Acs0614 Mesenteric Revascularization Procedures

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Acs0614 Mesenteric Revascularization Procedures

  1. 1. © 2005 WebMD, Inc. All rights reserved. ACS Surgery: Principles and Practice 6 VASCULAR SYSTEM 14 MESENTERIC REVASCULARIZATION PROCEDURES — 1 14 MESENTERIC REVASCULARIZATION PROCEDURES Scott E. Musicant, M.D., Gregory L. Moneta, M.D., F.A.C.S., and Lloyd M.Taylor, Jr., M.D., F.A.C.S. Mesenteric ischemia is encountered infrequently. To date, there ceral stenosis or occlusion and the extent of collateral develop- have been no randomized, controlled trials comparing treatment ment. In most cases, a transfemoral Seldinger technique is suit- modalities for either acute or chronic mesenteric ischemia. able, though in the setting of iliofemoral occlusive disease, a Consequently, decisions on how to treat this condition must be transaxillary approach is occasionally required. Between 60 and based on a few large case series in which a variety of operations 100 ml of contrast material is required for appropriate lateral and were used. anteroposterior views of the abdominal aorta. Visceral artery Overall evaluation and management of acute mesenteric lesions are usually ostial but may extend beyond the orifice of the ischemia are addressed more fully elsewhere [see 6:4 Acute vessel as a posterior plaque, especially in the superior mesenteric Mesenteric Ischemia]. In what follows, we focus specifically on the artery (SMA). Selective catheterization of the main intestinal operative techniques used to treat mesenteric ischemia (whether arteries is rarely necessary and may be dangerous. Appropriate chronic or acute) and discuss the available literature supporting magnification views generally allow characterization of the proxi- their use.The appropriate technique for a particular patient varies mal SMA beyond its origin, even without selective catheterization according to the individual anatomy and the particular intraoper- of the SMA. Intra-arterial digital subtraction techniques are usu- ative findings. ally adequate for lateral views, and they require less contrast mate- The relevant surgical procedures may be conveniently divided rial than other techniques. Arteriography also demonstrates coex- into those employed for chronic mesenteric ischemia and those isting lesions of the aorta and of the renal and iliac arteries that employed for acute ischemia. may be important in planning revascularization. OPERATIVE PLANNING Procedures for Chronic Intestinal Ischemia Essentially all patients with peripheral artery disease have con- comitant coronary artery disease (CAD). Although no symptoms PREOPERATIVE EVALUATION of CAD may be evident, care must still be taken to provide peri- J. E. Dunphy, in 1936, was the first to suggest that timely diag- operative cardiac protection. Perioperative beta blockade and anti- nosis and intervention for mesenteric artery occlusive disease may platelet therapy should be routinely employed in all patients prevent intestinal infarction.1 It is now clear that optimal treat- undergoing elective procedures. ment of mesenteric ischemia depends on prompt diagnosis and If the patient is undergoing a bypass procedure, the choice of that a high index of suspicion is vital. graft material should be addressed. In general, prosthetic grafts Patients with chronic intestinal ischemia generally, but not always, report experiencing colicky, dull, or aching abdominal pain, primarily located in the epigastrium but occasionally radiat- ing to the back. Symptoms typically begin 15 to 30 minutes after eating and may last as long as 3 hours. Peritonitis is not a charac- teristic of reversible intestinal ischemia; rather, it is indicative of intestinal infarction. Chronic postprandial abdominal symptoms result in markedly reduced food intake (so-called food fear),2 which generally leads to weight loss. Physical examination often yields no significant abdominal findings. Abdominal bruits may be audible, but they are a non- specific sign. Patients often, but not always, show evidence of ath- erosclerotic disease in other vascular territories. Bowel habits vary, ranging from normal elimination to diarrhea or constipation. Useful diagnostic tests include duplex ultrasonography, con- trast angiography, and magnetic resonance angiography (MRA). Duplex scanning is effective in detecting visceral artery stenosis [see Figure 1] and may allow earlier detection of visceral artery stenosis associated with chronic mesenteric ischemia.3,4 By itself, however, it is not sufficient for planning a mesenteric revascular- ization procedure. Arteriography is the primary imaging procedure employed in planning mesenteric revascularization for chronic intestinal ischemia [see Figures 2 and 3]. Lateral and anteroposterior views Figure 1 Example of duplex spectral waveform with compara- of the aorta are required for full evaluation of the severity of vis- tive arteriogram in patient with SMA stenosis.
