Mesenteric Duplex Examinations

Course

Mesenteric Arterial Disease

Mesenteric ischemia is hemodynamically significant blood flow reduction to the intestines or colon from arterial disease of two or more of the major arteries. This is a rare disease and is reported in < 1% of all United States hospital admissions. The three major mesenteric arterial branches that could be involved include the celiac, superior mesenteric and inferior mesenteric arteries. Mesenteric arterial disease can result from an acute or chronic process.

Acute mesenteric ischemia presents abruptly, is often the result of an embolus from the heart and has a high mortality rate. The superior mesenteric artery is the most common location for an embolic occlusion. An embolus is a clot that breaks loose and travels through the blood stream. It can lodge within one of the smaller vessels such as the superior mesenteric artery. Emboli frequently originate from the heart or aorta. Patients with dysrhythmias are predisposed to forming clots and the irregularity of their heart beat is usually the mechanism to dislodge the clot. The presenting symptoms include a sudden onset of severe abdominal pain, vomiting and diarrhea – sometimes described as gut emptying. Lab tests can show elevated white blood cell count and acidosis. Early diagnosis and treatment is essential because of the potential to develop life-threatening bowel infarction and death. A screening exam such as duplex is often bypassed for computed tomography angiography (CTA) to aid in rapid diagnosis.

Chronic mesenteric ischemia is the result of atherosclerosis of the aorta and mesenteric vessels. Atherosclerosis causes stenosis or occlusion of the arteries which reduces the blood flow to the affected organs and can cause ischemia and infarction. Atherosclerotic disease is the chronic formation of plaque within a vessel. Plaque formation begins with an injury to the intima, inner lining of a blood vessel. This injury then allows for lipids and cholesterol that are in the blood to lodge in the sub-intimal space. An inflammatory process then reacts and macrophages digest the sub-intimal deposits to form foam cells, which is the plaque. The plaque then can thicken and harden, which will narrow the lumen of the blood vessel and cause disruption of flow. Risk factors associated with the development of atherosclerotic disease include hypertension, hypercholesterolemia, obesity, diabetes, smoking, age and genetic predisposition. Since atherosclerosis is a slow developing process, it gives the body a chance to develop collateral circulation to aid in the distribution of oxygenated blood to the intestines. There are numerous pathways in the abdominal circulation making the symptoms difficult to recognize. The most commonly described symptoms are postprandial pain, sitophobia –which is the fear of food - and weight loss. These symptoms are associated with several other, more common diseases such as peptic ulcers and chronic cholecystitis. Chronic mesenteric ischemia can exist without symptoms for many years because of collaterals and long-term study reports have shown that a majority of people with asymptomatic chronic mesenteric ischemia will eventually develop symptoms. Those with asymptomatic chronic mesenteric ischemia are at a higher risk of developing an acute event. Diagnosis can be made through a mesenteric Doppler exam, magnetic resonance angiography (MRA) and CTA. The treatment for chronic mesenteric ischemia depends mostly on the extent and location of the disease. Percutaneuous angioplasty, stenting, embolectomy, and surgical bypass are some treatment options to be considered.

Mesenteric Duplex Exam

Equipment

High resolution ultrasound systems are necessary for evaluation of the mesenteric arteries. Ultrasound systems should have excellent B-mode color, power and spectral Doppler capabilities with a wide range of frequencies. Phased and curved array pulsed Doppler transducers should be available with frequencies ranging at least from 2-5MHz. The equipment should be able to sustain high frame rates with deep focal zones and show a high-resolution B-mode image simultaneously with both color and Doppler spectral analysis.

Patient Preparation

Visceral vessels lie posterior to the intestines and stomach; therefore, the exam needs to be scheduled with the patient fasting for 6-8 hours. The interpretation criteria used to diagnose mesenteric arterial disease is validated only with the patient fasted. There are different velocity criteria for patients in a post-prandial state.

Duplex Exam

The exam begins with the patient supine on the examination table and the anterior abdomen exposed. The transducer is then placed on the upper abdomen just below the xiphoid process in a transverse orientation to identify the short axis of the abdominal aorta and inferior vena cava. Once identified, sweep inferiorly until the iliac bifurcation of the aorta is identified. Observe the size and position of the abdominal aorta and document aneurysmal dilatation or atherosclerotic disease. Anterior-posterior and transverse aortic measurements are performed to document the maximum aortic diameter. Next, sweep superiorly to the proximal aorta and rotate to a sagittal orientation. Identify the long axis of the proximal aorta. Obtain proximal, mid and distal aorta Doppler spectral waveforms. Peak systolic and, where appropriate, end diastolic velocities should be measured with the spectral signal obtained center stream and the cursor angle aligned parallel to the vessel walls and sixty degrees or less.

