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Reperfusion injury after intestinal ischemia: pathophysiology and experimental models*
(Portuguese PDF version)

Marcelo Eduardo Ribeiro¹, Winston Bonetti Yoshida²

1. Master's degree student, Faculdade de Medicina de Botucatu (FMB), Universidade Estadual Paulista (UNESP), Botucatu, SP, Brazil.
2. Adjunct professor, Vascular Surgery, Department of Surgery and Orthopedics, FMB, UNESP, Botucatu, SP, Brazil.

* Study performed at the Vascular Surgery Course, Department of Surgery and Orthopedics, Faculdade de Medicina de Botucatu (FMB), Universidade Estadual Paulista (UNESP), Botucatu, SP, Brazil.

Correspondence:
Marcelo Eduardo Ribeiro
Faculdade de Medicina de Botucatu.
Department of Surgery and Orthopedics.
CEP 18618-970 - Botucatu, SP, Brazil
Phone: +55 (19) 3534.2474
E-mail: angiomed@linkway.com.br

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ABSTRACT

The intestinal ischemia is unusual in vascular surgery emergency. Its main causes are embolisms and arterial thrombosis. In addition to severe ischemia, reperfusion of the ischemic tissues can lead to several complications that may worsen the ischemic lesion and produce a life threatening situation caused by systemic alterations. Intestinal tissue injuries due to ischemia and reperfusion have been demonstrated in clinical and experimental studies, in which pathophysiology and adequate treatment were also studied. The great variety of experimental models used and results achieved reflect the need for an intestinal ischemia and reperfusion experimental model that is simple, reproducible and consistent, in order to search for treatments that can reduce the damage caused by this situation. In this review, the main pathophysiologic aspects and the most used experimental models will be reviewed.

Key-words: ischemia, reperfusion, intestines.

J Vasc Br 2005;4(2):183-94

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The intestinal ischemia is a disease that occurs in the absence or reduction of arterial and/or intestinal venous blood flow, due to severe or chronic obstruction of the arteries and/or visceral veins, that is, of the celiac trunk (CT), the superior mesenteric artery (SMA) and/or the inferior mesenteric artery (IMA) and/or corresponding veins.¹

The etiology of the arterial obstruction in the severe mesenteric ischemia may be due to embolism, thrombosis, low blood flow, extrinsic compression, and vasospasm induced by vasoactive drugs; on the other hand, the venous occlusion may be mainly caused by venous thrombosis, infectious and inflammatory processes, and alteration of coagulation.

ARTERIAL EMBOLISM

In the arterial embolism, the main emboligenic sources are mural thrombi of the cardiac cavities associated to the myocardial infarction and atrial fibrillation,² but the origin can also be in the valvar disease or prosthesis, vegetation in bacterial endocarditis, auricular myxomas or myocardiopathy (in Brazil, Chagas disease). Embolism, being less frequent, can have its origin in the proximal aneurysm with thrombi or embolization of atheroma plaque of the proximal artery and cholesterol emboli associated with angiographic procedures. The SMA is the most affected in 5% of all intestinal embolisms.^3-5

ARTERIAL THROMBOSIS

The arterial thrombosis is initiated after the atherosclerotic plaque, which can have a degenerative or, more rarely, inflammatory origin. The atherosclerotic plaque is the most frequent cause of thrombosis and it is usually located in the artery ostium. Associations with hypercoagulability states, low cardiac output, aortic dissection, abdominal trauma, angiographic management, and use of hormonal contraceptive are frequent.

VENOUS THROMBOSIS

The venous thrombosis is the most frequent cause of venous occlusion and the superior mesenteric vein is affected in 95% of cases.⁶ The venous thrombosis occurs in 10% of intestinal ischemia,⁶ usually as a consequence of several clinical complications, such as intra-abdominal infections, hypercoagulability states, portal hypertension, inflammations, postoperative trauma or state, other risk factors known for venous thrombosis, such as smoking, alcoholism, oral contraceptive, malignant intra-abdominal neoplasia, etc.²

