question about myocardial ischemia

Discussion in 'Pre-Medical - MD' started by jolu, Nov 28, 2002.

  1. jolu

    jolu Member
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    Can someone explain to me how tissue plasminogen activator (tPA) is useful in dissolving clots in coronary arteries and why it is important to use it only iwthin a short period (about 4 hours) after the onset of myocardial ischemia (heart attack)??

    thanks a lot!!
     
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  3. SCIronMike

    SCIronMike Member
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    Stuff like streptokinase and urokinase dissolves clots but this is only useful in the period right after an occlusion because after a period of time, the tissue will be permanently damaged from the ischemia.
     
  4. Gleevec

    Gleevec Peter, those are Cheerios
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    First of all a heart attack is myocardial infarction, ischemia is massive cell death that occurs if the heart is deprived of oxygen and glucose for an extended period of time.

    It dissolves blood clots by binding plasminogen (which itself is bound to fibrin in a clot). Thus, tPA converts plasminogen to plasmin, which digests the fibrin (that it was attached to, due to its proximity) in the clot and restores blood flow.

    I would assume it is important to use early because otherwise the myocardial cells will start dying otherwise. I think if you use it too early it causes some weird side effects involving even more cardiac arythmias and more profuse bleeding, it can also affect blood pressure in random ways.
     
  5. DALABROKA

    DALABROKA Raider Hater
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    Plasminogen is activated to yield the protease plasmin, which dissolves clots. Most acute MIs occur due to rupture of hard atherosclerotic plaques leading to the production of a thrombus. The thrombus can occlude one of the coronary arteries and cause the ischemia. If the thrombus is not broken down quickly, the myocardium will infarct. This is why it needs to be administered in a timely manner.

    Cheers,
    DALA
     
  6. jolu

    jolu Member
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    so then i would guess you'd only administer the tPA after sufficient time has been giving to allow the clots to form??? huh???

    thanks alot for great info...but i'm a bit confused about the last part..
     
  7. SCIronMike

    SCIronMike Member
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    You give it after you determine the patient is suffering from a MI. It's one of the drugs given in the ER.
     
  8. Gleevec

    Gleevec Peter, those are Cheerios
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    Like if the guy has some wounds or something, you need to wait. You also need to stabilize the person's blood pressure. The drug works quickly but takes about about an hour to subside

    (I really dont know the clinical aspects, Im just a measly undergrad with some knowledge of pharmacology, sorry)
     
  9. SCIronMike

    SCIronMike Member
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    tPA works within just a few minutes. It's practically instantaneous.
     
  10. souljah1

    souljah1 Attending
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    tPa is used to dissolve clots only in those who meet certain criteria. first of all, if the hospital has a cardiac catheterization lab on staff who can perform percutaneous transluminal coronary angioplasty, that should be the choice over tPa (so says the latest clinical studies after comparing tpa vs ptca and mortality). tPa is associated with negative side-effects. In order to even be considered for treatment with tPa's, you have to meet certain criteria such an indication of a bundle branch block and history suggesting acute MI, and an ECG finding that is very indicative of an MI (ST elevation for those that care). Fibrinolytics (Thrombolytics) are used not only for acute MI, but also in some instances for established, serious pulmonary embolism and deep vein thromoses.

    Also, ischemia is not massive cell death (as gleevec indicated above). Ischemia is descreased oxygen supply to tissue along with a decrease in clearance of metabolites from tissue. this is usually due to poor perfusion. Ischemia can be the result of angina, either due to stable plaque formation, unstable plaque ruptures, vasospasm, etc. A myocardial infarct is a heart attack, which is death of cardiac tissue. Ischemia usually precedes an infarct, though the duration and severity of ischemia can vary. Sometimes they are difficult to differentiate (i.e. unstable angina versus an infarct), but usually lab tests are ordered that help to narrow down your differential (there may also be distinctive changes in the ECG).

    tPa therapy was accurately described above, it just helps convert plasminogen to plasmin, which will degrade fibrin scaffolds and help to decrease clot formation. the recombinant tPa's a bit more specific compared to strep, for they also bind to fibrin (which aids in selective activation of plasmin only in (or most likely in)areas where there are clots).

    Sorry for this long winded reply, but we are neck-deep in our cardiovascular block and all of these thoughts are floating around my mind. Selfishly, I figured that an attempt at gathering my thoughts would be helpful to not only you, but also to me.
     
  11. jolu

    jolu Member
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    thanks alot for the info, i really appreciate this.... :)
     
  12. DALABROKA

    DALABROKA Raider Hater
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    You mean the cause of angina pectoris, not the result of, right.:)

    Cheers,
    DALA
     

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