FRCA Notes

Minimum alveolar concentration and potency

  • MAC is:

    • The minimum alveolar concentration of an anaesthetic vapour at equilibrium to prevent movement to a standardised surgical stimulus (midline laparotomy incision) in 50% of un-premedicated adult patients breathing 100% oxygen at sea level (1atm)

  • It is a measure of potency - drugs with a lower MAC are more potent

  • As most anaesthetic agents are given at 1atm, it is easy to think that it is their concentration that is important
  • However, it is the partial pressure of the agent that is the key measure - this just happens to approximate to concentration at sea level because 1atm = 101.3kPa

Agent MAC
Halothane 0.8%
Isoflurane 1.2%
Enflurane 1.7%
Sevoflurane 2%
Desflurane 6.6%
Xenon 71%
Nitrous oxide 105%
  • MAC is a proxy for suppression of spinal cord reflexes and therefore can't assume a lack of awareness
  • However, cases of awareness with a MAC >0.7 is rare

  • MAC is also additive
  • A mixture of agents with a cumulative MAC of 1.0 has the same effect as a single agent with a MAC of 1.0

  • The dose-response curve for MAC is very steep
    • At 0.8 MAC almost all patients move in response to a surgical stimulus
    • At 1.2 MAC <5% of patients move in response to a stimulus
    • This is very different to (say) propofol, where there is a much greater individual variation in effect-site concentration and clinical effect

MAC-awake

  • MAC-awake is the concentration of vapour in the lungs required to block voluntary reflexes and control perceptive awareness
  • At MAC-awake the eyes open on command during emergence from anaesthesia
  • 50% of patients respond to commands at MAC-awake
  • It is typically 0.3 x MAC-asleep
  • However, for sevoflurane it is 0.2 x MAC-asleep due to preferential blood flow to fat compartment and high fat solubility

MAC-BAR [block autonomic reflexes]

  • Is the concentration of vapour in the lungs required to block autonomic reflexes to noxious stimuli
  • Is 1.8 x MAC-asleep

  • MAC is typically increased by factors inducing hypermetabolic or sedative-tolerant states:
Factors increasing MAC
Decreasing age and infancy (peak value at 1 - 6months)
↑ temperature
↑ thyroid hormones
↑ Na+
↑ CO2
Catecholamine & sympathomimetic states
e.g. anxiety, acute amphetamine use, ephedrine
Chronic opioid and alcohol use
Ginger hair


  • MAC is reduced by factors reducing metabolism or those causing sedative effects:
Factors reducing MAC
↑ age (10% decrease for every 10yrs after 40yrs old)
Pre-term neonate
↓ in pregnancy by 40% due to the action of progesterone
Hypothermia
Hypothroidism
Hyponatraemia
Hypotension/hypovolaemia
Acute opioid and alcohol use
Sedatives including benzodiazepines, ɑ2-adrenoreceptor agonists
Other drugs: lithium, lidocaine, magnesium
Chronic amphetamine use


  • Height
  • Weight
  • Gender
  • Duration of anaesthesia