Reperfusion injury

Editors-In-Chief: ; C. Michael Gibson, M.S., M.D. [mailto:Mgibson@perfuse.org]

Overview
Reperfusion injury refers to damage to tissue caused when blood supply returns to the tissue after a period of ischemia. The absence of oxygen and nutrients from blood creates a condition in which the restoration of circulation results in inflammation and oxidative damage through the induction of oxidative stress rather than restoration of normal function.

Mechanisms of reperfusion injury
The damage of reperfusion injury is due in part to the inflammatory response of damaged tissues. White blood cells carried to the area by the newly returning blood release a host of inflammatory factors such as interleukins as well as free radicals in response to tissue damage .The restored blood flow reintroduces oxygen within cells that damages cellular proteins, DNA, and the plasma membrane. Damage to the cell's membrane may in turn cause the release of more free radicals. Such reactive species may also act indirectly in redox signaling to turn on apoptosis. Leukocytes may also build up in small capillaries, obstructing them and leading to more ischemia.

Mitochondrial dysfunction plays an important role in reperfusion injury. While the mitochondrial membrane is usually impermeable to ions and metabolites, ischemia alters permeability by elevating intro-mitochondrial calcium concentrations, reducing adenine nucleotide concentrations, and causing oxidative stress. This primes the mitochondrial permeability transition pore (MPTP), which opens when reperfusion occurs. This leads to an increased osmotic load into the mitochondrial body causing swelling and rupture, release of mitochondrial proteins which stimulate apoptosis. Mithochondrial function is disrupted and ATP is hydrolyzed, leading to the activation of degradative enzymes. Finally, excessive Poly ADP ribose polymerase-1 (PARP-1) activation impairs the function of other organelles and accelerates the production of reactive oxygen species.

In prolonged ischemia (60 minutes or more), hypoxanthine is formed as breakdown product of ATP metabolism. The enzyme xanthine dehydrogenase is converted to xanthine oxidase as a result of the higher availability of oxygen. This oxidation results in molecular oxygen being converted into highly reactive superoxide and hydroxyl radicals. Xanthine oxidase also produces uric acid, which may act as both a prooxidant and as a scavenger of reactive species such as peroxinitrite. Excessive nitric oxide produced during reperfusion reacts with superoxide to produce the potent reactive species peroxynitrite. Such radicals and reactive oxygen species attack cell membrane lipids, proteins, and glycosaminoglycans, causing further damage. They may also initiate specific biological processes by redox signaling.

The central nervous system
Reperfusion injury plays a part in the brain's ischemic cascade, which is involved in stroke and brain trauma. Repeated bouts of ischemia and reperfusion injury also are thought to be a factor leading to the formation and failure to heal of chronic wounds such as pressure sores and diabetic foot ulcers. Continuous pressure limits blood supply and causes ischemia, and the inflammation occurs during reperfusion. As this process is repeated, it eventually damages tissue enough to cause a wound.

The myocardium
Restoration of epicardial patency can be associated with reperfusion injury in the myocardium. This can manifest in a number of ways clinically, including arrhythmia, microvascular dysfunction, myocardial stunning, and myocyte death.

Arrhythmia is mediated by mitochondrial dysfunction, as discussed above. The mitochondrion is unable to restore its inner membrane potential, leading to destabalization of the action potential.

Microvascular dysfunction, or "no reflow," as well as myocardial stunning, are both possible consequences of reperfusion injury. Myocardial stunning, which results from persistent anearobic metabolism that continues after reperfusion, may to some extent be mediated by impaired microvascular function.

An area of ongoing study is how much damage, or myocyte death, is attributable to ischemia vs. reperfusion injury after vessel patency has been established. Animal studies suggest that up to 50% of of infarct size can be related to reperfusion injury. This opens the door for novel therapies that can attenuate myocyte death due to reperfusion injury.

Therapies
While many pharmacotherapies are successful in limiting reperfusion injury in animal studies or ex-vivo, many have failed to improve clinical outcomes in randomized clinical trials in patients.

Therapies Associated with Limited Success
Pharmacotherapies that have either failed or that have met with limited success in improving clinical outcomes include:


