Fa/Fi

[waterbottle10]

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Does anyone remember why the higher Fa is the faster the onset? And the lower the solubility the higher the induction onset? Most of the places I've looked just tells us to remember it but I actually like to understand the reason!

 

[Iso4ane]

Does anyone remember why the higher Fa is the faster the onset? And the lower the solubility the higher the induction onset? Most of the places I've looked just tells us to remember it but I actually like to understand the reason!

What fills up faster? A big bathtub or a small bathtub? Lower solubility = Smaller bathtub (less volatile can be dissolved). Fa/Fi curves are just a reflection of the filling times.

 

[vector2]

The solubility concept is counterintuitive because you'd think that the more agent dissolved in blood, the faster the induction would be. The key idea to remember is that you're interested in how quickly the partial pressure of volatile can equilibrate so that it reaches a level of brain effect (the Pb) quickly. The less an agent dissolves in blood, the more it's available to actually bind to the brain and spinal cord and cause an anesthetic effect.

 

[sigrhoillusion]

The solubility concept is counterintuitive because you'd think that the more agent dissolved in blood, the faster the induction would be. The key idea to remember is that you're interested in how quickly the partial pressure of volatile can equilibrate so that it reaches a level of brain effect (the Pb) quickly. The less an agent dissolves in blood, the more it's available to actually bind to the brain and spinal cord and cause an anesthetic effect.

That's the way I think of it. More gas to brain.

 

[Iso4ane]

Here's another way to think about it, think table salt vs chalk (calcium carbonate). Both are soluble in water, but one is much less so. So it takes less chalk (less soluble material) to fully saturate the water. Same way with gas. Iso is more soluble so it takes a lot of gas to saturate it; Des is less soluble so less gas is required.

 

[waterbottle10]

I get these ways of thinking about it to help make sense of it but does anyone actually know what happens? It feels like going in circles. Iso is more potent than des, but onset and offset is much slower. I'm guessing inhaled induction with 8% Iso is slower than 8% sevo...
If it is not dissolved in the blood, it exists as a gas in the blood and I guess that makes it easier to cross over to the brain?

 

[Iso4ane]

I get these ways of thinking about it to help make sense of it but does anyone actually know what happens? It feels like going in circles. Iso is more potent than des, but onset and offset is much slower. I'm guessing inhaled induction with 8% Iso is slower than 8% sevo...
If it is not dissolved in the blood, it exists as a gas in the blood and I guess that makes it easier to cross over to the brain?

Potency and Fa/Fi are not related. Thats like comparing two cars by there top speed (fa/fi) and trying to correlate to their MPG (potency) ratings.

The gases are dissolved in blood. That's the definition of blood:gas and oil:water solubility constants. How much is dissolved in one thing vs the other so the partials pressures are equal.

 

[vector2]

I get these ways of thinking about it to help make sense of it but does anyone actually know what happens? It feels like going in circles. Iso is more potent than des, but onset and offset is much slower. I'm guessing inhaled induction with 8% Iso is slower than 8% sevo...
If it is not dissolved in the blood, it exists as a gas in the blood and I guess that makes it easier to cross over to the brain?

I think you're getting hung up because you're thinking that volatile anesthetics follow the kinetics and dynamics of an IV induction agent. They do not, and your last statement is correct. Volatile has to enter the blood as a route to get to the brain, but that is not equivalent to saying you want it fully dissolved in the blood. If that were the case, a bunch of volatile would enter the brain in the blood and then pass right out the other side without any lipid bilayer or receptor interaction. Based on solubility, volatile will exist in the blood with some fraction dissolved and some fraction in gas form at a particular partial pressure. The faster the rise in partial pressure (the less soluble), the faster you see brain effect.

 

[drmwvr]

The faster the rise in partial pressure (the less soluble), the faster you see brain effect.

this.

OP, it's just a matter of partial pressure gradients. The term "Minimum Alveolar Concentration", in my opinion, should be abandoned for "minimum brain partial pressure".

The less soluble the agent, the faster a more steep pp gradient from alveoli to blood to brain exists and the faster the onset of desired effect.

 

[Iso4ane]

I get these ways of thinking about it to help make sense of it but does anyone actually know what happens? It feels like going in circles. Iso is more potent than des, but onset and offset is much slower. I'm guessing inhaled induction with 8% Iso is slower than 8% sevo...
If it is not dissolved in the blood, it exists as a gas in the blood and I guess that makes it easier to cross over to the brain?

How do you think soda works? Or decompression sickness? Gases are dissolved in the blood and they have to be dissolved in the other tissues as well. As the target tissues reaches the proper concentration you get the effect you want.

I feel like there is a fundamental misunderstanding somewhere that is limiting how you ask you question.

Try asking the question a different way. And again to stress Fa/Fi has no influence on potency.

And back to 8%iso vs 8%sevo, two things first iso vaporizers don’t go to 8% (as far as I am aware) and two that is not a good comparison as 8% iso is ~7-8MAC while 8% Sevo is only ~3-4MAC and it’d be hard to know which one would lead to faster induction.

If you are thinking about inhalation induction, sevo is used because at equipotent levels it is faster, but also not pungent (hence why we don’t use des).

 

[deleted162650]

Why do we keep entertaining this guy. He’s not a doc.

 

[Psai]

It is really funny that this guy pretends to be "almost an attending". The sheer lack of understanding of basic pharmacologic principles shows that there's no way he went to medical school.

 

[coffeebythelake]

Why do we keep entertaining this guy. He’s not a doc.

Maybe sRNa or cRNa

 

[pgg]

Why do we keep entertaining this guy. He’s not a doc.

Maybe he is, maybe he's not.

Regardless, it's a useful topic of discussion for med students and junior residents. And all of us have the odd weird hole in knowledge. I don't know if it's a good path for us to get in the habit of giving people **** for asking questions.

 

[deleted162650]

Maybe he is, maybe he's not.

Regardless, it's a useful topic of discussion for med students and junior residents. And all of us have the odd weird hole in knowledge. I don't know if it's a good path for us to get in the habit of giving people **** for asking questions.

Fair enough.

 

[anbuitachi]

Maybe he is, maybe he's not.

Regardless, it's a useful topic of discussion for med students and junior residents. And all of us have the odd weird hole in knowledge. I don't know if it's a good path for us to get in the habit of giving people **** for asking questions.

Nah we don't have gaps in knowledge on sdn

 

[Twiggidy]

I’ll give the benefit of the doubt that maybe @waterbottle10 is a CA3 and quite honestly it took me a minute to get this concept down.

First learn this curve and commit it to memory.

Then understand this chart and notice the correlation.

FA/FI is close to 1, which correlates to a low blood:gas coefficient, which means faster induction agent. So let's answer the question of why Sevo is a better (faster) induction agent than Iso. (A real world example)

Sevo has a blood: gas coefficient of 0.65 vs Iso of 1.4.
Higher blood: gas coeffcient = high lipophilicity = higher solubility
High solubility means MORE anesthetic needs to UPTAKE to the blood to get the brain to do it's thing. Therefore the onset is slower.

This why SEVO is a faster induction agent than Isoflurane (which is why when you do peds inhaled inductions you use Sevoflurane)

So to really answer the question @waterbottle10 is asking. Higher FA/FI means LOWER solubility (requires less uptake) and fast action to the brain and therefore faster induction.