4 Abdomen The initial euphoria about interventional treatment of renal artery stenosis has given way to a more sober, even skeptical assessment. Good results are still being reported (Patel et al 2009), with a significant decrease in complications (Schillinger and Zeller 2007). Yet recent studies indicate that the regulation of blood pressure and restoration of kidney function achieved with stent angioplasty are on average no better than the results achieved with medical treatment alone (Lenz 2010). However, one of these studies has been criticized for flaws in its methodology. Regardless of how this dispute is likely to be resolved, the radiologist should exercise restraint in determining whether the treatment is indicated. Close consultation with specialists in allied disciplines is essential. Renal artery stenosis is a major cause of impairment or loss of function. Yet renal artery stenosis is a probable cause of arterial hypertension in at most 5% of patients. The kidneys have an autonomous blood pressure regulation system. When one kidney detects too low pressure (e.g., due to renal artery stenosis) it secretes renin to increase pressure throughout the system (renal hypertension). This mechanism is referred to as Goldblatt hypertension in honor of the physician who first discovered it. Eliminating the causative stenosis can improve or eliminate the hypertension in this case. About 90% of renal artery stenoses are due to arteriosclerosis. The rest are caused primarily by fibromuscular dysplasia (FMD). Restenosis rates of 14% have been observed in arteriosclerotic renal artery stenoses treated by stent placement (Sapoval et al 2005). Suspicion of renal artery stenosis usually arises when hypertension is poorly controllable with medication, when it occurs at a particularly early age (< 30 years) or late age (> 50 years), and when it worsens under therapy with angiotensin-converting enzyme (ACE) inhibitors. Rapid deterioration of kidney function can also be a sign of renal artery stenosis. A stenosis grade of 70% or even 80% is generally regarded as the threshold defining the indication for intervention. Today it is felt that intervention is not usually indicated in patients with well controlled hypertension and in older hypertensive patients with normal kidney function. The reliability of magnetic resonance imaging (MRI) and computed tomography (CT) in detecting stenosis is highly dependent on the specific equipment and examination technique. Magnetic resonance angiography (MRA) in particular often produces false-positive results. Because of the toxicity of the contrast agent, both modalities are problematic in patients with impaired kidney function. Ultrasound is a relatively reliable method in a slender patient if the equipment is very good and the examiner is experienced. Sensitivity and specificity are reported to be ~ 90% (Henry et al 2008). Captopril-enhanced renal scintigraphy (too costly) and separate renin measurements for each side (too unreliable) are no longer recommended for routine clinical practice (Henry et al 2008). Angiography will occasionally fail to detect an ostial stenosis where the ostium cannot be visualized separately from the aorta. Renal insufficiency can be due to renal artery stenosis. Eliminating it can improve long-term function. However, in 20% of the cases with renal insufficiency it initially leads to worsening of the condition (Henry et al 2008). Therefore great care should be taken when determining whether treatment is indicated, and specialists from every discipline involved should be consulted, especially the nephrologist. Percutaneous Transluminal Angioplasty or Stent? Ostial stenoses are now usually treated by primary placement of a stent. This is because they are usually very rigid and because the balloon only temporarily presses the occluding material into the aortic lumen. From there it falls back into the ostium after dilation (Fig. 4.1). Because they have higher radial strength and can be placed with greater precision, balloon-expandable stents are used almost exclusively in the renal artery (average length 15 mm). Good results can often be achieved with dilation alone in stenoses distal to the ostium. FMD should only be treated with a stent in exceptional cases. Fig. 4.1 Percutaneous transluminal angioplasty (PTA) in ostial stenoses. In an ostial stenosis, the PTA balloon often only pushes the plaque material into the aorta. It then falls back into the ostium. a Ostial stenosis. b Dilation. The balloon pushes plaque material into the aorta. c Then it falls back into the ostium. d No improvement in findings. Deterioration of kidney function is the most important risk to be anticipated in an intervention in the renal arteries. The main reason for this is the nephrotoxicity of the contrast agent. Therefore the decisive prophylactic measure is to use contrast agent extremely sparingly. Note that a normal serum creatinine level does not exclude renal functional impairment. Serum creatinine increases over the normal value of 1 to 1.1 mg/dL only when the glomerular filtration rate (GFR) drops to 50%. Renal insufficiency can be the decisive indication for elimination of renal artery stenosis. Because the condition is usually progressive, even stabilization