Help me understand MAC...PLEASE!

[jihong]

CA-1 here, with a basic question about MAC that I JUST CAN'T SEEM TO WRAP MY HEAD AROUND!

I understand that MAC is "the concentration of the vapour (measured as a percentage at 1 atmosphere, i.e. the partial pressure) that prevents patient movement in response to a supramaximal stimulus (traditionally a set depth and width of skin incisions) in 50% of subjects".

When we refer to MAC as the definition above, we're talking about 1 MAC, correct? Using easy numbers, that's 2% for sevo and 6% for Des.

1) MAC awake/aware is .5MAC. Does that mean MAC awake/aware for Sevo would be 1% and Des would be 3% (roughly)?

2) MAC BAR is 2MAC, so MAC BAR for sevo is 4% and for des is 12%?

3) The concept that MAC is additive. .5MAC of sevo + .5MAC of Des = 1 MAC of any gas. I know you can't administer sevo and des at the same time due to some form of safety mechanism (if you know the name, please fill me in), but does that mean running 1% sevo and running 3% des is EQUAL to 2% sevo or 6% des or 105% nitrous?

4) How can the MAC of Nitrous Oxide be 105%? How can the partial pressure be greater than 100%?

Basic questions I know, I just haven't been able to find any sources that directly state this in a way that I can be sure of my answers. Is my general understanding as stated above correct??

THANKS!

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[criticalelement]

the mac of nitrous is 105% true. That should tell you something. Nitrous needs an adjunct. Using nitrous at 1 mac would make the patient hypoxic. use it 70% max.

When we or shall I say, when I speak of mac being additive I speak of intravenous adjuncts that lower the mac of the volatile agent. For example, if I was using Desflurane alone with oxygen for an anesthetic I would need close to 5-6 percent of the poison. But if I use versed, fentanyl, vecuronium; all those aforementioned add to the mac so to speak so you may need only 3 percent desflurane. Its balanced anesthesia brother.

Mac awake.. without looking it up there is no way that mac awake is .5 mac for any of the volatiles. I use .5 mac intraop as my anesthetic.
As you age MAC decreases. So in a septuagenarian 3 percent des is 1 mac.

.5 mac is just what it sounds. 1/2 mac. So yes 1/2 mac of des is roughly 3 %,

All these MAC figures go out the window if you are using a balanced technique.
I often see people overdosing folks with the poisons.

If you have a long case and gave 20 cc of fentanyl up front some vitamin v, nitrous. You may not need any inhalational agent at all..

 

[Ronin786]

Don't overthink it. 1 MAC is a reference number. It refers to a specific "dose" of anesthetic that prevents movement of 50% of patients in response to skin incision. It was created to easily compare different volatile anesthetics. 1 MAC = 2% sev0 = 6% des

The answer to your first three questions is yes. The reason for the 4th is precisely what you mentioned. You can't achieve 105% N2O, you can't even achieve >80% without killing the patient. As a result it's a theoretical MAC. The point of it is to explain that you can never achieve 1 MAC of N2O alone.

Only caveat is that MAC Awake is closer to 0.2 or 0.3. Which is 20% of 1 MAC of whatever gas you're using. They are completely additive. 1% Sevo with ~50% N2o = 1 MAC. Same if you had turned on some Sevoflurane then switched on Desflurane. 1% Sevo with 3% Des = 1 MAC.

Also, the interlock mechanism is what prevents you from using more than one vaporizer at once.

 

[jihong]

Thanks for the reply!

I understand that most of my questioning isn't fully applicable clinically, but I'm asking them in preparation for exams and questioning where they expect text-book answers (unfortunately). Most sources and texts state that mac-awake/aware is .3-.5MAC and MAC BAR is somewhere around 1.7-2MAC (this is without any other agents on board). But I was confused regarding the quantification of MAC, but you helped clarify that as well, thanks!

 

[jihong]

Don't overthink it. 1 MAC is a reference number. It refers to a specific "dose" of anesthetic that prevents movement of 50% of patients in response to skin incision. It was created to easily compare different volatile anesthetics. 1 MAC = 2% sev0 = 6% des

The answer to your first three questions is yes. The reason for the 4th is precisely what you mentioned. You can't achieve 105% N2O, you can't even achieve >80% without killing the patient. As a result it's a theoretical MAC. The point of it is to explain that you can never achieve 1 MAC of N2O alone.

