Historical Background
Embolectomy of the superior mesenteric artery for acute mesenteric ischemia was suggested by Ryvlin in 1943 and Klass in 1951, with the first successful embolectomy reported by Shaw and Rutledge in 1957. The feasibility of performing synchronous superior mesenteric artery embolectomy and small bowel resection was described a year later.
The manifestation of occlusive disease of the superior mesenteric artery and celiac axis as chronic abdominal pain and intestinal ischemia was recognized by Klein in 1921. Although a variety of surgical procedures were described for treatment of chronic mesenteric ischemia during the 1950s, including transection and reimplantation of the superior mesenteric artery and superior mesenteric artery endarterectomy, an aortomesenteric bypass for chronic mesenteric ischemia was first reported by Morris and DeBakey in 1961, with construction of Dacron bypass grafts to the superior mesenteric and hepatic arteries. A larger series of patients with chronic mesenteric ischemia treated with surgical bypass was published in 1962. The first case of an ileomesenteric bypass for chronic mesenteric ischemia was also reported in 1962 by Mavor and Lyall.
Indications
In the setting of acute mesenteric ischemia, impaired intestinal perfusion leads to mucosal ischemia, which can progress to bowel infarction with perforation and peritonitis. Emergent revascularization is warranted. Likewise, all patients with chronic mesenteric ischemia should undergo elective revascularization to prevent bowel infraction and by resolving characteristic food avoidance restoring normal nutritional status. There is no role for chronic parenteral alimentation and noninterventional therapies, even among patients at high risk for revascularization. The indication for revascularization in asymptomatic patients with visceral arterial occlusive disease remains unresolved. Thomas and associates reported that patients with significant occlusive disease in all three visceral vessels represent a “high-risk” group for bowel infarction, whereas other reports have documented adverse outcomes after aortic reconstruction among patients with untreated occlusive disease of the celiac axis and superior and inferior mesenteric arteries.
Endovascular therapy has evolved over the past several years and has become the first line of therapy for patients with chronic mesenteric ischemia in most institutions. An endovascular approach offers the advantages of a shorter length of hospital stay, reduced morbidity and mortality, and improved quality of life. However, long-term vessel patency is inferior to open revascularization with a requirement for secondary procedures. Nonetheless, recurrent stenosis after endovascular treatment does not necessarily equate to recurrence of symptoms, nor does it appear to precipitate acute mesenteric ischemia. The role of endovascular treatment for acute mesenteric ischemia has been limited by the need to assess the integrity of the bowel, as well as by an increased risk of embolization when recanalizing an acute embolic or thrombotic occlusion. However, a combined open and endovascular approach may be feasible, with endovascular revascularization performed at the time of laparotomy.
Preoperative Preparation
Acute Mesenteric Ischemia
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Patients with acute mesenteric ischemia from embolus or in situ thrombosis require emergent definitive treatment. Extensive preoperative evaluation is potentially harmful because of the narrow window for salvaging the bowel. There is no role for nonoperative treatment.
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Heparin anticoagulation should be initiated to limit clot propagation and broad spectrum antibiotics administered that are effective against enteric organisms.
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Patients are frequently hypovolemic, and resuscitation should be initiated before the induction of anesthesia.
Chronic Mesenteric Ischemia
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Patients with chronic mesenteric ischemia should undergo expeditious medical and cardiac evaluation. However, a careful preoperative assessment is required to exclude other potential causes of chronic abdominal pain.
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Operative planning is facilitated by a computed tomography (CT) angiogram of the aorta and visceral vessels. Imaging is obtained to identify the extent of atherosclerotic occlusive disease, as well as the suitability of the supraceliac aorta as a site to originate an antegrade bypass, if required.
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Ankle-brachial indices and duplex imaging of the saphenous and femoral veins as potential conduits should be obtained.
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Patients with minimal postprandial pain should be counseled to avoid large meals or the specific food types that exacerbate their symptoms. Those with continuous abdominal pain should avoid oral intake with the exception of medications. Although the operative procedure should not be delayed in an attempt to replete nutritional stores, malnourished patients may be started on total parenteral nutrition if a somewhat prolonged preoperative course is anticipated.
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Preoperative bowel preparation is unnecessary.