From this long axis view of the proximal aorta, the celiac and superior mesenteric arteries should be visualized arising from the anterior wall of the aorta, just below the diaphragm as shown in video 1. The first vessel is the celiac trunk and the second is the superior mesenteric artery. The celiac trunk is a short segment of vessel that travels anteriorly then quickly branches into the hepatic, splenic and left gastric arteries. The celiac distribution is responsible for circulation to supply the hepatobiliary system, stomach and spleen. The superior mesenteric artery arises off the anterior wall of the abdominal aorta about one centimeter inferior to the celiac trunk. It has several major branches including the inferior pancreatic artery, duodenal artery, colic artery, iliocolic artery and intestinal artery. All of these supply small intestine and proximal to mid colon. Spectral Doppler signals of the origin and proximal celiac and superior mesenteric arteries can be obtained from this view with angles at 60 degrees or less. In many cases, the angle of insonation will be close to zero degrees because the vessels travel anteriorly and directly toward the transducer. Care should be taken to sweep the spectral Doppler sample volume from the proximal aorta, through the origin of the celiac and superior mesenteric arteries to obtain the highest velocities. Documentation should include at least one celiac spectral Doppler waveform and one origin and proximal superior mesenteric artery Doppler waveform with peak systolic and end diastolic measurements. Flow throughout the celiac artery should show rapid systolic acceleration, high diastolic flow and have uniform velocities without turbulence. Flow in the superior mesenteric artery should also show rapid systolic acceleration, low diastolic flow compared to the celiac artery and have uniform velocities without turbulence. Image 1 demonstrates the comparison in diastolic flow of the celiac and superior mesenteric arteries.

Video 1. Visualization in the long axis view

Next, continue to assess spectral Doppler waveforms throughout the length of the superior mesenteric artery. There should be a minimum of four recorded Doppler spectral waveforms of the superior mesenteric artery making sure that the angle is always sixty degrees or less and aligned parallel to the vessel walls. These recorded signals should include an origin signal, proximal signal, mid signal and distal signal. In the presence of disease, several Doppler signals should be obtained to confirm the reproducibility of the highest peak velocity signal, as well as documenting post-stenotic turbulence.

Image 1: Comparison of diastolic flow in the celiac artery (left) and superior mesenteric artery (right)

Once assessment of the superior mesenteric artery is complete, return the transducer to a transverse view under the xiphoid process to show the celiac artery in short axis. From this view, the hepatic and splenic arteries can be identified traveling laterally to their respective organs. The common hepatic artery travels to the patient’s right side at almost a ninety degree angle. By ultrasound, it is seen as the upper border of the pancreas head. The common hepatic artery becomes the proper hepatic artery once the gastroduodenal artery branches from it and travels inferiorly. Then the proper hepatic artery, the hepatic duct and the portal vein ascend into the liver together, and the proper hepatic artery divides into right and left branches. The largest of the three celiac trunk branches is the splenic artery. It is markedly tortuous as it travels horizontally to the left and along the superior border of the pancreas. Assess the flow throughout the course of both vessels obtaining signals in the proximal, mid and distal segments. Distal signals should be obtained at the portahepatis for the hepatic artery and the splenic hilum for the splenic artery. This may be best accomplished from a lateral decubitus position. Flow throughout the hepatic and splenic artery should show rapid systolic acceleration, high diastolic flow and uniform velocities without turbulence.

The last major mesenteric artery to evaluate is the inferior mesenteric artery. It can be identified just superior to the iliac bifurcation. From a transverse distal abdominal aortic view, the inferior mesenteric artery can be seen branching off the aorta anteriorly at about 1 or 2 o’clock. It can also be identified by obtaining a long axis view of the distal abdominal aorta and then sweeping the inferior side of the transducer left lateral to view the IMA branch. This is demonstrated in image 2. In this image, the orientation is slightly off axis from the sagittal scan plane and angled to the left. Therefore, we see the distal aorta in long axis and the inferior mesenteric artery extending from the distal aorta also in long axis. The IMA is diving left lateral and into the pelvis; therefore, flow is directed away from the transducer. This spectral Doppler signal demonstrates this as a negative shift and is being displayed below the baseline. Some laboratories prefer to display arterial waveforms above the baseline regardless of whether or not the shift is positive or negative and some facilities leave the signals un-inverted as is demonstrated in image 2. The information can be accurately displayed in either direction; however, all exams should be performed consistently and this should be defined in the facility specific protocol. The normal IMA waveforms should be similar to the superior mesenteric artery signal with a rapid systolic upstroke, rapid deceleration and lower diastolic flow.

Image 2: Long axis view of inferior mesenteric artery.

Anatomic variants may be encountered during this exam. The most common one is a replaced right hepatic artery off of the superior mesenteric artery. Another variant could be the common hepatic artery arising from the superior mesenteric artery or the aorta. And finally, is a common celiac and superior mesenteric artery trunk. If there is a hepatic artery variant branching off the superior mesenteric artery, then the diastolic flow pattern in the proximal SMA will have an alternate appearance. Because the hepatic artery is branching off the SMA, the proximal SMA is now responsible for feeding the liver as well as the intestines and will have higher diastolic flow. The liver requires a larger blood supply regardless of the metabolic demands of digestion.

