ABSTRACT
The causes of clinically significant acute mesenteric ischemia are arterial embolus (50%), arterial thrombosis (20%), low flow state (20%), mesenteric venous thrombosis (5%), and other (5%). Included in the other category would be situations such as sigmoid ischemia due to inferior mesenteric artery occlusion as a result of abdominal aortic replacement (see Case 2); or gastrointestinal (GI) ischemia secondary to visceral vessel occlusion as a result of aortic dissection. The overall mortality rate of acute
mesenteric ischemia is about 50% and has not improved much in the past three decades, although admittedly the patients presenting today with this problem may be sicker and more complex than in the past.(5, 6)
Major mesenteric emboli usually involve the superior mesenteric artery (SMA) distribution perhaps because of a more favorable angle with the aorta than the celiac or IMA. The site of origin of the embolus is frequently the heart, and multiple previous (or future) episodes of embolization are not uncommon. Cardiac conditions associated with thromboembolic phenomena include arrhythmia (particularly atrial fibrillation), acute myocardial infarction (AMI), ventricular aneurysm, and valvular disease.(7) Most often mesenteric embolism from a cardiac source occurs spontaneously but it may occur after catheter-based cardiac treatment or after cardiac surgery. An aortic source is the second most common site of origin for SMA emboli. Usually these are spontaneous events, but occasionally they are seen following instrumentation of the aorta and presumed dislodgement of atheromatous material, for example, following left heart cardiac catheterization or coronary angiography, carotid stenting, or thoracic stent grafting. The typical SMA embolus lodges at the major branch point just distal to the middle colic artery, preserving the blood supply to the transverse colon and very proximal jejunum. Unless flow is restored promptly, bowel necrosis occurs from the proximal jejunum to the ascending colon,
Table 50.1 Risk Factors for Acute Mesenteric Ischemia.