  2. 2. © 2005 WebMD, Inc. All rights reserved. ACS Surgery: Principles and Practice 6 VASCULAR SYSTEM 14 MESENTERIC REVASCULARIZATION PROCEDURES — 2 A more versatile endarterectomy technique is transaortic endarterectomy.7 This procedure involves a posterolateral approach to the aorta, in which the aorta is exposed transperi- toneally with medial visceral rotation. Alternatively, a completely retroperitoneal approach may be taken.The main disadvantage of the retroperitoneal approach is that it restricts the surgeon’s abil- ity to assess the bowel at the completion of revascularization. Transaortic endarterectomy Step 1: incision and initial approach. A midline incision is recommended. A complete medi- al visceral rotation is performed, with the left kidney left in its bed. Step 2: exposure. The lateral aorta is exposed, and the celiac artery and the SMA may be identified anteriorly; the left renal artery lies posteriorly. Step 3: endarterectomy. A trapdoor incision is made in the aor- tic wall in such a way as to encompass the orifices of the SMA and the celiac artery. Partial occlusion of the aorta with a clamp is sometimes possible, but in most cases, complete aortic occlu- sion is required. If necessary, the aortotomy can be extended dis- tally and posteriorly to include the renal artery orifices as well. Among the advantages of this operation are that it permits simultaneous endarterectomy of the aorta and all visceral vessel orifices and that it does not require the use of prosthetic materi- al [see Figure 3]. The disadvantages include the potential risks associated with suprarenal clamping (e.g., cardiac overload, renal and lower-extremity embolization, and ischemia). Because of Figure 2 Lateral aortogram clearly shows moderate stenosis of proximal celiac artery and occlusion of SMA (arrow) in patient with intestinal ischemia. work well for mesenteric artery bypass. However, the entire abdomen and both legs should still be included in the operative field in case autologous vein proves necessary for the bypass con- duit. Autologous vein is often required in cases involving bowel resection and may also be preferable for bypasses to smaller vis- ceral vessels. If an autologous vein bypass procedure is planned, preoperative duplex scanning of the greater saphenous and femoral veins is recommended to facilitate selection of the best available vein for the conduit. OPERATIVE TECHNIQUE Visceral Endarterectomy Visceral endarterectomy for treatment of mesenteric ischemia was first described in 1958 by Shaw and Maynard,5 who per- formed endarterectomy of the SMA in a blind retrograde fash- ion through a distal arteriotomy. At present, retrograde endarte- rectomy cannot be recommended. The SMA can be approached directly once control of the suprarenal aorta has been obtained.6 A longitudinal incision is made across the origin of the SMA, and an endarterectomy is performed. In most patients, the exposure is limited. This direct approach may be considered when the SMA is widely separated from the renal arteries and the visceral aorta is relatively free of Figure 3 Lateral aortogram showing so-called coral reef athero- disease; however, this scenario is uncommon. ma involving visceral aorta with occlusion of origin of SMA.
