Ischemic Bowel

last updated: December 14, 2016

Orlando Debesa

MDCT/CTA of Mesenteric

Ischemia: Part 1

General:

Small bowel ischemia is an uncommon vascular disease resulting in not enough blood supply to the intestines. Mesenteric ischemia can be classified as either acute or chronic. Acute mesenteric ischemia can be subdivided into occlusive and nonocclusive mesenteric ischemia. The mortality rate is >50%. The most significant indicator of survival is the timeliness of diagnosis and treatment.

Etiology:

Approximately 80% of acute mesenteric ischemia have an occlusive etiology (arterial emboli and thrombosis occurs in 65% of the cases and venous thrombosis in 15% of the cases). Occlusive ischemia is a result of disruption of blood flow by an embolus or progressive thrombosis in a major artery supplying the intestine. Intestinal ischemia remains extremely challenging to diagnose.

Anatomy:

The superior mesenteric artery (SMA) supplies from the ampullary region of the 2nd part of the duodenum to the splenic flexure. The inferior pancreaticoduodenal artery (the first branch off the SMA or the first jejunal artery) supplies the head of the pancreas.

Acute Mesenteric Ischemia:

The difference between acute and chronic mesenteric ischemia is the timing; acute mesenteric ischemia comes on suddenly with unremitting severe abdominal pain out of proportion to the physical exam while chronic mesenteric ischemia comes on gradually.

Occlusive Acute Mesenteric Ischemia:

Emboli originate from the heart in >75% of cases and lodge preferentially just distal to the origin of the middle colic artery from the superior mesenteric artery. In the case of SMA occlusion where the embolus usually lies just proximal to the origin of the middle colic artery, the proximal jejunum is often spared while the remainder of the small bowel to the transverse colon will be ischemic.

Risk factors for acute arterial ischemia:

- atrial fibrillation

- recent myocardial infarction

- valvular heart disease

- recent cardiac or vascular catheterization

Mesenteric Venous Thrombosis (MVT)

Patients with MVT may present with a gradual or sudden onset. Symptoms include vague abdominal pain, nausea, and vomiting. Examination findings include abdominal distention with mild to moderate tenderness and signs of dehydration. Mesenteric Venous Thrombosis is associated with the presence of a hypercoagulable state including protein C or S deficiency, antithrombin III deficiency, polycythemia vera, and carcinoma.

The diagnosis of mesenteric thrombosis is frequently made on abdominal spiral CT with oral and IV contrast. Findings on CT include bowel-wall thickening and ascites. Intravenous contrast will demonstrate a delayed arterial phase and clot within the superior mesenteric vein. The goal of management is to optimize hemodynamics and correct electrolyte abnormalities with massive fluid resuscitation. Intravenous antibiotics as well as anticoagulation should be initiated. If laparotomy is performed and MVT is suspected, heparin anticoagulation is immediately initiated and compromised bowel is resected. Of all acute intestinal disorders, mesenteric venous insufficiency is associated with the best prognosis.

Physical Exam:

Associated symptoms may include nausea and vomiting, transient diarrhea, and bloody stools. With the exception of minimal abdominal distention and hypoactive bowel sounds, early abdominal examination is unimpressive. Later findings will demonstrate peritonitis and cardiovascular collapse.

 

Progressive thrombosis of at least two of the major vessels supplying the intestine is required for the development of chronic intestinal angina.

Non-Occlusive Acute Mesenteric Ischemia:

Nonocclusive mesenteric ischemia is caused by a low cardiac output state and is responsible for 20% of acute mesenteric ischemia cases. Nonocclusive ischemia is due to a disproportionate mesenteric vasoconstriction (arteriolar vasospasm) in response to a severe physiologic stress such as dehydration or shock. If left untreated, early mucosal stress ulceration will progress to full-thickness injury.

 

Physical Exam:

Nonocclusive or vasospastic mesenteric ischemia presents with generalized abdominal pain, anorexia, bloody stools, and abdominal distention. Often these patients are obtunded, and physical findings may not assist in the diagnosis.

Management:

  • Admit: monitored bed or intensive care unit

    • recommended for resuscitation and further evaluation​

  • General Surgery consultation

  • Diagnostic Tests

    • plain abdominal film

      • earlier features include bowel-wall edema, known as “thumbprinting”

      • if the ischemia progresses, air can be seen within the bowel wall (pneumatosis intestinalis) and within the portal venous system

      • may show evidence of free intraperitoneal air, indicating a perforated viscus and the need for emergent exploration​

      • calcification of the aorta and its tributaries, indicating atherosclerotic disease

    • CT (PO and IV contrast) with three-dimensional reconstruction is a highly sensitive test for intestinal ischemia

    • ​More recently, mesentery duplex scanning and visible light spectroscopy during colonoscopy have been demonstrated to be beneficial

    • mesenteric duplex scan demonstrating a high peak velocity of flow in the superior mesenteric artery (SMA) is associated with an ˜80% positive predictive value of mesenteric ischemia. More significantly, a negative duplex scan virtually precludes the diagnosis of mesenteric ischemia. Duplex imaging serves as a screening test; further investigations with angiography are needed.

