Volatile & Amensia

[turnupthevapor]

Anyone know how much gas (ie .3 mac) it takes to cause reliable amnesia.....hows about N2O?

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[behindthadeuce]

MAC awake w/ volatiles is around .3

Don't think there is a reliable MAC with nitrous (alone) for amnesia

don't quote me on those 👍

 

[aphistis]

MAC awake w/ volatiles is around .3

Don't think there is a reliable MAC with nitrous (alone) for amnesia

don't quote me on those 👍

Most of the staff I've worked with have claimed .5-.6 MAC will give you reliable amnesia. Can't quote any studies, though.

 

[behindthadeuce]

sorry. I need to clarify. MAC awake for volatiles is .3 MAC... so for instance that would be .6 or .7% for sevoflurane.

 

[Jeff05]

remember that MAC is variable. it's the 50th percentile. Lange states that without any other anesthetics - one needs 0.8 MAC to reliably prevent recall.
Nitrous oxide by itself will not reliably prevent recall. When i run high remi with nitrous (above 70%) i still run a touch of propofol (25-40) - as nitrous narcotic technique does not reliably prevent recall.

 

[Noyac]

1 MAC is reliable however, not absolute.

Anything less, not reliable.

Like someone said, these levels are 50th percentile.

 

[huktonfonix]

1 MAC is reliable however, not absolute.

Anything less, not reliable.

Like someone said, these levels are 50th percentile.

some of the awareness studies showed a 0.7 MAC level to be fairly reliable at least compared to BIS monitoring. However, the quality of the study probably wasnt optimal.

 

[Noyac]

some of the awareness studies showed a 0.7 MAC level to be fairly reliable at least compared to BIS monitoring. However, the quality of the study probably wasnt optimal.

Yes, I've seen the studies you mention. All I'm saying is that if you want reliable amnesia then 1 MAC is probably it. With that being said I rarely go above 1 MAC. But then again I have many other meds working as we all do. So the OP's question has many variables. Are you talking just volatile or volatile along with other meds? As we know MAC is changed as we add to the mixture.

For example, on a young healthy adult having a painful procedure I wouldn't rely one .3-.7MAC to adequately prevent awareness with out something else onboard.

Of course, my previous response was somewhat tongue-in-cheek, but still true nonetheless.

 

[Gern Blansten]

I am not aware that volatile anesthetics disrupt the menstrual cycle.😛

 

[Planktonmd]

I am not aware that volatile anesthetics disrupt the menstrual cycle.😛

It's a well known phenomenon and was discovered by studying the effects of volatile agents on the menstrual cycle of yellow baboons.
This is why you never see a female yellow baboon working in an OR.

 

[turnupthevapor]

thanks all

 

[coprolalia]

Ask your patients later how much they actually remember when they appear to be awake, extubated, talking to you, following commands, moving over to the stretcher, wheeled down the hall, into the PACU, etc., etc.

You'll be surprised just how much true "amnesia" is still going on, despite them having rapidly-approaching-zero-MAC volatile anesthetic on-board.

High MAC (> 0.7) pretty much assures amnesia, but doesn't guarantee it. Likewise, low MAC doesn't automatically mean that they're going to remember either. Just can't be certain they won't.

-copro

 

[aphistis]

Ask your patients later how much they actually remember when they appear to be awake, extubated, talking to you, following commands, moving over to the stretcher, wheeled down the hall, into the PACU, etc., etc.

You'll be surprised just how much true "amnesia" is still going on, despite them having rapidly-approaching-zero-MAC volatile anesthetic on-board.

High MAC (> 0.7) pretty much assures amnesia, but doesn't guarantee it. Likewise, low MAC doesn't automatically mean that they're going to remember either. Just can't be certain they won't.

-copro

I've noticed that my patients seem highly amnestic right after emergence, but I've always wondered whether their recall increases as they come back to their senses post-op. Are there any studies out there looking at interval changes in perioperative recall?

 

[Salt Creep]

I know we're taught that MAC awake is 0.3 MAC, but I doubt how accurate it is. Have you had patients reliably wake up with 2% Des or even 0.5% Sevo? My patients seem to never wake up with anything greater than 0.3% Sevo, irrespective of anything else on board, which I know would lower their MAC.

