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Old 06-12-2011
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Anesthesiology Pharmacokinetics of Inhalation Anesthetics

Which of the following is correct concerning the rise in alveolar (FA) anesthetic concentration toward the inspired (FI) concentration:

Pharmacokinetics of Inhalation Anesthetics-fafi.jpg
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1. Drug A is more potent
2. Drug D has the smallest blood-gas partition coefficient
3. The median alveolar concentration (MAC) of drug B is greater than drug C
4. The alveolar-to-venous anesthetic gradient (partial pressure difference) of drug C is greater than drug D
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I'm really bad at anesthetic pharmacology so I reviewed the Lionel Raymon videos on the topic

I believe the answer is 3. MAC of B is greater than C

My interpretation of this graph is that the y axis is telling us how much of the anesthetic is remaining in the alveoli as time passes.

Using this interpretation, Drug A has the highest concentration in the alveoli, meaning that it is not diffusing into the blood to a great extent. Thus the Minimal alveolar concentration for Drug A is the greatest. On the other hand, Drug D has the lowest maximum value meaning that it is diffusing into the blood the fastest, giving it a low MAC.

MAC is a measure of effective dose. At the MAC concentration, the drug is 50% effective (like ED50) so the MAC is a measure of potency. The lower, the MAC, the more potent the drug is, thus a lower required dose.

Blood-Gas Ratio on the other hand is a measure of induction time. "Blood" represents amount of anesthetic bound in the blood while "Gas" measures amount of the anesthetic in tissues. Thus the lower the B/G, the faster induction time (and also faster recovery), and vice versa.

So back to the question:

1. Drug A is actually the least potent using the above logic
2. We cannot make any conclusions about the B/G ratio from this graph because it does not tell us anything about concentration in the blood vs in the tissues, we are only given alveolar concentration
4. Not sure about this answer choice
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  #3  
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Btw Bebix, I believe MAC stands for Minimal Alveolar Concentration, not Median
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Quote:
Originally Posted by apx85 View Post
Btw Bebix, I believe MAC stands for Minimal Alveolar Concentration, not Median
MAC = minimum alveolar concentration = median alveolar concentration = median effective dose of the anesthetic.... they are all the same (ref. Millerīs Anesthesia & Lippincottīs Pharmacology)


btw,
"2. We cannot make any conclusions about the B/G ratio from this graph because it does not tell us anything about concentration in the blood vs in the tissues, we are only given alveolar concentration"...you can (and #4 too)...
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Hmm. I may need you to drop a knowledge bomb on me bebix. Not sure how to interpret the graph I guess
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Quote:
Originally Posted by apx85 View Post
Hmm. I may need you to drop a knowledge bomb on me bebix. Not sure how to interpret the graph I guess
I think this is a tough qs...but I had one in UW (asking about blood-gas partition coefficient (solubility), potency...etc)...so I guess we should know this (or at least the concept...)
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here I go...

1. Drug A is more potent:
Fa/Fi almost 1 = lower blood-gas partition coefficient = less potent

2. Drug D has the smallest blood-gas partition coefficient:
Drug D = lower Fa/Fi = increase uptake from the alveoli to the blood = highest blood-gas partition coefficient = more potent

3. The median alveolar concentration (MAC) of drug B is greater than drug C :
MAC A > MAC B > MAC C > MAC D (since looking at the graph, blood-gas partition coefficient A < B < C < D) = TRUE


4. The alveolar-to-venous anesthetic gradient (partial pressure difference) of drug C is greater than drug D :
mmmm....greater AV diff = drug is not in the blood - is entering the tissues (e.g. fat) = blood return to the lung with lower partial pressure => Drug with the highest blood-gas partition coefficient should have the greatest AV gradient.
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Quote:
Originally Posted by bebix View Post
I think this is a tough qs...but I had one in UW (asking about blood-gas partition coefficient (solubility), potency...etc)...so I guess we should know this (or at least the concept...)
Do you have the question Id? I would like to look it up if you have it
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Quote:
Originally Posted by apx85 View Post
Do you have the question Id? I would like to look it up if you have it
give me a couple of min...
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Quote:
Originally Posted by apx85 View Post
Do you have the question Id? I would like to look it up if you have it

q.id 660

A new inhaled anesthetic has been developed...
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  #11  
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Correct Answer

btw, the correct answer is C.

here is the explanation:
Aneshthetics and the arteriovenous gradient!