NONOCCLUSIVE INTESTINAL ISCHEMIA

In the nonocclusive intestinal ischemia, a mechanical obstacle cannot be seen, that is, there is no evidence of arterial or venous occlusion, but a reduction in the intestinal blood perfusion, due to severe splanchnic vasoconstriction. It corresponds to 20 to 30% of acute mesenteric ischemia⁶ and affects patients with severe cardiopathies, receiving digitalis, who are frequently hospitalized due to recent worsening of such cardiopathy or due to a severe event (infection, trauma or major surgery). It can also occur after exhausting exercises (long-distance runners), use of vasopressor drugs in the postoperative of cardiac surgery,^7,8 intoxication due to cocaine, ergotamine, and diuretics.^9-11 In case of low cardiac output, low perfusion (shock of any type of etiology) or severe peripheral hypoxia, the cerebral circulation and the vital organs are given priority, to the detriment of the splanchnic circulation. Those alterations are the cause of the fall in the perfusion pressure on the splanchnic capillary bed and the collapse of the capillaries. The increase in catecholamine, angiotensin II, and circulating vasopressin contributes to the deterioration of the clinical picture and is intensified by the use of alpha-adrenergic vasoconstrictors and digitalis.⁶

Although intestinal ischemia is not a frequent disease, it presents high mortality rates, which may range from 60 to 100% of cases.^12-16 It is diagnosed in 2% of all necropsies and occurs in 800 out of 10,000 hospital admissions.¹⁴ The incidence of intestinal ischemia is likely to be higher than what is admitted. In our environment, it is difficult to obtain data from the Data SUS system (Database of the Brazilian Health Care System), since it does not classify the disease specifically as intestinal ischemia, but as generalized circulatory disorders. In a retrospective study performed in the epidemiological surveillance in the town of Rio Claro (SP, Brazil), with a population of 200,000 inhabitants, an average rate of 0.36% of deaths related to intestinal ischemia was found from January 1997 to November 2002. Such data only concern death certificates, since the town does not have a death verification service available.

High mortality rates may be related to the difficulty of making an early diagnosis of the intestinal ischemia, as well as to the lack of specificity of the abdominal pain and available complementary examinations. If an early re-establishment of arterial flow does not occur, it causes wide resections of intestinal segments, which is usually responsible for the short bowel syndrome. This syndrome is characterized by diarrhea, fluid and electrolyte abnormalities, malabsorption, and weight loss. It causes inconveniences of parenteral and enteral feeding and can lead to sepsis and death.

Recent progresses and improvements in work-up methods, along with advances in the maintenance of the nutritional state and the hydro-electrolytic and acid-basic balance of patients are improving the prognostic for this disease, which depends on an early diagnosis. In the first 8 hours, the artery repair using embolectomy or thromboendarterectomy and revascularization through aorto-mesenteric shunts are indicated. Between 8 and 22 hours, the risk for such operations is highly increased, due to metabolic alterations, mainly hyperpotassemia.¹⁷

Complementary examinations which may help in the diagnosis are blood count, simple X-ray, duplex scanning, angiography, computed tomography, and resonance angiography. Among those, the selective angiography is the gold standard examination.

The patient's initial approach is made by support measures, such as gastric drainage using a probe, hydroelectrolytic reposition, antibiotic therapy, cardiac insufficiency compensation, and shock treatment.

The treatment of the arterial ischemia consists of recognizing the lesion as early as possible and re-establishing blood flow. This early treatment rarely occurs, because the disease is not suspected by the surgeon or clinician most of the times. If there is still doubt as to the intestinal viability after the removal of the embolus or thrombus, the abdominal wall can be closed with total stitches, performing a new abdominal exploration 24 hours later (second look), as suggested by Shaw & Maynard¹⁷ for assessing viability of intestinal loops. Regarding the mesenteric venous thrombosis, in case there are no signs of peritoneal irritation, it can be conservatively treated with anticoagulants or with the via intra-arterial infusion of thrombolytic agents, performed through catheterization of the SMA. In case there are signs of peritoneal irritation, a surgery for attempting thrombectomy is indicated, which is an uncommon operation, due to the diagnostic delay^18,19 or, more frequently, resection of the nonviable intestine and its mesentery.

