REPERFUSION INJURY

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REPERFUSION INJURY

• Frank Nami, M.D.
• Saint Barnabas Medical Center

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Ischemia and Reperfusion

• Ischemic tissue will ultimately progress to
  cellular death if restoration of blood flow does
  not occur in a timely manner.
• Restoration of blood flow and oxygenation to
  ischemic tissues can result in a paradoxical
  enhancement of tissue injury.

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Reperfusion Injury

• Often more severe than damage incurred during the
  ischemic period itself.
• Characterized by cellular edema, intracellular
  Ca2 overload with subsequent activation of Ca2-
  dependent autolytic enzymes, disruption of lipid
  membranes, and perturbations in mitochondrial
  structure and function.

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Reperfusion Injury

• Great relevance to the practice of surgery
• Vascular Surgery
• Cardiac Surgery
• Transplant Surgery
• Restoration of blood flow to the ischemic limb,
  heart or transplanted organ.

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Mediators of Reperfusion Injury

• Endothelial Cell
• Oxygen Free Radicals
• Polymorphonuclear Cells (PMNs)

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Endothelial Cell

• Thin monolayer of cells, resting on a basement
  membrane, surface area 5000 m2 but comprises
  only 1 of total body weight.
• Exerts influence over blood vessel tone,
  permeability, cell adhesion, coagulation, and
  growth by regulating the production of a battery
  of molecules and cell surface proteins.

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Endothelial Cell-Mediated Vasomotor Tone

• Vasodilation - Prostacyclin (PGI2) via
  cyclooxygenase pathway, activates adenylate
  cyclase and protein kinase A, also inhibits
  platelet aggregation by increasing cAMP -
  promotes microcirculatory flow.

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Endothelial Cell-Mediated Vasomotor Tone

• Vasodilation - Nitric Oxide (NO) also inhibits
  platelet aggregation, decreases vascular smooth
  muscle cell proliferation. Produced from
  L-arginine and oxygen in endothelial cytosol.
• Deficiency of NO synthesis reported in study of
  human volunteers with hypertension.

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Endothelial Cell-Mediated Vasomotor Tone

• Patients with diabetes, atherosclerosis,
  hypercholesterolemia, or cigarette smoking also
  exhibit deficient NO synthesis.
• Adenosine - also a vasodilator, inhibits platelet
  and neutrophil aggregation.

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Endothelial Cell-Mediated Vasomotor Tone

• Vasoconstriction - Thromboxane A2 (TXA2) via
  thromboxane synthase, opposes prostacyclin and
  produces platelet adherence.
• Endothelin-1, most potent vasoconstrictor known,
  counteracts NO.

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Endothelial Cell-Mediated Cell Adhesion

• Mediators formed during reperfusion induce
  endothelial cells to express intercellular
  adhesion molecules (ICAM 1 and 2), endothelial
  leukocyte adhesion molecule (ELAM) and selectins.
• These receptors bind the CD11/CD18 complex on
  activated, facilitating PMN adherence to and
  migration across endothelium.

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Endothelial Cell

• Secretes an abundance of soluble factors which
  promote vasoconstriction, platelet aggregation,
  PMN plugging of capillaries, and increased
  vascular permeability.
• Factors include Platelet aggregating factor
  (PAF), LTB4, TXA2 and endothelin.

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Endothelial Cell

• End result perfusion of the microcirculation is
  severely compromised, which manifests as the
  classic no-reflow phenomenon of reperfusion
  injury.

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Oxygen Free Radicals

• Three different molecules to be aware of
• Superoxide anion O2-
• Hydrogen peroxide H2O2
• Hydroxyl radical .OH

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Oxygen Free Radicals

• Reperfusion stimulates xanthine oxidase which is
  activated in ischemic endothelial cells to
  generate superoxide radicals.
• PMNs also generate oxygen free radicals.

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Oxygen Free Radicals

• These toxic moieties are rapidly generated at the
  onset of reperfusion and cause widespread damage
  to cellular macromolecules.
• Peroxidation of lipid membranes, protein
  degradation, nucleic acid damage, cytochrome
  inactivation and neutralization of nitric oxide.

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Oxygen Free Radicals

• Most damaging effect is on lipid membranes,
  impairs normal fluidity and permeability of cell
  membranes leading to cellular edema, massive Ca2
  and Na overload and cell lysis.

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Oxygen Free Radicals

• Oxygen free radical scavengers and antioxidants
  have been shown both experimentally and
  clinically to ameliorate reperfusion injury.

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Oxygen Free Radicals

• Natural protective enzyme systems to reduce free
  radical damage include superoxide dismutase,
  catalase, and glutathione peroxidase.
• Most important endogenous antioxidant is
  glutathione. N-acetylcysteine is an artificial
  glutathione precursor.

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Activated PMNs

• Inflict damage to reperfused endothelial and
  parenchymal cells.
• Release a host of destructive proteolytic
  enzymes, including elastase, collagenase,
  gelatinase, lysozyme, and cathepsin G.

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Activated PMNs

• Source of oxygen free radicals by virtue of a
  superoxide generating NAD oxidase.
• Produce hypochlorous acid by activity of
  myeloperoxidase.

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Reduction of Reperfusion Injury

• Allopurinol - inhibitor of xanthine oxidase has
  been shown to have protective effects.
• Desferrioxamine - an iron chelator, removes an
  essential cofactor for the generation of hydroxyl
  radical.

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Reduction of Reperfusion Injury

• Vitamin E - prevents neutrophil accumulation and
  attenuates tissue damage in ischemic-reperfused
  human skeletal muscle.
• N-acetylcysteine - pretreatment 30 minutes before
  infrarenal aortic clamping may help prevent
  reperfusion injury.

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Ischemia, Reperfusion Injury and Compartment
Syndrome

• Elevated pressure within a confined tissue space.
• High energy injuries.
• Pain out of proportion to injury.
• Most commonly occurs in the leg.

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Ischemia, Reperfusion Injury and Compartment
Syndrome

• Four compartments in leg
• Anterior anterior tibial artery, deep peroneal
  nerve, extensor muscles of toes and foot
• Lateral superficial peroneal nerve, peroneal
  brevis and longis muscle

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Ischemia, Reperfusion Injury and Compartment
Syndrome

• Deep posterior tibial nerve, posterior tibial
  artery, peroneal artery, deep toe and foot flexor
  muscles
• Superficial posterior superficial foot flexor
  muscles
• Examining leg, document sensation at first web
  space (deep peroneal nerve), dorsum of foot
  (superficial peroneal nerve) and plantar surface
  of foot (tibial nerve)

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Ischemia, Reperfusion Injury and Compartment
Syndrome

• Pressure threshold at which fasciotomy is
  indicated has been debated.
• 30-40 mm Hg
• Can use arterial pressure transducer, IV tubing,
  3 way stopcock, 20 ml syringe, a 16Ga needle.
• Four compartments should be measured