Ultrasound signs of abdominal aortic occlusion

Among the causes of occlusive diseases of the abdominal aorta, atherosclerotic lesions occupy one of the leading places. According to the results of color Doppler scanning, the following degrees of aortic lesions can be distinguished: early atherosclerotic changes; stenosis; occlusion.

Early atherosclerotic changes in the aorta are limited to the localization of the process only in the aortic wall. When examining in B-mode, uneven thickening of the aortic walls, the presence of hyperechoic inclusions corresponding to calcium deposition in the form of individual lumps, an uneven internal contour of the aorta with an intact lumen of the vessel and a main type of blood flow according to the Doppler spectrogram may be noted.

Aortic stenosis diagnostics is based on B-mode visualization of echogenic masses that reduce the aortic lumen. Echogenic masses may be caused by the presence of either local, less commonly prolonged atherosclerotic plaques and/or the presence of parietal thrombosis. Atherosclerotic lesions are most often localized in the infrarenal region, in the bifurcation area of the abdominal aorta, and in a significant number of observations - along the posterior wall. The atherosclerotic process of this localization is characterized by the presence of high-intensity echo signals in homogeneous and heterogeneous plaques, in some cases accompanied by the presence of an acoustic shadow morphologically corresponding to calcification. Parietal thrombotic masses are hypoechoic, predominantly homogeneous in structure formations, which are usually located along the aortic wall and have an echogenicity slightly higher than that of the vessel lumen. The extent and shape of the plaque, as well as the mural thrombotic masses, can be clearly determined when working in the CDC and/or EDC mode. The degree of aortic damage is diagnosed based on the results of recording an ultrasound image with calculation of the percentage of stenosis using a computer program and supplemented by data from spectral analysis of blood flow. According to Th. Karasch et al., a local increase in the systolic linear velocity of blood flow (SLV) of more than 200 cm/s indicates the presence of hemodynamically significant aortic stenosis. In some cases, stenotic aortic damage can be combined with its deviation, especially in patients with arterial hypertension.

According to localization, there are three types of abdominal aortic occlusion:

1. low occlusion - occlusion of the bifurcation of the abdominal aorta distal to the origin of the inferior mesenteric artery;
2. middle occlusion - proximal to the origin of the inferior mesenteric artery;
3. high occlusion - at the level of the renal arteries or within 2 cm distal.

Ultrasound diagnostics of abdominal aortic occlusion is based on the following criteria:

1. The presence of echogenic masses obstructing the lumen of the aorta and the absence of blood flow in the lumen according to the data of color Doppler imaging and/or EDC and the spectrum of Doppler frequency shift.
2. Decreased systolic and diastolic blood flow velocity in the aorta proximal to the occlusion.
3. Registration of collateral blood flow in arteries distal to the occlusion.

Compensation of blood flow in case of abdominal aortic occlusion is realized by collateral circulation along various anatomical pathways, the course of which, according to color Doppler scanning (CDS), is not always possible to trace. However, in this situation, CDS allows us to obtain information about individual components of the collateral circulation system, in particular the inferior mesenteric artery, lumbar arteries and superior mesenteric artery.

Depending on the examination conditions, visualization of the inferior mesenteric artery (IMA) is possible in 56-80% of cases. Visualization of the IMA in the initial segment is performed during examination in the sagittal or transverse scanning planes 50-60 mm proximal to the aortic bifurcation at the level of the III-IV lumbar vertebrae. Normally, the IMA diameter is 2-3 mm. Qualitative characteristics of the blood flow spectrum indicate high peripheral resistance in the artery involved in the blood supply to the left part of the transverse and descending colon, sigmoid colon and proximal rectum. High peripheral resistance in the IMA is one of the ultrasound criteria for differential diagnosis of the IMA and renal arteries, which are characterized by low peripheral resistance.

Lumbar arteries are paired vessels located in the infrarenal aorta. Qualitative spectrum assessment indicates the presence of high peripheral resistance. When performing the function of collateral vessels in the lumbar arteries, the level of blood circulation increases, which improves the possibility of ultrasound imaging of these arteries.

In occlusive lesions of the abdominal aorta, the inferior, superior mesenteric arteries, and lumbar arteries bear a compensatory load, resulting in an increase in blood flow velocity with a gradual increase in their diameter. A feature of compensatory blood flow is the registration of an increase in the linear velocity of blood flow throughout the entire length of the vessel, accessible to ultrasound visualization, while in hemodynamically significant stenosis of the inferior or superior mesenteric arteries, local changes in hemodynamics are detected at the site of arterial narrowing.

Another cause of occlusive diseases of the abdominal aorta is nonspecific aortoarteritis. According to A.V. Pokrovsky et al., depending on the localization of aortic stenosis, there are three variants of damage to the thoracoabdominal segment of the aorta. In variant I of the lesion, only the descending thoracic aorta is involved in the process. This type occurs in 4.5% of observations. For variant II of the lesion, localization of the process in the supra-, inter- and infrarenal segments of the aorta is characteristic with almost obligatory simultaneous involvement of the visceral and renal arteries in various combinations. This most typical and frequently encountered type of aortic damage was noted by the authors in 68.5% of observations. In type III - 27% of observations - the descending thoracic aorta, its supra-, inter- and infrarenal segments, as well as the visceral and renal arteries are simultaneously involved in the process.

When performing color Doppler scanning in this group of patients, it is advisable to adhere to the following methodological points:

1. To optimize the aorta image and study the area of interest in detail, which in this case is the aortic wall, it is necessary to use the ultrasound device function that allows you to obtain an image of the area of interest in an enlarged size. In addition, to improve the quality of the ultrasound image of the aorta in B-mode, it is advisable to use the tissue harmonic mode.
2. The percentage of aortic stenosis based on ultrasound imaging should be measured based on cross-sectional area calculation.

An increase in the thickness of the posterior and/or anterior walls of the aorta indicates the presence of nonspecific aortoarteritis. However, the quantitative assessment of the wall thickness is not a constant value and may change depending on the activity of the inflammatory process. Ultrasound examination allows diagnosing the extent of changes in the aorta, which are characterized by prolonged damage, gradually moving into unchanged areas of the aorta. The echogenicity of the aortic wall may be normal or increased.

Information on the degree of aortic stenosis is important for determining the tactics of patient management and deciding on indications for surgical treatment. We distinguish two options: hemodynamically insignificant stenosis, which is characterized by the presence of wall thickening, the percentage of stenosis according to ultrasound imaging does not exceed 70%, normal values of the LBFV indicators in the abdominal aorta are preserved; hemodynamically significant stenosis, which is characterized by thickening of the aortic wall in combination with an increase in blood flow velocity, the percentage of stenosis according to ultrasound imaging exceeds 70%.

In addition, the obtained data can be supplemented with information about the role of the inferior and superior mesenteric arteries, lumbar arteries in the development of collateral circulation.