In the postoperative of arterial and venous thrombosis, patients must be maintained anticoagulated for a long time, in order to avoid risk of recurrence.²⁰

Despite the advancements in the treatment and support measures, the disease still presents a high morbidity and mortality rate.

To improve this situation, it is necessary to have a better knowledge of the pathophysiology of alterations that are a consequence of intestinal ischemia and reperfusion, as well as of combined therapeutic measures. In this sense, the experimental study in animals has provided a valuable contribution, and the knowledge acquired from such studies will be described in this review.

ISCHEMIA AND REPERFUSION IN THE INTESTINAL ISCHEMIA

Several mechanisms could be involved in the production of intestinal lesions after ischemia and reperfusion of splanchnic organs, which are usually attributed to the circulatory shock that is present in this situation.^21,22

During ischemia, there is lesion of intestinal mucosa, increase in microvascular permeability, fluid loss in the intestinal lumen, release of lysosomal hydrolases, increase in proteolysis and release of myocardial depressant factor in the circulation and circulatory shock, creating a vicious cycle in which such alterations cause a depression of cardiac function, which, in its turn, provokes progressive deterioration of the intestinal perfusion.

Those alterations are aggravated by reperfusion, since it triggers the accumulation of free radicals, which attack and damage the cellular membranes, attract neutrophils, and stimulate the release of inflammatory mediators.^21-26 Among all these alterations, the increase in capillary permeability and production of oxygen free radicals are the main determining factors of hemodynamic instability and subsequent mortality.²⁷

One of the most important factors that cause intestinal lesion after reperfusion is the production of oxygen free radicals through the hypoxanthine-xanthine oxidase system.^21-24,28 During ischemia, there is a reduction in the oxygen transport to the affected tissue, leading to the inhibition of the oxidative phosphorilation in the mitochondria and to the loss in the production and storage of adenosine triphosphate (ATP). However, the storage of ATP would still be consumed and would be decomposed to adenosine diphosphate (ADP) and adenosine monophosphate (AMP), and, afterwards, in adenosine, inosine, and hypoxanthine.²⁹ The lack of cellular energy would cause the failure of the sodium-potassium pump (Na⁺/K⁺). Due to the failure of the pump, there would be a higher accumulation of intracellular Na⁺ and loss of K⁺ in the cell, with consequent edema of the cell and its organelles. Concurrently, there would be a Ca⁺⁺ and chloride inflow to the intracellular environment, accumulating Ca⁺⁺ in the cytosol (Figure 1).

Figure 1 - Sequence of chemical events that occur during ischemia and reperfusion.

This accumulation of Ca⁺⁺ in the cytosol would provoke the activation of the protease calpain that, in its turn, would break the peptide bond of the enzyme xanthine dehydrogenase (XD), leading to the formation of the enzyme xanthine oxidase (XO).³⁰ Differently from the XD, the XO needs oxygen to make the conversion of hypoxanthine to xanthine. During ischemia, however, there would be an accumulation of those two substances.^31,32 With reperfusion, hypoxanthine would then be oxidized to xanthine, which would be oxidized to uric acid, having the superoxide anion formation as a byproduct (Figure 1). The superoxide, which is unstable, is transformed into hydrogen peroxide, spontaneously or by the action of the superoxide dismutase (SOD). Hydrogen peroxide, in its turn, is transformed into water by the action of catalase and glutathione peroxidase (Figure 1). Since the substrate (hypoxanthine) for xanthine oxidase was accumulated during ischemia, the production of free radicals in the reperfusion blocks the neutralization ability of the endogenous antioxidants, so those radicals start performing deleterious actions.^21,31,32 The superoxide radical causes the ferrous ion release from ferritin, which reacts with the hydrogen peroxide, forming the highly toxic hydroxyl radical.³¹

Free radicals and particularly the hydroxyl radical initiate the peroxidation of cellular membranes, releasing arachidonic acid and lipid peroxyl free radicals. The arachidonic acid is metabolized by the cyclooxygenase in thromboxanes, prostaglandins PGE1 and PGI2, or by lipoxygenase in leukotrienes LTB4, C4, D4, and E4.²⁹ The peroxyl radical causes additional lipoperoxidation, removing one hydrogen from the fatty acid, originating a chain reaction. The final product is the malonyldialdehyde (MDA), whose measure is used as a marker for ischemia and reperfusion lesions^{21,23,24,33,34} (Figure 2).