  3. 3. © 2005 WebMD, Inc. All rights reserved. ACS Surgery: Principles and Practice 6 VASCULAR SYSTEM 14 MESENTERIC REVASCULARIZATION PROCEDURES — 3 these risks, the need for more extensive dissection, and the unfa- rograde if they originate from the infrarenal aorta or a common miliarity of most surgeons with this procedure, arterial bypass iliac artery. Antegrade bypass from the supraceliac aorta, using procedures are generally preferred for treatment of chronic either prosthetic material or autologous vein, has certain advan- mesenteric ischemia. tages, including a straight graft configuration that minimizes tur- bulence and graft kinking and, typically, reduced atherosclerotic Mesenteric Arterial Bypass calcification in the supraceliac aorta.11 The disadvantages of ante- Technical considerations Single-vessel versus multiple-vessel grade bypass are similar to those of visceral endarterectomy and revascularization. There are two schools of thought on the extent derive from the need to clamp the supraceliac aorta for the prox- of revascularization for chronic mesenteric ischemia. Proponents imal anastomosis. As with visceral endarterectomy, partial occlu- of so-called complete revascularization advocate revascularization sion clamping is theoretically possible but not always practical. of both the celiac artery and the SMA and suggest that this Clamping of the supraceliac aorta may increase the risk of cardiac approach makes recurrent ischemia less likely should one graft or events, visceral or renal emboli, and ischemia. One prerequisite graft limb undergo thrombosis.8 In a 1992 study, overall graft for use of the supraceliac aorta in an antegrade bypass is that the patency and survival were better in patients who underwent mul- vessel must be angiographically normal to ensure that it can safe- tiple-vessel bypass than in those who underwent single-vessel ly be clamped. It should also be kept in mind that reoperation on bypass. The investigators concluded that multiple-vessel bypass the supraceliac aorta is difficult: once this site has been used, reex- patients were likely to remain asymptomatic because of the pres- posure generally is not safe. ence of additional grafts or graft limbs that remained patent.8 Others maintain that the critical vessel involved in chronic Antegrade bypass Step 1: incision and initial approach. mesenteric ischemia is the SMA and argue that bypass to the SMA Supraceliac aorta–visceral artery bypass is performed through an alone is a relatively simple procedure that relieves symptoms of upper midline incision. Self-retaining retractors are helpful. mesenteric ischemia. In a 2000 study evaluating 49 patients who underwent bypass to the SMA alone, the 9-year primary assisted Step 2:exposure. The dissection begins with division of the gas- graft patency rate was 79% and the 5-year survival rate was 61%9— trohepatic ligament and retraction of the left lobe of the liver to the results equivalent to those noted in contemporary studies of multi- right, followed by incision of the diaphragmatic crus and ex- ple-vessel revascularization for chronic intestinal ischemia.10 posure of the anterior aspect of the aorta. Antegrade versus retrograde bypass. Mesenteric bypass grafts Step 3: choice of graft. In clean cases with no intestinal necro- may originate either above or below the renal arteries. Bypass sis or perforation, we use woven Dacron bifurcated grafts. If a sin- grafts are considered antegrade if they originate on the anterior gle-vessel bypass is to be performed, a single limb is cut from the surface of the abdominal aorta cephalad to the celiac artery, ret- bifurcated graft. Autologous vein grafts are usually reserved for a b Figure 4 Arterial bypass: antegrade. Shown is bypass from supraceliac aorta to SMA alone (a) or to hepatic artery and SMA (b).28
  4. 4. © 2005 WebMD, Inc. All rights reserved. ACS Surgery: Principles and Practice 6 VASCULAR SYSTEM 14 MESENTERIC REVASCULARIZATION PROCEDURES — 4 contaminated cases. The femoral vein is an excellent autogenous conduit for mesenteric arterial bypass. Step 4: anastomosis of graft to supraceliac aorta and visceral artery. If the celiac artery alone is to be revascularized, the usual procedure is to perform an end-to-side proximal anastomosis to the aorta, fol- lowed by an end-to-side distal anastomosis to the common hepat- ic artery. If the SMA alone is to be revascularized, it is generally necessary to tunnel the graft beneath the pancreas to the inferior border of the pancreas, then perform an end-to-side anastomosis to the SMA at that level [see Figure 4a]. Extreme care must be exer- cised in developing the retropancreatic tunnel. If this area appears too narrow or is scarred as a result of previous pancreatic inflam- mation, the graft should be tunneled anterior to the pancreas to ensure that it is not compressed and to avoid causing bleeding from disrupted pancreatic veins.12 If a prepancreatic tunnel is required, an autogenous conduit should be considered because the graft will be lying adjacent to the posterior wall of the stomach. If both the celiac artery and the SMA are to be revascularized from the supraceliac aorta, a bifurcated prosthetic graft is attached to the supraceliac aorta proximally, with one distal limb anastomosed to the hepatic artery and the other to the SMA [see Figure 4b]. Retrograde bypass In a retrograde bypass, the infrarenal aorta or a common