MAC how Deep is your Depth?

Specialties CRNA

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What is the general understanding and use of the information that folks are getting about MAC these days?

What I mean is ......what type of education about patient specific adjustments are students being educated on about MAC?

i.e. For it has been established that Inhibitory actions on movement with GA occur on multiples planes - vol. mvt loss at lower concentrations and actual response abolishing at the dorsal and ventral root ... hence the supposed role of central control of somatic reflexes under GA is a biasness , and has remained the sine qua non for years. Since the understanding/acceptance of unconsciousness is a central mechanism than surely central motor depression also goes hand in hand. Right? - No....on the contrary, as per Rampil (2003) numerous attempts have been made to find a correlation with LOC and movement - ..none has been reliably identified. But we persist in using these correlates...Why?

Furthermore.....we persist in over overpressuring patients - many times to hemodynamic instability, in chasing a specific MAC number. It certainly cannot be in an attempt to gain/maintain unconsciousness .... esp. when amnesia has been repeatedly found to occur at significantly lower concentrations ( Eger - see Mawake studies) and on top of that Mawake is AGE adjusted - in which the most direct and simple log regression calculation being MAC40 AGENT * 10-.00269(age-40) - sorry can't write exponentials here but you know the calc (or should)...

So for many elderly - this equates to over anesthetizing - This cal can easily help any clinician to see an approximate level to achieve. But I see daily the misuse of non age adjusted MAC monitors - with ET PIA levels being titrated based on that .... in many case without regard to hemodynamics (again in disregards of the principles of Snow, Guedel, and Woodbridge) - requiring pressor drips to maintain that MAC number. In light of the possible mortality and depth issues (Weldon, Lenmarken, and Monks) this seems ridiculous. If low ET readings are the issue - bispectral analysis can assist in maintaining an appropriate level of amnesia. I am not a fan of BIS - but is does have its uses.

The use of AGE specific ET MAC monitors are in use in the UK but in the none are in use in the US and there is only one software that will actually calculate various Age adjusted MAC multiples (I think at http://www.gasshead.com or something like that). Strange for these concepts have been around for several years but still remain "fringe" to many CRNAs

In speaking with new grads and older practitioners - I have yet to find one that can speak in depth about agents that are used daily - at least locally- My posting is to see if this is actually the case in other institutions and what students are being educated on.

My concern is simple....... These are not esoteric concepts... if CRNAs are to call themselves experts -should they not be incredibly well versed on the most basic and commonly used agents at their disposal?

Hence my question.....So what exactly is being taught at the master level these days?

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I'm not sure what your question is and believe is seems to be a ranting more than an actual question or topic for discussion. What's the deal and why the aggression on a theoretical topic like MAC? You can use formulas all you want but MAC values only apply to 50% of the population.

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Thanks your your reply.

What numbers and research are you basing 50%? If you are using age old MAC numbers - do you know that they apply to an average age of 40 (meta analysis research - but you know that right?) - hence the spread..

Additionally - if you apply your 50% application ....if that is your feeling..... how do you clinically safely quantify depth in your paralyzed patients?

The question is actually clear - its not aggression - its frustration at the lack of understanding on some very base fundamental research/science that many either don't know or dismiss out of hand- hence cannot have a clear and concise discussion.

The question stands - thanks for your insight ..

OS

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Specializes in Anesthesia.
........ These are not esoteric concepts... if CRNAs are to call themselves experts -should they not be incredibly well versed on the most basic and commonly used agents at their disposal?........

I too find your rant rather bizarre, Oldsalt.

You are quite right, however -- anesthesia isn't rocket surgery: if the patient should move, that is probably an indication for increasing the inhaled agent concentration. Emperically, the answer to 'How much do you give?' is always 'You give ENOUGH.'

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I too find your rant rather bizarre, Oldsalt.

You are quite right, however -- anesthesia isn't rocket surgery: if the patient should move, that is probably an indication for increasing the inhaled agent concentration. Emperically, the answer to 'How much do you give?' is always 'You give ENOUGH.'