Figure 2 - Scheme of the lipid peroxidation in the fatty acids of the cellular membrane. The free radical OH" removes a hydrogen from the fatty acid, originating the lipid radical. The lipid radical, since it is unstable, suffers a molecular rearrangement and forms a conjugated diene, which may react with an oxygen molecule and form a peroxyl radical. The peroxyl radical removes a hydrogen from the fatty acid, originating a chain reaction. (Modified from Del Maestro ³⁵).

Free radicals can also act indirectly, attracting and activating neutrophils in involved tissues. Activated neutrophils secrete proteolytic enzymes (myeloperoxidase, elastases, proteases, etc.), synthesize prostaglandins, release free radicals, besides occluding the microcirculation during reperfusion, making blood flow difficult. This occlusion on the microcirculation is called the no-reflow phenomenon.³²

The increase of such products of lipoperoxidation in the plasma and intestine correlates to the epithelial desquamation, formation of ulcers and hemorrhage.²² Histological alterations in the intestine after ischemia and reperfusion may vary from desquamation of the epithelial covering of the villi (particularly their apex) to epithelial necrosis and the decomposition of the lamina propria, with subsequent ulceration and hemorrhage^28,36 (Figure 3).

Figure 3 - Histopathologic grading for intestinal ischemia and reperfusion lesions in rats, according to Chiu et al.³⁷

Clinical and experimental studies

Clinical studies

Considering the severity of the disease, there are only few clinical studies, besides not being conclusive, since the sample was small, certainly due to the fact that it is not a frequent disease. Most existing studies are case reports, from which it is not possible to standardize a consensual and adequate conduct.

In the clinical studies performed to evaluate the presence of lesions as a consequence of intestinal ischemia and reperfusion, the authors focused on the thoracic and abdominal aortic clamping. Based on this, they studied the action of antioxidant solutions for attenuating the lesions caused by that situation, using parameters such as measurement of plasmatic levels of free radicals, MDA, lipoperoxides, and control of hemodynamic stability.^33,38

Experimental studies

Experimental studies are a major reference source in the search for solutions for this serious problem, despite the variability in types of animals, ischemia time, parameters used for assessing variables and treatments.

Generally speaking, studies on ischemia and reperfusion are characterized by, at least, two different experimental stages: one ischemia stage, with occlusion of a vessel or feeding vessels of a certain organ or tissue to be studied, or by the reduction in the circulating flow, as in the case of ischemia by controlled hypovolemic shock; and the reperfusion stage, with the reopening of the previously occluded vessel through correction of the shock, in a way that there is an adequate reperfusion in the ischemic tissue.

Types of animals

In a bibliographic survey in MEDLINE, Lilacs, and SciELO databases with the key words ischemia, reperfusion, intestinal, experimental, and animal, 861 references were found in the period from 1980 to 2004. The distribution of animals can be seen in Figure 4.

Figure 4 - Types of animals used in the studies. Intestinal ischemia/reperfusion.

The rat was most frequently used, because it is easy to work with, besides presenting protocol adequacy and availability. The visceral anatomic constitution of the rat is quite similar to men's, which makes it possible to extrapolate, to a certain extent, much of the knowledge acquired for men. Moreover, it is an animal resistant to anesthesia, both inhalation and intravenous, small-sized, presenting an easy surgical management for approaching the arteries, as well as having a low cost. Those characteristics provide the grouping of a higher number of individuals, making the statistical evaluation easier. On the other hand, cats,³⁶ pigs,^39,40 and dogs,⁴¹ for being large-sized, need a more sophisticated surgical technique, more surgical time and cost, restricting the number of individuals by studied group. Besides, some researchers do not feel at ease to use pets, such as dogs and cats, in experimental studies. However, such animals have the advantage of having the macroscopic lesions easier to be observed.

Ischemia production

The most used method for producing ischemia was the arterial obstruction by vascular clamping, followed by stenosis or extrinsic vascular compression, surgical tapes to controlled release of blood flow, hypovolemic shock, and hypothermia.