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  #12  
Old 07-30-2011
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Post A

1. Drug A is more potent
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hmmm,i just saw this. and im thinking a and c could both be justified. any thoughts on this?
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Quote:
Originally Posted by bandar View Post
hmmm,i just saw this. and im thinking a and c could both be justified. any thoughts on this?
drug A has the higher FA/FI (almost 1)....this is equal to say that the blood-gas partition coefficient is very tiny (think of Desflurane or Nitrous oxide)...the MAC for these drugs are 7.3 and 104, respectively....Now, drug D has the lower FA/FI...this could be Halothane. The MAC of Halothane is 0.74.

MAC is inversely proportional to Potency

More information here
http://www.usmle-forums.com/usmle-st...-gradient.html


Last edited by bebix; 07-30-2011 at 06:34 AM. Reason: typo
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Quote:
Originally Posted by bebix View Post
drug A has the higher FA/FI (almost 1)....this is equal to say that the blood-gas partition coefficient is very tiny (think of Desflurane or Nitrous oxide)...the MAC for these drugs are 7.3 and 104, respectively....Now, drug D has the lower FA/FI...this could be Halothane. The MAC of Halothane is 0.74.

MAC is inversely proportional to Potency

More information here
http://www.usmle-forums.com/usmle-st...-gradient.html

sorry bebix they say page is nt available
by ruling out i cme to ans C i dnt knw hw v cn calculate MAC value on basis of these graphs also i ve issue in understanding the concept of gas tension tht is influenced by rate and depth of ventilation
hw cn v correlate the changes in PAO2 or PACO2 with the solubility of inhalation anesthesia??
i hope i cn get a lot of help
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Quote:
Originally Posted by qurat21 View Post
sorry bebix they say page is nt available
by ruling out i cme to ans C i dnt knw hw v cn calculate MAC value on basis of these graphs also i ve issue in understanding the concept of gas tension tht is influenced by rate and depth of ventilation
hw cn v correlate the changes in PAO2 or PACO2 with the solubility of inhalation anesthesia??
i hope i cn get a lot of help
please check post #11...the link is there
This has nothing to do with PAO2 and PACO2 and you donīt need to learn how to calculate the MAC value
This graph only shows the relationship between the [gas] in the alveoli versus the [gas] that was administered (inspired).
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Originally Posted by bebix View Post
please check post #11...the link is there
This has nothing to do with PAO2 and PACO2 and you donīt need to learn how to calculate the MAC value
This graph only shows the relationship between the [gas] in the alveoli versus the [gas] that was administered (inspired).
yes i knw it has nothing to do with it but i wnt to knw the concept of gas tension
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Originally Posted by qurat21 View Post
yes i knw it has nothing to do with it but i wnt to knw the concept of gas tension
ohhh....OK.

Curve A is almost identical to O2....(actually O2 has the highest FA/FI). Please read the other post...I think is well explained there.
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This concept always confuse me..so I am writing it here so that i can remember..Please correct me if i am wrong..

If we give for example 2ml of an anesthetic..If its blood gas concentration is high ,it means that it is soluble in blood more and its concentration in alveoli is low,so it means delayed onset of action and vice versa.
So it would be having High MAC value because a greater amount of the drug will be required to achieve the effect.

On the other hand,If a drug has Large AV gradient,it means that there is a difference in arteriolar(alveolar) and venous concentration..so,a large amount of drug will be required to saturate the blood with anesthetic,so that it can achieve the effect..so its action will be slow.e.g Halothane.
Regarding the concept of FA/FI..its the concentration in alveoli to the concentration in the inspired gas..
SO,IF THE CONCENTRATION IS HIGH in alveoli as compared to inhaled..it means that its blood conc is low as its high is alveoli..So ,the blood gas coefficient will be low..
Am i right???please correct me if i am wrong..

Regarding this question,graph A has approached the Fa/FI of 1..it means that its concentration will be the same in alveoli and inspired gas with small administration of the drug..so its Blood:gas ration is low,it means that alot of drug is required to achieve a concentration in blood..so it is less potent although it has rapid onset of action.
OPTION B..Graph D has the lowest FA/FI..i means that its concentration in alveoli is lower than that of the inspired gas ..so its Blood gas coefficient will be high as it will be having greater amount in blood as compare dto alveoli..
OPTION C..MAC value is the amount required to achieve the affect in 50%..so
the greater the drug is soluble in alveoli,the rapid the onset of anesthesia but a lot of amount is required to achieve the effect..so as Graph B has greater FA/FI value as compared to graph C,it means that it has greater MAC value as it is less soluble in blood..so its MAc value will be greater than Graph C..
OPION D..AV gradient means the amount in alveoli as compared to veins..So if a drug has higher AV gradient ,it means that a lot of drug is required to saturate the blood,,but we can see that drug C achieve greater effect with the same dose as compared to Drug D..so the AV gradient for Graph C is less than that of Graph D.