Let me apologize - since my original post was a bit overstated and over the top...

The question that I wanted to ask - is how much application of fundamental understanding should we demand of ourselves and our partners?

For the rest is superfluous if it doesn't matter - Right?

It boils down to the process of the individual's ability to find the "enough" level - as you rightly put.. for example - is the movement voluntary or involuntary (latter being part of the NRM - nociceptive reflex arch or mechanism) - what if your ET% of ISO is at 3% and this happens ? ???hmmm??? Do you then give more? This is rhetorical - yes - but you get the picture...

Are our actions based on feelings/generalizations or science or both - hence are we able to quantify and validate our actions?

It is the process in how we get to the final action is the core of my question....

How important are the published MAC numbers/calculations in your everyday practice and how do you adjust for age and temp variations?

I do thank you for your time and comments. And again apologize to all on this post if I seem more on the attack rather than wanting a discussion.

OS

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Specializes in Trauma/Surgical ICU, L&D.

Forgive my ignorance (starting CRNA school this fall) but, it seems that you are looking for evidence that CRNAs are using MAC (and the earlier posting re 50% of the population...I assume he/she was talking about the ED50 or effective dose in 50% of the population)...MAC related numbers/calculations which effects are definately variable in regards to age/temp/pulm functioning (and lots, I am sure, that I haven't a clue about yet). So....I think that getting a theoretical background, good training/mentoring and being intuitive enough to know and learn how much is enough.....should be enough...right??? Unless you are promoting more research re BIS monitoring???

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We all know that MAC is just a number. There are far too may influences on MAC. Therefore, I would answer your question this way. There is a tremendous amount of fundamental understanding involved in MAC anesthetic. We use the many different ingredients (narcotics, muscle relaxants, temp, age, etc) that makes up our anesthetic to determine our MAC level. We use vitals signs an dat times monitors like the BIS. Then it becomes a "feeling" if you wish. I get a feeling as to whether or not my pt will move on incision or not. But my feeling is based on fundamentals, solid fundamentals at that.

Is that what you are looking for?

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Forgive my ignorance (starting CRNA school this fall) but, it seems that you are looking for evidence that CRNAs are using MAC (and the earlier posting re 50% of the population...I assume he/she was talking about the ED50 or effective dose in 50% of the population)...MAC related numbers/calculations which effects are definately variable in regards to age/temp/pulm functioning (and lots, I am sure, that I haven't a clue about yet). So....I think that getting a theoretical background, good training/mentoring and being intuitive enough to know and learn how much is enough.....should be enough...right??? Unless you are promoting more research re BIS monitoring???

Thanks for your question.

MAC is defined as minimum alveolar concentration to achieve specific end points (Awake levels, Movement, BAR) - first described by Eger over 40 years ago - it was initially used as a point that defined movement but as more research was done - further applications on the range of various concentrations help identify specific levels in which the clinicans could utilize PIAs.

The common method is to used specific published ET % (MAC), which has specific application for an average age of around 40 (n20 104, D - 6, S- 2.05, I - 1.15, E - 1.68, H -0.74- note Enfl and Hal are not commonly used in the US anymore) -

Generally used to describe the possibility of movement in a patient - Using the number one as the constant - a MAC of 1 describes about 50% of the patients moving to a surgical stim - Furthermore, Surgical MAC is defined as 1.3 (Hence the above numbers would change increase by the above number X 1.3 - simple yes?) There are other "levels' that can apply to MAC

so... if you are shooting for MAC awake - where the constant is .34 - .37 for PIA but .55 for n20...it would be also applied to the above numbers...

Ok Hang in there...there is more...Variability applies to many things in medical science - but it is how to adjust for them is the key - hence there has been specific research to this end - starting in 1997...basic log regression calculations - as per my previous posts...

GA requires we always keep ET% monitored and appropriate - hence the use of MAC - but folks will decide what MAC numbers to use ..or not to use

Dont get me wrong - calculating a specific number will burn you - but understanding how to do it and when it can apply, how to use, and further adjust...- that is the key to understanding what one is doing..