The methods proved to be efficient, but the preference by the vascular clamping might be because it represents more accurately the clinical situation. Moreover, other methods may not simulate that situation, since they depend on the researchers when they control the force applied in the compression and the control of the release, which may not be equal in all animals, generating different levels of pressure on the arteries and structures, leading to alterations in the variables to be analyzed.

Vascular territories studied

The temporary occlusion of the CT and SMA was the most used. However, in many models the occlusion of the CT, SMA or IMA was made separately, as well as the CT, SMA and IMA simultaneously, besides the occlusion of the thoracic aorta and abdominal aorta.^42-49

Most authors made the obstruction of the CT or SMA,^27,50-57 but the obstruction of other arteries was necessary in some experiments to better compare the collateral circulation, which varies according to the species, according to the experiment performed by Deune & Khouri,⁵⁸ in which a comparison between two difference races of rats (Sprague-Dawley and Lewis) was made, with epigastric flaps submitted to ischemia from 10 to 16 hours, followed by spontaneous reperfusion, without the use of antioxidant drugs. The authors observed a statistically significant difference in the survival rate of flaps between the two races for ischemia times of 12 and 14 hours, showing that the Lewis rat could stand ischemia better than the Sprague-Dawley rat.

Ischemia and reperfusion time

The choice of ischemia and reperfusion times has a major importance, since the intestinal mucosa presents alterations in its microvascular permeability with 1 hour of ischemia, characterized by edema and fluid loss in the intestinal lumen,²³ and, after 1 hour of ischemia, the mucosa is seriously damaged, presenting ulcerations and hemorrhage. According to Schoenberg & Berger,²³ an ischemia of more than 2 hours would cause irreversible damage to the intestinal mucosa after the reperfusion. Thus, depending on the ischemia time, alterations can be predominantly molecular or biochemical, progressing to histological alterations that, in their turn, can vary from minor lesions to tissue necrosis. The tolerance of tissues and organs to ischemia is variable, multifactorial, and depends on the ischemia time, metabolic needs of each tissue cells, transport of collateral circulation, and local humoral factors. Thus, when standardizing an experimental model, ischemia time cannot make the tissue lesions become irreversible, not allowing the repair of involved organs and tissues.

The determination of a short, medium or long ischemia time would be related to the type of alterations one wishes to observe. Times too short may not cause measurable alterations, besides not stimulating a real practical condition of critical ischemia time, causing false results. On the other hand, alterations would be irreversible in times that are too long.

Parameters

Several parameters were used aiming at evaluating and adequately estimating alterations produced by ischemia and reperfusion, among which we can cite: hemodynamic (mean blood pressure, heart rate), biochemical (arterial pH, plasmatic MDA,^21,33,34 tissue MDA,^21,34 lipohydroperoxides, measurement of antioxidant enzymes, free radicals, ATP, myeloperoxidase, thromboxanes, etc.) and histological,^36,55,59-61 considering that the different variables were studied according to the main focus of the work, whether towards a biochemical, morphological or hemodynamic study.

In case of a study with a more therapeutical approach, an evaluation with all those parameters would be theoretically justified, since alterations and impacts would be expected in all aspects of the clinical practice. Nevertheless, a more directed approach could deepen the specific knowledge on a determined pathophysiologic aspect, as well as contribute for searching subsides for strategies in the attenuation of lesions caused by ischemia and reperfusion in several organs and tissues.

Treatment

Several potential alternatives have emerged from the researches, as a consequence of the great number of investigations. Some have already been clinically used, as an attempt to attenuate lesions. Among those, we can cite: enzymatic water-soluble endogenous antioxidants (SOD, catalase, glutathione peroxidase), fat-soluble antioxidants (tocopherols, carotenoids, quinones), allopurinol (xanthine oxidase inhibitor), mannitol, calcium channel blockers, specific antagonist of leukotrienes, platelet-activating factor (PAF) receptors, iron chelators, leukocyte filters, and different forms of leukocyte depletion, polynitroxyl-albumin (PNA), pyruvate, L-arginine, combination of drugs, controlled reperfusion, ischemic preconditioning, etc.⁶²

The use of a single drug or procedure did not cause any impact on the clinical practice. A wider approach to the ischemia and reperfusion process, using several treatments with different action levels, might present a more satisfactory response than the use of an isolated drug, due to its multifactorial pathophysiology.

