Vascular Reperfusion Injury

Etiology

Ischemic cerebral stroke, acute coronary syndrome/myocardial infarction, traumatic limb injuries or organ lacerations, and organ transplants are the most commonly encountered situations associated with the possibility of ischemia and hypoxia. The first line of treatment is always thrombolytic therapy or an invasive revascularization procedure. Quite often, there are instances when post revascularization, greater damage is done to the already damaged cells and membranes. This is also sometimes called the “second hit” phenomenon. The main concern remains that early revascularization should be achieved in order to prevent initial extensive cellular damage.

Epidemiology

The prevalence of reperfusion injury is directly proportional to the duration for which ischemia remains. It has been shown through studies that those receiving thrombolytic therapy within 1 hour had a 51% infarct size reduction compared to the 31% reduction seen in those receiving treatment after 1 to 2 hours.[1] Rates of symptomatic intracerebral hemorrhage after a cerebral infarct are generally higher in intra-arterial lytic trials (e.g., 10%) than in intravenous lytic trials (e.g., 6.4%). The rates of symptomatic ICH following revascularization with a device are even lower and range from 4% to 2%.

Pathophysiology

Mechanisms of injury involved in vascular reperfusion are three-fold:

• Increased formation of reactive oxygen species
• Microvascular vasoconstriction
• Adhesion of neutrophils to endothelial lining, their activation, and release of cytokines.

Whenever there is impairment of blood flow to the tissues, there is tissue hypoxia, which triggers anaerobic respiration resulting in depletion of cellular ATP reserves. Ionic pumps like Na/K ATPase pumps start malfunctioning due to low ATP stores leading to ionic imbalance. The Na/Ca exchange pumps are activated, resulting in increased intracellular calcium levels. These increased calcium levels also explain the hyper-contractile myocardial tissues seen in post –MI cardiac tissues.[2] In certain studies, it is also postulated that post revascularization there is the opening of the mitochondrial permeability transition pore (mPTP). It is responsible for mitochondrial and cardio-myocyte death. Platelet activation has also been shown to play a role in reperfusion injury, especially in myocytes.[3] Whether this cellular injury is irreversible or reversible will depend on the duration for which the cell remained in a hypoxic state.[4][5]

Once blood flow to a hypoxic tissue is restored, an increased supply of molecular oxygen leads to activation of pathways resulting in increased production of ROS. The enzymes involved in these pathways are NADPH oxidase, xanthine oxidase, and nitric oxide synthase. Primary ROS, such as superoxide and secondary such as hydroxyl, peroxynitrite, and hypochlorous, are formed. This damages the micro-vasculature as well as membranes, including those of the mitochondria. Damaged mitochondrial membranes lead to the release of caspases and cytochromes and activation of the process of apoptosis.[6]

Further, increased activity of nitric oxide synthase forms a nitric oxide which reacts with superoxide to give peroxynitrite that potentiates the damage to nucleic acids/proteins and lipids. Hypoxia causes the conversion of NAD-reducing xanthine dehydrogenase into oxygen radicle, producing xanthine oxidase. During decreased oxygen supply, ATP is broken down into hypoxanthine. Reperfusion introduces oxygen, which reacts with hypoxanthine in the presence of xanthine oxidase forming hydrogen peroxide and superoxide anions. These further react with iron to give hydroxyl radicles.[7]

Propylhydroxylase is a set of enzymes that are oxygen dependent because they need oxygen as a co-factor. In the hypoxic state, there is inhibition of these enzymes, causing post-translational changes in nucleic acids, protein oxidation as well as lipid peroxidation. Studies have been done to show that structural and functional damage can occur to glycocalyx, which is the endothelial lining on the luminal side.  

There is evidence to state that ROS formed by one enzymatic pathway goes onto activate and speed up the production of more ROS by other pathways. The rate at which ROS are formed exceeds the rate at which they can be detoxified. These ROS then go onto activate neutrophils.[8] Adherence of these neutrophils to endothelial lining causes the release of cytokines and damage to the membrane along with activation of the entire inflammatory and complement cascade.

History and Physical

Once reperfusion injury has occurred, it can present as acute heart failure, cerebral dysfunction, SIRS (systemic inflammatory response system), and multi-organ failure. The most common presentation of reperfusion in the heart is ventricular arrhythmias, myocardial wall rupture, microvasculature abnormalities, and myocyte death. Myocardial stunning occurs post percutaneous coronary intervention (PCI) and is thought to be due to the oxidative stress and high levels of intracellular calcium.

In cases of cerebral ischemic injury, hemorrhagic transformation of the infarct or development of cerebral edema, which is evident by focal neurological deficits and decreased neurology is indicative of reperfusion injury. These changes are usually picked up on repeat CT scans of the brain. With limb ischemia, pain that is out of proportion to the physical findings, decreased pulses, skin changes, and decreased sensations are all indicative of a possible reperfusion injury. Patients can occasionally present with nasal or oral bleeding indicative of deranged coagulation or increased drowsiness or delirium suggestive of possible hepatic and renal shut-down. These are most often seen in cases of multi-organ failure due to reperfusion injury.

Evaluation

Following an episode of ischemia that has been treated with revascularization; the patient should be placed under strict observation. All baseline investigations such as complete blood count showing increased white blood cell count decreased platelets, renal function tests showing elevation in urea and creatinine levels, and liver function tests especially post-liver transplant or resection; all give clues towards a diagnosis of reperfusion injury. Coagulation profiles, electrolyte levels, and strict fluid input and output monitoring should be done to assess renal function.

