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Using Pullback Pressure Measurements to Identify Venous Stenoses Persisting After Successful Angioplasty in Failing Hemodialysis Grafts

¹ Department of Radiology, The University of Chicago Hospitals, MC2026, 5841 S. Maryland Ave., Chicago, IL 60637.
² Racine Radiologist Group, 3803 Spring St., Rm. 208, Racine, WI 53405.

Received June 13, 2001; accepted after revision November 6, 2001.

 
Address correspondence to B. Funaki.

Abstract

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OBJECTIVE. We used pullback pressure measurements to identify venous stenoses persisting after angioplasty of failing hemodialysis grafts.

MATERIALS AND METHODS. Fifty angioplasty procedures were performed in 32 patients with elevated venous pressures at dialysis. Grafts were initially evaluated on digital subtraction angiography, and all stenoses measuring greater than 50% on angiography underwent angioplasty. In successful cases (residual stenosis < 30%), pullback pressure measurements were obtained from the superior vena cava to the graft to identify hemodynamically significant (> 10 mm Hg) stenoses. These lesions were then treated with repeated angioplasty.

RESULTS. Hemodynamically significant stenoses with a gradient range of 10-27 mm Hg (mean, 16 mm Hg) were found in nine (18%) of 50 procedures. All gradients occurred at sites of previous angioplasty. Repeated angioplasty of these stenoses performed with larger angioplasty balloons reduced gradients to less than 3 mm Hg in six stenoses and to 5 mm Hg in three stenoses. In this subgroup, primary patency was eight (89%) of nine stenoses at 1 month and 2 months and five (56%) of nine stenoses at 6 months. Using life table analysis, we found that primary patency of the entire population was 84% at 1 month, 66% at 2 months, and 47% at 6 months. The mean time between interventions was 6 months, and the thrombosis rate was 0.32 per year.

CONCLUSION. Pullback pressure measurements are a useful adjunct to angiography to evaluate the hemodynamic results of angioplasty in patients with failing hemodialysis grafts.

Introduction

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A large body of literature shows the efficacy of using pressure measurements during dialysis to identify failing hemodialysis grafts and fistulas [1,2,3]. Early detection of outflow stenoses, the most common cause of graft failure, enables percutaneous treatment of lesions using balloon angioplasty. Recommended by the Dialysis Outcomes Quality Initiative [4], this treatment prolongs graft patency and overall access life [1, 2, 5, 6]. Despite validation in the arterial system, to our knowledge there are few reports of using pressure measurements at the time of angiography to evaluate the hemodynamic significance of venous stenoses [7]. Instead, the results of angioplasty are usually evaluated by fluoroscopic monitoring of balloon dilatation and either uni- or biplane digital subtraction angiography. Other studies have shown that more than 20% of patients with failing dialysis grafts have a poor response to angioplasty as measured by parameters recorded during dialysis [8]. These studies have reported that this subset of patients is overlooked at the time of digital subtraction angiography. The purpose of our study was to identify hemodynamically significant venous stenoses in failing hemodialysis grafts that persist after successful angioplasty (as defined by digital subtraction angiography) and to determine whether these lesions could be successfully treated with repeated angioplasty using larger balloons.

Materials and Methods

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Thirty-two patients (age range, 20-77 years; mean age, 54.2 years; 10 men, 22 women) with synthetic hemodialysis grafts were referred to our interventional radiology section for fistulography to evaluate elevated venous pressures found during dialysis. Fifty percutaneous angioplasty procedures were performed in this group over 22 months. The group consisted of patients with older grafts; mean graft age at the time of interventions was 888 days or 2.4 years. A range of zero to 19 angioplasty procedures (mean, 4.5 procedures) and a range of zero to six declotting procedures (mean, 0.9 procedures) had been performed on grafts before the interventions performed in the study. Graft locations were right forearm (n = 3), right upper arm (n = 5), right thigh (n = 1), left forearm (n = 9), left upper arm (n = 22), and left thigh (n = 10). Eighteen patients underwent one intervention, 10 patients underwent two interventions, and four patients underwent three interventions during the study.

The graft was punctured using a 21-gauge needle (Micropuncture set; Cook, Bloomington, IN) near the arterial anastomosis directed toward the venous outflow. The needle was exchanged over a 0.018-inch guidewire for a 5-French dilator, and uniplanar digital subtraction angiography was performed from the arterial anastomosis to the central veins. Digital subtraction angiography was performed in an orthogonal plane (when possible) in areas of stenosis less than 50%. According to the Dialysis Outcomes Quality Initiative recommendation [4], any stenosis that measured greater than 50% on venography underwent angioplasty. Balloon sizes 1 mm larger than the upstream vein were chosen. Because all grafts were composed of 6-mm straight expanded polytetrafluoroethylene, 7-mm-diameter high-pressure angioplasty balloon catheters (Blue Max; Boston Scientific, Natick, MA) were used preferentially for anastomotic lesions. In two procedures, patients had long-segment strictures of the graft material in addition to anastomotic strictures. In these patients, 6-mm angioplasty balloons were used. In one patient with severe graft degeneration due to pseudoaneurysm formation, an 8-mm-diameter angioplasty balloon was used for an anastomotic stenosis.