Even the kids having an I&D of an abscess, who I don't give anything at all other than sevo via a mask, don't seem to wake up until less than 0.6% sevo - which in those cases is good, because they're protecting their airway but it gives you time to get the PACU before the screaming for mommy begins. 🙂

 

[RussianJoo]

I am only a 4th year student, but I read that 1 MAC is the 50th percentile and that a standard deviation is 0.1 MAC so at 1.2MAC you'll have 95% of patients with reliable anesthesia and at 1.3MAC you'll have 99% of the patiants covered. hope this helps.

 

[proman]

I am only a 4th year student, but I read that 1 MAC is the 50th percentile and that a standard deviation is 0.1 MAC so at 1.2MAC you'll have 95% of patients with reliable anesthesia and at 1.3MAC you'll have 99% of the patiants covered. hope this helps.

You're right but the concept is better thought of as the ED50, ED95, ED99 of a drug (assuming the definition of effective dose = immobility to clamping your rat's tail).

 

[RussianJoo]

You're right but the concept is better thought of as the ED50, ED95, ED99 of a drug (assuming the definition of effective dose = immobility to clamping your rat's tail).

thank you for that correction. So is this what the OP was looking for? or are there different MACs that people use in real life? i.e. evidence based medicine is different from "textbook" medicine?

 

[polkadotcap]

remember what MAC actually is- the concentration of volatile required to render a patient immobile to painful stimulus, or surgical incision (in 50% of the population as you mentioned)- the OP, I believe, was asking about concentration required to reliably ensure amnesia.

 

[MDEntropy]

I am only a 4th year student, but I read that 1 MAC is the 50th percentile and that a standard deviation is 0.1 MAC so at 1.2MAC you'll have 95% of patients with reliable anesthesia and at 1.3MAC you'll have 99% of the patiants covered. hope this helps.

MAC as a concept, by itself, doesn't involve recall, only movement to a painful stimulus.

A few subtle points...

I think the std for a young adult is 0.15 to 0.17 mac (as opposed to 0.1 mac).

Also, you only care about the upper end of the (assumed normal) distribution in this case...that is, the folks that will move...so with 1.3 mac, 2.5% may still move move with a painful stimulus. (1/2 of those that are outside the 95th percentile. The other 2.5 percent are on the other end of distribution curve and won't move even at .7 mac). So 97.5% won't move with a painful stimulus at 1.3 MAC.

To be sure, not all painful stimuli are created equally.

MAC assumes you are only using vapor which you almost never do, so even though most won't move at 1.3 mac, you rarely even need this much.

Amnesia is most important in paralyzed patients because MAC is consistently higher than MAC-aware (or the anesthetic level at which new memories can be formed). So patients who are not paralyzed will move long before they are at a low enough anesthetic level to have recall. This movement will give you a clue to make changes to your anesthetic. You will make these changes and recall will never have been an issue.

 

[RussianJoo]

thank you guys for the explinations.

 

[Pilot Doc]

I've noticed that my patients seem highly amnestic right after emergence, but I've always wondered whether their recall increases as they come back to their senses post-op. Are there any studies out there looking at interval changes in perioperative recall?

The recent BIS vs End Tidal Sevo study interviewed the study participants at 3 time periods, I think it was 1, 7 and 30 days. not sure about the first two, but the last was definitely 30 days. They picked up several people at the 30 day interview.

So to answer your question - yes - awareness or at least teh willingness to report it increases over time.

 

[proman]

I haven't come across anything on a "MAC-anmesia".

I've found MAC, MAC-awake, MAC-intubation and MAC-bar (beta adrenergic response). The B-Unaware (NEJM 3/13/08) study probably gives the best evidence for a MAC-aware, but I haven't found anything specifically testing awareness/amnesia and MAC.

 

[MDEntropy]

I haven't come across anything on a "MAC-anmesia".

I've found MAC, MAC-awake, MAC-intubation and MAC-bar (beta adrenergic response). The B-Unaware (NEJM 3/13/08) study probably gives the best evidence for a MAC-aware, but I haven't found anything specifically testing awareness/amnesia and MAC.

The term MAC "aware" may not be used, but the concept of the MAC needed for memory formation supression is certainly in Miller. I believe you can find more depth in Eger's 'Pharmacology of Inhaled Anesthetics'.

We really only use inhaled anesthetics for one reason...the amnestic effects. They help with other things but we have other drugs we use regularly that will do those other things more reliably.