However, despite all the basic knowledge acquired on free radicals, few practical benefits have emerged up to the present moment.⁶³ The basic knowledge provides an evidence that the protection of post-ischemic tissues must be directed towards the neutralization of free radicals, neutrophils in the inactivation of enzymes (xanthine oxidase), chelation of transition metals, and blocking the chain reactions of the lipid peroxidation.

The use of SOD has caused controversial results, mainly in studies on the limitation of the myocardial infarct area^64,65 and has been of little value for that type of patient.⁶³ The inefficiency in practice is attributed to the difficulty of the SOD to enter the intracellular space, which forces its use in high doses.

To reduce the problems of continuous infusion, both of SOD and catalase, such enzymes have been combined to inactive macromolecules, such as Ficoll, Dextran, polyethylene glycol or packaged in liposomes. It increases their half-life to 30-40 hours. Polyethylene glycol, in particular, can promote its penetration and fixation in the intracellular environment.²³ Such modifications are being object of study.²³

Other antioxidants have shown variable cytoprotective effects in experimental models. Vitamin E, carotenoids, propanolol, calcium channel blockers, lipoxygenase inhibitors (nafazatrom), trimetazidine, and mercapto-propionyl-glycine are part of this group. In our environment, minimizations of ischemic and reperfusion lesions in posterior limbs of rats using vitamin E have been demonstrated.⁶⁶ However, there is still no consensus on whether any of those agents offers a significant protection to the oxidative damage in the clinical practice.^67,68

The metabolic interactions among the vitamin E, selenium, and sulfuric amino acids have, for a long time, interested researchers who study antioxidant nutrients.⁶⁹ The discovery of the participation of the selenium in the constitution of the glutathione peroxidase enzyme has risen other interaction possibilities with the vitamin E. According to Hoekstra,⁷⁰ the vitamin E could act as a trap for the free radical, preventing the formation of hydroperoxides in the membranes, and the selenium, via glutathione peroxidase, could neutralize any lipid hydroperoxide formed by the peroxidation of polyunsaturated fatty acids that escaped from the protective action of the vitamin E. In a study made in experimental model of ischemia and reperfusion in abdominal visceral organs of rats, Yoshida et al.⁵² tested the vitamin E, the sulfuric amino acid taurine, and the selenium. Only the selenium was able to reduce the alterations originated from the oxidant lesion in those animals.

Among the blockers of the hydroxyl radical, the mannitol has been used in clinical practice with this objective for many years.⁷¹ Moreover, mannitol has the advantage of inhibiting the synthesis of thromboxane B2⁷² through its antioxidant action. It can also offer renal protection after revascularization of the limbs due to its known diuretic action. When used in the dose of 0.20 g/kg, it was able to reduce permeability and pulmonary edema after the surgery of the abdominal aortic aneurysm, independently from its osmotic diuretic action.⁷² In our environment, it was experimentally demonstrated that the aortic occlusion for 60 minutes in dogs, followed by reperfusion, caused an increase in the levels of malondialdehyde and the formation of oxygen free radicals in this procedure.⁷³

The allopurinol, due to its inhibiting action of the xanthine oxidase, is a medication traditionally used with good results in the treatment of gout. The allopurinol has a structural conformation similar to the hypoxanthine and, therefore, competitively inhibits the xanthine oxidase, reducing the production of the superoxide anion.⁶⁸

