·  1cm surgical incision of the forearm

Response criteria

·  Movement must be purposeful

What’s it good for?

·  Measure of anaesthetic depth

·  Correlates with very high likelihood of hypnosis and amnesia

·  Allows comparison of drug potency




·  Block adrenergic response (BAR) to surgical stimulus in 50%

·  Variability: iso 1.3x, des 1.3x, halo 1.45x, sevo 2.2-3.5x


·  Prevent eye opening to voice in 50%

·  Variability: iso 0.34, sevo 0.36, des 0.4, halothane 0.53x, N2O 0.64x


·  Prevent recollection of noxious stimulus in 50%


Factors ↓MAC:


·   Age highest age 6 months; ↓6% per decade after age 20

·   Pregnancy: ↓30% (progesterone)

·   Obesity: inflammatory cytokines


·   ↓mAP (<40mmHg)

·   ↓pO2 (<40mmHg)

·   ↑pCO2 (>60mmHg sedation, >80mmHg anaesthesia if acute)

·   ↓Temp

·   ↓pH

·   ↓[Na+]

Drug interactions

·   Acute depressants: propofol, barbiturates, benzodiazepines, opioids, local anaesthetics, Mg2+

·   Chronic stimulant: (e.g. amphetamines)


Factors ↑MAC:


·   Age: max at 6 months

·   Red hair


·   ↑Temp (>38°C)

·   ↑[Na+]


·   Acute stimulant

·   Chronic depressant



No effect

·   Sex

·   Body weight

·   Duration of anaesthetic

·   Prior exposure to anaesthetic

·   Physiological variables within normal limits e.g. mAP, pO2, pCO2

Unclear effect

·   Thyroid disease

·   Race



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