iliac artery is used as the inflow vessel. One clear advantage of this procedure is that the approach to the infrarenal aorta is more familiar to most surgeons. Another is that dissection and clamping of the infrarenal aorta are less risky than dissection and clamping of the supraceliac aorta. Yet another is that the surgeon can work within a single operative field. Once the self-retaining retractor is placed, the operation on the infrarenal aorta and the SMA can be performed without further adjustment of the retractor. Figure 5 Arterial bypass: retrograde. Shown is bypass from iliac artery to SMA.28 Step 1:incision and initial approach. Here too, a midline incision and a transperitoneal approach are preferred.The transverse meso- colon is retracted upward, and the ligament of Treitz is divided. to the SMA is passed cephalad, turned anteriorly and inferiorly 180°, and anastomosed to the anterior wall of the SMA just Step 2: exposure. After division of the ligament of Treitz, the beyond the inferior border of the pancreas.12 In this manner, a duodenum and the small bowel are retracted to the right. The gentle C loop is formed that, if placed correctly, keeps the graft SMA may then be identified arising from beneath the inferior bor- from kinking when the viscera are restored to their anatomic posi- der of the pancreas. The retroperitoneum is divided distally along tion after retractor removal [see Figure 5]. The ligament of Treitz the aorta to a point just beyond the level of the aortic bifurcation. and the parietal and mesenteric peritoneum are closed over the The distal aorta and both common iliac arteries are assessed graft to exclude it from the peritoneal cavity. to allow determination of the proper location for the proximal Endovascular Techniques anastomosis. Early reports describing the use of percutaneous transluminal Step 3: choice of graft. As a rule, grafts made of Dacron or of angioplasty (PTA) to treat visceral atherosclerotic lesions indicat- ringed, reinforced expanded polytetrafluoroethylene (ePTFE) ed that initial technical success rates were as high as 80% but that are preferred. Problems may arise when retrograde bypasses are recurrence rates ranged from 20% to 40%.12,13 In a 1996 study of performed with autologous vein grafts, in that such grafts are PTA in 19 patients who were considered high risk, the initial suc- prone to kinking when the viscera are replaced. When a retro- cess rate was 95%, and the recurrence rate was 20% at 28 grade vein bypass is performed, the graft may be brought straight months.14 In a subsequent study of 25 patients who underwent up from the right iliac artery so that it lies between the aorta and angioplasty and stenting of the celiac artery or the SMA for the duodenum, then anastomosed to the posteromedial wall of chronic visceral ischemia, the initial technical success rate was the SMA. 96%, and the initial clinical response rate was 88%.15 At 6 months, 92% of the stents remained patent. Step 4: anastomosis to infrarenal aorta or common iliac artery and COMPLICATIONS SMA. Our preference is to use the area near the junction of the aorta with the right common iliac artery for the proximal anasto- mosis. (Short grafts originating from the midportion of the Technical infrarenal aorta, though commonly used, are prone to kinking The main technical complication of mesenteric bypass is acute when the viscera are returned to their normal position.) The graft graft thrombosis. This event is rare, but when it occurs, prompt
  5. 5. © 2005 WebMD, Inc. All rights reserved. ACS Surgery: Principles and Practice 6 VASCULAR SYSTEM 14 MESENTERIC REVASCULARIZATION PROCEDURES — 5 recognition is essential to prevent intestinal infarction. Kinking based contrast angiography is optimal for evaluating the bypass and compression of the graft are the most common causes of this graft and the distal vasculature, allowing identification of anasto- condition. If the retrograde graft is too long, the redundancy motic stenoses, kinking, or, in the case of autologous grafts, nar- makes it more susceptible to kinking. Similarly, if the graft is not rowing caused by valves [see Figure 6a]. If a technical defect is dis- positioned so as to form a gentle C loop, it is at risk for kinking covered, reoperation and correction are required to ensure pro- when the viscera are returned to their normal position. An ante- longed patency. In the past few years, we have started evaluating grade graft that is too long is equally at risk for kinking and occlu- selected patients perioperatively with CT angiography. This sion.When an antegrade bypass is tunneled behind the pancreas, modality is less invasive than traditional contrast angiography, but an adequate amount of space must be present to ensure that the it still requires administration of contrast material and exposure to graft is not compressed. In general, prosthetic grafts are more radiation [see Figure 6b]. resistant to kinking and compression than vein grafts are. Duplex ultrasonography has been used for postoperative graft Identification of perioperative graft occlusion is hindered by surveillance after mesenteric revascularization.18 At present, there postoperative incisional pain, fluid shifts, fever, and leukocytosis, are no validated criteria for determining what constitute normal all of which are common in the postoperative period and may velocities within a mesenteric bypass