of renal function may be regarded as a successful outcome. The more severe the insufficiency, the more likely it will improve with treatment. Yet at the same time the risk to residual function posed by the contrast agent increases with the severity of the functional impairment. The average rate of worsening of renal function secondary to intervention ranges from 15% (Schillinger and Zeller 2007) to 21% (Henry et al). It can probably be reduced to less than 5% (Sos and Trost 2008) by good preparation and very sparing use of contrast agent, possibly substituting CO2 (see also Chapter 2, CO2 as a Contrast Agent, p. 36). Kidney function should invariably be evaluated before the patient is discharged! Aside from contrast toxicity, the embolization of cholesterol particles within the kidney is also an important causative factor in possible deterioration of kidney function. This is triggered by mechanical action on plaques. Therefore there is an increased risk of cholesterol embolization in an aorta with severe changes in the form of unstable plaques. Cholesterol emboli can occur up to 3 days after the intervention (frequency up to 3%, Schillinger and Zeller 2007). Protection systems such as those used in the interventional treatment of carotid stenosis are currently being tested. There have not yet been any definitive recommendations for their use. The mechanical risks associated with the treatment of renal artery stenosis include incorrect placement of the stent, dissection, perforation of the kidney with a guidewire, rupture, and avulsion of the renal artery. The most important measures for avoiding mechanical complications include the following: • Slender catheter systems • Correctly matching the balloon and stent size to the vessel diameter, which requires precise angiographic measurement • Slowly inflating the balloon while closely observing the patient: Is there a sensation of pressure? Pain? • Correct placement of the stent • Angiographic monitoring prior to deploying the stent • Avoiding any uncontrolled movement of the wire (a monorail system should be preferred) Incorrect placement of the stent. A stent placed in the ostium should project 1 to 2 mm into the aortic lumen. This prevents plaque material from falling back into the ostium after dilation. The most important step in avoiding incorrect placement of the stent is to visualize the ostium in an optimal projection (Fig. 4.2). This is best achieved by using cross-sectional MR or CT images as the basis for planning the procedure. A stent that projects too far into the aorta (> 2 mm) will render any reintervention difficult. The guidewire will tend to enter the mesh of the stent rather than its lumen (Figs. 4.3 and 4.4). A renal artery dissection occurs significantly more often than a rupture. The dissection most commonly occurs during dilation in the vicinity of a rigid plaque, although it can also be caused by a guidewire. It can usually be managed by placing a stent. Perforation of the kidney with a guidewire (usually a glide wire). One must have the guidewire in view during any manipulation. Should one have neglected to do so and the patient complains of a stabbing pain in the kidney region, then one must check the wire immediately. It may be that it has entered the parenchyma or even perforated it. If that is the case, do not immediately withdraw the wire. First administer protamine to neutralize the heparin. Then advance a 4 French catheter over the wire to the end of the vessel. Remove the wire, and inject contrast agent through the catheter to demonstrate any bleeding. In the event there is bleeding, a 2 mm coil is placed in the wound canal. A CT image is later obtained to exclude a hematoma. Rupture of a renal artery. This will most likely occur where the balloon is too large for the renal artery and where the stenosis is severely calcified. The best prophylaxis is to precisely measure vessel diameter and avoid excessive dilation. When in doubt, use a smaller balloon or stent. If this proves to be too small, one can always dilate again with a larger balloon. If the patient complains of intense pain during dilation, deflate the balloon and perform angiography via the sheath. If any extravasation is detected, inform the vascular surgeon immediately. Fig. 4.2 If the ostium is not correctly visualized perpendicular to the beam, a stent that lies too far distal (shown here in cross section) will be projected onto the aortic lumen on the angiographic image. a Incorrect projection. b Correct projection. Fig. 4.3 If the stent lies too far within the aortic lumen, the wire will pass through the mesh of the stent when reintervention is attempted. a Caudal approach. b Cranial approach. Fig. 4.4 Where the renal artery arises at an acute angle, the caudal aspect of the stent must project farther into the aortic lumen. a Eccentric calcifications in a stenosis near the ostium. b The caudal aspect of the stent projects farther into the aortic lumen than its cranial aspect. Then one can attempt to tampon the bleeding with the balloon (at low pressure). At the same time, neutralize the heparin with protamine. (Caution: There is a risk of anaphylactic reaction, especially in patients who are allergic to fish protein.) If this does not succeed, place