Mesenteric Ischemia

34 Mesenteric Ischemia







Pathophysiology


Any patient with advanced age, atherosclerosis, thromboembolic disease, atrial fibrillation, and processes leading to chronic low-flow states is at risk for the development of arterial mesenteric ischemia (Tables 34.1 and 34.2).1,2,4,5 Mesenteric venous obstruction carries its own separate risk factors, which are similar to those for venous thrombosis anywhere in the body.



Table 34.2 Incidence of Ischemic Bowel Diseases






















DISEASE INCIDENCE (%)*
Superior mesenteric artery (SMA) embolism: 50
 The SMA is susceptible to embolism because of large vessel caliber and a narrow angle of departure from the aorta.
 The proximal SMA is most commonly obstructed within 6-8 cm of the aorta.
Nonocclusive ischemia 25
SMA thrombosis 20
Mesenteric venous thrombosis 5

* Percentage of all cases of acute mesenteric ischemia.


Acute mesenteric ischemia is a result of the precipitous onset of hypoperfusion caused by occlusive or nonocclusive obstruction of either arterial or venous blood flow. Acute hypoperfusion occurs in 65% of cases and carries a mortality rate exceeding 60%. Occlusive arterial obstruction is most commonly caused by embolic or thrombotic obstruction of the superior mesenteric artery (SMA). NOMI is often due to vasoconstriction of the splanchnic system. Occlusive venous obstruction occurs with thrombosis or segmental strangulation. Mesenteric venous thrombosis is the main cause of mesenteric ischemia in younger patients without cardiovascular disease.


Acute arterial embolism causes a dramatic cessation of blood flow, with rapid progression from ischemia to infarction. As the bowel wall necroses, contamination with intraluminal bacteria leads to peritonitis, sepsis, and toxin-mediated hypotension.


Nonocclusive infarction, which represents 25% of all cases of acute ischemia, is most often caused by splanchnic hypoperfusion and vasoconstriction. Risk factors for nonocclusive disease include advanced age, acute myocardial infarction (AMI), acute cardiac decompensation, and heart failure. Diuretics contribute to a decrease in splanchnic perfusion in patients with profound heart disease, whereas medications such as digoxin and alpha-blockers cause regional vasoconstriction and may add to a low-flow state. Cocaine can cause splanchnic vasoconstriction and should be suspected as a cause of mesenteric ischemia in younger patients.


Bowel perfusion is generally preserved during periods of hypotension; therefore, NOMI represents failure of the normal autoregulatory systems.2,6,7 Patients with chronic renal failure may have bowel ischemia after hemodialysis, probably from hypoperfusion, which promotes preferential shunting of blood from the splanchnic circulation to preserve flow to the cardiac and cerebrovascular systems.


Although acute mesenteric vein thrombosis accounts for a small proportion of cases of ischemic bowel disease (5% to 10%), the ease of diagnosis with computed tomography (CT) has allowed identification of a greater number of patients with venous thrombosis. Symptoms are even less specific than those of arterial obstruction and are manifested over a longer period before bowel infarction occurs. Thrombus secondary to hypercoagulable states develops first in the smaller vessels and later progresses into the larger veins; clots associated with cirrhosis, neoplasm, or local injury (operative, trauma) start at the site of obstruction and evolve distally.4


Thrombotic arterial ischemia occurs late in the course of severe mesenteric atherosclerotic disease and involves the three major sources of intestinal blood supply: the celiac artery, the SMA, and the inferior mesenteric artery (IMA). Symptoms are typically manifested when two of the three vessels are significantly stenosed or completely obstructed.


A review of the anatomy of arterial blood flow to the intestines is helpful in understanding the pathophysiology of mesenteric ischemia.


The celiac artery arises anteriorly from the abdominal aorta at the level of the 12th thoracic vertebra. The celiac artery branches into the common hepatic, splenic, and left gastric arteries. These vessels supply their corresponding organs with significant redundancies, so ischemia in these areas is rare.


The SMA comes off the aorta 1 cm below the celiac artery and terminates as the ileocolic artery. This latter vessel supplies the majority of the blood delivered to the small intestine, as well as some flow to the pancreas, right colon, and transverse colon.


The IMA originates from the aorta 7 cm distal to the SMA. It provides blood to the distal transverse colon, descending colon, and rectum.


There is a significant array of collateral blood vessels and flow patterns. The small intestine is especially vulnerable to ischemia, however, because the terminal arterioles enter the intestinal wall without collateral pathways.4 Splanchnic blood flow requirements vary continuously but can account for up to 35% of cardiac output.


Venous drainage of the system occurs via the superior mesenteric vein, which empties into the portal vein.



Presenting Signs and Symptoms



Classic Presentation


Soon after ischemia begins, patients have complaints of severe abdominal pain that is clearly out of proportion to the findings on physical examination, such as a soft abdomen that is not very tender to palpation. The description and location of the pain vary over time. As the disease progresses, infarction develops and the symptoms may temporarily remit. Over the next several hours, bowel necrosis leads to signs of peritonitis: the abdomen becomes rigid, distended, and very painful with decreased bowel sounds. The intestinal mucosa begins to slough, and rectal bleeding occurs. At this point the stool contains occult blood in 60% of patients. The bowel may perforate, as signaled by findings of hypotension and sepsis.


Clues to diagnosis of the various ischemic bowel diseases are as follows:



Chronic mesenteric ischemia, or “intestinal angina,” refers to a pattern of pain typically brought on after eating that is usually episodic and recurrent, is sometimes constant, and lasts for up to 3 hours at a time. Mesenteric arterial atherosclerotic disease is generally the cause of chronic mesenteric ischemia, with a process similar to that of coronary artery disease and resultant angina pectoris.


Colonic ischemia occurs much less frequently than small bowel ischemia. Colonic ischemia often resolves spontaneously and without sequelae but can lead to significant morbidity and, in some cases, death.


Jun 14, 2016 | Posted by in EMERGENCY MEDICINE | Comments Off on Mesenteric Ischemia

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