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.

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

MAC-BAR
(1.5-2x)

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

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

MAC-awake
(0.33x)

·  Prevent eye opening to voice in 50%

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

MAC-amnesia
(0.25x)

·  Prevent recollection of noxious stimulus in 50%

Physiological

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

·   Pregnancy: ↓30% (progesterone)

·   Obesity: inflammatory cytokines

Pathological

·   ↓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)

Physiological

·   Age: max at 6 months

·   Red hair

Pathological

·   ↑Temp (>38°C)

·   ↑[Na⁺]

Drugs

·   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