It is not often in the clinical realm we can adjust be calculating at least a "reasonable" adjustment - hence the theoretical.....

My questions is how much are people actually doing these days in terms of the recent application of newer theoretical thought to the practical - are they using more recent research or not.

In the US we dont have monitors that do this for us - we only have one software for PALM (see previous post) that can do it...

That alls - I could go on - but it requires that you attend one of my classes ;)

Did that help?

Thanks for your post

OS

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We all know that MAC is just a number. There are far too may influences on MAC. Therefore, I would answer your question this way. There is a tremendous amount of fundamental understanding involved in MAC anesthetic. We use the many different ingredients (narcotics, muscle relaxants, temp, age, etc) that makes up our anesthetic to determine our MAC level. We use vitals signs an dat times monitors like the BIS. Then it becomes a "feeling" if you wish. I get a feeling as to whether or not my pt will move on incision or not. But my feeling is based on fundamentals, solid fundamentals at that.

Is that what you are looking for?

Thanks for your post.

Absolutely -

In terms of those fundamentals - how do you adjust for age variations, temp, and MAC multiples?

Example: Say youre doing a LE Vasc. Case (Fem-Pop - something to that nature) - on a 86 yo, 68KG, - good EF (>50%) cardiopulm checks out - HTN on Betas, Ok METs level >4, using a 50:50 o2/n20 - your ET of ISO is .3 , On Vecur. 1/4TOF, Soft BP w/ high pulse pressure variability (>13%) ranging low 80's systolic (represents of over 50% reduction from resting state), HR in low 90's, temp is 34.8. Gases come back with HB at 9.7, ICA - 3.76 (temp adjusted) Exchange is Ok, BE trending downward, Electrol K 3.1, MG 1.5 - UOP @ .3cc/kg/hr - (creat @ 1.4) , 4 hours now into the case with 3 more to go.....

Very common scenario right? 20 ways to Sunday in approaching this - Is this an appropriate level for this patient? - what's the min. MAC - What makes you nervous - or not? How would you personally proceed (skin this cat) - from a fundamental prospective?

No right answer - just exchanging knowledge....

thank for your attention

OS

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Oldsalt,

With experience, all your concerns will be addressed.

If you're looking for a "perfect formula" that will tell you what MAC should be used on a pt based on his age, ASA, history, etc, ... there is none.

That's why CRNA training is so intense and the pay is so high, b/c YOU are the one making the right decision about MAC. :monkeydance: It's a high wire act.

"It takes ability to reach the top, but it takes character to stay there."

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Oldsalt,

With experience, all your concerns will be addressed.

If you're looking for a "perfect formula" that will tell you what MAC should be used on a pt based on his age, ASA, history, etc, ... there is none.

That's why CRNA training is so intense and the pay is so high, b/c YOU are the one making the right decision about MAC. :monkeydance: It's a high wire act.

"It takes ability to reach the top, but it takes character to stay there."

Thank you for your comment.

No - no perfect formula - no such animal - but again - its foundational knowledge (the depth - no pun here intended) by which critical thinking can be based.

Training - is the key - how much "foundation" are we talking about - I have been an involved researcher and educator of both residents and CRNAs - and the concern is that the "intense training" (forget the monetary gains- that is superfluous in this discussion) for many CRNA's just scratches the surface (this coming from very involved discussions with new graduates and delicate discussions with my peers).......

MAC is just not smoke and mirrors - but understanding the nuances of the multiplicity of factors that have shown to have "measurable" impact - ....that is the core of the question.......How and what degree do people take that into account? Do they use tools available to them (ie software in my first post - or if you are overseas - do you use the AGE MAC calculations?)

For example....MAC and depth have direct and lasting impact even after the operation - What are your views on the studies by Monk, or Weldon, or Lennmarken? How does that change your view on how you manage your anesthetic?

Do me a favor - dissect the above scenario systematically (for it is a very common situation) - I'm just interested the depth of critical thinking out there...no correct answer - but actions are revealing ...yes?

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