Help with a basic concept

[Ender]

So I recently finished my intern year and started in anesthesia! It is fantastic! Now lets get to the point of this post... Sometimes I get hung up on basic concepts. I am trying to understand what a MAC of a volatile anesthetic really means. Here is where I am having trouble:

Every source I could find states that the MAC of a volatile anesthetic is mean alveolar concentration at which 50% of patients will fail to move to surgical stimuli. MAC of volatile anesthetics are as follows: Nitrous Oxide 104, Halothane 0.75, Enflurane 1.63, Isoflurane 1.17, Desflurane 6.6, and Sevoflurane 1.80. Then sources also state that a MAC of 1 prevents movement of 50% of patients during surgical stimulation and a MAC of 1.3 prevents movement of 95% of patients, but they do not specify which anesthetic this is (this should not be true for Desflurane -MAC 6.6). Am I to understand that MAC of 0.75 for Halothane actually equals a MAC of 1? That does not make much sense to me if that is true. Are there two definitions of MAC (one specific to the drug and one general usage for all drugs, i.e. the universal MAC of 1)?

Usually a concentration has units, does any one know what the units of MAC are?

I know that when you use two different volatile anesthetics (i.e. Nitrous oxide and Isoflurane) at once you reduce the MAC, but how exactly is this calculated?

Sorry if these questions are confusing, it reflects the difficulty I am having with the lack of a good explanation (maybe I have been thinking about this too long).

Thanks in advance for your help.

Ender

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[huktonfonix]

wow thats a lot of questions. to start with a few

1. The 6.6 for des, 0.75 for halothane, etc... equals the CONCENTRATION of each agent that is 1 MAC. Each agent has a different potency and therefore a different MAC. In other words it takes a concentration of 6.6 of desflurane to prevent movement while it only takes a concentration of 0.75 of halothane (halothane being much more potent). MAC is simply what the definition states, whatever concentration it takes to produce a desired effect (e.g. MAC to blunt adrenergic response, MAC awake, etc....). A MAC of 1.3 means that it will be the MAC of that particular agent multplied by 1.3. So MAC 1.3 of DES = 6.6 x 1.3 or 7.8 , MAC 1.3 of Halothane is 0.75 x 1.3 and so on. Think of it like drinking tequila (halothane) vs beer (desflurane) with the endpoint being to get drunk (MAC). Tequila is more potent (stronger alcohol content) than beer so it will take less tequila to mess you up. The end result (drunkeness or MAC) will be the same, but it will take less tequila to get you there.

2. I dont think MAC really has units. Concentration does.

3. MAC is additive. 60% Nitrous (0.6 MAC) + 1% Sevoflurane (approx 0.5 MAC) = 1.1 MAC


Hope this helps

 

[Trisomy13]

is there anything we CAN'T learn from beer and tequila?

 

[FuzzyBK]

Your definition of MAC is correct. By defintion, 1.0 MAC is the amount (end-tidal concentration) of an inhalational agent that keeps 50% of people from moving. The numbers you listed - Halothane 0.75%, Iso 1.2%, etc. - are the concentrations of expired gases that corresponds to 1.0 MAC for that gas. These numbers are derived from experiments.

If 100 people have end-tidal halothane concentrations of 0.75%, then theoretically only 50% will move when the scalpel comes down. Likewise if the end-tidal Iso concentration was 1.2% in those people, again only 50 of them would move when the hurting starts.

Where you state that a MAC of 0.75 for Halothane equals of MAC of 1 would actually mean - an end tidal concentration of 0.75% Halothane is the amount of agent that prevents movement in 50% of people, i.e. a MAC of 1.0. (The true 0.75 MAC of Halothane would be 0.75% x .75 = 0.56%).

The MAC number is unitless since it expresses a fraction (current gas concentration) / (Experiementally determined concentration inhibiting movement in 50% of people, aka the numbers in the charts in textbooks).

I hope this helps.

