Definitions

Second Gas Effect

  • This is the phenomenon whereby the inhalation of a second gas, present at high concentration, will speed up the rate of rise in FA/Fi of the first gas, and is due to two mechanisms:
    • Concentration effect
    • Augmentation of ventilation

MAC

  • MAC is the minimum alveolar concentration of a volatile anaesthetic agent expressed as a percentage, at 1 atm ambient pressure, after 15 min delivery to allow equilibration of alveolar partial pressure with the brain effect site that will prevent gross, purposeful movement in 50% of young, healthy, unpremedicated human subjects or laboratory animals, in response to a standardised, supramaximal stimulus, such as a surgical skin incision or tail clamping

MAC95

  • MAC is the minimum alveolar concentration of a volatile anaesthetic agent expressed as a percentage, at 1 atm ambient pressure, after 15 min delivery to allow equilibration of alveolar partial pressure with the brain effect site that will prevent gross, purposeful movement in 95% of young, healthy, unpremedicated human subjects or laboratory animals, in response to a standardised, supramaximal stimulus, such as a surgical skin incision or tail clamping

MAC-BAR

  • MAC is the minimum alveolar concentration of a volatile anaesthetic agent expressed as a percentage, at 1 atm ambient pressure, after 15 min delivery to allow equilibration of alveolar partial pressure with the brain effect site that will block the adrenergic response in 50% of young, healthy, unpremedicated human subjects or laboratory animals, in response to a standardised, supramaximal stimulus, such as a surgical skin incision or tail clamping

MAC-awake

  • MAC is the minimum alveolar concentration of a volatile anaesthetic agent expressed as a percentage, at 1 atm ambient pressure, after 15 min delivery to allow equilibration of alveolar partial pressure with the brain effect site that will prevent eye opening in 50% of young, healthy, unpremedicated human subjects, in response to verbal command

MAC-hr

  • A measure of drug exposure, useful for standardising comparisons between volatile anaesthetics or evaluating cumulative effects such as toxicity from fluoride
  • 1 MAC-hr = 1 hour exposure at 1 MAC = 2 hour exposure at 0.5 MAC




Minimum Alveolar Concentration

Types of MAC

MAC

  • MAC is the minimum alveolar concentration of a volatile anaesthetic agent
    • expressed as a percentage,
    • at 1 atm ambient pressure,
    • after 15 min delivery, to allow equilibration of alveolar partial pressure with the brain effect site
  • that will prevent gross, purposeful movement in 50% of
    • young
    • healthy
    • unpremedicated
  • human subjects or laboratory animals, in response to a standardised, supramaximal stimulus, such as a surgical skin incision or tail clamping
  • In my view, the factors that are commonly cited to affect MAC, such as age, medications and pathological conditions are technically incorrect as they are excluded by the very definition of MAC - but don't be a smart ass in the exam! Just reproduce the list that is in all the textbooks

MAC95

  • The same as MAC, except preventing the endpoint in 95% of subjects, instead of 50%
  • MAC95 is approx 1.3 MAC - differs slightly for individual agents
  • MAC95 is the minimum alveolar concentration of a volatile anaesthetic agent
    • expressed as a percentage,
    • at 1 atm ambient pressure,
    • after 15 min delivery, to allow equilibration of alveolar partial pressure with the brain effect site
  • that will prevent gross, purposeful movement in 95% of
    • young
    • healthy
    • unpremedicated
  • human subjects or laboratory animals, in response to a standardised, supramaximal stimulus, such as a surgical skin incision or tail clamping

MAC-BAR

  • The same as MAC, except the endpoint is blocking the adrenergic response, instead of gross, purposeful movement
  • MAC-BAR is approx 1.7-2.0 MAC - differs slightly for individual agents
  • MAC-BAR is the minimum alveolar concentration of a volatile anaesthetic agent
    • expressed as a percentage,
    • at 1 atm ambient pressure,
    • after 15 min delivery, to allow equilibration of alveolar partial pressure with the brain effect site
  • that will block the adrenergic response, as measured by catecholamine concentration in venous blood, in 50% of
    • young
    • healthy
    • unpremedicated
  • human subjects or laboratory animals, in response to a standardised, supramaximal stimulus, such as a surgical skin incision or tail clamping
  • Initially proposed as thought that it was desirable to prevent the stress response

MAC-awake

  • The same as MAC, except the end point is preventing eye opening in response to verbal command
  • MAC-awake is approx 0.3-0.5 MAC - differs slightly for individual agents
  • MAC-awake is the minimum alveolar concentration of a volatile anaesthetic agent
    • expressed as a percentage,
    • at 1 atm ambient pressure,
    • after 15 min delivery, to allow equilibration of alveolar partial pressure with the brain effect site
  • that will prevent eye opening in 50% of
    • young
    • healthy
    • unpremedicated
  • human subjects, in response to verbal command

MAC-hour

  • A measure of drug exposure, useful for standardising comparisons between volatile anaesthetics or evaluating cumulative effects such as toxicity from fluoride
  • 1 MAC-hr = 1 hour exposure at 1 MAC = 2 hour exposure at 0.5 MAC




Comparison of Agents

Comparison of Agents

  • Click here to download a Word doc with a table comparing volatile agents
  • It includes a blank version
  • You should memorise the key points, then fill the details into the blank version once per day until you have it memorised
  • It's a grind, but it doesn't take that long to do