After angioplasty of all significant stenoses, a 5-French end-hole catheter (Multipurpose Angled Catheter; Boston Scientific) was advanced from the puncture site into the superior vena cava over a guidewire. A pressure monitor (Namic Perceptor Morse Manifold; Boston Scientific) was then attached to the catheter, and pressure measurements were obtained from the superior vena cava into the graft. If a focal pressure change of greater than 10 mm Hg (mean pressure) was detected, measurements were repeated across the site, and a mean gradient greater than 10 mm Hg was considered a hemodynamically significant stenosis. When a residual hemodynamically significant stenosis was found, a guidewire was advanced to the central veins, and the end-hole catheter was exchanged for a high-pressure angioplasty balloon catheter that was then used to dilate the stenosis. If the gradient was at a site of previous angioplasty, the area was dilated using a larger angioplasty balloon (usually 1 mm larger than that used initially), and pressure measurements were repeated. In two patients with residual gradients of 5 mm Hg after repeated angioplasty, additional dilation was performed with a 1-mm-larger angioplasty balloon catheter (2 mm larger than the initial balloon used). Angioplasty was performed in an attempt to reduce the gradient across the lesion to less than 5 mm Hg.

According to the Dialysis Outcomes Quality Initiative recommendations [4], primary patency was defined as the time after angioplasty until any additional intervention, percutaneous or surgical, was performed. Technical success on initial angioplasty was defined as a residual stenosis of less than 30%. Reference vessels were upstream veins or graft in all stenoses.

Results

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Angiographically significant stenoses were initially found and successfully dilated in all 50 procedures before obtaining pullback pressure measurements. No occlusions or complications occurred. Hemodynamically significant stenoses were found in nine (18%) of 50 grafts after initial angioplasty (Table 1), with associated gradients ranging from 10 to 27 mm Hg (mean, 16.4 mm Hg) (Fig. 1A,1B,1C,1D). All lesions were at the sites of prior angioplasty. After angioplasty, gradients were successfully reduced to less than 3 mm Hg in six procedures. In three procedures, gradients could not be reduced below 5 mm Hg.

View larger version (121K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1A. —48-year-old woman with left upper arm synthetic dialysis graft referred for angioplasty because of elevated venous pressure. Initial venogram shows significant anastomotic stenosis (arrows) that is partially obscured by adjacent basilic vein.

View larger version (90K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1B. —48-year-old woman with left upper arm synthetic dialysis graft referred for angioplasty because of elevated venous pressure. Fluoroscopic image shows balloon angioplasty of anastomotic stenosis.

View larger version (146K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1C. —48-year-old woman with left upper arm synthetic dialysis graft referred for angioplasty because of elevated venous pressure. Repeated venogram obtained after angioplasty using 7-mm angioplasty balloon catheter reveals residual stenosis having 10-mm Hg pressure gradient.

View larger version (156K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1D. —48-year-old woman with left upper arm synthetic dialysis graft referred for angioplasty because of elevated venous pressure. Repeated venogram obtained after angioplasty using 8-mm angioplasty balloon catheter shows mild residual stenosis having no significant pressure gradient.

For the entire population, primary patency using life table analysis was 84% at 1 month, 66% at two months, and 47% at 6 months. Mean interval between interventions was 6 months, and the thrombosis rate was 0.32 per year. For the group with stenoses found using pressure measurements that underwent repeated angioplasty, primary patency was eight (89%) of nine stenoses at 1 month and 2 months and five (56%) of nine stenosis at 6 months.

