4.1 Clinical Presentation

Several intra-abdominal disease processes can present in a fashion identical to that of mesenteric ischemia; thus, the key to diagnosis lies in a high index of suspicion. Patients with advanced ischemia present with diffuse peritonitis, shock and severe metabolic derangements. In these patients it is clear that a catastrophic event has occurred. However, ischemia is only one of a few possible diagnoses. In most situations these patients will come to surgery, and in those where the diagnosis was not confirmed preoperatively, the diagnosis will become obvious at the time of surgery. Often these patients can not be salvaged; the mortality is reported to be between 70 and 90%. It must be stressed that the patient with early ischemia is far more challenging to diagnose. Given that the mortality rate is extremely high in advanced ischemia, the best chance of successfully treating a patient with this condition depends on early diagnosis and treatment.

The typical patient is usually over 50 years of age and often has a history of cardiac and peripheral vascular disease. In the early stage of ischemia the patient complains of severe abdominal pain (due to vasospasm) in the absence of peritoneal findings. This scenario has been described by clinicians as "pain out of proportion to the physical findings." Other nonspecific symptoms such as nausea, vomiting and altered bowel habit may be present, but they are usually not particularly helpful in the diagnosis.

4.2.1 LABORATORY

Unfortunately, there is no serum marker that can reliably predict early intestinal ischemia. Many studies have attempted to identify such a biochemical marker. Creatinine kinase, alkaline phosphatase, lactate dehydrogenase, diamine oxidase and inorganic phosphate are among those biochemical markers that have been examined. Although all of these will eventually become altered with advanced disease, their alteration with early ischemia is too varied to provide any clinical usefulness. Although biochemical changes with advanced intestinal ischemia are nonspecific, one can expect to find a leukocytosis. Due to third-space loss of fluid into the abdomen, electrolyte and renal function abnormalities secondary to dehydration are also often seen. Hyperamylasemia may occur secondary to amylase leaking from the infarcted bowel into the abdominal cavity, which may then enter the systemic circulation. In some situations this hyperamylasemia can be misinterpreted as an indication of pancreatitis. Finally, in advanced ischemia blood gas analysis will usually show a metabolic acidosis.

Initial radiological investigations are aimed at ruling out other causes of abdominal pain and peritonitis. All patients should have an upright and supine plain film of the abdomen. Although these films may support a diagnosis of ischemia, as indicated by bowel wall thickening and "thumb-printing," the main purpose of the films is to rule out visceral perforation or bowel obstruction. In many centers CT scan is being used as a first-line investigation in patients with abdominal pain. Several markers of intestinal ischemia have now been described by radiologists with expertise in CT scans. These include bowel wall thickening, mucosal edema, pneumatosis and mesenteric and portal vein gas. Using large injections of peripheral venous contrast, mesenteric arterial and venous occlusion can now also be identified in some patients. Of course, many of these findings are not specific, and thus we do not at present advocate the CT scan as a diagnostic test for intestinal ischemia. However, the CT scan can play a role in ruling out other intra-abdominal disease processes. For example, differentiating acute pancreatitis from abdominal ischemia on clinical grounds can sometimes be difficult. Both can present with hyperamylasemia and/or peritonitis. This is one situation where an abdominal CT scan may be useful to rule out retroperitoneal inflammation.

Ultrasonography combined with Doppler assessment of blood flow in the splanchnic arterial and venous system is now being used in some centers to screen for mesenteric ischemia. Our personal experience with this technique is limited and the exact role this technique will play is not clearly defined. There is experimental evidence, using a rabbit model of ischemia, that magnetic resonance (MR) scanning may also be of significant use in the diagnosis of mesenteric ischemia. Certainly, both arterial and venous abnormalities as well as the extent of the collateral circulation can be identified in some patients using MR technology; however, further clinical experience is required before this technique can be completely evaluated.

Angiography remains the gold standard in the diagnosis of mesenteric ischemia (Figure 3), and as will be discussed, it may play a significant role in the treatment of such patients. It is our belief that all patients with suspected mesenteric intestinal ischemia should undergo angiography to confirm the diagnosis and plan treatment. Wherever possible this approach should include even those patients presenting with peritonitis. Often there is a tendency to take patients with peritonitis straight to the operating room without performing angiography. These patients need to be treated in an expedient fashion. However, the short delay to obtain an angiogram may prove to be beneficial. Not only will it identify those patients who may require embolectomy or vascular reconstruction, but it will also provide a means to treat vasospasm in the perioperative period. This type of treatment policy has two implications: First, in order for management to be effective, an invasive radiologist must be available at all times and a system must be in place that will allow the angiography suite to be functioning with a short lead time. Second, the physician must realize that an appreciable number of negative angiograms should be expected with this low angiography threshold.

