Arterial embolism is the most common etiology of AMI and accounts for 40–50 % of patients with AMI. The thrombus usually originates in the heart of a patient with atrial fibrillation, recent myocardial infarction, congestive heart failure, left ventricular aneurysm, or valvular heart disease and lodges in the superior mesenteric artery (SMA) a few centimeters distal to the orifice near the origin of the middle colic artery. On CTA the proximal SMA is often patent and without calcification with a filling defect seen at the site of the embolus (Fig. 15.1). Thus, the affected area of bowel is often limited to the distribution of the occluded vessel with clear demarcation that will often spare the proximal jejunum. Bowel wall findings on CTA may include increased thickening, dilatation, and attenuation. Pneumatosis intestinalis, portal venous air, mesenteric edema, and ascites are other CTA findings associated with bowel ischemia. At exploratory laparotomy, the entire bowel is inspected carefully. Visual inspection of normal color and peristalsis alone is sometimes misleading and other modalities to assess bowel viability include palpable pulses in the mesentery, dopplerable arterial signals on the antimesenteric border of the bowel, bleeding from the cut ends, inspection for perfusion under a Wood’s lamp after fluoroscein injection, surface oximetry, and laser tissue flowmetry [29]. Balloon catheter thromboembolectomy is performed with removal of the thrombus from the SMA, and flow is restored to the bowel. Segments of obviously necrotic bowel are resected, and bowel continuity is restored only after revascularization is completed and vascularity of the ends has been determined to be satisfactory. In most instances, it is prudent to preserve all equivocal segments of bowel for reassessment at a second look laparotomy in 24–48 h.

Acute arterial thrombosis superimposed on preexisting severe atherosclerotic disease and preexisting mesenteric arterial stenoses is the second most common cause of AMI, accounting for 25–30 % of cases [30]. Prior symptoms of chronic mesenteric ischemia can be elicited in 25–50 % of patients. Patients frequently present with a slower progression of symptoms, and the acuity and severity of the situation depend on the extent of preexisting arterial stenoses and degree of collateralization. Bowel infarction is more insidious in onset because extensive collaterals are able to maintain viability until there is final closure of a critically stenotic vessel or collateral. In fact, the natural history of mesenteric artery stenosis in the general population is benign, as described by Wilson et al. [31]. Asymptomatic mesenteric artery stenosis is not associated with either death or adverse cardiovascular events unless there is a history of prior bowel resection, which could result in loss of collaterals. As with arterial embolism, severely symptomatic patients with an acute abdomen should be taken directly to the operating room with the same strategy outlined above. Patients stable enough to undergo a CTA should have the study done; in acute mesenteric arterial thrombosis this will usually reveal ostial SMA occlusion in a calcified artery. In this setting, there is no sparing of the proximal jejunum, because the thrombosis extends to the origin of the SMA and across the proximal jejunal branches (Fig. 15.2). In the patients presenting with subacute symptoms and well-developed collateral vessels on CTA, mesenteric arteriography with stenting of the SMA is a viable option with potentially lower morbidity and mortality [4]. This approach is less optimal in severely symptomatic AMI patients due to the unavoidable delay in assessment of the bowel for possible necrosis. Intraoperative mesenteric bypass or retrograde stenting of the SMA during laparotomy are viable options in this situation, the latter being easier and superior especially when bowel resection is necessary [32, 33].