So,option C is the correct one..
SORRY FOR WRITING LENGTHY DETAILS..
Any feedback would be appreciated.
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[QUOTE=struggle;57233]This concept always confuse me..so I am writing it here so that i can remember..Please correct me if i am wrong..

If we give for example 2ml of an anesthetic..If its blood gas concentration is high ,it means that it is soluble in blood more and its concentration in alveoli is low,so it means delayed onset of action and vice versa.
So it would be having High MAC value because a greater amount of the drug will be required to achieve the effect.


HIGH coef. = High potency = low MAC (Halothane!) = "slow action"
LOW coef = Low potency = high MAC (N2O) = "fast action" (easy laugh!)



------------


So, first:
The product of three factors determines anesthetic uptake: solubility (λ), cardiac output (Q), and alveolar-to-venous partial pressure difference (PA - PV).

Uptake = [(λ) Ũ (Q) Ũ (PA − PV)]/Barometric pressure

1.- Solubility (λ) : here we have the blood-gas partition coefficient (i.e., blood solubility [λ]), which describes the relative affinity of an anesthetic for two phases and therefore the partitioning of that anesthetic between the two phases at equilibrium.

For example, Isoflurane has a blood-gas partition coefficient of 1.4, indicating that at equilibrium, isoflurane's concentration in blood is 1.4 times its concentration in the gas (alveolar) phase.

A larger blood-gas partition coefficient produces a greater uptake and hence a lower alveolar gas concentration/inspired gas concentration or FA/FI ratio.

2.- Cardiac output : greater pulmonary blood flow removes more anesthetic and thereby lowers the FA/FI ratio.

3.- (PA − PV) : the alveolar-to-venous anesthetic partial pressure difference results from tissue uptake of anesthetic. Were there no tissue uptake, the venous blood returning to the lungs would contain as much anesthetic as the arterial blood that left the lungs. The alveolar (which equals arterial)-to-venous partial pressure difference would be zero.

So:

If solubility is small (as with N2O, Desfrurane or Sevoflurane = less potency = MAC increases) = Fa/Fi aproach one = rapid onset of anesthesia.
If cardiac output approaches zero (as in profound myocardial depression) = Fa/Fi aproach one = rapid onset of anesthesia (and complications...hipotension....)
If large AV diff = small Fa/Fi = slow onset of anesthesia
If the alveolar-to-venous difference becomes almost 0 (as can occur after an extraordinarily long anesthetic), uptake would be minimal, and FA/FI would equal 1.0.
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@ struggle
"Regarding this question,graph A has approached the Fa/FI of 1..it means that its concentration will be the same in alveoli and inspired gas with small administration of the drug..so its Blood:gas ration is low,it means that alot of drug is required to achieve a concentration in blood..so it is less potent although it has rapid onset of action"
[B]

PERFECT!!!!

Last edited by bebix; 07-30-2011 at 11:19 AM.
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@bebix..thanks alott..
I always mess up this concept..

So,it means that if FA/Fi =1..or all concentration in alveoli..rapid induction,rapid recovery,high MAC,and Low B:G ratio..right??
But one thing i want to ask that will it be less potent or highly potent ?because as far as i understand it should be less potent..then how we require less medication to achieve the same affect (because as we give alot of N2O to achieve the effect,beacuse of high MAC and rapid recovery)..

So sorry for bothering..:sorry:
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Originally Posted by struggle View Post
@bebix..thanks alott..
I always mess up this concept..

So,it means that if FA/Fi =1..or all concentration in alveoli..rapid induction,rapid recovery,high MAC,and Low B:G ratio..right??
But one thing i want to ask that will it be less potent or highly potent ?because as far as i understand it should be less potent..then how we require less medication to achieve the same affect (because as we give alot of N2O to achieve the effect,beacuse of high MAC and rapid recovery)..

So sorry for bothering..:sorry:
You are absolutely right!

The potency is defined quantitatively using the MAC value.

More potent drug = less MAC to achieve the same effect at equilibrium (no how fast or slow)

(I edited a previous post, because I think was a little bit ambiguous)
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