a covered stent. This can be difficult because the covered stent is less flexible than a regular stent. Avulsion of a renal artery is a very rare event and is thought to occur more often in the presence of a severely calcified ostium. In this case one must occlude the aorta proximal to the renal artery with a large occlusion balloon and prepare for immediate vascular surgery. Material for emergency treatment (according to Schillinger and Zeller 2007) includes the following: • Covered stents 4 to 7 mm in diameter, 15 to 20 mm long • Large balloons to seal off the aorta • Embolization coils 2 to 6 mm • Aspiration catheter that fits a 0.014 in. wire • Thrombolytic agent • Protamine ampoules containing 1,000 or 5,000 IU for intravenous (IV) administration: 1,000 IU of protamine; deactivate 1,000 IU of heparin (risk of anaphylactic reaction; see earlier mention) Table 4.1 Complications of intervention in the renal arteries Complication Avoidance Treatment Contrast-induced nephropathy Hydration Preparation using cross-sectional images of the aorta Catheterize renal artery without contrast Small doses of contrast with high image frequency Co2 as contrast agent where feasible Incorrect placement of stent Correct projection (cross-sectional images!) Angiographic monitoring Stiff wire: bend Perforation Do not use glide wire Monorail system Embolization (surgery) Dissection Stent Rupture Precisely measure vessel diameter Caution where calcification is present Be alert to pain sensation! Balloon tamponade Covered stent (surgery) Cholesterol embolization Atraumatic technique Slender systems Thrombosis, embolus Spasm Aspirin, clopidogrel, heparin Verapamil, nitroglycerin The complication rate in large series used to be ~ 10 to 14%. Since the introduction of long guiding sheaths and smaller systems (0.014 or 0.018 in. wires, monorail) it has decreased to less than 3% (Schillinger and Zeller 2007, Table 4.1). • Measure urea and creatinine serum levels, creatinine clearance, wherever possible GFR. • Measure blood pressure over 24 hours. • Determine medication. • Perform duplex ultrasound scan. Any error or omission in renal percutaneous transluminal angioplasty (PTA) can have serious consequences for the patient, including dependence on dialysis, loss of a kidney, and death. This justifies any additional expense that improves safety. Whatever can be resolved beforehand or would require additional contrast agent to resolve during the intervention should be resolved beforehand. This includes the anatomy of the abdominal aorta and the iliac arteries (MRI or CT) and, particularly important, the direction of the origins of the renal arteries (where feasible, without contrast agent). If this has not been resolved beforehand, one will end up groping toward a halfway decent projection of the renal artery on the basis of two or three aortograms. Cross- sectional CT or MR images usually show this with perfect clarity. Therefore make an effort to obtain previous imaging studies (CT, MRI, angiography). Where no such studies are available, obtain at least a noncontrasted CT scan of the kidney region prior to the intervention. Aside from the direction of the origin, these images will also show major calcifications. If the angular adjustment is off by only a few degrees, then the ostial stenosis will be obscured by the contrasted aortic lumen. Where the renal arteries arise at different angles (Fig. 4.5), two separate image series are needed for the left and right renal arteries. On the day of the intervention begin with hydration of the patient: Administer 1 to 1.5 mL per kg body weight per hour of a normal or half normal saline solution 12 hours before and 12 hours after the intervention. In patients with heart failure, the fluid must be balanced with the volume of urine. Also begin therapy with acetylsalicylic acid and clopidogrel on the day before the procedure (300 mg on the first day, thereafter 75 mg daily). Oral acetylcysteine, a strong antioxidant, is often administered as prophylaxis against kidney damage (600 mg each the day before and on the day of the intervention). Before the advent of stent treatment it was customary to work with only one catheter via a short sheath. The balloon catheter had to be withdrawn and replaced by a diagnostic catheter to verify the results of treatment. The development of the stent gave rise to the need to verify the stent’s precise position angiographically during the procedure. This was initially done by introducing a pigtail catheter via the contralateral inguinal artery and advancing it into the aorta parallel to the first one (Fig. 4.6). This can no longer be recommended. The quantity of contrast agent required is significantly higher than when injected via a long guiding sheath. Additionally, catheterization of the contralateral groin increases the risk of complications. Fig. 4.5 Differences in the direction of the origins of the left and right renal arteries. Fig. 4.6 Once a common procedure with two catheters, now no longer recommended. The decisive advance came with the introduction of long guiding sheaths (Fig. 4.7). These devices provided a significantly more reliable means of advancing the stent up to and into the renal artery. They allowed angiographic control with very