Only caveat is that MAC Awake is closer to 0.2 or 0.3. Which is 20% of 1 MAC of whatever gas you're using. They are completely additive. 1% Sevo with ~50% N2o = 1 MAC. Same if you had turned on some Sevoflurane then switched on Desflurane. 1% Sevo with 3% Des = 1 MAC.

Thanks Ronin786, COMPLETELY clarifies this for me, especially regarding my question on N2O. Much appreciated!

 

[deansrv72]

the mac of nitrous is 105% true. That should tell you something. Nitrous needs an adjunct. Using nitrous at 1 mac would make the patient hypoxic. use it 70% max.

When we or shall I say, when I speak of mac being additive I speak of intravenous adjuncts that lower the mac of the volatile agent. For example, if I was using Desflurane alone with oxygen for an anesthetic I would need close to 5-6 percent of the poison. But if I use versed, fentanyl, vecuronium; all those aforementioned add to the mac so to speak so you may need only 3 percent desflurane. Its balanced anesthesia brother.

Mac awake.. without looking it up there is no way that mac awake is .5 mac for any of the volatiles. I use .5 mac intraop as my anesthetic.
As you age MAC decreases. So in a septuagenarian 3 percent des is 1 mac.

.5 mac is just what it sounds. 1/2 mac. So yes 1/2 mac of des is roughly 3 %,

All these MAC figures go out the window if you are using a balanced technique.
I often see people overdosing folks with the poisons.

If you have a long case and gave 20 cc of fentanyl up front some vitamin v, nitrous. You may not need any inhalational agent at all..

If we figure that a septuagenarian is anywhere from 70-80 yrs old, then age adjusted MAC would be in the range of 4.7-5% Des, using 6% as one MAC and 6% decrease per decade after age 40. Are you using a different formula or literature?

 

[caligas]

I thought you were going to ask the difference between MAC, conscious sedation, and sedation.

 

[nimbus]

I thought you were going to ask the difference between MAC, conscious sedation, and sedation.

I still don't understand this....lmao

 

[Mman]

my least favorite phrase is "room air general" as a description for a heavily sedated patient. It's not room air, they generally have a high concentration of oxygen applied to keep their sat up. It probably does qualify as general anesthesia, though. More accurate term would be "general anesthetic with uncontrolled airway".

 

[deleted162650]

my least favorite phrase is "room air general" as a description for a heavily sedated patient. It's not room air, they generally have a high concentration of oxygen applied to keep their sat up. It probably does qualify as general anesthesia, though. More accurate term would be "general anesthetic with uncontrolled airway".

Ah yes, the MACeral:

When done correctly can be quite nice, but when done poorly can be veeeery fishy.

 

[DocFocker]

my least favorite phrase is "room air general" as a description for a heavily sedated patient. It's not room air, they generally have a high concentration of oxygen applied to keep their sat up. It probably does qualify as general anesthesia, though. More accurate term would be "general anesthetic with uncontrolled airway".

I like term general with a native airway better because it doesn't imply that your not in control.

 

[deleted171991]

I like term general with a native airway better because it doesn't imply that your not in control.

General anesthesia implies a level of airway support, even if just head repositioning. Anesthesia is lack of feeling, general means in the entire body. One cannot have that without needing airway support.

 

[pgg]

General anesthesia implies a level of airway support, even if just head repositioning. Anesthesia is lack of feeling, general means in the entire body. One cannot have that without needing airway support.

As long as we're splitting hairs, that's not really true. ASA definitions are specific:

The only absolute is that the patient be unarousable to painful stimuli. Airway support is often needed, but not always. I often provide general anesthesia for brief painful cases (e.g. D&C) with nothing more than a ketofol infusion and supplemental oxygen via facemask with CO2 sampling. Most times I never touch the patient after the monitors or O2 go on.

 

[deleted171991]

As long as we're splitting hairs, that's not really true. ASA definitions are specific:

The only absolute is that the patient be unarousable to painful stimuli. Airway support is often needed, but not always. I often provide general anesthesia for brief painful cases (e.g. D&C) with nothing more than a ketofol infusion and supplemental oxygen via facemask with CO2 sampling. Most times I never touch the patient after the monitors or O2 go on.

I meant GA vs MAC, not levels of anesthesia. Deep sedation can qualify as GA, if airway support is involved (hence the debate whether propofol GI procedures are GA or MAC). Also, one can have response to painful stimuli even with general anesthesia (e.g "The patient is moving!").

It's a longstanding debate. There is not one single criteria that defines GA, although airway support usually implies it. But you are right, I should have said that GA usually requires airway support.

P.S. I do my D&C's like you, except with low-dose fentanyl instead of ketamine. And I call them GA.