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(Radiology. 2001;220:168-173.)
© RSNA, 2001

Ultrasonography

Renal Artery Stenosis: Duplex US after Angioplasty and Stent Placement1

Melhem J. A. Sharafuddin, MD, Carl A. Raboi, MD, Monzer Abu-Yousef, MD, William J. Lawton, MD and Joel A. Gordon, MD

1 From the Departments of Radiology (M.J.A.S., C.A.R., M.A.Y.) and Internal Medicine, Division of Nephrology (W.J.L., J.A.G.), University of Iowa Hospitals and Clinics, 200 Hawkins Dr, 3889 JPP, Iowa City, IA 52242; and the Department of Radiology, Iowa City Veterans Administration Medical Center, Iowa (M.J.A.S.). Received January 14, 2000; revision requested March 4; final revision received October 31; accepted December 4. Address correspondence to M.J.A.S. (e-mail: mel-sharafuddin@uiowa.edu).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
PURPOSE: To evaluate the hemodynamic outcome of technically successful percutaneous transluminal renal artery angioplasty and stent placement (PTRAS) with duplex ultrasonography (US).

MATERIALS AND METHODS: Eighteen patients who underwent PTRAS in 22 renal arteries were prospectively examined. All had abnormal preprocedural duplex US findings. Those who had significant renal artery stenosis (>70%) at angiography and underwent technically successful percutaneous interventions were enrolled. Standard intrarenal duplex US parameters (acceleration index [AI], acceleration time, waveform morphology grade, and resistive index) were compared before and after interventions.

RESULTS: A significant AI increase occurred after PTRAS (9.02 m/sec2 ± 4.85 [SD]), as compared with before intervention (2.34 m/sec2 ± 2.03; P < .001). Acceleration time significantly decreased from 0.084 second ± 0.049 to 0.032 second ± 0.008 (P < .01). There was also a significant resistive index increase from 0.69 ± 0.12 to 0.79 ± 0.12 (P < .01). Abnormal waveform morphology (modified Halpern waveform grade 3–6) was present in 19 (86%) of 22 intrarenal arteries prior to intervention, as compared with one (5%) after PTRAS (P < .001). In the instance in which an abnormal waveform persisted after intervention, waveform morphology improved from grade 6 to grade 3, with a concomitant AI increase from 0.96 to 5.1 m/sec2.

CONCLUSION: The findings suggest an important potential role for duplex US in noninvasive assessment of the immediate hemodynamic outcome and long-term follow-up of PTRAS.

Index terms: Hypertension, renovascular, 961.723 • Renal arteries, stenosis or obstruction, 961.721 • Renal arteries, transluminal angioplasty, 961.1268 • Renal arteries, US, 961.12984 • Stents and prostheses, 961.1268 • Ultrasound (US), Doppler studies, 961.12984


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
With the recent advances in intravascular stent technology, the spectrum of renovascular lesions amenable to percutaneous treatment has expanded markedly. Percutaneous transluminal renal artery angioplasty and stent placement (PTRAS) allows effective and consistent treatment of ostial atherosclerotic stenoses (1). In addition, it can be an effective technique for suboptimal or complicated percutaneous transluminal renal balloon angioplasty (2). Data suggest that PTRAS may be used to lower the rate of restenosis, as compared with percutaneous transluminal renal artery angioplasty (PTRA) alone (3,4). Concomitantly, there has been increased awareness among clinicians in general of percutaneous therapy for various forms of renal artery stenosis (RAS) (5).

Despite remarkable short-term results, the main drawback of PTRAS is a substantial incidence of restenosis, which varies widely from 2% to 36% at 6–12 months (1–4,6–17). Although a trend toward a favorable restenosis rate has been noted with the use of stents, as compared with that with use of balloon angioplasty alone (3,4), restenosis remains the main concern after PTRAS. Restenosis often results in symptom recurrence and may progress to occlusion and kidney loss. Early restenosis detection is thus important to allow effective secondary intervention.

Duplex ultrasonography (US) is emerging as a screening modality for RAS (18–20). It is widely available, inexpensive, and noninvasive and does not require iodinated contrast media. Although the value of duplex US in RAS screening is well established, its role in assessing the hemodynamic outcome of percutaneous intervention and routine follow-up after successful intervention has not been well evaluated.