Discussion

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Angiographically inconspicuous stenoses in patients with synthetic hemodialysis grafts may be attributed to patient positioning, graft type, lesion configuration, or a combination of these factors. Anastomotic stenoses may be masked by superimposition of adjacent outflow veins, particularly in the upper arm and thigh, where these veins may be larger than synthetic dialysis grafts. True eccentric stenoses may also occur before and after angioplasty [9]. These lesions may be angiographically subtle on routine fistulography because obtaining orthogonal images of the outflow veins in the upper arm, chest, or thigh is not always possible. The hemodynamic significance of eccentric lesions may be difficult to assess on angiography because lesions are either over- or underestimated, depending on the angiographic image obtained. Furthermore, there is intra- and interobserver variability in the estimation of degree of stenosis when it is based on digital subtraction angiography, particularly after percutaneous angioplasty. In the arterial system, one study estimated that measurements based on angiography were only 45% sensitive and 65% specific for determination of suboptimal angioplasty, compared with pressure gradient measurements [10]. Secondary signs such as the presence of collateral veins or slow inflow rate may provide valuable clues to the presence of a hemodynamically significant venous stenosis but are difficult to quantify. We believe that pullback pressure measurements more accurately evaluate the hemodynamic effect of subtle venous stenoses. Our study shows that angiographically inconspicuous, hemodynamically significant venous stenoses persist in approximately 18% of failing hemodialysis grafts after angioplasty, and most of these lesions can be successfully treated with repeated angioplasty. Thus, pullback measurements help to optimize treatment of these lesions.

The primary reason for hemodynamic screening of dialysis grafts is to enable early percutaneous interventions that reduce access thrombosis and increase graft longevity. Surveillance is predicated on successful treatment of venous stenoses that are responsible for graft thrombosis in the most patients [11,12,13]. Percutaneous transluminal angioplasty has been effective in dilating venous stenosis, thereby increasing blood flow through grafts and prolonging patency. However, hemodynamically significant lesions that are overlooked on digital subtraction angiography do occur, and if left untreated, presumably adversely affect patency after percutaneous interventions. Schwab et al. [8] studied angioplasty of failing grafts and noted that 21% of dialysis graft angioplasty procedures failed to improve flow greater than 20%. These researchers concluded, "Not all angioplasty procedures are successful despite the impression of the physician performing the procedure." Ahya et al. [14] compared the angiographic assessment of a venous stenosis with the change in graft blood flow after angioplasty. These investigators found no significant correlation between change in blood flow and the change in percentage of a stenosis as measured by digital subtraction angiography. Thus, the angiographic assessment of the lesion after angioplasty failed to predict the hemodynamic success of the procedure. Both studies highlight weaknesses inherent to angiographic evaluation of dialysis-related stenoses. Our results resemble those of Schwab et al. [8]: 8% of our patients had persistent stenoses after angioplasty that were not seen on digital subtraction angiography.

Improved detection of venous stenoses may reduce the incidence of early graft failure after percutaneous interventions. Murray et al. [15] evaluated the usefulness of repeated thrombolysis of grafts that clotted within 1 month of percutaneous declotting. Repeated thrombolysis uncovered unsuspected causes of graft failure in seven (18%) of 39 patients. Murray et al. attributed this finding to the increased vigilance of interventional radiologists for detecting subtle lesions that were missed at the time of initial declotting. In 41% of cases, larger angioplasty balloons were used to treat venous outflow stenoses, and in 18% of cases, stents were deployed. These repeated interventions resulted in 35% patency at 3 months. The implications of this study are twofold. First, incompletely treated subtle lesions may cause early thrombosis after declotting; and second, when correctly identified, these lesions can be successfully treated by percutaneous interventions.

Our study differs from previous work in one important respect. Whereas previous studies have used parameters measured at dialysis to show the limitations of digital subtraction angiography or have followed patients with early graft failure after percutaneous interventions, the use of pressure measurements in our study enabled us to identify the limitations of angiography at the time of fisulography. Thus, we could provide additional treatment to this subset of patients and measure patency of these interventions over time. To our knowledge, this type of additional therapy has not previously been evaluated.

Sullivan et al. [7] used systolic pressure ratios to quantify the hemodynamics of failing dialysis grafts. These researchers found that a stenosis greater than 40% causes a statistically significant rise in graft pressure and that graft pressures can help determine the hemodynamic importance of a stenosis and the need for intervention. Rather than using pressure ratios, we adopted a simplified approach using pullback pressure measurements to identify lesions. Because all the patients in this series were referred for angiography with elevated venous pressures, this practice seemed to be the most straightforward approach to identify hemodynamically significant lesions. Pullback pressures are commonly used to evaluate arterial stenoses although the pressure ratio approach described by Sullivan et al. offers a better overall assessment of graft function. Although we believe that pressure measurements may offer increased accuracy compared with angiographic assessment when applied properly, hemodynamic measurements also have limitations. Changes in blood pressure, venous spasm, and pressure variability across arteriovenous grafts may confound lesion evaluation. Furthermore, merely identifying a stenosis does not ensure that the lesion can be optimally treated. In three of our patients, angioplasty only partially alleviated gradients that could not be reduced to less than 5 mm Hg. None of the interventions in these three patients led to a primary patency of greater than 3 months.

