REPERFUSION INJURY

Frank Nami, M.D.

Saint Barnabas Medical Center

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.

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.

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.

Mediators of Reperfusion Injury

• Endothelial Cell

• Oxygen Free Radicals

• Polymorphonuclear Cells (PMNs)

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.

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.

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.

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.

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.

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.

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.

Endothelial Cell

• End result : perfusion of the microcirculation is severely compromised, which manifests as the classic “no-reflow”

phenomenon of reperfusion injury.

Oxygen Free Radicals

• Three different molecules to be aware of:

• Superoxide anion O2-

• Hydrogen peroxide H2O2

• Hydroxyl radical .OH

Oxygen Free Radicals

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

• PMNs also generate oxygen free radicals.

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.

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.

Oxygen Free Radicals

• Oxygen free radical scavengers and antioxidants have been shown both

experimentally and clinically to ameliorate reperfusion injury.

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.

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.

Activated PMNs

• Source of oxygen free radicals by virtue of a superoxide generating NAD oxidase.

• Produce hypochlorous acid by activity of myeloperoxidase.

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.

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.

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.

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

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)

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