graft. Undoubtedly, duplex- mask signs of intestinal ischemia. Patients with chronic mesen- derived peak systolic velocities and end-diastolic velocities depend teric ischemia often have symptoms only when eating and thus on the caliber of the graft, whether the graft supplies both the celi- may be asymptomatic in the postoperative period until they ac artery and the SMA, and the length of the graft.The lack of val- resume oral feeding. For these reasons, we advocate evaluating the idating data notwithstanding, we routinely use postoperative graft early in the postoperative period with either conventional duplex scanning to establish baseline values and to permit com- contrast angiography or computed tomographic angiography [see parisons for follow-up evaluation of graft patency. If markedly ele- Outcome Evaluation, below]. vated, focal peak systolic velocities are recorded—especially if they Additional technical complications may occur as a result of increase on serial examinations—a contrast angiogram should be clamp placement. Clamping of the supraceliac aorta can lead to obtained to confirm graft stenosis. Duplex scanning can be diffi- renal atheroemboli or ischemia. These problems can be mini- cult in the early postoperative period because of incisional tender- mized by using a supraceliac clamp only on an angiographically ness and the increased intra-abdominal gas associated with post- normal aorta. operative ileus. Systemic Myocardial infarction is the most common cause of mortality Procedures for Acute Intestinal Ischemia in patients treated for mesenteric ischemia. Pulmonary compro- PREOPERATIVE EVALUATION mise is also a common systemic complication of mesenteric revas- cularization. Renal failure after mesenteric revascularization is As in the evaluation of patients with possible chronic mesen- more common in patients with preoperative renal insufficiency.16 teric ischemia, a high index of suspicion is of primary importance Mortality is markedly increased when renal failure occurs post- in the evaluation of patients with possible acute mesenteric operatively.16 Postoperative renal insufficiency can be minimized ischemia [see 6:4 Acute Mesenteric Ischemia]. Most cases of acute by administering mannitol, furosemide, and, possibly, vasodilators intestinal ischemia result either from thrombosis of a preexisting intraoperatively. stenotic lesion or from embolization19 (most frequently to the Patients who undergo mesenteric revascularization occasional- SMA). Cardiac emboli are the most common variety, though ly experience a profound reperfusion syndrome manifested by aci- tumor emboli20 and atheroemboli are seen as well. Atheroemboli dosis, pulmonary compromise, and coagulopathy.We recommend generally result from iatrogenically induced cholesterol emboliza- administering sodium bicarbonate (to minimize the effects of tion caused by aortic catheterization. The prognosis for acute metabolic acidosis) and mannitol (for its free radical–scavenging intestinal ischemia of embolic origin is more favorable than that properties) before restoring intestinal perfusion. for ischemia of thrombotic origin. Emboli typically lodge distally in the SMA distribution, and therefore, the proximal intestine is OUTCOME EVALUATION still partially perfused.19 In contrast, thrombotic occlusion occurs Restoration of pulsatile flow to the small bowel usually results at the origin of the vessel, resulting in complete interruption of in immediate active peristalsis and intestinal edema. The techni- midgut perfusion. cal success of surgical revascularization is assessed intraoperative- Acute, severe abdominal pain that is out of proportion to the ly through visual examination of the intestine and continuous- physical findings is the classic manifestation and is strongly sug- wave Doppler examination of the distal mesenteric vasculature gestive of intestinal ischemia.The duration of symptoms does not and the bowel wall. Doppler signals should be heard along the appear to correlate with the degree of intestinal infarction.21 antimesenteric border, and pulses should be palpable in the Peritonitis is initially absent, but vomiting and diarrhea may be mesentery. Intraoperative duplex scanning may also be used to present, and occult gastric or rectal bleeding may be identified in visualize anastomotic sites directly.17 as many as 25% of patients.21 Electromagnetic flow measurements can be helpful in evaluat- There are no reliable serum markers for acute intestinal ing the adequacy of mesenteric revascularization. Such measure- ischemia. Leukocytosis, hyperamylasemia, or elevated lactate lev- ments must be made after all packs and retractors have been els may be present, but these findings are insensitive and incon- removed. In most cases, the flow rate through the graft should be sistent. Abdominal radiographs may reveal dilated bowel loops between 500 and 800 ml/min, but flow rates as high as 1,000 and, occasionally, thickened bowel wall, but these findings are ml/min may be recorded.12 similarly inconsistent. In theory, duplex ultrasonography may be To confirm technical success after mesenteric revasculariza- helpful, but in practice, its applicability is often limited by the tion, we advocate routine postoperative imaging of the graft. gaseous visceral distention frequently associated with acute Ideally, this is done early in the postoperative period. Catheter- intestinal ischemia.