Metal chelating agents inhibit the reaction of Fenton catalyzed by the metals, preventing the formation of the hydroxyl radical (OH). Deferoxamine (DFO) is a drug which derivates from the bacterium Streptomyces pilosus. It has been approved for use in the treatment of acute and chronic iron intoxications.^23,59 DFO has proven to have a beneficial effect as an antioxidating agent, both experimentally and clinically, when used in situations of ischemia and reperfusion.^61,74-76 The potential benefit of the DFO was clinically demonstrated in patients submitted to extracorporeal circulation, in which the DFO was administered via IV and in the cardioplegic solution. In those patients, isolated blood leukocytes in the right atrium produced significantly less superoxide radicals than the control group patients.⁷⁷ In a subsequent study, the DFO was administered to the same type of patients, resulting in a reduction in the susceptibility of circulating low-density lipoproteins to peroxidation. Despite those studies, the administration of DFO has limitations, once it has a rapid excretion and several toxic side effects, such as myocardial depression and hypotension, which occur mainly when high doses are used.⁵⁹ To avoid such problems, several experimental studies have been made, combining DFO with high molecular weight polymers, such as the hydroxyethyl starch or Dextran.⁵⁹ Such combinations increase the half-life of the drug in the intracellular space.⁷⁸ In spite of that, several experimental studies have showed beneficial effects of those derivates in situations of ischemia and reperfusion.⁵⁹

Neutrophil adherence can be inhibited with the use of PAF antagonists and 5-lypoxygenase inhibitors.⁷⁹ To achieve the same objective, monoclonal antibodies against the CD11/CD18 complex have already been successfully tested in experimental models of cardiac ischemia and reperfusion in dogs.⁸⁰ However, there are still no prospective clinical studies able to support the routine use of such products in ischemia and reperfusion. More clinical studies are required for the use of the TGF-B (which inhibits the neutrophils adherence to the endothelium), adenosine (which inhibits the production of free radicals by neutrophils), perfluorochemicals (which inhibits chemotaxis of neutrophils and lysosome degranulation), and antiproteases.³¹

There is still the possibility to reduce the ischemic lesion of the cell through hypothermia, a frequently used procedure in cardiac surgery, neurosurgery, and urology. The reduction in the metabolism preserves the ATP and, therefore, reduces the production of active species after reperfusion.³¹

Moreover, in several previous studies, methods capable of reducing the biophysical and biochemical alterations after ischemia and reperfusion in the splanchnic territory were tested. In general, the administration of antioxidants, xanthine oxidase, iron, and leukocyte inhibitors offered a minimization of the effect of free radicals in that model.^{34,37,42,45,46-50,52,60,61,63,81-83}

Models

The improvement of an experimental model to understand the pathophysiology and clear the role of the oxygen free radicals in the pathogenesis of ischemic and reperfusion lesions, as well as to search for ways of minimizing the hemodynamic and pathophysiologic impacts in the post-ischemic vascular repair in several tissues and organs is a constant search, and the use of experimental models in animals has been frequent.

Experimental models with occlusion and reperfusion of the celiac arteries and/or SMA in rats simulate all the biochemical and structural alterations mentioned above, besides causing a major hemodynamic disorder called "splanchnic artery occlusion shock" - SAOS.^25,50

Despite the great variety of models, animals, and parameters, experimental models have been very useful in the study of the pathophysiology and treatment of the intestinal ischemia and reperfusion syndrome. However, they have been little used in the clinical practice,⁵⁹ since the heterogeneity of the population and, consequently, the need for large casuistics cause difficulties to perform clinical trials. There is also the tendency of not publishing negative results and the inadequate design of some clinical research that do not take into consideration the "therapeutic window",⁸⁴ such as, for instance, the allopurinol used in cardiopulmonary bypass in rats, which offers cardiac protection during reperfusion after 30 minutes of ischemia and any protection in lower or higher periods of ischemia.⁸⁴ However, there is still the need to create a simple and reproducible model. The model in rats, as previously mentioned, is the one that most approaches the ideal model. Even so, more studies are necessary to solve this problem.

CONCLUSION

In the surgical practice, there is still no effective adjuvant medication therapy that can be introduced and improve the alterations caused by ischemia and reperfusion. In the experimental models of intestinal ischemia and reperfusion, the use of some medications, mainly those that interfere or block free radicals, reactive oxygen species and neutrophils, showed a capacity to inhibit the tissue lesion, at least partially. Some authors suggest that an antioxidant cocktail might be necessary to neutralize all the active oxygen species generated during ischemia and reperfusion.⁶⁶ Nevertheless, new studies with substances that have presented promising results and also with new substances are necessary, as an attempt to evaluate ideal doses, medication interactions, and the ideal moment for starting and ending the treatment.

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