Interpretation

Normal fasting flow velocity parameters are not universal for the celiac, superior mesenteric or inferior mesenteric arteries. The celiac artery and its hepatic and splenic artery branches feed the liver and spleen. As stated, flow patterns of the superior mesenteric and the inferior mesenteric arteries are directly affected by whether or not the patient has fasted because they supply the intestines. There are peak systolic velocity criteria for the celiac and superior mesenteric arteries; however, there is currently no validated criteria for the inferior mesenteric artery. The superior mesenteric artery should show peak systolic velocities of less than 275cm/s and the celiac artery should be less than 200cm/s. Even though there is no specific velocity for the inferior mesenteric artery, signs of an arterial stenosis would include a focal increase in peak systolic velocity, post-stenotic turbulence, and a delayed systolic upstroke, or tardus parvus signal. Tardus parvus is Latin and is defined as slow to rise and slow to fall and this occurs distal to a hemodynamically significant stenosis or occlusion. If all three of these characteristics are clearly identified, then they most likely represent a hemodynamically significant stenosis. Complete occlusion should be documented with color, power and spectral Doppler demonstrating absence of flow signal. In many cases, multiple collateral pathways can be visualized distal to the occlusion. In cases of celiac artery occlusion, retrograde hepatic flow may be seen feeding the splenic artery. This occurs as a result of retrograde flow through the gastroduodenal artery to back fill the celiac and splenic arteries as well as fill the proper hepatic artery with antegrade flow. This collateral channel is known as the pancreatic duodenal arcade. Another collateral channel is through the arc of Riolan where the middle colic and the left colic arteries, which are branches off the SMA and IMA, connect to form an arc. The last major collateral pathway is the marginal artery of Drummond where it also connects the superior mesenteric and inferior mesenteric arteries through multiple colic branches. The marginal artery is farther distal in the mesentery as opposed to the proximal location of the arc of Riolan. Increase in velocities may be noted in the remaining patent splanchnic vessels due to collateralization. Refer to table one to summarize the duplex criteria for diagnosis of greater than seventy percent mesenteric arterial stenosis or occlusion.

Mesenteric Artery Diagnostic Criteria

Differential Diagnosis – Median arcuate ligament compression

Sometimes alternate pathology is identified as part of the duplex exam. Median arcuate ligament compression is intermittent compression of the celiac artery by the median arcuate ligament of the diaphragm. It may cause hemodynamically significant stenosis of the celiac artery with the patient supine and at rest and be relieved with deep inspiration or with the patient in an upright position. When elevated velocities are obtained in the celiac artery, the spectral Doppler signals should be recorded with the patient in the supine positon at rest, with deep inspiration and in an upright position. This will help differentiate if the elevated velocities are the result of median arcuate ligament compression. If the compression is present, the celiac artery will show high velocities and post-stenotic turbulence just like it would in the presence of stenosis. There will be no evidence of atherosclerotic disease by gray scale imaging. Have the patient take a deep breath in or have them sit up and resample the vessel. Either of these can pull the ligament off of the celiac artery. Re-sampling may confirm to the examiner that the celiac artery is being compressed by the median arcuate ligament.

Conclusion

Mesenteric duplex examinations can be performed in most patients with suspected chronic mesenteric ischemia. The exam requires careful evaluation of all the aorta and all major mesenteric branches with b-mode, color and spectral Doppler analysis. It has been proven effective because it is non-invasive and can aid in earlier detection of mesenteric ischemia since symptoms are historically unreliable and common to other abdominal conditions.

References

1. Daigle, R (2009). Techniques in noninvasive vascular diagnosis: An encyclopedia of vascular testing 3rd edition. Littleton, CO: Summer Publishing.
2. Lim H, Lee W, et al. Splanchnic Arterial Stenosis or Occlusion: Diagnosis at Doppler US. Radiology May 1999; 211: 405-410
3. Moneta GL, et al. Duplex ultrasound criteria for diagnosis of splanchnic artery stenosis or occlusion. J Vasc Surg 1991; 14:511-520.
4. Moneta GL, et al. Mesenteric duplex scanning: A blinded prospective study. J Vasc Surg 1993; 17:79-86
5. Pellerito, J, (2012). Introduction to Vascular Ultrasonography 6th Edition. Philadelphia, PA: Elsevier.
6. Size, G (2013). Inside Ultrasound’s Vascular Reference Guide. Pearce, AZ
7. Zierler, RE (2010). Strandness’s Duplex Scannning in Vascular Disorders Fourth Edition. Philadelphia, PA: Lippincott Williams & Wilkins.
8. Zwolak RM, et al. Mesenteric and celiac duplex scanning: a validation study. J Vasc Surg 1998; 27: 1078-1087; discussion 1088.