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Common Congenital Heart Lesions - page 108 / 126

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Pathophysiology:

  • Complications from ischemia distal to stenosis.

  • Imbalance between oxygen supply and demand: exercise raises demand, stenosed/obstructed artery can’t supply.

  • Rest improves symptoms as supply-demand balance is restored.

  • Amount of blood flow reduction is very closely linked to the amount of vessel narrowing, length of stenosis, and viscosity of blood…

  • Poiseuille’s Law: volume rate of flow is Q = (Pπr4) / (8ηL)…

    • o

      Where P is pressure difference between the ends of the stenosis, L is the length of the stenosis, r is the radius or the vessel, and η is the viscosity of the blood

      • o

        The most important component of the system is the radius (r) as it is present in the calculation of blood flow to the power of 4! So, when the vessel’s radius is decreased by ½, the vessel’s flow is decreased by (½)4 = 1/16.

      • o

        Note that higher flow (Q) rates across a stenosis with a fixed length (L) and radius (r) indicate a larger pressure gradient (P) – blood turbulence results in a loss of energy and thus a decreased perfusion pressure post stenosis

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        What’s the point? We can use this law to show how blood flow is limited to structures distal to the stenosis.

  • In PAD, obstructed arteries can’t respond to vasodilating stimuli during exercise – blood flow is further limited.

  • Dysfunctional atherosclerotic endothelium doesn’t release normal amounts of vasodilators – blood flow limited again.

  • Due to ischemia, can see changes in physical and biochemical state of muscles distal to the stenosis.

    • o

      Denervation and dropout of muscle fibers Æ reduced muscle strength and atrophy

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        Abnormalities of mitochondrial oxidative metabolism in viable muscle fibers Æ muscle weakness

Diagnostic tests/Imaging:

  • Duplex Ultrasonography: might be used to visualize and assess extent of arterial stenoses and corresponding

reductions in blood flow

Treatment:

  • Lifestyle/Risk Factor Modification…

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      Quit smoking

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        Lower lipids

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        Control diabetes

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        Control hypertension

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        Start an exercise program

  • Drug Therapies: goals are symptomatic relief and improved exercise capacity; in severe PAD, goals are healing ulcers and preventing limb loss

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        Anti-platelet therapy: not actually proven to reduce symptoms or prevent thrombotic complications, but given to all PAD patients

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        Phosphodiesterase inhibitor (cilostazol): thought to improve claudication by inducing vasodilatation and inhibiting platelet aggregation (note, however, that most vasodilators are not helpful in relieving claudication)

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        Drugs that improve RBC and WBC deformability (pentoxifylline): may improve symptoms

  • Medical Procedures:

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      Mechanical revascularization: indicated when drugs didn’t relieve disabling claudication or when have severe leg ischemia

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        Catheter-based interventions: percutaneous transluminal angioplasty and stent implantation can be done on selected patients with few complications

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        Bypass operations: go around occluded arteries with prosthetic or saphenous vein grafts

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        Amputation: needed in severe limb ischemia if blood flow can’t be reestablished

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