Post myocardial infarction, repeat ECGs, and echo-cardiograph should be done to assess the myocardial function and for any possible arrhythmias. Any worsening seen in baseline and relevant investigation post-ischemic episode is usually warranted to reperfusion injury.

Treatment / Management

The main aim of treatment modalities is to decrease the formation of ROS, introduce monoclonal antibodies that will prevent the binding of inflammatory mediators to endothelial linings, and to decrease the neutrophilic activation. Ischemic pre or post-conditioning, antioxidants, and controlled reperfusion appear to decrease the degree of reperfusion injury. Pre-conditioning involves exposing the organ to small periods of ischemia before the actual ischemic insult to the organ takes place. On the contrary, post-conditioning involves partial or episodic, limited reperfusion of the ischemic organ; this helps to slow down the washout of adenosine, allows opioids and nitric oxide to form, as well as down-regulate tissue factor production.

Various studies are being done on animals to ascertain the efficacy of these treatment modalities. Therapeutic hypothermia is postulated to have a protective role in ischemia due to its effect on microRNA. Hypothermia also correlates with the upregulation of inhibitor of apoptosis stimulating protein of p53 (iASPP) and decreasing its target organs, which, in turn, has a neuroprotective effect. Hyperbaric oxygen therapy appears to reduce neutrophils and endothelial adhesion by preventing intercellular adhesion molecule-1 (ICAM-1) binding to CD18 due to inhibition of CD 18 polarisation. [9]This is mediated via nitric oxide and requires nitric oxide synthase. The role of peptides in protecting against cellular injury due to ROS due to their properties like low toxicity, good solubility, immunogenicity, and distinct tissue distribution pattern is also a topic of study.[10](B3)

Clinical trials are underway to assess the protective effect of hypercapnic acidosis in ischemic-reperfusion injuries, mainly involving the retina but also the central nervous system, lungs, and myocardium. Concluding evidence of these studies has shown that hypercapnic acidosis has an attenuating effect on the inflammatory, oxidative, and apoptotic processes. [11]

Further studies done regarding retinal ischemia-reperfusion injuries have shown beneficial effects of pioglitazone.[12] Its use has been associated with the inactivation of glial cells and, in turn, the process of gliosis and fibrosis as well as inhibiting apoptosis. The NF-κB pathway is possibly involved in the process mentioned above. Superoxide dismutase, catalase, and glutathione are some of the free radicle scavengers involved in the detoxification of the reactive oxygen species. The safety and efficacy of using recombinant; monoclonal antibodies against CD18 subunit of beta-2 integrin receptors in reducing leukocyte adhesion to endothelial linings are being studied. Animal studies using various methods of modulating the cytokine response have shown beneficial effects from modulation of IL-1 and TNF.[13][14][13](B3)

So far, treatment modalities have not shown promising clinical outcomes due to the following restraints such as multiple mechanisms involved in injury; mechanisms like neutrophilic activation cannot be entirely blunted due to its very essential role in healing; multiple co-morbid like diabetes, hypertension, and elevated lipid levels; and the inability to administer treatment at the optimal time.[15](B3)

Activated platelets and their aggregation along with P- selectins are thought to play a role in reperfusion injury; hence the use of glycoprotein 2b/3a inhibitors are said to have a protective role against reperfusion injuries. Comparative studies between using a combination of hydrogen and carbon monoxide vs. usage of only hydrogen gas to reduce the oxidative stress and reactive oxygen species have also been conducted. These studies concluded that dual gas therapy is more beneficial.[16] Adenosine has been shown to have protective effects against reperfusion injury by replenishing ATP reserves, causing vasodilation, and attenuating the adverse effect of ROS by neutrophilic and platelet aggregation inhibition.[17](A1)

Differential Diagnosis

Whenever there is an ischemic insult of an organ, its management involves revascularization or thrombolytic therapy. Following this management, if there is progressive clinical deterioration it is indicative of reperfusion injury. It is imperative to diagnose this as early as possible and to differentiate it from other post-ischemic complications such as arrhythmias due to possible electrolyte imbalance, activation of latent infections due to ischemic stress, blood loss resulting in hypovolemia in addition to ischemia resulting in acute renal shutdown or hypoxia-induced brain injury resulting in neurological and possible autonomic deficits.

Prognosis

The most important prognostic factor remains the duration of ischemia which goes on to determine whether the cellular injury is reversible or irreversible. In addition, the presence of other co-morbid as well as the history of prior ischemic insult to the same organ is associated with a worse prognosis. These risk factors include stenosis of >80%, pre-morbid hypertension, and poor collaterals.

In patients with ischemic stroke and embolism, endarterectomy can be used as a modality to restore blood flow. After blood flow restoration these people are at a danger of developing hyperperfusion or reperfusion injury. The potential risk of developing this can be estimated by transcranial doppler USG (TCD) or by estimating the cerebrovascular reactivity to CO2 by giving acetazolamide. Use TCD both post and pre-op to regulate cerebral blood flow.

Complications

Ischemia-reperfusion (I/R) injury is associated with numerous retinal diseases, such as diabetic retinopathy, acute glaucoma, and other vascular retinopathies. In the case of coronary ischemia, it can result in septal or ventricular wall rupture or arrhythmias. Cerebral ischemia can be complicated by the development of cerebral edema or hemorrhagic transformation of the infarct. When limbs are involved it can exacerbate the damage to endothelial lining, increasing capillary permeability and leading to edema. This can eventually result in acute compartment syndrome. Occasionally reperfusion injury is seen to activate the systemic inflammatory process and eventually result in multi-organ failure.