    • In acute embolic disease, mesenteric angiography is best performed intraoperatively

    • Endoscopic techniques using visible light spectroscopy can be used in the diagnosis of chronic ischemia

    •  “gold standard” for the diagnosis and management of acute arterial occlusive disease is laparotomy. Surgical exploration should not be delayed if suspicion of acute occlusive mesenteric ischemia is high or evidence of clinical deterioration or frank peritonitis is present. The goal of operative exploration is to resect compromised bowel and restore blood supply. Intraoperative or preoperative arteriography and systemic heparinization may assist the vascular surgeon in restoring blood supply to the compromised bowel. The entire length of the small and large bowel beginning at the ligament of Treitz should be evaluated. The pattern of intestinal ischemia may indicate the level of arterial occlusion.

The surgical management of acute mesenteric ischemia of the small bowel is attempted embolectomy via intraoperative angiography or arteriotomy. Although more commonly applied to chronic disease, acute thrombosis may be managed with angioplasty, with or without endovascular stent placement. If this is unsuccessful, a bypass from the aorta to the superior mesenteric artery is performed.

Treatment:

Aggressive fluid resuscitation may be necessary. To optimize oxygen delivery, nasal O2 and blood transfusions may be given. Broad-spectrum antibiotics should be given to provide sufficient coverage for enteric pathogens, including gram-negative and anaerobic organisms. Frequent monitoring of the patient's vital signs, urine output, blood gases, and lactate levels is paramount, as is frequent abdominal examination. All vasoconstricting agents should be avoided; fluid resuscitation is the intervention of choice to maintain hemodynamics.

 

  • Ensure hydration

  • Give antibiotics

  • Reverse acidosis

  • Optimize oxygen delivery

  • Support cardiac output

  • Avoid vasoconstrictors

  • broad spectrum antibiotics (including anaerobic coverage)

 

Laparotomy for nonocclusive mesenteric ischemia is warranted for signs of peritonitis or worsening endoscopic findings and if the patient's condition does not improve with aggressive resuscitation. Ischemic colitis is optimally treated with resection of the ischemic bowel and formation of a proximal stoma. Primary anastomosis should not be performed in patients with acute intestinal ischemia.

Chronic Intestinal Ischemia:

Chronic intestinal ischemia presents with intestinal angina or abdominal pain associated with need for increased blood flow to the intestine. Patients report abdominal cramping and pain following ingestion of a meal. Weight loss and chronic diarrhea may also be noted. Abdominal pain without weight loss is not chronic mesenteric angina. Physical examination will often reveal the presence of an abdominal bruit as well as other manifestations of atherosclerosis. Duplex ultrasound evaluation of the mesenteric vessels has gained in popularity. In the absence of obesity and an increased bowel gas pattern, the radiologist may be able to identify flow disturbances within the vessels or the lack of a vasodilation response to feeding. This tool is frequently used as a screening test for patients with symptoms suggestive of chronic mesenteric ischemia. The gold standard for confirmation of mesenteric arterial occlusion is mesenteric angiography. Evaluation with mesenteric angiography allows for identification and possible intervention for the treatment of thrombus within the vessel lumen and will also evaluate the patency of remaining mesenteric vessels. The use of mesenteric angiography may be limited in the presence of renal failure or contrast allergy. Magnetic resonance angiography is an alternative if the administration of contrast dye is contraindicated.

 

Treatment:

The management of chronic intestinal ischemia includes medical management of atherosclerotic disease by lipid-lowering medications, exercise, and cessation of smoking. A full cardiac evaluation should be performed before intervention. Newer endovascular procedures may avoid an operative intervention in selected patient populations. Angioplasty with endovascular stenting in the treatment of chronic mesenteric ischemia is associated with an 80% long-term success rate. In patients requiring surgical exploration, the approach used is determined by the mesenteric angiogram. The entire length of the small and large bowel should be evaluated, beginning at the ligament of Treitz. Restoration of blood flow at the time of laparotomy is accomplished with mesenteric bypass.

Determination of intestinal viability intraoperatively in patients with suspected intestinal ischemia can be challenging. After revascularization, the bowel wall should be observed for return of a pink color and peristalsis. Palpation of major arterial vessels can be performed as well as applying a doppler flowmeter to the antimesenteric border of the bowel wall, but neither is a definitive indicator of viability. In equivocal cases, 1 g of IV sodium fluorescein is administered and the pattern of bowel reperfusion is observed under ultraviolet illumination with a standard (3600 A) Wood's lamp. An area of nonfluorescence >5 mm in diameter suggests nonviability. If doubt persists, reexploration performed 24–48 h following surgery will allow demarcation of nonviable bowel. Primary intestinal anastomosis in patients with ischemic bowel is always worrisome, and reanastomosis should be deferred to the time of second-look laparotomy.

 

References:

Harrisons principles

Acute on Chronic Mesenteric Ischemia:

You may also see acute on chronic disease which may come on more slowly. Signs and symptoms of chronic disease include abdominal pain after eating, unintentional weight loss, vomiting, and being afraid of eating.

Labs/Radiographic evaluation:

- WBC count: elevated in most cases of mesenteric ischemia

- Lactic acid: SMA emboli causes metabolic acidosis in 42% of cases

- serum amylase: elevated in 50% of the patient

- plain radiograph: exclude free intraperitoneal air, bowel obstruction. Rare to see pneumatosis intestinalis, portal venous gas, or thick bowel wall with thumbprinting - plain film is normal in a majority of patients.

- mesenteric angiography: gold standard for the diagnosis of AMI. Can also be used therapeutically to infuse papaverine (vasodilator)

Management

  • ostomy output goal

... but I would more especially commend the clinician who, in acute diseases, by which the bulk of mankind are cutoff, conducts the treatment better than others.
 
Hippocrates, 400 BC