How is a PDA enough to be cardioprotective for...

Discussion in 'Medical Students - MD' started by CBG23, Jul 30, 2011.

  1. CBG23

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    Transposition of the Great Arteries? I understand that ultimately any shunt (ASD, VSD, or PDA) is just a temporary solution until the the patient can get corrective surgery. It seems like a PDA will only cause significant mixing downstream of the origin of the Left Subclavian artery and basically your head (and more importantly, brain) and UEs would be screwed. Am I missing something here?
     
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  3. Gheorgia

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    Aren't there collateral arteries and internal mammary arteries and intercostals vessels upstream of the left subclavian?
     
  4. BigRedBeta

    BigRedBeta Why am I in a handbasket?
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    "Screwed" is all relative. You're really only looking to get a minimal venous sat >60-65%. Spend some time in the peds CVICU and you'll see lots of kiddos doing just fine with pulse ox readings of 70%. In the hypoplastic left heart syndrome kids who are status post stage 1 or 2 palliation surgeries, you actually want to avoid overcirculating the pulmonary vasculature, so sats above 90% are actually
    somewhat frowned upon...
     
  5. isoquin

    isoquin Allopathetic
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    It turns transposition from two completely separate parallel systems to cardiopulmonary communication. Not a lot, but better than zippo.
     
  6. OveractiveBrain

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    So I'll get more at the anatomy and logic than others have stabbed at.

    Blood goes from the Right Heart to the Lungs, Back to the left heart, than up and around the aorta to the rest of the body. You got that.

    In the case of a PDA, oxygenated blood going into the aorta goes BACK into pulmonary circulation through the little connection. It is a "left to right shunt." Afterall, the aorta is a high pressure system, the pulmonary artery is a low pressure system. So, if blood is going to flow anywhere, its from a high pressure system to a low pressure system.

    In transposition, the RIGHT ventricle-aorta-RIGHT atria is just one continuous circuit. Its a low pressure system (because the right heart is weak), and the blood never sees the lungs to get oxygenated. Meanwhile, The LEFT ventricle - pulmonary artery - LEFT atrium is also just one continuous circuit. It is a high pressure system (because of the left heart is strong). It ONLY oxygenates, and never perfuses the body.

    Now put transposition and PDA together. Pay very close attention to right vs left (low and high pressure systems respectively). The HIGH pressure, left ventricle pushes most of its blood through the lungs to get oxygenated (yay!). Then, because of a persistent PDA, there is a connection between the high pressure oxygenated blood and the low pressure unoxygenated blood. The oxygenated content flows down its pressure gradient and into the systemic system.

    See what happened? In PDA alone it flows left to right (from the aorta to the pulmonary arter), because the aorta is a high pressure system because the left ventricle is high pressure. But in a TGA, the aorta is connected to the right ventricle. So its a low pressure sytem because the right ventricle is low pressure, and now the pulmonary artery is a high pressure system because its connected to the left ventricle. So, when you put PDA and TGA together you have again have a "left to right" shunt. "Left" is oxygenated, "Right" is unoxygenated. But "left" in this new case means "from the pulmonary artery" and "from the fully oxygenated" while "right" means "to the aorta" and "to the unoxygenated periphery"

    This mixing of the circulations gives some oxygenation to the RV-Aorta-RA system. This blood gets circulated again and again, which means that there is at least oxygen coming out of the right ventricle in to the aorta even BEFORE the PDA.

    of course, if you don't buy that, then you could try:
    1. Left ventricle is a higher pressure system so can fill BACKWARDS from the PDA because there is no pressure resisting it
    2. There are collaterals

    and if you don't buy that:
    1. Who cares why. Understanding physics isnt that important. babies live when you keep the PDA open. Maybe you can't see why it works, but the fact is that it does. So we do it.
     
    #5 OveractiveBrain, Jul 31, 2011
    Last edited: Jul 31, 2011
    awared2 likes this.
  7. ArcGurren

    ArcGurren only one will survive
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    The PDA isn't long-term effective to be cardioprotective but it is in the short term so that the surgeon can go in and fix the TGV. Often they'll use a cath to perforate the atrial septum temporarily to allow for more time.
     
  8. CaptainSSO

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    You explained this very well. I haven't even started med school yet but yours was the only post I was able to mostly follow.

    However, one part I'm not following, is if there is this hole between the aorta and the pulmonary artery, wouldn't the blood flow from the pulmonary artery (high pressure) and into the aorta (low pressure) before it is oxygenated?

    Edit: After reading your post again, I guess I'm not fully understanding the TGA. So by continuous circuit, you mean that the right and left sides of the heart aren't connected at all? ...which would mean the left circuit is always oxygenated, so it's already oxygenated when it's shunted to the aorta?
     
    #7 CaptainSSO, Jul 31, 2011
    Last edited: Jul 31, 2011
  9. CuriousGeorge2

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    Wait a second, I thought that the only reason that LV pressure is greater than RV pressure is because it normally has to pump blood to the systemic circulation. If the RV is pumping blood into the systemic vasculature, shouldn't its pressure exceed that of the LV??? I could be wrong, but it was my understanding that the LV only becomes so strong AFTER birth; it's not intrinsically thicker than the RV in the fetus...

     
  10. Subaru

    Subaru ASA Member
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    I agree, at least that is what I was taught in class and always made sense to me that I never looked into it much more. Regardless blood shall mix and apparently enough of the Sat O2 will make it systemically until corrective surgery. :)
     
  11. CaptainSSO

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    Doesn't really make sense that they would mix if that were the case, though, since like OverReactiveBrain said the blood will always move from the area of high pressure to the area of low pressure.

    http://www.mottchildren.org/congenital/services/patient_con_tran.html

    Looking at the picture, seems like the left ventricle is larger (although it may not have been drawn to scale).
     
    #10 CaptainSSO, Aug 1, 2011
    Last edited: Aug 1, 2011

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