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