Acute Mesenteric Ischemia In The Elderly
Acute Mesenteric Ischemia (AMI) leading to bowel infarction is a relatively common catastrophic condition of the elderly. It is one of those conditions where the diagnosis is impossible, the prognosis hopeless, and the treatment almost inconsequential if diagnosed too late. AMI is a true surgical emergency. Because time is of the essence, prompt diagnosis is essential. And because there really is no classical presentation for the condition, in order to make the diagnosis physicians must maintain a high index of suspicion.
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AMI can be occlusion or non-occlusive. Occlusive AMI (OAMI) occurs when the blood flow through the mesenteric artery to the intestine is blocked. The specific causes of OAMI are acute thrombotic and acute embolic mesenteric artery occlusion. The sources of emboli are from the heart secondary to blood stasis due to atrial fibrillation, valvular lesions or a mural thrombus after a myocardial infarction. Thrombosis typically occurs at the artery origin, resulting in more structures affected by the occlusion, thus causing more damage and a lower survival rate than embolization.
Nonocclusive AMI (NAMI) can occur without vascular abnormalities. Usually there is underlying atherosclerosis of the mesenteric artery and there is poor perfusion to the intestine because of congestive heart failure, myocardial infarction, or hypovolemia. It is typically a syndrome of vasospasm and constriction that occurs in critically ill patients who are in "low-flow" states from septic shock, cardiogenic shock, burns, or hypovolemic shock.
When blood flow to the small or large intestine is decreased, ischemia, bowel infarction, tissue necrosis, sepsis and death can result. Within 4-6 hours after ischemia begins, the intestinal mucosa becomes necrotic and full thickness infarction occurs thereafter. If left untreated, patients will hemorrhage into their bowel, perforate, become septic, and die.
Risk factors for AMI include atherosclerosis, arrhythmias, hypovolemia, congestive heart failure, recent myocardial infarction, valvular disease, advanced age, and intra-abdominal malignancy.
AMI accounts for 0.1% of all hospital admissions. In fact, mesenteric artery stenosis is found in 17.5% of elderly adults. Emboli and thrombosis account for about 70-80%% of AMI, while NAMI accounts for 20-30%.
History and Physical:
The diagnosis of AMI may be overlooked because of the vague nature of the patient's symptoms. The constellation of symptoms and signs include: a history of postprandial pain (pain after eating), either acute or sub-acute onset of generalized abdominal pain, diarrhea or constipation with occasional blood in the stool, the pain is typically out of proportion to findings (that is, there are no peritoneal signs).
Usually the white blood cell count will be elevated with an increase in immature cells (so called "left shift"), amylase and lipase are often elevated, and a sedimentation rate will be increased. Also, the patient will usually have metabolic acidosis.
Imaging studies will make the diagnosis of AMI. Plain films will suggest the diagnosis when there is the so-called "thumb print" sign representing submucosal hemorrhage and edema in the colon. Also, free air from colon perforation can be seen. CT scanning has 95% specificity, but the gold standard for the diagnosis is arteriography.
Nonocclusive mesenteric ischemia is treated medically, while acute and chronic occlusive ischemia is correctable with surgery. If a patient is found to have vasospastic (NAMI) disease, direct injection of papaverine into the SMA may resolve the vasospasm. If hypovolemia is considered likely, fluid resuscitation is required. All patients with possible bowel ischemia should be started on broad-spectrum antibiotics to cover the possibility of bowel necrosis and infection.
Surgical therapy involves having the surgeon examine the bowel during surgery to identify the affected areas. Every effort is made to retain viable bowel. Patients with emboli are treated definitively with embolectomies. Removing thrombi can be more challenging because of multiple locations and greater mass.
Because of the high prevalence of atherosclerosis, one of the most common complications involves myocardial infarction. This can be prevented by identifying correctable coronary artery disease before the patient enters the operating room. A Swan-Ganz catheter should be used to monitor fluid and cardiac function. The anesthesiologist can use myocardial protective maneuvers and afterload reduction to maximize cardiac output.
Acute renal failure in the immediate postoperative period can be prevented by keeping the patient well hydrated and administering mannitol before the aorta is cross-clamped.
Other possible complications include bleeding, infection, bowel infarction, prolonged ileus, and graft infection.
Outcomes and Controversies:
Because of the delay in diagnosis, mesenteric artery ischemia is typically a lethal disease, with a mortality rate of 45-65%. When more than half the bowel is removed, mortality rates of up to 80% have been reported. A recent review of 45 studies has shown that the prognosis for patients with acute mesenteric ischemia differs when one looks at the etiology. Mortality rates are highest for patients with arterial thrombosis (70-87%), followed by nonocclusive mesenteric ischemia (70 -80%), and arterial embolism (66-71%).
Over the past 20 years, diagnosis and treatment of AMI has advanced only minimally. The following treatments are presently being evaluated in multiple studies: Percutaneous transluminal angioplasty with stenting, thrombolytic therapy if patients can be treated within 8 hours of presentation and do not have signs of bowel necrosis or peritonitis, and local tissue plasminogen activator to reduce the amount of bowel requiring resection.
Missed or delayed diagnosis of AMI with all its complications is a major cause of medical malpractice litigation. Delayed clinical and radiologic diagnosis leads to greater risk of complications, particularly infarction, with associated high mortality rates and poor outcomes. Physicians must maintain a high index of suspicion for AMI when faced with patients who meet the clinical, laboratory and radiographic criteria for the condition.
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