We do not advocate the use of pullback measurements in all venous stenoses. Measurements are not helpful in severe stenoses because these lesions are obvious on angiography and the catheter used for measurements will limit flow, exaggerating pressure gradients. Obtaining pressure measurements in these lesions simply prolongs procedures unnecessarily. Before angioplasty, all significant stenoses in this series were readily identified by initial digital subtraction angiography. No hemodynamically significant lesions were undetected on angiography; rather, pullback measurements simply identified stenoses that were not optimally treated by angioplasty. Routine pullback pressure measurements used after initial treatment of angiographically apparent venous stenoses are advantageous because they may eliminate the need for biplane angiography, limiting the contrast material needed to perform interventions and reducing radiation exposure to patients and physicians. Since performing this study, we use pressure measurements in most patients in lieu of routine biplane angiography. We do not believe that pressure measurements should be substituted for angiographic assessment of dialysis grafts; rather, they are simply a useful adjunct to identify stenoses and to assess the results of angioplasty. Selective use of pressure measurements may be more appropriate than generalized application in all patients. Specifically, we believe that pullback measurements are most helpful to evaluate venous stenoses in the upper arm, chest, or pelvis. Although 12 (24%) of 50 procedures in this series were performed on patients with forearm grafts, no persistent stenoses were found in forearm veins or at the venous anastomosis in these patients. Further studies should be performed to fully evaluate the cost benefit of routine pressure measurements after venous angioplasty.

There are several weaknesses of our study. First, we did not prove that persistent lesions, if left untreated, would adversely affect graft patency. This presumption is based on the established principle that venous stenoses cause thromboses that limit the life span of the graft. To determine the effect of angioplasty on additional stenoses compared with digital subtraction angiography, a randomized study with a control arm would be required. We believe this is worthy of further investigation. Our 6-month primary patency of 47% was similar to that of many other investigators who have found that patency after percutaneous transluminal angioplasty consistently ranges from 40% to 50%. Nonetheless, our population comprised patients with older synthetic grafts, averaging 2.4 years old, with an average of 5.4 previous percutaneous interventions (angioplasty and thrombolysis procedures). In general, old grafts are expected to have lower primary patency after angioplasty compared with new grafts [12].

Second, no consensus exists as to what constitutes a hemodynamically significant pressure gradient in the venous system. Neither the Dialysis Outcomes Quality Initiative [4] nor the Society of Cardiovascular & Interventional Radiology guidelines [11] regarding dialysis grafts defined a specific threshold. Other researchers have suggested using thresholds between 5 and 20 mm Hg [14, 16, 17]. We chose 10 mm Hg, although a better threshold may be lower or higher and will require further study. We chose not to deploy stents in three patients with 5-mm residual gradients because of the lack of an established threshold. Even in the arterial system, no clear consensus exists regarding hemodynamic criteria. Kamphuis et al. [18] noted in the Dutch Iliac Stent Trial that various thresholds for stent placement reported in the literature would have resulted in stent placement in anywhere from 4% to 87% in their patients. Regardless of the criteria used to define a hemodynamically significant stenosis, we believe that reducing pressure gradients as much as possible during percutaneous interventions equates with the best hemodynamic result angioplasty can offer and protects against overlooking hemodynamically significant subtle lesions that are amenable to percutaneous angioplasty. Hemodynamic measurements also help to identify patients who have not responded to angioplasty and may benefit from surgical intervention or stent placement.

The clinical usefulness of pressure measurements is debatable. One advantage of using pressure measurements is that these measurements, when used in conjunction with digital subtraction angiography, enable treatment of additional hemodynamic stenoses missed on angiography. A second advantage is that these measurements can be substituted for repeated oblique angiograms and will limit contrast material and radiation doses to patients. An argument against routine use of measurements is that no further therapy will be necessary for approximately 80% of patients, and an additional 7% of patients may have lesions that are not completely resolved with repeated angioplasty. Thus, improvement in overall graft patency of the patient population may not be large. A second disadvantage of using pressure measurements is the cost of the pressure transducer. We recommend using pressure measurements routinely except in anastomotic stenoses in patients with forearm grafts. Pressure measurements are also suggested in equivocal stenoses, particularly in lieu of repeated oblique digital subtraction angiograms.

In summary, we have found that angiographically subtle, hemodynamically significant stenoses occur in nearly one of five failing dialysis grafts. These lesions can be identified using pullback pressure measurements, and additional angioplasty successfully alleviates gradients in two thirds of the cases. Pullback pressure measurements are a useful adjunct to angiography when identifying venous stenoses and assessing the results of angioplasty in these lesions.

References

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