Experimental Treatment in Non-Occlusive Mesenteric Ischaemia
Bailey et al. have demonstrated that mesenteric vasoconstriction is mediated in large part by the renin-angiotensin axis. Bailey and colleagues showed in pigs subjected to prolonged pericardial tamponade, that blockade of this pathway either by intravenously infused angiotensin converting enzyme inhibitor (ACEi) or by bilateral nephrectomy protected the small bowel from ischaemia. Further, phenoxybenzamine, an α -adrenergic inhibitor was not protective against the development of ischaemic changes; this finding contradicts the theory that the sympathetic nervous system primarily regulates mesenteric vasoconstriction. These findings may be difficult to extrapolate to hypovolemic-induced ischaemia given the different pathophysiology of tamponade-induced cardiogenic shock. A subsequent experimental study supported this study's findings: intra-arterial captopril given at the beginning of shock reduced the extent of tamponade-induced mesenteric ischaemia in piglets. The narrow window of time in which the ACEi must be administered to be effective, however, limits its clinical application, although the prophylactic use of ACEi in high-risk patients is an intriguing idea. The risk of systemic hypotension with an ACEi also needs to be considered in the clinical setting. Glucagon, histamine and perhexiline also have been shown to restore blood flow and oxygen consumption via vasodilation in dogs with digoxin-induced mesenteric vasoconstriction.
Iloprost, a synthetic derivative of prostacyclin, has been shown to be beneficial in patients with peripheral vascular disease; it is a potent inhibitor of platelet aggregation and may also promote fibrinolysis by decreasing an inhibitor of plasminogen activation. Additionally, in low doses, it has been shown to increase SMA blood flow and oxygen delivery in septic shock. Kang et al. recently conducted an experimental study to examine the effect of intravenous iloprost in NOMI secondary to cardiac tamponade: compared with a control group given normal saline, the iloprost group was found to have increased SMA blood flow, decreased intestinal mucosal hypercarbia and increased pHI; interestingly, mucosal oxygen consumption did not change significantly during its use. Unfortunately, the intestine was not studied at the conclusion of the experiment, thus making the significance of these findings less clear.
Ovine corticotropin releasing factor, sauvagine and urotensin I are three chemically related peptides that have been shown to be selective mesenteric vasodilators in a canine model. Each intravenously administered peptide improved mesenteric blood flow and decreased mesenteric vascular resistance in animals subjected to mesenteric hemodynamic changes induced by intravenous digoxin. In a follow-up canine study, intravenous urotensin I was found to be as effective as intra-arterial papaverine in correcting mesenteric blood flow and oxygen kinetics. Systemic side effects were not observed with intravenous urotensin I, but as expected, hypotension was marked when papaverine was given intravenously at sufficient doses to restore mesenteric blood flow to baseline values. While these studies are certainly intriguing, these peptides have yet to be examined for a possible clinical role in the treatment of NOMI in humans.
Finally, gastric inhibitory peptide (GIP) is a member of the secretin-glucagon-vasoactive intestinal polypeptide family of gastrointestinal hormones and has been shown to increase SMA blood flow selectively in a canine model. GIP has yet to be studied in NOMI.Mesenteric Venous Thrombosis
MVT may have an acute, subacute or chronic presentation. Patients with MVT are typically younger than those with other types of mesenteric ischaemia. MVT is associated with a myriad of hypercoaguable states (including protein C, protein S or antithrombin III deficiency, factor V Leiden mutation, anticardiolipin antibodies, malignancy, estrogens and pregnancy, and peripheral deep vein thrombosis), portal hypertension, intra-abdominal inflammation and sepsis, post-operative states, and trauma. MVT secondary to cirrhosis or neoplasm tends to start at the site of obstruction and propagate peripherally, while the opposite sequence is observed in hypercoaguable states. If collateral blood flow does not permit venous drainage around the obstructed vessel, the bowel will become congested and oedematous. Coincidental arterial vasoconstriction may occur and cause bowel infarction. Standard CT is the current diagnostic test of choice for MVT, unlike its use in patients with arterial occlusive or non-occlusive mesenteric ischaemia.