Pitfalls and Danger Points
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Both acute and chronic mesenteric ischemia are life-threatening problems frequently associated with diagnostic delays before referral to a surgeon. A high index of suspicion and an expedited evaluation are mandatory to reduce adverse outcome.
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Acute mesenteric ischemia may be the first manifestation of visceral artery occlusive disease.
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Major operative procedures, particularly aortic reconstructions, can precipitate acute mesenteric ischemia in patients with visceral artery occlusive disease. Preemptive visceral revascularization of the asymptomatic patient may be indicated selected patients.
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Preoperative evaluation should include a CT angiogram and lower extremity duplex vein survey to assess the supraceliac aorta and variability in visceral vessel configuration, as well as the saphenous and femoral veins, respectively.
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Adequate exposure of the supraceliac aorta is mandatory during antegrade bypass to avoid injuring the esophagus and to assure precise suture placement.
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Caution should be exercised while creating the retropancreatic tunnel during antegrade bypass to avoid injuring the splenic vein.
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A retrograde aorto-superior mesenteric artery bypass courses both caudal to cephalad and posterior to anterior. The propensity of this bypass to kink may be reduced by using an externally supported graft.
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Resection of ischemic bowel that is not infarcted should be delayed until after revascularization. A “second look” operation the following day is warranted should there be any doubt regarding the viability of the bowel.
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The postoperative course after visceral vessel revascularization may be complicated by multiple organ dysfunction, which requires supportive therapy.
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Acute mesenteric ischemia secondary to an early graft occlusion can be confused with multiple organ dysfunction. A high index of suspicion will necessitate imaging of the bypass graft.
Operative Strategy
Surgical Anatomy
There is an extensive collateral network among the celiac axis, superior mesenteric artery, inferior mesenteric artery, and internal iliac arteries. The celiac axis and superior mesenteric artery collateralize through the superior and inferior pancreaticoduodenal arteries, respectively, with the direction of flow contingent on the location of the stenosis. The superior and inferior mesenteric arteries collateralize through both the meandering artery and the marginal artery of Drummond. The meandering artery is the most significant collateral vessel and connects the ascending branch of the left colic artery with the middle branch of the middle colic artery. It lies at the base of the mesentery and is at risk of being ligated along with the inferior mesenteric vein during exposure of the infrarenal aorta. The inferior mesenteric artery communicates with the internal iliac artery via the hemorrhoidal branches and may represent a more important collateral than originally appreciated. This collateral pathway may be disrupted during sigmoid colectomy or infrarenal aortic aneurysm repair.
A significant amount of variability exists in the configuration of the visceral vessels. The “classic” three-vessel pattern for the celiac axis, consisting of the splenic, left gastric, and common hepatic arteries, occurs in approximately 75% of patients. Replaced or accessory hepatic arteries can originate from the superior mesenteric artery, and the left hepatic artery can arise off of the left gastric artery and course through the gastrohepatic ligament. A thorough review of preoperative imaging studies should be performed because both exposure and surgical planning for revascularization can be affected.
Acute Mesenteric Ischemia Secondary to Embolization, in Situ Thrombosis, and Dissection
Emboli responsible for acute mesenteric ischemia originate in the heart, usually as a result of atrial fibrillation, an acute myocardial infarction, or a ventricular aneurysm, and lodge in the superior mesenteric artery. Patients frequently have had a history of prior embolic events. Notably, an embolism to the superior mesenteric artery is quite large, in contrast to the micron-sized atheroembolic particles that cause blue toe syndrome. As a consequence, flow is easily restored in most situations by superior mesenteric artery embolectomy.
Acute mesenteric ischemia secondary to in situ thrombosis is superimposed upon the symptoms of chronic mesenteric ischemia in more than 50% of the patients. Patients with acute mesenteric ischemia secondary to in situ thrombosis require a mesenteric bypass. Although antegrade and retrograde bypasses are both options, retrograde bypass from the infrarenal aorta or iliac artery is the optimal procedure in the acute setting because of the ease of exposing the inflow source.