small volumes of contrast, and they are also required for the use of monorail systems. A long sheath without a bend (Fig. 4.8) allows the use of a monorail system. However, it does not aid in advancing the wire or catheter and requires equally large volumes of contrast as the pigtail catheter technique described earlier. In addition, the spread of contrast along the aorta is even more pronounced than in the use of a pigtail, resulting in bad image quality. This means it cannot be recommended at all. Fig. 4.7 Intervention via a long sheath. Angiographic monitoring prior to deploying the stent. Guiding sheaths and guiding catheters are supplied with various curvatures (Fig. 4.9). The width and curvature of the aorta and the angle of the renal artery origin are the decisive criteria for selecting the sheath. For a renal artery that arises nearly horizontally it is best to choose a sheath the tip of which is angled perpendicular to the aortic wall. Where the renal artery courses caudally from its origin, it is best if the sheath has a more pronounced bend. Fig. 4.8 Long sheath without a bend. The quantity of contrast agent required is higher than with a pigtail catheter. Image quality is poor due to superimposition! Where the sheath is to be advanced into the renal artery, a model with a less pronounced bend is recommended. A sheath with a sharp bend can be required to compensate for a bend in the infrarenal aorta (Fig. 4.10, left). For the contralateral side one will need for the same constellation a straight sheath with a short bend at its tip (Fig. 4.10, right). The popular Renal Double Curve (RDC) model is not recommended. The RDC sheath has a sharp bend near its tip that can prevent the passage of a stent. This risk is significantly reduced where the sharp bend is replaced by a harmonious curve (Fig. 4.11). Catheters are specified by their outer diameter but sheaths by their inner diameter (see Chapter 2, Size Specifications for Cannulas, Guidewires, Catheters, and Sheaths, p. 7) (i.e., according to the catheter that fits in the sheath). This explains why a 6 French guiding sheath has the same inner and outer dimensions as an 8 French guiding catheter. When an 8 French guiding catheter is used, the 8 French sheath that is also required will naturally create a larger puncture wound in the wall of the femoral artery. Transbrachial catheterization (Fig. 4.12) may be the better approach in the presence of a slender aorta, acute-angle renal artery origin, or greatly elongated (tortuous) iliac arteries. However, this approach is associated with a higher risk, especially where placing a larger stent requires the use of a 6 French sheath that can occlude the lumen of the brachial artery. Thrombi mobilized by the sheath can cause a cerebellar insult. The intervention begins with abdominal aortography with a high degree of collination (obtained in the optimal projection determined on the basis of prior CT or MRI studies). This image serves to verify the indication and determine the position of the ostia. Where no cross-sectional images are available, an initial attempt with a 20° left anterior oblique (LAO) projection is recommended. Using the smallest possible volumes of contrast to optimal effect for the aortography requires that the contrast agent be deposited over a very short segment of the aorta. The pigtail catheters commonly used distribute the contrast over a long segment of the aorta through too many side holes. Another problem is that the end of the pigtail points cranially and therefore separates up to 50% of the contrast agent from the bolus. For this reason it is better to use a catheter whose tip points caudally and has only a few side holes close to it (Fig. 4.13). (The most elegant solution would be a catheter without an end hole and with side holes that expel contrast agent only in the direction of the renal arteries.) If the level of the renal artery origin is not known, then the side holes and end hole of the catheter should lie at the level of the T12–L1 vertebrae. Fig. 4.9 Guiding sheaths of various shapes. Fig. 4.10 Adapting the shape of the sheath to a curved aorta. Fig. 4.11 The unnecessarily sharp bends in the Renal Double Curve (RDC) sheath (left) can interfere with the passage of a stent. The sheath on the right exhibits the same angle between shaft and tip but with a harmonious curve. a RDC sheath. b Harmonious curve. Fig. 4.12 Transbrachial approach in the presence of a slender aorta and acute-angle renal artery origin. Sos and Trost (2008) recommend using contrast agent with 150 mg iodine/mL. The disadvantage of this is that blood invariably dilutes the contrast agent anyway. If half the volume of a contrast agent with 300 mg of iodine per mL is used, no greater contrast burden is placed on the kidneys but better contrast will briefly be achieved in the vicinity of the catheter tip. The important thing is that at small contrast volumes (5 to 8 mL) one needs a high image frequency (4/s) and a summation image generated from a series of individual images. The patient’s position and the projection must remain unchanged during the intervention. Then an overlay image from the angiography performed beforehand is the most reliable guide for catheterizing the renal artery. Alternatively, an