The purpose of our study was to evaluate the hemodynamic outcome of technically successful PTRAS by performing duplex US.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Between July 1998 and August 1999, all patients referred to our angiography department for percutaneous renal artery intervention were followed up prospectively. Patients were selected for the study if they met the following criteria: They (a) underwent preintervention dedicated RAS duplex US in our department; (b) had preintervention angiographically significant (>70%) atherosclerotic RAS (excluding fibromuscular dysplasia); and (c) underwent angiographically successful percutaneous intervention (either PTRA or PTRAS), as determined by the interventional radiologist, according to the following criteria: smooth lumen, less than 20% residual stenosis, and angiographically brisk flow.

Eighteen patients (10 men and eight women; mean age, 64 years ± 11 [SD]; age range, 42–82 years) who underwent 22 renal artery interventions met the inclusion criteria and were enrolled in the study. In all patients with bilateral renal artery stenoses, both renal arteries were treated during a single procedure. All patients underwent follow-up RAS duplex US within 2 days after intervention. The study was reviewed and approved by our institutional review board. Appropriate informed consent was obtained from all patients.

Indications for intervention were renovascular hypertension in nine patients, chronic renal insufficiency in two, hypertension and chronic renal insufficiency in six, and planned abdominal aortic aneurysm surgery with a solitary kidney RAS in one. The cause of RAS was atherosclerosis in all patients, with an ostial pattern in 16 arteries and a main renal artery pattern in six. RAS was bilateral in four patients, right-sided in nine, and left-sided in five. All interventions were performed in the angiography suite by using standard technique and equipment. Intervention consisted of 21 PTRAS procedures in 17 patients. The remaining patient underwent balloon PTRA only. The stent was balloon-expandable (Palmaz; Cordis, Warren, NJ) in 19 arteries, self-expanding (Symphony; Boston Scientific, Natick, Mass) in one, and balloon-expandable (Intra Stent; Intra Therapeutics, St Paul, Minn) in one. The mean expanded stent diameter was 6.4 mm ± 0.85. Stents were chosen according to procedural necessity and the preference of the interventional radiologist.

All duplex US studies were performed in a similar manner in the radiology department by using modern equipment (Sequoia 512; Acuson, Mountain View, Calif) with phased-array sector transducers. A majority of examinations were performed by one operator (M.A.Y.). Duplex examinations were performed with 3.5- or 4-MHz phased-array transducers. Spectral tracings were obtained from the segmental and interlobar arteries in the upper and lower poles of the kidney. The sweep time was set to the highest possible value (100 mm/sec) with the lowest spectral filter. The gain was set such that background echoes were barely visible. The Doppler gate width was kept small, and the angle of insonation was maintained at or lower than 60°. Patients were examined in the lateral decubitus position.

Quantitative duplex waveform analysis included the velocity and time changes of early systole (velocity change [{Delta}V] and acceleration time change [{Delta}T]), the acceleration index (AI), and the resistive index. {Delta}V was calculated from the onset of the systolic upstroke to the early systolic peak, whereas {Delta}T was the time change between these two points. AI was calculated by dividing {Delta}V by {Delta}T. The resistive index was calculated by subtracting the end-diastolic velocity from the peak-systolic velocity and dividing the result by the peak-systolic velocity. Measurements were made with machine software and electronic calipers. Only one waveform tracing from each examination was chosen for analysis. Because multiple measurements were obtained, the tracing with the most normal waveform was selected for analysis, in the following order: most normal waveform morphology, highest AI, and lowest {Delta}T. All interventions were performed in single renal arteries. Since no significant renal artery branch disease was present in the study patients, we did not separate upper- from middle- or lower-pole values.

Pre- and postintervention Doppler waveform tracings were qualitatively analyzed by two experienced radiologists (M.J.A.S. and M.A.Y.) who were blinded to patient name and pre- or postprocedural status, according to a classification modified from the study by Halpern et al (18): Waveforms were assigned one of six grades (1-2, normal; 3-6, abnormal) according to the identification of an early systolic peak, or "tardus-parvus" configuration. A schematic representation of the various waveform classes was provided during the review process. In cases of discrepancy, a third radiologist (C.A.R.) provided an opinion, and the majority opinion was considered. Interobserver agreement between the two primary observers was verified with congruence analysis.