4.3.1 RESUSCITATION AND ASSESSMENT

It must be strongly stressed that if a diagnosis of mesenteric ischemia is being questioned, the subsequent investigation and management must proceed in an efficient and aggressive fashion if morbidity and mortality are to be reduced. Initial management of all patients consists of resuscitation. The degree of resuscitation required varies widely with the degree and extent of ischemia. Patients with early ischemia will require very little resuscitation, whereas those with infarcted intestine may require admission to a critical care unit for invasive monitoring. Insertion of a Swan-Ganz catheter with central pressure monitoring can be very useful in resuscitating the shocked patient with underlying cardiac disease. It must be kept in mind that in patients with extensive and advanced infarction, complete "stability" may never be obtained and thus investigation and treatment should proceed without extensive delay. However, ongoing patient "instability" is no doubt an ominous sign. As a general rule vasopressors to support blood pressure should be avoided, as they may further increase the degree of intestinal ischemia. The role of antibiotics is not clear-cut. Our policy is to administer broad-spectrum antibiotic coverage as soon as possible to those patients presenting with peritonitis. In those without peritonitis, antibiotics are used in the perioperative period, should surgery be required.

The treatment algorithm we recommend is outlined in Figure 4. Essentially, patients are divided into two groups: those with peritonitis and those without. Although all patients with peritonitis will require laparotomy, the exact treatment plan for both groups of patients will be dictated by the angiographic findings. Angiographic findings fall into four major categories:

Thrombotic occlusion. This finding is usually identified with an aortic flush of contrast dye; however, it can sometimes be difficult to differentiate from a proximal arterial embolus. The other pitfall with this finding is that sometimes it represents a chronic obstruction that is not necessarily related to the patient's present symptoms and findings. In most cases, these patients require arterial reconstruction, although the final treatment plan will be based on the exact vascular anatomy and degree of collateral circulation. Patients with peritonitis will almost always require a bowel resection. Perioperative papaverine infusion in these patients may indeed be useful; however, depending on the site of vascular obstruction, it may not be possible to secure a catheter for infusion.

Major embolus. Major emboli are usually located in the proximal portion of the superior mesenteric artery. The majority of these patients should be referred to surgery for consideration of embolectomy regardless of the presence or lack of peritoneal findings. Papaverine infusion in the perioperative period should be used to reduce vasospasm-induced ischemia.

Minor embolus. These emboli are limited to the branches of the superior mesenteric artery or to that portion of the vessel distal to the origin of the ileocolic artery. Unless peritoneal signs are present, these patients should be managed with papaverine infusion and observation.

Vasospasm (nonocclusive ischemia). This finding may occur in response to a mechanical arterial obstruction; however, when it represents the sole finding it is diagnostic of nonocclusive ischemia. The recommended management is papaverine infusion.

Papaverine infusion has been recommended as a major component of the medical therapy for mesenteric ischemia. Although we support its use, it must be stressed that the efficacy has not been absolutely proven by proper clinical trials.

Papaverine is a smooth-muscle relaxant. Administered systemically it will nonspecifically dilate the vascular tree. However, since it is virtually completely metabolized by a single pass through the liver, selective administration into the mesenteric circulation results in very few systemic effects. This allows vasodilation in the mesenteric circulation to occur without a drop in the systemic blood pressure. Typically, papaverine is infused into the mesenteric circulation (usually the superior mesenteric artery) after angiographic-guided selective catheterization of an arterial trunk. Papaverine is dissolved in normal saline to a concentration of 1mg/mL, although a higher concentration can be used. Heparin should not be added to the solution, as it will crystallize. The infusion is started at 30 mg/hour and may be increased to 60 mg/hour. In most cases the papaverine infusion is maintained for 24 hours. The catheter is then flushed with normal saline for 30 minutes and the angiogram is then repeated. If vasospasm persists, the cycle should be repeated every 24 hours for a maximum of 5 days. During the papaverine infusion the patient's systemic vital signs must be monitored. A sudden drop in the blood pressure usually suggests that the catheter has slipped out of the mesenteric circulation into the aorta. A repeat angiogram at the bedside can be performed to confirm this. Generally, papaverine infusion is quite safe and major complications are usually related to the initial passage of the arterial catheter. Complications include injury to the femoral artery, dislodgement of atherosclerotic plaques with embolic accidents in the lower extremities and the formation of a false aneurysm after the catheter is removed.

The role of surgery is to evaluate the viability of ischemic bowel, to resect if necessary and if possible to alleviate or bypass a vascular obstruction. If at all possible the vascular surgery should be performed first so that its effect on intestinal viability can be assessed.

One of the most difficult decisions the surgeon has to make is to decide if the bowel injury is reversible or not. Subjective criteria such as the bowel wall color, the presence of peristalsis and the presence of palpable mesenteric pulses are often used. Unfortunately, these criteria can lead to an inaccurate assessment in over 50% of cases. This has led surgeons to adopt a second-look approach. With this approach only the most obviously infarcted gut is resected and any questionable bowel is left in situ. A second look within 24 hours is then used to decide on the necessity for further resection. More recently, several objective measurements have been employed in the assessment of bowel viability. These include fluorescence staining, laser Doppler flowmetry, surface oximetry and intramural pH measurements. At present, no single technology has been widely adopted, but as these techniques are further refined they may eventually play a valuable role in the assessment of intestinal viability.

A second difficult situation for the surgeon is the management of patients with near-total intestinal infarction. Even with resection, the mortality rate in this group of patients is very high, and survivors will be dependent on total parental nutrition indefinitely. In elderly patients with other underlying medical problems, many surgeons elect to close without resections. The approach in a younger patient with a catastrophic vascular accident tends to be more aggressive, particularly in the hope that with increasing advances in bowel transplantation, in some years the resected bowel can be replaced.