Acute mesenteric ischemia: Clinical sciences

Acute mesenteric ischemia, or AMI, occurs when there is a sudden decrease of blood flow within the mesenteric vasculature, which can lead to infarction or necrosis of the bowel wall. Depending on the blood vessels involved, mesenteric ischemia can affect either the small or large bowel. Mesenteric ischemia is divided into 3 types based on the type of vasculature involved: arterial occlusion, venous occlusion, and non-occlusive arterial vasospasm. While each type can be acute or chronic, acute presentations require timely diagnosis and treatment, as they can rapidly progress to bowel necrosis, perforation, sepsis, and even death.

Now, here’s a high-yield fact to keep in mind! The superior mesenteric artery, or SMA, supplies the GI tract from the small intestine to the proximal transverse colon, while the inferior mesenteric artery, or IMA, supplies the distal one-third of the transverse colon to the rectum. The celiac artery, which mainly supplies the stomach and the proximal portion of the duodenum, can provide some collateral flow to the proximal small bowel and transverse colon via communications called confluences. The intestines can actually survive off of only one of these major vessels. Venous drainage of the gut is quite similar to the arterial supply. The superior mesenteric vein, or SMV, drains the midgut organs, while the inferior mesenteric vein, or IMV, drains the hindgut.

When approaching a patient who presents with signs and symptoms suggestive of acute mesenteric ischemia, your first step is to do an ABCDE assessment to determine if the patient is stable or unstable.

If the patient is unstable, you should initiate acute management to stabilize their airway, breathing, and circulation. This means you might need to obtain IV access, administer IV fluids, begin empiric broad-spectrum antibiotics, make sure the patient is NPO, and insert an NG tube if needed to decompress the bowel. These measures are important, as there's a high risk for bowel necrosis and sepsis.

Once you have initiated acute management, your next step is to obtain a focused history and physical examination. Typically, patients with acute mesenteric ischemia report sudden onset of severe abdominal pain, which may be out of proportion to physical exam, and is usually associated with nausea and vomiting, as well as diarrhea and bloody stools. Additionally, the history might reveal risk factors for AMI, such as atrial fibrillation, diabetes mellitus, hypercoagulable disorders, or hypertension.

Finally, on a physical exam, you might find abdominal distension, diffuse abdominal tenderness with rebound and guarding, as well as tachycardia and hypotension. The abdominal exam may also be completely normal despite severe pain, which is typically described as pain out of proportion to the physical exam.

Alright, if you see these findings, obtain an abdominal x-ray to look for signs of bowel infarction or perforation. The x-ray might reveal pneumatosis intestinalis, or gas within the intestinal wall, which indicates bowel infarction and necrosis; or pneumoperitoneum, which means that perforation occurred. If you see any of these findings, obtain an emergent surgical consultation for an exploratory laparotomy, which will be both diagnostic and therapeutic. Remember, the only treatment for a necrotic bowel is surgical resection.

Okay, now that the treatment for unstable patients is complete, let’s go back to the ABCDE assessment and discuss how to manage stable patients.

If the patient is stable, you should first obtain a focused history and physical exam to identify the nature of the presentation, as well as the patient’s risk factors. Patients might report varying degrees of abdominal pain, ranging from prior episodes of postprandial colicky abdominal pain, sometimes referred to as intestinal angina, which indicates that the blood flow is not meeting the bowel’s demands, to sudden onset severe constant pain that may indicate progression to complete vessel occlusion. Keep in mind that postprandial pain is more likely in chronic mesenteric ischemia. They may also experience nausea and vomiting, as well as diarrhea and bloody stools.

Make sure to perform a thorough risk assessment of underlying conditions that may increase the risk of AMI, like arrhythmias such as atrial fibrillation, prior myocardial infarction, uncontrolled hypertension, venous thromboembolism, hypercoagulable disorders, malignancy, diabetes, smoking, or oral contraceptive use. Patients with a history of prior blood clots are at a higher risk of developing clots within the mesenteric vasculature.

Now, the physical exam is typically not as severe as their pain. You might find a soft, non-distended, non-tender, or mildly tender abdomen, without guarding or rebound. In fact, if the patient complains of abdominal pain that is out of proportion to the physical exam findings, you should have a high suspicion of acute mesenteric ischemia.

Be sure to order labs for lactate, which will be needed for serial monitoring; and start supportive care, including IV fluids, broad-spectrum IV antibiotics, and bowel rest.

Now that the supportive care is initiated, your next step is to obtain a CT or CTA of the abdomen and pelvis to visualize the mesenteric vessels.