Relevant numeric variables ({Delta}T, AI, and resistive index) before and after intervention were compared by performing the paired Student t test. Corresponding discrete variables ({Delta}T > .07 second, AI < 3.0 m/sec2) and abnormal Doppler waveforms were compared by performing the McNemar test. Preliminary analysis of all tested variables was performed after random deletion of one side in instances in which bilateral renal artery interventions were performed. This resulted in no alteration in the significance of any of the evaluated variables. Pearson correlation analysis was performed between the various numeric variables before and after intervention. Fisher R to Z transformation was performed to determine the corresponding probability levels. Cohen {kappa} statistics were obtained to compare interobserver congruence. Statistical analysis was performed with computer software (SYSTAT 8.0; SPSS Science, Chicago, Ill).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
After technically successful PTRAS, the mean {Delta}T decreased from 0.084 second ± 0.049 to 0.032 second ± 0.008; a significant difference was noted with the paired t test (mean difference, -0.052; P < .01). The mean AI increased from 2.34 m/sec2 ± 2.03 to 9.02 m/sec2 ± 4.85 after intervention, with a significant difference at paired t test analysis (mean difference, 6.68; P < .001) (Table 1, Fig 1).


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  		TABLE 1. Intrarenal Duplex US Parameters before and after Intervention

 


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  		Figure 1a. (a) Flush anteroposterior digital subtraction angiogram of abdominal aorta demonstrates high-grade right RAS (arrow). (b) Corresponding longitudinal duplex US image. Doppler waveform from a lower-pole segmental artery demonstrates a tardus-parvus pattern (AI and {Delta}T were 0.18 second and 0.99 m/sec2, respectively). (c) Flush anteroposterior digital subtraction angiogram of the abdominal aorta after PTRAS shows restoration of a widely patent right renal artery ostium and lumen (arrows). (d) Corresponding longitudinal duplex US image. Doppler waveform from a lower-pole segmental artery, obtained after PTRAS, demonstrates marked improvement in waveform morphology, with a well-defined early systolic peak (arrow). AI and {Delta}T were 0.04 second and 7.38 m/sec2, respectively.

 


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  		Figure 1b. (a) Flush anteroposterior digital subtraction angiogram of abdominal aorta demonstrates high-grade right RAS (arrow). (b) Corresponding longitudinal duplex US image. Doppler waveform from a lower-pole segmental artery demonstrates a tardus-parvus pattern (AI and {Delta}T were 0.18 second and 0.99 m/sec2, respectively). (c) Flush anteroposterior digital subtraction angiogram of the abdominal aorta after PTRAS shows restoration of a widely patent right renal artery ostium and lumen (arrows). (d) Corresponding longitudinal duplex US image. Doppler waveform from a lower-pole segmental artery, obtained after PTRAS, demonstrates marked improvement in waveform morphology, with a well-defined early systolic peak (arrow). AI and {Delta}T were 0.04 second and 7.38 m/sec2, respectively.

 


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  		Figure 1c. (a) Flush anteroposterior digital subtraction angiogram of abdominal aorta demonstrates high-grade right RAS (arrow). (b) Corresponding longitudinal duplex US image. Doppler waveform from a lower-pole segmental artery demonstrates a tardus-parvus pattern (AI and {Delta}T were 0.18 second and 0.99 m/sec2, respectively). (c) Flush anteroposterior digital subtraction angiogram of the abdominal aorta after PTRAS shows restoration of a widely patent right renal artery ostium and lumen (arrows). (d) Corresponding longitudinal duplex US image. Doppler waveform from a lower-pole segmental artery, obtained after PTRAS, demonstrates marked improvement in waveform morphology, with a well-defined early systolic peak (arrow). AI and {Delta}T were 0.04 second and 7.38 m/sec2, respectively.

 


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  		Figure 1d. (a) Flush anteroposterior digital subtraction angiogram of abdominal aorta demonstrates high-grade right RAS (arrow). (b) Corresponding longitudinal duplex US image. Doppler waveform from a lower-pole segmental artery demonstrates a tardus-parvus pattern (AI and {Delta}T were 0.18 second and 0.99 m/sec2, respectively). (c) Flush anteroposterior digital subtraction angiogram of the abdominal aorta after PTRAS shows restoration of a widely patent right renal artery ostium and lumen (arrows). (d) Corresponding longitudinal duplex US image. Doppler waveform from a lower-pole segmental artery, obtained after PTRAS, demonstrates marked improvement in waveform morphology, with a well-defined early systolic peak (arrow). AI and {Delta}T were 0.04 second and 7.38 m/sec2, respectively.

