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  1. #1
    tommyk is offline USMLE Advisor
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    HY Concept 302 (Wonderful Question)

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    I love this case because it involves chemistry and you MUST be aware of it. Two scenarios:

    Patient A is getting a large lidocaine dose for LP:

    Patient B is working with aniline dyes in a factory:

    Both present to your clinic the same way, with tachycardia, and symptoms of CYANOSIS like lip/skin discoloration (hint).

    1) What is the disease?
    2) Mech of disease
    3) Treatment
    4) Mech of Action of tx?
    "All USMLE cases are original and are expressly not from questions seen, recalled, paraphraphrased from the real USMLE, the material is for the purpose of the education of future physicians and the safety of their patients."

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  2. #2
    tommyk is offline USMLE Advisor
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    great!

    1)Both of these cases are of methemoglobinemia!

    2) Any oxidizing agent converts Fe +2 to Fe +3.

    3) As such the hemoglobin cannot bind the oxygen in this ferrous form.

    4) Give methylene blue and LATER bicarb and hydration for the acidosis. Long term tx for chronic cases is Vit C! The methylene blue acts as a cofactor in the NADPH-dependent metHb reductase system and reduces the iron to ferric form which can bind the oxygen.

    How's that for combining the sciences!
    "All USMLE cases are original and are expressly not from questions seen, recalled, paraphraphrased from the real USMLE, the material is for the purpose of the education of future physicians and the safety of their patients."

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  3. #3
    Anonymous is offline Unregistered Guest
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    Lidocain and metHb

    Lidocain and MetHb ???? What is the source Pl????

  4. #4
    tommyk is offline USMLE Advisor
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    Great Question!

    Thanks so much for looking over my posts. You see the mech is that Lidocaine as well as a thousand other compounds like Dapsone, Nitrites, even SPINACH are OXIDING AGENTS. Anything that pushes the Fe up in oxidation will do this.! Thanks!

    Interestingly, with Lidocaine poisoning, first you get an excitatory rxn from blockage of Na channels, then with higher doses you get overall CNS depression!!!! as the lidocaine moves on and blocks the excitatory pathways. Well done!
    "All USMLE cases are original and are expressly not from questions seen, recalled, paraphraphrased from the real USMLE, the material is for the purpose of the education of future physicians and the safety of their patients."

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  5. #5
    Anonymous is offline Unregistered Guest
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    Ref Pl

    Pl give your source of information i.e proper ref That lidocain produces Met Hb.It is not mentioned anywhere in its poisoning .Thanks.

  6. #6
    tommyk is offline USMLE Advisor
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    This is merely in my notes...BUT EXCELLENT QUESTION AGAIN>

    I probably should not have made up that example because usually, interns and doctors don't face scenarios where they could overdose someone with lidocaine, although I have heard it happen.

    But the theme of the question is that any oxiding agent like a Nitrate, a Chlorate, etc. can pull on outer electron off the Fe +3 to a +2 state (Including Lidocaine). Even abnormal Hb also can cause metHb. Seven types of Hb exist and are designated as HbM. They are named HbM because they are associated with methemoglobinemia. In most of them, a tyrosine replaces the histidine residue, which binds heme to globin. This displaces the heme moiety and permits oxidation of the iron to the ferric state. Patients are cyanotic and susceptible to hemolytic anemias. Guest you are making a good point in that a more common scenario would be a child eating mothballs or something akin to that...thanks again!
    "All USMLE cases are original and are expressly not from questions seen, recalled, paraphraphrased from the real USMLE, the material is for the purpose of the education of future physicians and the safety of their patients."

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  7. #7
    Anonymous is offline Unregistered Guest
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    Link to Methemoglobinemia

    I found a link to support Tommy, there may be more:

    http://www.pharmacy.umaryland.edu/~m...s/June2000.pdf

    Best wishes,



    QMS

  8. #8
    Anonymous is offline Unregistered Guest
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    please see table 1 of the link. I copied it here. Tommy is right on this topic.

    Agents That Cause Methemoglobinemia

    Acetonitrile (nail remover)
    Anesthetics (benzocaine, lidocaine, prilocaine)
    Aniline (dyes)
    Chlorates (matches, explosives, weed killers)
    Naphthalene (mothballs)
    Volatile nitrites (amyl, butyl, isobutyl)
    Nitroprusside
    Sodium nitrite
    Phenacetin
    Phenazopyridine (PyridiumŪ)
    Quinones (chloroquine, primaquine)
    Sulfonamides (sulfamethoxazole)
    Dapsone

    Best wishes to everyone.

    QMS

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