Ischemic Nephropathy

Age >55 years (considering that ischemic nephropathy predominantly interests to age >55/60 years old)

Positive family history for coronary disease (mainly in the male patients in the age >50 years old)

Smoking

Hypercholesterolemia

Obesity

Diabetes mellitus (DM)

Atherosclerotic vasculopathy

Table 3.2

Indicative sign of renal ischemic pathology

Renal asymmetry

Severe hypertension state after 55 years old

Rapid worsening of arterial hypertension previously well controlled

Sudden appearance of severe hypertensive state that may be treatment resistant to medical therapy, defined as an inability to reach a good control of pressure in a patient who is under full-dose antihypertensive therapy with at least three drugs

Malignant hypertension or hypertension with evidence of acute renal damage or acute renal insufficiency

Recurrent episodes of pulmonary edema without anomaly or heart dysfunction

Hypertensive retinopathy III or IV

Continuous abdominal systolic-diastolic bruit (can be inaudible in the case of subtotal stenosis), it is a sign that presents a positive predictive value and especially is related to renovascular hypertension

A rise in serum creatinine following the administration of ACEI or ARBs

The presence of the pathology related to atherosclerosis or peripheral vasculopathy disease (e.g., femoral iliac stenosis, intermittent claudication, etc.)

The ideal imaging method for diagnosing of ischemic nephropathy should allow identifying the main renal artery as well as the accessory branches, to localize the stenosis site, to identify significant hemodynamic lesion, to suggest the type of the treatment (medical therapy or angioplasty), to identify the associated pathology (renal mass or abdominal aorta aneurysm), and to distinguish chronic ischemic damage induced by nephroangiosclerosis or thromboembolism [2, 3].

The method should be able to prove or exclude the diagnosis of RAS or eventually to evaluate if ischemic nephropathy is due to an involvement of atherosclerosis without renal artery stenosis [4, 5].

Angiography, which in the past was the gold standard test for arterial diagnosis, is an invasive method, expensive with some severe complications such as adverse reaction to iodinated contrast, dissection, and atheroembolism.

For all these reasons, angiography is not more used like screening method, but like therapeutic instrument. In the last years, many less invasive imaging methods were introduced, such as renal scintigraphy with captopril test, Doppler ultrasound, computed tomography (CT), or magnetic resonance angiography (MRA). In many centers, Doppler ultrasound was accepted as the main screening instrument for identifying RAS [6].

3.2 Technical Study of the Renal Artery

Depth of the arteries, respiration movement, and intestinal meteorism are the factors that can complicate the exam. To decrease the intestinal meteorism, it is important to perform examination in the early morning time, after 12 h of fasting. In clinical practice, the duration of exam is about 15–20 min and depends on the experience of the operator, the number of performed exams, and the ultrasound platform performance.

If the exam would be lasting for more than 20–30 min, it should be repeated with adequate intestinal preparation; otherwise the operator’s performance tends rapidly to decrease.

The procedure starts with the patient in supine position and the head upward of 30° in order to relax the abdominal wall with the use of multifrequency convex probe (2.5─6.0 MHz). Numerous scans let us study the abdominal aorta (AA) and renal artery (RA). Both of the RA arise off the anterolateral side of the AA (Fig. 3.1). Generally in correspondence of the superior border of the second lumbar vertebra, immediately 12 cm below the superior mesenteric artery, this artery is divided to form the segmental arteries that irrigate different segments of the kidney. Segmental arteries divide within the renal sinus to form the interlobar arteries which originate the arcuate arteries located between the cortex and medulla. Arcuate arteries give off interlobular or cortical radial artery branches.

Fig. 3.1

Renal arteries via transverse anterior abdominal approach. Both renal arteries rise off the anterolateral side of the aorta. The right RA passes behind the inferior vena cava (IVC) and the right renal vein to reach the renal hilum. The left RA tends to originate from the posterolateral side of the aorta and courses posteriorly and obliquely to lumbar region reaching the renal hilum

Two principal scans to study the RA are sagittal and coronal scan through anterior and lateral abdominal wall. Scan choice depends on the anatomical location of renal vascular system to be investigated.

In the most cases, anterior scan is used to evaluate proximal portion of the RA, while lateral scan with the patient in lateral decubitus can be used to evaluate intrarenal vascularization and main branches of the RA (Fig. 3.2). Each scan has limits that depend on the patient’s habits and other many parameters such as ability to breath hold.

Fig. 3.2

Kidney’s lateral scan identifying intrarenal vascularization, main branches, and the renal artery. The lateral decubitus position is essential, because the kidney itself acts as a sonographic window making visible intrarenal vascularization, main branches of the renal artery, and the renal artery. Doppler angle is optimal and US beam is pointed in the line of arterial flow

The right RA originates from lateral antrum of the AA and passes behind the inferior vena cava (IVC) and the right renal vein to reach the renal hilum. Indeed in transversal scan, proximal portion of the right RA is deep and perpendicular to ultrasound’s wave and difficult to intonate with an angle lesser than 60°. In this condition, the best approach is subcostal view with the patient in left lateral decubitus position. In fact, blood flow of the RA in this scan runs parallel over the ultrasound’s wave [7] (Fig. 3.3).