 
Threshold values also showed a significant difference (Table 2). By using {Delta}T greater than 0.07 second as the abnormal threshold, 10 (45%) intrarenal renal arteries were abnormal before intervention, as compared with none (0%) afterward (P < .001). Likewise, by using a value of AI less than 3.0 m/sec2 as abnormal, 17 intrarenal arteries (77%) had abnormal values before the procedure, as compared with none afterward (P < .001). Of note, in all five patients in whom the preprocedural AI was within the normal range, the AI increased after intervention and more than doubled in four. Congruence analysis revealed good agreement in waveform morphology classification between the two primary observers (Cohen {kappa}, 0.76). Abnormal waveform morphology (Halpern waveform grade 3–6) was present in 19 (86%) of all intrarenal arteries before intervention, as compared with only one (5%) after PTRAS (P < .001). Of note is that in the instance in which an abnormal waveform persisted after intervention, waveform morphology improved from grade 6 to grade 3, and the AI increased from 0.96 to 5.1 m/sec2. All but one vessel had at least a one-grade waveform improvement after PTRAS, and 19 (86%) had at least a two-grade improvement (Fig 2).


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  		TABLE 2. Analysis of Threshold-type Intrarenal Arterial Duplex US Variables

 


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  		Figure 2. Graph shows the frequency of Doppler waveform grade change after PTRAS. Twenty-one (95%) of the 22 treated arterial segments improved at least two to five Halpern grades in Doppler waveform morphology, with a majority (17 [80%] of 22) of procedures resulting in three to five improvement grades.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
There is little consensus on what constitutes the ideal follow-up modality after renal artery revascularization. Although conventional angiography is considered the reference standard, it is not ideal because of its invasiveness, high cost, and risk of contrast material–induced nephrotoxicity in a population at high risk for renal failure. Clinical follow-up, which has traditionally been used in large series of percutaneous and surgical renal artery revascularizations, may not enable detection of asymptomatic restenosis before it becomes symptomatic or progresses to kidney loss. Magnetic resonance angiography after stent placement is limited by the ferromagnetic artifacts caused by most stents (21).

Duplex US as a follow-up modality offers several advantages: being widely available, noninvasive, relatively inexpensive, and well tolerated by patients. Duplex US in the evaluation of initial patency and subsequent surveillance after renal artery revascularization has been recommended (22–24). Few studies (20,25–29) included a comparison of duplex US parameters immediately before and after renal artery revascularization. Such comparisons were available in only small subgroups of patients, which raised concern about selection bias. Likewise, the numbers of comparisons were too small to arrive at meaningful conclusions, and the timing of follow-up varied widely. It is important that a majority of patients in these studies underwent PTRA without stent placement, and the degree of angiographic success after revascularization was not taken into account.

Direct evaluation of the main renal artery provides a reliable means of evaluating for RAS (18,30). However, it is time-consuming, difficult to reproduce, technically difficult in large patients, and limited by individual variability in the course of the main renal artery and by excessive bowel gas or respiratory motion (18,30).

Indirect evaluation of RAS through its hemodynamic effects in the downstream hilar and segmental intrarenal arteries has emerged as a more reproducible yet sensitive means for detecting significant RAS (18,25,31–33). This method relies on two main components for RAS identification: (a) quantitative assessment of {Delta}T and AI (increased systolic upstroke time and decreased acceleration) and (b) qualitative waveform pattern recognition (pulsus tardus-parvus and loss of early systolic peak) (18,33,34). A low resistive index relative to the contralateral kidney also has been reported as specific for RAS (35). However, the resistive index can be affected by underlying parenchymal renal disease and contrast material nephrotoxicity.

Analysis of intrarenal quantitative Doppler US parameters has been shown to be highly reliable for detecting severe stenoses (>75%), albeit less reliable for moderate stenoses (50%–75%), which may or may not be hemodynamically significant (36). The "early systolic peak," a high-velocity peak occurring at the top of the initial systolic upstroke, which is a more sensitive indicator of RAS (37), may be helpful in detecting moderate stenoses undetected with quantitative parameters (AI, {Delta}T, and resistive index) alone (36). In addition, recent advances in color Doppler US and intravascular US contrast agents promise improved accuracy of duplex US in intermediate-severity RAS stenosis as a result of improved reproducibility of spectral waveforms, with enhanced accuracy and shortened examination time (38).

