Mesenteric Duplex Scanning
In healthy individuals, fasting blood flow velocity waveforms differ in the SMA versus the CA. Arterial waveforms reflect end organ vascular resistance. The liver and spleen have relatively high constant metabolic requirements and are therefore low resistance organs. As a result, CA waveforms are generally biphasic, with a peak systolic component, no reversal of end systolic flow, and a relatively high end-diastolic velocity. The normal fasting SMA velocity waveform is triphasic, reflecting the high vascular resistance of the intestinal tract at rest. There is a peak systolic component, often an end-systolic reverse flow component, and a minimal diastolic flow component.
Changes in Doppler-derived arterial waveforms in response to feeding are also different in the CA and SMA. Because the liver and spleen have basically fixed metabolic demands, there is no significant change in CA velocity waveform after eating. Blood flow in the SMA, however, increases markedly after a meal reflecting a marked decrease in intestinal arterial resistance. The waveform changes in the SMA postprandially include a near doubling of systolic velocity, tripling of the end diastolic velocity, and loss of end-systolic reversal of blood flow. In addition, there is a detectable increase in the diameter of the SMA after eating. The diameter of the SMA has been shown to be 0.60 ± 0.09 cm in the fasting state and 0.67 ± 0.09 cm after a meal (p < 0.0001). These changes are maximal at 45 minutes after ingestion of a test meal and are dependent on the composition of the meal ingested. Mixed composition meals produce the greatest flow increase in the SMA when compared with equal caloric meals composed solely of fat, glucose, or protein.
Duplex ultrasound can detect hemodynamically significant stenoses in splanchnic vessels. In 1986, investigators at the University of Washington found that flow velocities in stenotic SMA and CA were significantly increased when compared with normal controls. Quantitative criteria for splanchnic artery stenosis were first developed and validated at Oregon Health & Science University.
In a blinded prospective study of 100 patients who underwent mesenteric artery duplex scanning and lateral aortography.. A PSV in the SMA of 275 cm/sec or more indicated a ³70% SMA stenosis with a sensitivity of 92%, a specificity of 96%, a positive predictive value of 80%, and a negative predictive value of 99% and an accuracy of 96. In the same study, a PSV of ³200 cm/sec identified a ³70% angiographic celiac artery stenosis with a sensitivity of 87%, a specificity of 80%, a positive predictive value of 63%, a negative predictive value of 94%, and an accuracy of 82%.
Other duplex criteria for mesenteric artery stenoses are also in use. A SMA end diastolic velocity (EDV) greater than 45 cm/sec correlates with a ³50% SMA stenosis with a specificity of 92% and a sensitivity of 100%, while a CA EDV of 55 cm/sec or greater predicts a ³50% CA stenosis with a sensitivity of 93%, specificity of 100%, and accuracy of 95%.
Post prandial mesenteric duplex scanning as an adjunct to the diagnosis of mesenteric stenosis was evaluated in a study of 25 healthy controls and 80 patients with vascular disease. Pre-prandial SMA PSVs in controls and patients with <70% SMA stenosis did not differ and post prandial SMA PSVs in controls and patients with <70% SMA stenosis did not differ. The percent increase in SMA PSV was lower in patients with ³70% SMA stenosis than in controls. In normal patients with <70% SMA stenosis, the post prandial SMA PSV increases more than 20% over baseline.
The specificity for the combination of fasting SMA PSVs and post prandial PSVs was only marginally improved over that provided by a fasting duplex scan alone. Therefore, while theorectically attractive, post prandial duplex scanning offers no significant improvement over fasting mesenteric duplex scanning and does not need to by routinely utilized as part of ultrasound assessment of mesenteric artery stenosis.