  6. 6. © 2005 WebMD, Inc. All rights reserved. ACS Surgery: Principles and Practice 6 VASCULAR SYSTEM 14 MESENTERIC REVASCULARIZATION PROCEDURES — 6 a b Figure 6 Routine postoperative imaging is performed to confirm technical success after revascu- larization. (a) Postoperative arteriogram shows iliac artery–SMA saphenous vein graft with kink (arrow).This problem was asymptomatic and was corrected by reoperation on postoperative day 5. (b) Postoperative CT arteriogram shows retrograde iliac artery–SMA prosthetic graft. C (hook) configuration of distal anastomosis provides antegrade flow into SMA. The use of preoperative arteriography to diagnose acute ments of clearly viable bowel are often interspersed with segments ischemia is controversial. Acute intestinal ischemia is a true surgi- of marginally viable bowel and segments of necrotic bowel. cal emergency, and delaying treatment to perform arteriography Acutely ischemic bowel that is not yet necrotic may appear decep- could result in further intestinal infarction. Angiography may be tively normal. Mildly to moderately ischemic bowel may exhibit considered in patients who have abdominal pain without any other signs or symptoms of systemic illness [see Figure 7]. In patients who have rebound tenderness, rigidity, or evidence of toxicity or shock, emergency exploration is indicated. OPERATIVE PLANNING Patients with acute intestinal ischemia who present with evi- dence of toxicity must be resuscitated expeditiously to ensure that surgical intervention is not delayed. Once it is determined that surgery is indicated, no further delay is justified. The patient is placed supine on the operating table, and the entire abdomen and both legs are prepared. As in operative treatment of chronic intes- tinal ischemia, the possibility that autologous vein will be needed for bypass grafting must be anticipated. OPERATIVE TECHNIQUE Intraoperative Considerations Mesenteric revascularization and bowel resection The goals of surgical therapy are to restore normal pulsatile inflow, to ensure that questionably viable bowel is adequately perfused, and to resect any clearly nonviable bowel. During abdominal explo- ration, the viability of the intestine and the status of the blood flow to the SMA are assessed with an eye to determining the appro- priate treatment. The surgeon should be prepared to perform Figure 7 Preoperative arteriogram shows embolic occlusion of both intestinal revascularization and intestinal resection. Seg- SMA distal to its origin.
  7. 7. © 2005 WebMD, Inc. All rights reserved. ACS Surgery: Principles and Practice 6 VASCULAR SYSTEM 14 MESENTERIC REVASCULARIZATION PROCEDURES — 7 SMA Divided Ligament of Treitz IMA Figure 8 SMA embolectomy. Exposure of infrarenal aorta, proximal right common iliac artery, and proximal SMA is achieved by intestinal retraction and division of pos- terior peritoneum, ligament of Treitz, and base of small bowel mesentery.29 loss of normal sheen, absence of peristalsis, and dull-gray discol- Restoration of normal flow to the SMA can produce remark- oration. Other objective signs of ischemia are the absence of a pal- able changes in an ischemic bowel. Because these changes do not pable pulse in the SMA or in its distal branches, the absence of always occur immediately, it is often necessary to preserve ques- visible pulsations in the mesentery, and the absence of flow on tionably viable portions of the bowel initially and then perform a continuous-wave Doppler examination of the vessels of the bowel second-look laparotomy within 12 to 36 hours. If the questionably wall. The small bowel may be deeply cyanotic yet still viable. In viable bowel is not in significantly better condition at the time of most cases, if there is any doubt, bowel resection should not be the second-look operation, it should be resected. Occasionally, performed until after revascularization. however, even a third look is prudent. Revascularized intestine The distribution of ischemic changes provides valuable informa- that was profoundly ischemic may swell dramatically. Temporary tion about the cause of the ischemia. SMA thrombosis often results abdominal closure with mesh may permit tension-free abdominal in ischemia to the entire small bowel, with the stomach, the duode- closure, prevent abdominal compartment syndrome, and perhaps num, and the distal colon spared; in severe cases, the entire foregut even improve intestinal perfusion by reducing intra-abdominal may be ischemic. In contrast, ischemia secondary to SMA embo- pressure. lism generally spares the stomach, the