Can someone explain this FA anesthetic principle to me?

[Old Style Nanny]

What does this line mean: (under properties of anesthetics) FA2012 p475

In tissue (eg. brain): incr A/V concentration gradient = incr solubility = incr gas required to saturate tissue = slower onset of action


Increased solubility where? In blood? If so, why mention this line again when the previous line talks about solubility in blood already?

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[Stellar Clouds]

What does this line mean: (under properties of anesthetics) FA2012 p475

In tissue (eg. brain): incr A/V concentration gradient = incr solubility = incr gas required to saturate tissue = slower onset of action


Increased solubility where? In blood? If so, why mention this line again when the previous line talks about solubility in blood already?

With inhaled anesthetics there are 2 partition coefficients you are worried about:
1. Blood-gas partition coefficient: where an increased coefficient (higher blood solubility) means that you need to dissolve a larger amount of gas in the blood before the arterial partial pressure will equal the alveolar partial pressure.

2. Tissue-blood partition coefficient: relative solubility between tissue (e.g. brain) and blood. Increased solubility in blood (relative to tissue) means that you need more anesthetic to saturate tissue. Keep in mind that the blood should be thought of as a pharmacologically inactive reservoir of the anesthetic.


They put the second line in because (I don't have FA 2012, but I am assuming) they are referring to the blood-gas partition coefficient in the first line and the tissue-blood partition coefficient in the second line.

 

[Old Style Nanny]

With inhaled anesthetics there are 2 partition coefficients you are worried about:
1. Blood-gas partition coefficient: where an increased coefficient (higher blood solubility) means that you need to dissolve a larger amount of gas in the blood before the arterial partial pressure will equal the alveolar partial pressure.

2. Tissue-blood partition coefficient: relative solubility between tissue (e.g. brain) and blood. Increased solubility in blood (relative to tissue) means that you need more anesthetic to saturate tissue. Keep in mind that the blood should be thought of as a pharmacologically inactive reservoir of the anesthetic.


They put the second line in because (I don't have FA 2012, but I am assuming) they are referring to the blood-gas partition coefficient in the first line and the tissue-blood partition coefficient in the second line.

Thank you. That is detailed. However, I still have my confusion. Before posting the question, I reasoned out that they would not be talking about solubility in tissue because then the increased solubility would mean quicker onset of action, wouldn't it?

But the line reads: In tissue (eg. brain): incr A/V concentration gradient = incr solubility = incr gas required to saturate tissue = slower onset of action

Also, what is the A/V gradient?

 

[Phloston]

This concept is one that you'll grab in bits and pieces as you do more and more questions.

If the A/V gradient is high, it means the gas is diffusing quite well out of the arteries, so there isn't much left in the veins. The Vd of this gas would be high because it is distributing across greater volume.

FA makes it confusing. If the A/V gradient is high and there's increased solubility in the periphery, then there's increased gas required to saturate tissue of the brain = slower onset of action.

 

[Old Style Nanny]

This concept is one that you'll grab in bits and pieces as you do more and more questions.

If the A/V gradient is high, it means the gas is diffusing quite well out of the arteries, so there isn't much left in the veins. The Vd of this gas would be high because it is distributing across greater volume.

FA makes it confusing. If the A/V gradient is high and there's increased solubility in the periphery, then there's increased gas required to saturate tissue of the brain = slower onset of action.

Awesome, thanks. So, the correct way should be:

In tissue (eg. brain): incr A/V concentration gradient -> incr solubility in the peripheral tissue = incr gas required to saturate brain = slower onset of action

Right?

 

[Phloston]

Yeah. That's how I eventually came to understand it.

 

[Old Style Nanny]

Yeah. That's how I eventually came to understand it.

Thanks!