Fig. 3.3

The right renal artery in subcostal view. A transverse anterior abdominal approach with the patient in lateral decubitus can help in recognizing of the renal artery behind the corresponding vein. RA flow is in the parallel direction to the Doppler beam

Additional aid in locating the RA is given by the “banana peel” view, which consists of imaging the IVC and AA with a longitudinal scan and moving anterior to posterior direction until the RA arising from the AA is identified (Fig. 3.4). This approach is particularly useful to insonate ostial areas, where most stenosis occur in elderly patients. The patient is in lateral decubitus position opposite from the vessel being examined, and the transducer is oriented longitudinally [7, 8]. When the AA and RA are detected together, it can be the appearance of half-peeled banana with skin curved alongside. This scan lets to detect the RA with optimal angle (<60°) and to notice the presence of accessory RA branches, approximately in 20–30 % of the patients (Fig. 3.5) [7, 9].

Fig. 3.4

“Banana peel” view. The patient is turned to the opposite decubitus position from the vessel we want to examine, and transducer is oriented longitudinally. The operator locates the aorta first and then moves the transducer in anterior to posterior direction until renal arteries are identified. The right RA moving toward the probe in red and the left RA moving away in blue color confer to this image the half-peeled banana aspect. Doppler beam angle is close to zero and gives optimal vision of ostial region. Moreover, this maneuver allows precise calculation of RAR and clear identification of accessory renal arteries

Fig. 3.5

Accessory renal artery. (a) B-mode imaging of double left RA: when the main renal artery diameter is less than 4.5 mm, the presence of the accessory RA is extremely possible. Accessory arteries are seen approximately in 25–30 % of the cases, but their stenosis occur in less than 1 % of cases. (b) Color Doppler enhances the vision of double right (in red color) and left (in blue color) renal arteries

The left RA tends to originate from the posterolateral of the aorta and courses posteriorly and obliquely to the lumbar region; an aid to locating the left RA is to first identify the left renal vein, which is usually large and easy to find. Once the vein is identified, the artery will often be apparent as a smaller vessel directly behind it, coursing in the opposite direction. The inferior mesenteric artery (IMA) origin should not be mixed up for left RA origin; in fact the IMA tends to have a high-resistance pattern, which is quite different from the low-resistance pattern of the left RA [7]. The IMA also originates much lower than the left renal artery, unless arises from an atypical location.

Another approach to image the left RA is placing the patient in right lateral decubitus and locate the RA between the kidney and AA with a very close angle. The lateral decubitus position is essential, because the kidney acts as its own window [8].

3.3 Color Doppler Study

The current platform ultrasound lets to find the RA in more than 90 % of patients, with overcoming problems caused by the obesity or intestinal meteorism. Direct visualization of both main RAs is possible in 84 % of the cases, while visualization of the right RA or left RA is possible in 91 % and in 85 % of the cases. In 5 % of cases, lack of diagnosis is related to the total occlusion of the RA without intrarenal Doppler signals.

The origin of the RA especially appears in B-mode station with successive modification in box color: with correct adjustment of Doppler function such as pulse repetition frequency (PRF), wall filters, and gain, colorimetric map should be uniform and without artifacts.

The flow of the first segment of the right RA is directed toward the transducer depicted in red, while immediately after, the color changing to blue shows the reverse direction of the blood flow that directed posteriorly, away from the probe.

If the origin of the RA is imaged in the longitudinal section, the right RA passes directly toward the transducer so the color is red, whereas the left RA is directed away from the transducer and the color is blue.

The normal waveform of the main renal artery demonstrates a low-resistance pattern similar to that found in all parenchyma (Fig. 3.6). Although the main RA may be imaged from an anterior approach, the deep location in the abdomen often limits the resolution of the transducer that may be applied.

Fig. 3.6

Normal features of the renal artery. Duplex ultrasound image of the right renal artery in a normal subject. At the end of systolic spike is a recognizable small spike called early systolic peak (ESP)

Lower-frequency transducers will have better sonographic penetration, but there is a trade-off of decreased spatial resolution. The highest-frequency transducer that allows good demonstration of arterial waveforms is preferable. Doppler gain should be optimized to detect flow by increasing the gain to a level just below color artifact visualization in adjacent structures.

Pulse repetition frequency (PRF) should be carefully adjusted to avoid to fall in frequency ambiguity if the PRF is very low (overturning of the superior portion of the V/t curve) or space ambiguity if the PRF is very high (the presence of the “ghost vessels”).

Volume sample dimension should be set including the entire artery lumen and angled with the direction of the flow. The angle of insonation should be maintained at 60° or less [7, 8, 10]. The PRF depends on the angle between the vessel and the ultrasound (US) beam and even depends on the frequency of transducer used. If the course of the main RA is well recognized, velocity angle corrected can be calculated.

The peak systolic velocity (PSV) in the main RA and its branches should be less than 100 ± 20 cm/s [15] and decreases slowly in the intrarenal arteries as they branch into the kidney.

The resistive index (RI) that measures the degree of intrarenal arterial impedance is calculated by this formula [PSV − telediastolic velocity]/PSV. RI values in healthy subjects show significant dependence to age and area of the sampling. It is suggested to determine RI in interlobar artery level because it lets to get the optimal color signal and because it is easily reproducible. The normal RI values in the main RA are higher in the hilar region (0.65 ± 0.17) than in the more distal small arteries, and they are lowest in the interlobar arteries (0.54 ± 0.20). Some renal pathology such as nephroangiosclerosis, hypertension, tubular interstitial disease, diabetes mellitus, and severe bradycardia can cause an increase of RI, even in the presence of serum creatinine in the normal range [11]. In clinical practice 0.7 is the cutoff distinguishing between normal and pathologic state.

3.4 Doppler Criteria for Diagnosis of RAS (Renal Artery Stenosis)

Doppler US criteria of RAS can be divided into two groups:

(a)

Direct criteria or extrarenal (Doppler changes in the site of stenosis)
(b)

Indirect criteria or intrarenal (flow changes distal to the site of stenosis)