partial pressure of an inhaled anesthetic in blood

Discussion in 'Step I' started by voicesinmyhead, 09.22.14.

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  1. voicesinmyhead

    voicesinmyhead 2+ Year Member

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    Is the partial pressure of an inhaled anesthetic in blood, the partial pressure of the anesthetic caused by dissolving in blood? (similar to PaO2 being partial pressure of O2 dissolved in blood?)

    If thats the case, why does the partial pressure rise very quickly (causing early saturation) for a poorly soluble gas anesthetic?

    Shouldn't it rise slowly ?
     
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  3. EazyE1907

    EazyE1907 2+ Year Member

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    So for an inhaled gas anesthetic that is poorly soluble in blood and lipid (tissues) like N2O for ex will have a low arteriovenous concentration gradient. This basically means that it has a low tissue solubility. It is readily taken up by the pulmonary arteries of the lungs (arterial end) and taken up less by the Peripheral Tissues (venous end). More of the gas remains in the blood and less of it is taken up by tissue, which allows it to reach equilibrium concentration in arterial blood alot faster which then allows for faster induction times.

    In contrast, a gas like halothane will have increased blood/lipid solubility which will have an increase arteriovenous concentration gradient. more of the gas will be taken up by peripheral tissue, mianly being the CNS and less of the drug will be in blood, causing slower induction times.

    This is the way i think of it and make sense of it. Hope this helped.
     
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  4. voicesinmyhead

    voicesinmyhead 2+ Year Member

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    thanks for the reply! im sorry if i didnt understand you well and thought you didnt answer the qn.

    Im actually talking about the blood / gas coefficient that is dependent on the solubility of the anesthetic in blood and nothing else.

    anesthetics with high blood gas coeff are very soluble in blood - and apparently this leads to a slow rise in partial pressure.

    my qn is why is this slow? SHouldnt the increased solubility lead to a fast rise in partial pressure (similar to paO2 of O2)?
     
  5. SBR249

    SBR249 7+ Year Member

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    So partial pressure of a gas dissolved in a liquid can be thought of as the pressure exerted by the dissolved gas molecules that bounce around in the solvent.

    The solubility of a gas can be thought of as how energetically favorable it is for the gas molecules to stay in solution. Chemically speaking, the more non-covalent bonds that gas molecules can form (H bonds, van der waals, etc.) with the surrounding solvent molecules, the more favorable it is to stay in solution and thus the higher the solubility.

    So a good visualization of the process is the higher the solubility, the more bonds tie the gas molecules down, and the less they like to bounce around and exert pressure. Thus, for the same number of molecules, those of a low solubility gas will bounce around more (because they have fewer bonds tying them down) and lead to higher partial pressure than those of a high solubility gas. Based on this, if you add molecules at the same rate for both gases, the one with a lower solubility would see a faster rise in partial pressure.

    As for O2 and PaO2. When you are comparing different concentrations of the same gas, then solubility (and concentration) of that gas is directly proportional to partial pressure. That's because the more molecules, the more likelihood of collisions with the solution surface, and thus higher pressure exerted.

    tl;dr:

    1. Different concentrations of the same gas: partial pressure is proportional to concentration

    2. Same concentration of different gases: partial pressure is inversely proportional to solubility
     
    Last edited: 09.22.14
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  6. voicesinmyhead

    voicesinmyhead 2+ Year Member

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    thanks! makes a lot of sense. You should be a teacher!
     
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  7. Transposony

    Transposony Do or do not, There is no try 5+ Year Member

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    SBR248 is on a roll.........he should take the test already.:claps:
     
  8. SBR249

    SBR249 7+ Year Member

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    Already took the test back in June, I just lurk in this forum to help me retain a modicum of this stuff seeing as how I won't be touching it for the next 4 years :nod:
     
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  9. Transposony

    Transposony Do or do not, There is no try 5+ Year Member

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    Say it, you are missing all the fun Step 1 forum has to offer :highfive:
    Seriously, you should be contributing author to some Step 1 review book like FA.
    That way you will be able to stay in touch with the subject material.
    Not that we are complaining about your contribution here at all:corny:
     
    Last edited: 09.22.14

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