unsubtracted or partially subtracted image that clearly visualizes the skeleton can be displayed the whole time on a reference monitor for orientation purposes. The aortography provides the basis for the final decision as to whether intervention is indicated. PTA or stent placement is generally regarded as indicated where a stenosis of over 70% (some authors say 80%) is demonstrated (i.e., measured) on the image. Estimations according to the visual impression are unreliable and therefore inadmissible; in one experiment (Subramanian et al 2005) stenoses of 56.6 + 10.8% were estimated as 74.9 + 11.5%. The reference value for determining the severity of the stenosis is not the diameter in the region of the poststenotic dilation (Fig. 4.14)! Fig. 4.13 How catheter shape influences quantity of contrast required. a Contrast distribution patterns with pigtail and Omni-Flow catheter (Sos). b Aortography with Omni-Flow catheter. If the indication cannot be determined unequivocally on the basis of morphological criteria, measurement of the pressure gradient can be the deciding factor. The measurement is performed simultaneously distal to the stenosis (preferably with a 0.014 in. pressure wire) and in the aorta (via the sheath). A gradient of > 10% at peak systolic pressure is regarded as conclusive; experiments have demonstrated increased renin production in the kidney above this gradient (Sos and Trost 2008). Fig. 4.14 Measurements to determine the severity of stenosis. a, diameter of the stenosis; b, reference diameter distal to the poststenotic dilation. Because the wire for the pressure measurement is expensive, operators usually use a 4 French catheter instead. Yet this leads to overestimation of the severity of the stenosis because the catheter itself occludes part of the cross section within the stenosis. The 4 French catheter will turn a 60% stenosis in an artery 5 mm in diameter into a stenosis of 70%. However, this will keep one on the safe side at least for excluding an indication for intervention. The hemodynamic effectiveness of a stenosis depends on the flow rate and therefore on the peripheral resistance in the kidney. In advanced nephrosclerosis, it is possible that even a high-grade stenosis will not create a significant pressure gradient because of the low flow. Then intervention would be pointless (Sos and Trost 2008). • Select and introduce the appropriate sheath. • Administer heparin (usually 5,000 IU). To avoid spasms, 0.1 mg of nitroglycerin (short acting) is usually injected into the renal artery, or 2.5 mg of verapamil (longer acting). It is recommended to flush the sheath continuously during the intervention and to combine this with continuous blood pressure measurement. Otherwise the sheath must be flushed at least every 2 minutes. The technique described by Thomas Sos employs a modified Sidewinder catheter (AngioDynamics, Latham, NY, USA) in combination with a 0.035 in. Bentson wire. The distal segment of the Sos Omni catheter (AngioDynamics) is shorter than that of the Sidewinder I. This makes it easier and less traumatic to pull it into the renal artery. It is also possible to turn the catheter by simultaneously advancing and rotating it within the aorta beneath the aortic arch (Fig. 4.15). This does not require catheterizing the subclavian artery. Fig. 4.15 Changing the direction of the Sos Omni catheter by advancing the catheter and simultaneously rotating it. a Catheter is advanced and simultaneously rotated. b Redirected catheter. The Bentson wire has a soft tip 16 cm long that does not reextend the catheter. When it projects several centimeters beyond the catheter tip, it makes it possible to pull the curved tip of the catheter tip smoothly downward. Below the ostium one withdraws the wire until just less than 1 cm projects out of the catheter (Fig. 4.16b). When the catheter is then slowly advanced past the ostium, the end of the wire snaps into the renal artery (“flick”). Once the tip of the wire lies within the ostium, it is advanced further to secure its position (Fig. 4.16d). Then the catheter is carefully pulled into the renal artery (Fig. 4.16e). Fig. 4.16 Sos flick technique. a Beneath the ostium withdraw the wire to < 1 cm. b Slowly advance the catheter to the ostium. c “Flick”: The tip of the Bentson wire snaps into the renal artery. d The wire is advanced further to secure its position. e Carefully advance the catheter into the renal artery. f The sheath is “parked” in the iliac artery. g Once the wire and catheter lie securely within the renal artery, the sheath is advanced to the ostium under fluoroscopy (keep a grip on the wire and catheter!). Fig. 4.17 Angiography, measurement, preliminary dilation. a A suitable guidewire is inserted before the Sos catheter is removed. Angiography is performed and the vessel diameter measured. b Perform preliminary dilation where indicated.
Renal Arteries
Indication
Risks and Complications
Renal Insufficiency
Mechanical Risks
Preparation for Intervention
Preparing the Patient
Intervention Technique
Fundamentals
Intervention Technique in Detail
Guiding Sheaths and Guiding Catheters
Aortography
Measuring Pressure
After Determining the Indication
Sos Flick Technique
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