duodenum, and the proximal jejunum because the emboli tend to lodge at the level of the mid- Superior Mesenteric Artery Embolectomy dle colic artery rather than at the origin of the SMA.The choice of Step 1: incision and initial approach Again, a midline inci- operation for revascularizing the bowel depends on the underlying sion is made, and a transperitoneal approach is taken. causative condition. Embolectomy is indicated for arterial embo- lism, whereas bypass is indicated for thrombotic occlusion. Step 2: exposure of SMA at root of mesentery The SMA is exposed after division of the ligament of Treitz at the base of the Revascularization of the acutely ischemic intestine In transverse colon mesentery. The duodenum and the small bowel patients with very advanced intestinal ischemia, widespread bowel are retracted to the right [see Figure 8].The visceral peritoneum is necrosis may be obvious.This situation invariably proves fatal, and incised above the ligament of Treitz, just cephalad to the third thus, revascularization is not indicated. In many patients, howev- portion of the duodenum. The SMA should be readily palpable er, substantial portions of the bowel are ischemic but not frankly in this location as it crosses over the third portion of the duode- necrotic. Whether such bowel segments can be restored to viabil- num. The dissection is continued to obtain sufficient proximal ity cannot be accurately predicted. In most instances, therefore, and distal control of the vessel. Heparin is administered, and the revascularization should precede resection. vessel is clamped proximally and distally.
  8. 8. © 2005 WebMD, Inc. All rights reserved. ACS Surgery: Principles and Practice 6 VASCULAR SYSTEM 14 MESENTERIC REVASCULARIZATION PROCEDURES — 8 a b c Figure 9 SMA embolectomy. (a) Location of embolus within SMA is identified. (b) Transverse (as shown) or lon- gitudinal arteriotomy is performed, and embolus is extracted with balloon catheter. (c) Arteriotomy is closed. Primary closure (as shown) suffices for transverse arteri- otomy, but vein patch is usually required for closure of lon- gitudinal arteriotomy.29 Step 3: arteriotomy An arteriotomy is then made in the patients have fluid within the peritoneal cavity.This finding is not, SMA. The incision may be either transverse or longitudinal. We in itself, a contraindication to the use of a prosthetic graft. prefer to perform a longitudinal arteriotomy if there is any possi- However, if the patient has necrotic bowel that must be resected bility that a bypass graft may be needed. The arteriotomy should or if perforation has occurred, a prosthetic graft should not be be made approximately 2 to 3 cm distal to the origin of the SMA, used. In these situations, an autologous vein graft is preferred. A though alternative placements may be appropriate on occasion, good-quality vein is mandatory; if the saphenous vein is inade- depending on the anatomy and the estimated location of the quate, the femoral vein may be used instead. occlusion [see Figures 9a, b]. The techniques of mesenteric bypass for acute intestinal isch- emia are identical to those for chronic intestinal ischemia. Because Step 4: embolectomy Proximal embolectomy should be these patients are often acutely ill, it is vital to perform the opera- performed first to ensure adequate inflow. A 3 or 4 French balloon tion rapidly and efficiently. In the acute setting, bypass to the SMA catheter is sufficient in most cases. If very good pulsatile inflow is alone is strongly preferred [see Figure 10]. As a rule, a retrograde not achieved after embolectomy, then thrombosis of a stenotic approach, using the infrarenal aorta or a common iliac artery for lesion is likely to be the underlying cause of the acute intestinal inflow, is best; the supraceliac aorta is used for inflow only if the ischemia, and a bypass graft should be placed. Even when inflow infrarenal vessels are unsuitable for this purpose. Even highly cal- is apparently adequate, a bypass should be strongly considered if cified iliac arteries can be used for inflow, provided that there is no the proximal SMA is palpably abnormal. significant pressure gradient and that the surgeon is familiar with The narrowness and fragility of the distal SMA and its branch- intraluminal balloon occlusion techniques for proximal and distal es can make distal embolectomy particularly challenging. It is control. best to use a 2 French embolectomy catheter for this procedure. The catheter must be passed gently, without undue force. Endovascular Techniques It would seem