Ischemia-reperfusion injury (IRI) is a syndrome frequently encountered in veterinary medicine. It is characterized by a period of reduced or absent oxygen delivery to the tissue. The insult is therefore a two-step process that leads to both focal and systemic disturbances, ultimately leading to organ dysfunction and contributing to morbidity and mortality.

Clinical Scenarios

A broad range of diseases may lead to hypovolemia, which then may lead to system-wide ischemia. While those states are already potentially life-threatening, reperfusion injury may also develop following resuscitative efforts. Such examples would include hemorrhagic shock, hypovolemia due to severe vomiting, gastric dilation volvulus, etc.

Emergency and critical care personnel are also familiar with focal ischemia due to thrombosis, which may be followed by reperfusion upon clot lysis or removal. Aortic thrombosis in dogs and cats is a prime example.

Physiopathology

Mitochondria, small organelles, play a key role in normal cell function. They are responsible for energy production via the electron transport chain and ATP synthesis. Mitochondria are also responsible for the control of oxidative stress and cell apoptosis. Mitochondria play a key role in the initiation and propagation of IRI.

During the ischemic phase, due to reduced tissue oxygen delivery, ATP production is reduced. This leads to the accumulation of sodium and calcium inside the cells. This is also accompanied by acidosis, which further disrupts the cell machinery. This original injury culminates in cell swelling and membrane disruption.

After reperfusion, oxygen delivery contributes to the formation of reactive oxygen species in the face of dysfunctional antioxidant systems, leading to oxidative stress and further tissue injury. There is also concomitant inflammatory response. All of those effects combined explain the severe systemic derangements in patients suffering from IRI.

The process of oxidation describes the removal of electrons from an atom or molecule. The chemical species responsible for removal of the electron(s) is the oxidizing agent. The process of gaining an electron is called reduction, and the overall reaction is commonly referred to as a redox reaction. Molecular oxygen (O₂–dioxygen) is a very stable molecule and only a weak oxidizing agent, but it can be readily transformed into highly toxic, unstable substances known collectively as reactive oxygen species (ROS). Oxygen is reduced to water in the final step of the electron transport chain within the mitochondria. This requires four reactions and the addition of four electrons and four protons (hydrogen ions), as shown in the reaction below. When metabolic function is normal, approximately 98% of the oxygen metabolites are successfully converted to water, and only 2% escape into the cytoplasm. Molecular oxygen is reduced by the sequential addition of electrons to form the superoxide radical O₂, hydrogen peroxide H₂O₂, the hydroxyl radical OH, and, finally, water H₂O.

Figure 1

Some of these substances (O₂ and OH) are free radicals; an unpaired electron in their outer shell makes the molecules highly reactive and unstable. Every time a free radical interacts with a non-free radical molecule, it will make another free radical, perpetuating the damage. The only way the free radical state can be stopped is by two free radicals reacting with each other to form a non-radical end product. The most toxic free radical is the hydroxyl radical; it is capable of oxidizing any molecule in the body. Aside from the electron transport chain, enzymatic sources of free radicals include the xanthine oxidase system, NADPH oxidase system, and uncoupled nitric oxide synthase (NOS) system. Less important, non-enzymatic sources include myoglobin and hemoglobin.

Aside from mitochondria, neutrophils are the other major source of ROS in the body. Neutrophils use the enzyme myeloperoxidase to chlorinate hydrogen peroxide to produce hypochlorous acid (2HOCl), a very potent germicidal agent. When activated, neutrophils convert a large amount of oxygen into hydrogen peroxide, a process called the 'respiratory burst'. Only about 40% of this H₂O₂ is used to make hypochlorite; the remainder produces hydroxyl radicals.

Endogenous Antioxidants

Reactive oxygen species and other free radicals are constantly produced by mitochondria and at times by neutrophils. Numerous endogenous antioxidant systems exist to reduce the formation and facilitate the removal of these toxic substances. These include superoxide dismutase, catalase, glutathione peroxidase, vitamin E, vitamin C and albumin. When the production of ROS exceeds the capability of the endogenous antioxidant systems, oxidative tissue damage will result. This can occur due to an increase in the quantity of ROS produced, a decrease in the levels of endogenous antioxidants or a combination of both.

Tissue Damage

The three main targets of ROS in the body are deoxyribonucleic acid (DNA), proteins and lipids. Lipid peroxidation by free radicals damages the molecular structure of the lipid and generates new free radicals, further propagating the insult. The result is cell membrane disruption. Sulphydryl-containing proteins are also prone to oxidation, leading to inactivation of numerous cell functions. The overall result of ROS-mediated damage is cellular dysfunction/death (via apoptosis, necrosis, autophagy, or necroptosis) and inflammation.

Treatment

Overall, minimizing ischemia duration is likely to mitigate the severity of IRI. Cell preservation during the ischemic phase is the root of numerous research efforts, including medically induced hibernation or therapeutic hypothermia. Conventional efforts should also be implemented to treat electrolyte imbalance, hypovolemia, and vasoplegia. Whenever possible, reperfusion parameters, especially oxygen concentration, should be carefully titrated. Ongoing research efforts focus on mitochondrial rescue to control the severity of IRI. Likely, a comprehensive approach will ultimately be needed to reduce morbidity and improve patient outcomes.