Treatment of MVT generally involves surgery, anticoagulation or both. Surgical intervention includes resection of necrotic bowel and/or thrombectomy; thrombolytics have been used successfully in case reports. Papaverine and second-look operations are advocated as appropriate, using similar guidelines as for arterial causes of AMI. Traditionally it has been taught that peritoneal signs warrant immediate surgical intervention, but Brunaud et al. in a retrospective review of patients with acute MVT found that peritoneal signs do not always indicate transmural necrosis, and may be seen with only mucosal necrosis, a situation which may be reversible with anticoagulation alone. Based on his comparison of patients who were treated surgically with those who were treated medically, he suggested that non-operative management of acute MVT may be a reasonable option provided the initial CT diagnosis is unequivocal and if bowel infarction has not led to transmural necrosis and bowel perforation.
Current recommendations for continued anticoagulation are based on whether an underlying hypercoaguable state is discovered, in which case lifelong coumadin is advocated. If no underlying thrombotic state is found, then anticoagulation is generally recommended for 3-6 months. Chronic MVT is primarily found in the setting of portal hypertension and is generally asymptomatic or presents with gastrointestinal bleeding from varices; treatment is aimed at control of haemorrhage.Focal Segmental Ischaemia
FSI is generally not a life-threatening condition as only short segments of bowel are involved and adequate collateral blood flow generally limits transmural necrosis. The aetiology of FSI is varied and includes atheromatous emboli, strangulated hernias, immune complex disorders and vasculitis, blunt abdominal trauma, segmental thrombosis, radiation therapy and oral contraceptives. Partial necrosis may manifest as acute enteritis, chronic enteritis (often difficult to distinguish from Crohn's disease) or a stricture. Definitive treatment of FSI is resection of the involved bowel.Chronic Mesenteric Ischaemia
CMI, also known as intestinal angina or claudication, almost always is caused by severe mesenteric atherosclerotic disease; over 90% of patients have occlusion of at least two of the major splanchnic vessels, and over 50% have occlusion of all three. Abdominal pain results from ischaemia in the small intestine as blood is 'stolen' from this organ to meet the heightened need for gastric blood flow as food enters the stomach. Classically, patients complain of abdominal pain that begins approximately 30 min after eating and gradually remits over the next 1-3 h; fear of eating (sitophobia) often leads to food avoidance and consequent weight loss. Diagnosis is made by clinical history, angiographic findings of at least two-vessel occlusion and the absence of any other cause for symptoms.
The mainstay of treatment for CMI is surgical bypass, although percutaneous angioplasty and stenting procedures have also been studied. Therapeutic outcomes may be difficult to assess as success has been defined in different ways, including graft patency, relief of symptoms and long-term survival. As reviewed in one recent article, means of 85% for long-term pain relief, 86% for graft patency and 7% for mortality rate were found for surgical revascularization. Presently, only retrospective reviews of percutaneous angioplasty and stenting are available and are hampered not only by their inherent suboptimal study design but also by lack of homogeneity in terms of patient population, procedural technique, post-procedural assessment and length of follow-up. Additionally, it may be difficult to compare surgery with percutaneous procedures as usually more vessels are bypassed in surgery than are treated percutaneously; further, generally only stenosed and not occluded vessels are treated percutaneously. Although initial success rates for percutaneous angioplasty and stenting are reported to range between 63 and 100%,[1,62] long-term efficacy is generally less than that for surgery; one recent study reported recurrent symptoms in 34% of patients at 3 years. More recently, a study looking at patients treated with only angioplasty plus stenting found success rates equivalent to those of surgery (83% symptomatic relief at 15 months, 92% stent patency at 6 months and a 10% complication rate). More data are likely needed before definitive conclusions can be reached.Experimental Treatment of Reperfusion Injury
As mentioned previously, reperfusion injury may be more detrimental than primary ischaemic injury, especially when ischaemia is short-lived. Reperfusion injury is likely multifactorial, but oxygen radicals and polymorphonuclear cells (PMNs) are thought to be integral. Xanthine oxidase-derived oxygen radicals stimulate leukotriene B4 (LTB4) and platelet activating factor (PAF) production, which in turn promote neutrophil adherence and migration; these leukocytes mediate microvascular injury by the release of proteases and physical disruption of the endothelial barrier. Because the two main effectors of reperfusion injury are reactive oxygen metabolites and PMNs, it follows that they are also the major foci of experimental studies aimed at reducing reperfusion injury. Printer- Friendly Email This
Aliment Pharmacol Ther. 2005;21(3):201-215. ©2005 Blackwell Publishing
This is a part of article Diagnosis and Management of Mesenteric Ischaemia Taken from "Metronidazole Flagyl 500Mg" Information Blog