23.1 Acute Mesenteric Ischemia

The major causes of acute mesenteric ischemia are embolic obstruction thrombosis of the superior mesenteric artery (SMA), mesenteric venous thrombosis and nonocclusive ischemia. The congenital hypercoagulable states due to protein C or S antithrombin III deficiency can also cause thrombosis of the superior mesenteric vein. Embolic obstruction of the superior mesenteric artery is usually associated with cardiac arrhythmias, valvular disease, recent myocardial infarction or mycotic aneurysm. When an embolus lodges at the origin of the superior mesenteric artery, the entire small bowel and proximal colon are affected. Mesenteric venous thrombosis usually involves the superior mesenteric vein or its branches and the portal vein. It can be "primary" or "secondary" to a variety of hypercoagulable states (e.g., polycythemia rubra vera, carcinomatosis, oral contraception); to intra-abdominal sepsis (e.g., cholangitis, diverticular abscess); or to a condition in which blood flow is impaired (e.g., cardiogenic shock).

Nonocclusive bowel ischemia is the most common and lethal form of intestinal vascular insufficiency, accounting for at least 50% of all cases, with a mortality rate approaching 100%. It is commonly associated with reduced cardiac output, intra-abdominal sepsis and advanced malignant neoplasms. Digitalis constricts the splanchnic circulation and may aggravate or even precipitate mesenteric ischemia.

The typical patient is over 50 years of age, with arteriosclerotic or valvular heart disease, poorly controlled long-standing congestive heart failure, hypotension, recent myocardial infarction or cardiac arrhythmias. Abdominal pain is characteristically periumbilical and crampy. In the early stages, physical signs are often minimal. The abdomen is soft, sometimes slightly distended, with mild tenderness on palpation. Abdominal pain of any degree of severity associated with minimal abdominal findings and a high WBC (often over 20,000/mm²) is an important early clue to the correct diagnosis. Signs of advanced ischemia include nausea, vomiting, peritoneal irritation, leukocytosis and a progressive metabolic acidosis. In a minority, unexplained abdominal distention or gastrointestinal bleeding, or the rapid onset of confusion and acidosis in an elderly patient, may be the first manifestation of small bowel ischemia.

Initial resuscitation is directed at correcting the predisposing or precipitating cause(s). Restoration of cardiac output with IV fluid is paramount. Digitalis, diuretics and vasoconstrictors should be discontinued if possible. Plain radiographs, ultrasound or CT scans as appropriate should exclude other radiologically diagnosable causes of acute abdominal pain. After volume repletion, the key step in the management of acute mesenteric ischemia is abdominal angiography. Remember that angiography in a hypovolemic or hypotensive patient frequently shows mesenteric vasoconstriction; for such patients the technique loses its usefulness as a diagnostic tool. Also, angiography in a volume-depleted patient may precipitate renal failure. If the angiogram is normal, the patient should be carefully observed, and a diagnostic laparotomy performed only if peritoneal signs develop. If the angiogram shows a minor arterial occlusion and clinically there is no peritoneal irritation, papaverine can be infused into the superior mesenteric artery through the catheter used for angiography at a rate of 60 mg/hour. (The role of angioplasty or other angiographic techniques remains unproven.) If peritoneal signs occur at any time, a laparotomy with resection of the ischemic segment is indicated. If the angiogram shows a major obstruction at the origin of the superior mesenteric artery, laparotomy should be carried out immediately. An embolus can usually be easily removed, while thrombotic obstruction requires a bypass graft from the aorta to an area of the artery distal to the site of obstruction. After revascularization, any nonviable bowel should be resected. It is advisable to save all bowel that may be viable and to re-explore the patient 24 hours later. The decision to perform a "second look" operation is made at the initial laparotomy and should not be changed on the basis of a favorable postoperative course. Since acute occlusion of the superior mesenteric artery is associated with prolonged vasospasm, the artery should be perfused with papaverine for 24 hours postoperatively.

If nonocclusive splanchnic vasoconstriction is present, intra-arterial papaverine infusion should be started. If, in spite of the infusion, abdominal pain persists and signs of peritoneal irritation appear, a laparotomy must be performed without delay. Venous thrombosis is characterized on the angiogram by a prolonged arterial phase and a lack of opacity in the venous system. If a firm diagnosis of venous thrombosis has been made, anticoagulants are appropriate. However, if the patient develops peritoneal signs, immediate laparotomy and resection are indicated.

This systemic approach to the management of ischemia originating in the superior mesenteric artery results in earlier diagnosis and avoidance of surgery. The overall mortality rate has been reduced to about 50%; 90% of the patients who have no peritoneal signs at the time of angiography survive.

This uncommon condition occurs in elderly patients with partial occlusion of at least two of the three principal mesenteric vessels (the celiac axis and the superior and inferior mesenteric arteries). Epigastric or periumbilical abdominal pain beginning after a meal and lasting for one to three hours ("intestinal angina") is the most characteristic clinical feature, although it is not often elicited. The pain may lead to a reduction in food intake (sitophobia) and secondarily a significant loss of weight. Bloating, flatulence and diarrhea are common, and steatorrhea is present in 50% of patients. This is the case because chronic mesenteric ischemia can cause mucosal damage. The physical examination is usually not diagnostic. A systolic abdominal bruit is present in 50% of patients but is not pathognomonic. (Epigastric bruits are common in normal persons.) Patients in whom the syndrome is suspected, and who have no other demonstrable abnormality to explain their symptoms, should have abdominal angiography. If angiography shows greater than 90% occlusion of at least two vessels, aorto-SMA graft is the procedure of choice. The mortality rate for this procedure is less than 10% and the majority of patients will be relieved of their postprandial intestinal angina. It is important to identify and to treat chronic mesenteric ischemia because of the high risk of thrombosis of the SMA.