Dissections can occur in visceral vessels as an extension of an aortic dissection or, less commonly, as an isolated event. Mesenteric revascularization may be appropriate for patients with an acute aortic dissection and visceral malperfusion, but this approach is complicated by the underlying aortic pathology. Isolated or spontaneous dissections can occur in any visceral vessel, although the superior mesenteric artery appears to be the most common. The underlying cause or etiology remains unknown, although atherosclerosis, medial degeneration, trauma, fibromuscular disease, pregnancy, and a host of arteriopathies have been implicated. Patients’ symptoms may include abdominal pain. However, in one of the largest series in the literature, most patients were asymptomatic. The optimal treatment for an isolated dissection remains unresolved. Intervention is indicated for mesenteric ischemia, aneurysmal degeneration, and rupture. Both open and endovascular treatments have been reported, although the latter may be optimal given that the dissection can extend distally within the vessel. Expectant, conservative treatment with anticoagulation is likely adequate for asymptomatic or minimally symptomatic patients. Patients should be followed with serial CT imaging because of the risk of late aneurysmal degeneration. Similar to other vascular beds, these dissections can “heal” or resolve over time.
Selection of an Inflow Source
Mesenteric bypass, either antegrade from the supraceliac aorta or retrograde from the infrarenal aorta or common iliac artery, is indicated for patients with chronic mesenteric ischemia and those with acute mesenteric ischemia secondary to in situ thrombosis. The advantages of an antegrade bypass include the observation that the supraceliac aorta is usually free of atherosclerosis and the graft limbs follow a direct path while maintaining prograde flow. However, a retrograde bypass from the infrarenal aorta and common iliac artery is easier and faster to perform. Furthermore, although it is possible to partially occlude the supraceliac aorta while performing the proximal anastomosis of an antegrade bypass, the hemodynamic and ischemic impact of clamping the infrarenal aorta or iliac artery is minimal. One major disadvantage of a retrograde bypass is its potential to kink, which is particularly problematic for venous conduits. The graft must transition from the aorta that sits posterior in the abdomen to the superior mesenteric artery that sits more anterior. A ringed expanded polytetrafluoroethylene (ePTFE) conduit is a preferred choice. However, should a vein graft be required in the setting of enteric contamination, a relatively short conduit originating from the aorta, which is positioned to account for the normal lie of the superior mesenteric artery, should be considered.
Inherent to the debate about the type of bypass procedure is the number of vessels to be revascularized. Multivessel revascularization offers the advantage that if one graft limb occludes; the patient will not necessarily develop recurrent symptoms or acute mesenteric ischemia. Indeed, Hollier and colleagues reported that the rate of recurrent symptoms after open revascularization was inversely related to the number of vessels revascularized. Proponents of an isolated retrograde bypass to the superior mesenteric artery emphasize that the procedure revascularizes the primary vessel of concern, multivessel reconstruction significantly increases the complexity of the procedure, and more recent series have not demonstrated a clinical advantage for multivessel bypass.
Selection of the Celiac Artery Outflow SITE: Main Celiac Trunk or Common Hepatic Artery
Either the main celiac artery trunk or the common hepatic artery may be used as the outflow site for an antegrade bypass. The atherosclerotic occlusive process is essentially “aortic spillover” disease that affects the proximal portion of the vessel and usually extends to the first branch. Both options have relative advantages and disadvantages. Using the main celiac trunk facilitates an end-to-end anastomosis with a generous anastomosis given its larger size. In contrast, the common hepatic is more superficial to expose, although it is more difficult to orient the graft and the artery to configure the anastomosis.
Approach to the Superior Mesenteric Artery
The superior mesenteric artery may be exposed using one of three approaches as dictated by the underlying etiology and the planned configuration of the bypass ( Fig. 40-1 ). First, it may be exposed caudal to the inferior border of the pancreas by entering the lesser sac and incising the retroperitoneum ( Fig. 40-1 , A ). Second, it may be exposed at the transverse base of the mesocolon by elevating this segment of the colon and incising the proximal mesentery horizontally ( Fig. 40-1 , B ). Lastly, the superior mesenteric artery can be approached laterally by completely mobilizing the fourth portion of the duodenum after incising the ligament of Treitz and the other peritoneal attachments ( Fig. 40-1 , C ). The approaches through the lesser sac and the base of the transverse mesocolon are useful for antegrade bypass, whereas the lateral approach is preferred when a retrograde bypass is contemplated. An embolectomy can be performed most expeditiously by exposing the superior mesenteric artery at the base of the transverse mesocolon.
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