 * 1) Beta-blockade
 * 2) GIK (glucose-insulin-potassium infusion) (Studied in the Glucose-Insulin-Potassium Infusion in Patients With Acute Myocardial Infarction Without Signs of Heart Failure: The Glucose-Insulin-Potassium Study (GIPS)-II and other older studies
 * 3) Sodium-hydrogen exchange inhibitors such as cariporide (Studied in the GUARDIAN and EXPIDITION  trials)
 * 4) Adenosine (Studied in the AMISTAD I and AMISTAD II trials as well as the ATTACC trial ). It should be noted that at high doses in anterior ST elevation MIs, adenosine was effective in the AMISTAD trial. Likewise, intracoronary administration of adenosine prior to primary PCI has been associated with improved echocardiographic and clinical outcomes in one small study.
 * 5) Calcium-channel blockers
 * 6) Potassium–adenosine triphosphate channel openers
 * 7) Antibodies directed against leukocyte adhesion molecules such as CD 18 (Studied in the LIMIT AMI trial )
 * 8) Oxygen free radical scavengers/anti-oxidants, including Erythropoietin   , estrogen  , heme-oxygenase 1 , and hypoxia induced factor-1 (HIF-1).
 * 9) Pexelizumab, a humanized monoclonal antibody that binds the C5 component of complement (Studied in the Pexelizumab for Acute ST-Elevation Myocardial Infarction in Patients Undergoing Primary Percutaneous Coronary Intervention (APEX AMI) trial )
 * 10) KAI-9803, a delta-protein kinase C inhibitor (Studied in the Intracoronary KAI-9803 as an adjunct to primary percutaneous coronary intervention for acute ST-segment elevation myocardial infarction trial or DELTA AMI trial).
 * 11) Human atrial natriuretic peptide (Studied in the Human atrial natriuretic peptide and nicorandil as adjuncts to reperfusion treatment for acute myocardial infarction (J-WIND): two randomised trials.)
 * 12) FX06, an anti-inflammatory fibrin derivative that competes with fibrin fragments for binding with the vascular endothelial molecule VE-cadherin which deters migration of leukocytes across the endothelial cell monolayer (studied in the F.I.R.E. trial (Efficacy of FX06 in the Prevention of Myocardial Reperfusion Injury)
 * 13) Magnesium, which was evaluted by the Fourth International Study of Infarct Survival (ISIS-4) and the MAGIC trial.
 * 14) Hypothermia
 * 15) Hyperoxemia, the delivery of supersaturated oxygen after PCI (Studied in the AMIHOT II trial ).

Therapies Associated with Improved Clinical Outcomes
Therapies that have been associated with improved clinical outcomes include:


 * 1) Post conditioning (short repeated periods of vessel opening by repeatedly blowing the balloon up for short periods of time).
 * 2) *Mechanisms of protection include formation and release of several autacoids and cytokines, maintained acidosis during early repercussion, activation of protein kinases, and attenuation of opening of the mitochondrial permeability transition pore (MPTP)
 * 3) *One study in humans demonstrated an area under the curve (AUC) of creatine kinase (C) release over the first 3 days of reperfusion (as a surrogate for infarct size) was significantly reduced by 36% in the postconditioned versus control group
 * 4) *Infarct size reduction by PCI postconditioning persisted 6 months after AMI and resulted in a significant improvement in left ventricular (LV) function at 1 year
 * 5) Inhibition of mitochondrial pore opening by cyclosporine.
 * 6) *Specifically, the study by Piot et al demonstrated that administration of cyclosporine at the time of reperfusion was associated with a reduction in infarct size
 * 7) *Infarct size was measured by the release of creatine kinase and delayed hyperenhancement on MRI
 * 8) *Patients with cardiac arrest, ventricular fibrillation, cardiogenic shock, stent thrombosis, previous acute myocardial infarction, or angina within 48 hours before infarction were not included in the study #*Occlusion of the culprit artery (TIMI flow 0) was part of the inclusion criteria.

Limitations to applying strategies that have demonstrated benefit in animal models is the fact that reperfusion therapy was administered prior to or at the time of reperfusion. In the management of STEMI patients, it is impossible to administer the agent before vessel occlusion (except during coronary artery bypass grafting). Given the time constraints and the goal of opening an occluded artery within 90 minutes, it is also difficult to administer experimental agents before reperfusion in STEMI.

There are several explanations for why trials of experimental agents have failed in this area:
 * 1) The therapy was administered after reperfusion and after reperfusion injury had set in
 * 2) The greatest benefit is observed in anterior ST elevation myocardial infarctions (as demonstrated in the AMISTAD study), and inclusion of non anterior locations minimizes the potential benefit
 * 3) There are uninhibited redundant pathways mediating reperfusion injury
 * 4) Inadequate dosing of the agent

Future Directions
While there has been significant progress made in understanding repercussion injury, from molecular mechanisms to bedside clinical interventional, there are several areas that warrant further study. Some of these topics were outlined by the 2010 National Heart, Lung, and Blood Institute Workshop on Cardioprotection, and include:
 * 1) Identifying the molecular and subcellular mechanisms responsible for postconditioning, remote conditioning, and preconditioning.
 * 2) Determining whether the protective mechanism triggered by remote conditioning is humoral, neural, or both.
 * 3) Investigating whether age, obesity, and diabetes mellitus may attenuate the beneficial effects of cardioprotective strategies.
 * 4) Developing additional pharmacological strategies that mimic, synergize, or augment the protection exerted by conditioning protocols in conjunction with repercussion.
 * 5) Establishing a cardioprotective clinical trial network concurrent with the existing and complementary preclinical network (CAESAR) to test promising cardioprotective agents and strategies in patients in the setting of both acute myocardial infarction and cardiac surgery.