A number of recognized conditions can limit the accuracy of intrarenal duplex US in RAS evaluation, such as multiple (more than two) renal arteries or only mild to moderate stenosis (19). Patients who are young or have highly compliant arteries may have an absent early systolic peak, especially when distal interlobar arteries are sampled, which may yield false-positive results (39). Conversely, noncompliant vessels may not display the parvus-tardus response and may result in false-negative results (40). Several extraneous factors may also affect the accuracy of intrarenal duplex US, including valvular heart disease, left ventricular contractility disorders, and the effect of vasoactive drugs.

Results of the current study demonstrated significant improvement in all of the evaluated pertinent intrarenal Doppler waveform morphologic and velocimetric parameters after unequivocally successful PTRAS. These findings highlight the value of comparing pre- and postprocedural Duplex US findings. Normalization of previously abnormal Doppler US parameters after intervention indicates hemodynamic success. Such findings are of key importance, especially when only less than optimal morphologic results are achieved during intervention and may determine the need for additional treatment. Since our study included only patients with angiographic success (<20% residual stenosis), this argument is only speculative. Investigators in one small study (41) demonstrated good correlation between transstenotic peak systolic Doppler US velocity and degree of RAS (r = 0.84). Investigators in another small study (22) found a high incidence of significant residual stenosis at duplex US in poor respondents to intervention.

Results of the current study showed that after successful percutaneous renal artery revascularization, marked improvement is seen in quantitative and qualitative intrarenal duplex US parameters (AI, {Delta}T, and waveform grading). Hence, a new baseline can be established with which follow-up examination findings can be compared.

Investigators in one study (25) have reported the use of duplex US for follow-up after surgical or percutaneous revascularization for RAS. This study had variable timing of follow-up examinations and a lack of angiographic validation. Investigators in another more recent study (27) evaluated the value of intrarenal Doppler US velocimetric indices for assessing the results of percutaneous intervention for RAS. However, a majority of patients underwent PTRA without stent placement, with a wide range of residual stenosis. Investigators in two other studies (26,28) used serial duplex US to follow up patients who underwent PTRAS. However, duplex US was not routinely compared with a reference standard examination, and significant post-PTRA residual stenosis was present in a large number of study subjects.

The current study had a number of limitations. There was a relatively small number of patients. Correlation of Doppler US variables with the degree of either angiographic or clinical outcome variables was not possible, since only patients with angiographic success (<20% residual stenosis) were examined. Despite the establishment of a new normal baseline after successful PTRAS, the current study did not address the likely insufficient sensitivity of duplex US for detecting moderate degrees of restenosis. However, by combining subjective with quantitative waveform analysis, detecting moderate degrees of restenosis (approximately 50%–70%) may be possible, thus allowing timely percutaneous reintervention.

Potential error sources in the current study include the waveform selection method for analysis, since only one tracing was selected from each examination, with a potential for selection bias. Qualitative waveform analysis and AI and {Delta}T determination are subject to interobserver variability (42). We used similar guidelines for waveform selection in all examinations. Since investigators in prior studies of intrarenal duplex US analysis (18,25,31–33) rarely mentioned if and how analysis waveforms were selected, we thought that consistently choosing the waveform with the most normal appearance would be a conservative and controlled approach. We also required observers to compare for scoring the waveforms according to a predetermined pattern recognition scheme, which has been shown to enhance agreement in the interpretation of waveform morphology (43).

Results of our small prospective study suggested an important potential role for duplex US in the assessment and follow-up of percutaneous renal artery revascularization. In our practice, we now routinely perform duplex US before and after PTRAS. The role of duplex US should be further evaluated in a larger long-term study evaluating the value of duplex US in the early detection of restenosis.


   FOOTNOTES
 
Abbreviations: AI = acceleration index, {Delta}T = acceleration time change, {Delta}V = velocity change, PTRA = percutaneous transluminal renal artery angioplasty, PTRAS = percutaneous transluminal renal artery angioplasty and stent placement, RAS = renal artery stenosis

Author contributions: Guarantor of integrity of entire study, M.J.A.S.; study concepts and design, M.J.A.S., M.A.Y.; literature research, M.J.A.S., C.A.R.; clinical studies, M.J.A.S., W.J.L., J.A.G.; data acquisition and analysis, M.J.A.S., C.A.R., M.A.Y.; statistical analysis, M.J.A.S.; manuscript preparation, M.J.A.S., C.A.R., M.A.Y.; manuscript definition of intellectual content and editing, M.J.A.S., C.A.R., W.J.L., J.A.G.; manuscript review, M.J.A.S., M.A.Y., W.J.L., J.A.G.; manuscript final version approval, all authors.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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