reasonable that endovascular therapies might Step 5: closure Once all possible thrombus has been re- come to play a role in the treatment of acute intestinal ischemia, moved, the arteriotomy is closed. A transverse arteriotomy may be given that a preoperative angiogram is usually feasible in stable pa- closed primarily with interrupted monofilament sutures [see Figure tients. Several groups have reported treating acute arterial embol- 9c]; however, a longitudinal arteriotomy frequently must be closed ism with intra-arterial thrombolysis22,23; others have reported with an autologous vein patch. If adequate flow is not restored treating acute embolism, as well as thrombotic occlusion, with after the clamps are removed, the arteriotomy is used as the distal PTA.24,25 Although a degree of anecdotal success with these tech- anastomotic site of a bypass graft. niques has been achieved in selected cases, it should be kept in mind that reliance on endovascular therapy alone for presumed Superior Mesenteric Artery Bypass acute intestinal ischemia runs the risk of missing bowel necrosis. Patients with SMA thrombosis who are seen early enough and After endovascular therapy, frequent clinical reevaluation is neces- who have no intestinal necrosis may undergo SMA bypass graft- sary to identify patients with persistent intestinal ischemia. ing with a prosthetic conduit. At exploration, many of these Abdominal exploration should be very strongly considered in
  9. 9. © 2005 WebMD, Inc. All rights reserved. ACS Surgery: Principles and Practice 6 VASCULAR SYSTEM 14 MESENTERIC REVASCULARIZATION PROCEDURES — 9 a b Figure 10 SMA bypass. (a) Iliac artery–SMA bypass with prosthetic graft is suitable for cases in which SMA thrombosis produces ischemic but salvageable bowel. (b) Iliac artery–SMA bypass with saphenous vein is suitable for cases in which some segments of necrotic or perforated bowel must be resected.29 most cases, even if the angiographic result of endovascular treat- pulsatile flow in the mesenteric arcades, peristalsis, bleeding from ment is good. cut surfaces, and, of course, color. In one study, clinical parame- ters were found to be 82% sensitive and 91% specific for bowel TROUBLESHOOTING viability.26 Occasionally, patients present with emboli that have lodged in We routinely use a sterile continuous-wave Doppler ultrasound the small arterial branches of the SMA.These vessels are often too flow detector to evaluate pulsatile flow on the bowel surface. small to allow the passage of embolectomy catheters, and bypass Grossly discolored bowel with no Doppler signal after a period of beyond the point of obstruction frequently is not possible. In these observation should be resected; marginal bowel with no Doppler situations, resection of marginally viable bowel is the best option. signal is an indication for second-look laparotomy. As noted (see above), avoidance of graft kinking is crucial for With the fluorescein fluorescence method, 10 to 15 mg/kg of preventing early graft failure. Graft failure can have an even fluorescein is injected intravenously, and the intestine is inspected greater adverse effect on bowel viability in the setting of acute with a Wood lamp. A complete absence of fluorescence is diagnos- ischemia than in the setting of chronic intestinal ischemia. tic of nonviability; rapid, confluent, bright fluorescence is diagnos- Recovery after revascularization is often prolonged. Early and tic of viability. There is, however, a large gray area between these prolonged parenteral nutrition may be necessary in patients with two extremes in which interpretation is subjective. In one study, extensive bowel infarction. Only rarely, however, is lifelong par- the I.V. fluorescein method was found to be 100% sensitive and enteral nutrition required. specific for detecting nonviable bowel.27 The disadvantages of this technique are that it requires special equipment and that it expos- OUTCOME EVALUATION es the critically ill patient to the risk of an adverse reaction to the The techniques employed to evaluate the success of mesenteric dye. Other assessment methods (e.g., surface oximetry, infrared revascularization for acute ischemia include clinical inspection, photoplethysmography, and laser Doppler velocimetry) are avail- continuous-wave Doppler ultrasonography, and I.V. administra- able, but at present, they are mostly experimental and are not in tion of fluorescein. Clinical inspection entails visual assessment of general use for evaluation of bowel viability.
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