 

[dfk]

i think in more precise terms, MAC is defined as the response (or lack thereof) to PURPOSEFUL movement.

as for units, MAC unit is whatever you're using it for.

 

[Ender]

Your definition of MAC is correct. By defintion, 1.0 MAC is the amount (end-tidal concentration) of an inhalational agent that keeps 50% of people from moving. The numbers you listed - Halothane 0.75%, Iso 1.2%, etc. - are the concentrations of expired gases that corresponds to 1.0 MAC for that gas. These numbers are derived from experiments.

If 100 people have end-tidal halothane concentrations of 0.75%, then theoretically only 50% will move when the scalpel comes down. Likewise if the end-tidal Iso concentration was 1.2% in those people, again only 50 of them would move when the hurting starts.

Where you state that a MAC of 0.75 for Halothane equals of MAC of 1 would actually mean - an end tidal concentration of 0.75% Halothane is the amount of agent that prevents movement in 50% of people, i.e. a MAC of 1.0. (The true 0.75 MAC of Halothane would be 0.75% x .75 = 0.56%).

The MAC number is unitless since it expresses a fraction (current gas concentration) / (Experiementally determined concentration inhibiting movement in 50% of people, aka the numbers in the charts in textbooks).

I hope this helps.

Hukton and fuzzy,

Thanks for the explanations. That really clears things up. Now I can move to the next chapter ;-)

 

[fakin' the funk]

Related question:

So we always say that "opioids and benzos etc reduce MAC." But by how much? How much does a typical opioid regimen (e.g. 25-100mcg fentanyl boluses) knock down MAC? 0.01 ? 0.1? 0.5? Anyone seen any numbers on this?

 

[Planktonmd]

Related question:

So we always say that "opioids and benzos etc reduce MAC." But by how much? How much does a typical opioid regimen (e.g. 25-100mcg fentanyl boluses) knock down MAC? 0.01 ? 0.1? 0.5? Anyone seen any numbers on this?

No exact numbers.
Guys, always remember that the human physiology does not always work the same way in every patient and the best we can do is estimate how things will be in the average patient.
That applies to MAC and every other number they teach you, we are making educated estimates, that's all.

 

[jwk]

Think of MAC as an ED50. By definition, that's exactly what it is.

Then think - what else in medicine would we care about if it only worked half the time? Not much.

In the end, it's a marginally useful concept, except when you get to 1.4 MAC, which is closer to an ED90-95, which at that point is a much more useful number.

 

[cchoukal]

Think of MAC as an ED50. By definition, that's exactly what it is.

Then think - what else in medicine would we care about if it only worked half the time? Not much.

In the end, it's a marginally useful concept, except when you get to 1.4 MAC, which is closer to an ED90-95, which at that point is a much more useful number.

This is an excellent point (about the ED50, a concept many med students understand). I would also point out that MAC can be a very fuzzy topic. MAC for an abdominal incision is very different from MAC for other stimuli (as one of my attendings likes to say, "I mean, what's the MAC of your balls?"). It just means that "a MAC" of anesthetic is no guarantee of anything. I would add, also, that the MAC number is derived from very old studies with less precise measures of gas concentration utilizing Dixon's up-and-down method (not as sexy as it sounds!), which generally utilized very small sample sizes.

Is MAC completely useless? No, but as you progress in your training, you'll realize that there's a lot more to using anesthetics than MAC and you'll probably rely less on the concept of MAC as you move along.

Finally, let me add that the beer/tequila analogy, and getting drunk as an analogy for MAC, was sheer brilliance. I will definitely use that concept when teaching our new CA1s.

 

[dfk]

Think of MAC as an ED50. By definition, that's exactly what it is.

Then think - what else in medicine would we care about if it only worked half the time? Not much.

In the end, it's a marginally useful concept, except when you get to 1.4 MAC, which is closer to an ED90-95, which at that point is a much more useful number.

i thought it was 1.3 MAC = 95%.
yes, i know, that whole 0.1 is a whopper of a difference...