Hemodynamic Screening and Early Percutaneous Intervention Reduce Hemodialysis Access Thrombosis and Increase Graft Longevity

State of the Art I

, Hemodynamic Screening and Early ' Percutaneous Intervention Reduce , Hemodialysis Access Thrombosis and Increase Graft ~ongevityl Kevin L. Sullivan, MD Anatole Besarab, MD

Index terms: Dialysis, shunts Fistula, arteriovenous Grafts, intewen- tional procedures Grafts, stenosis or thrombosis Veins, stenosis or obstruction Veins, transluminal angioplasty

JVIR 1997; 8:163-170

Abbreviations: AVF = arteriovenous fistula, PBG = prosthetic bridge graft

From the Thomas Jefferson University HospitaWJefferson Medical College, 111 S 11th St, Suite 5390 G, Philadelphia, PA 19107 (K.L.S.), and the Division of Ne- phrology and Hypertension, Department of Medicine, Henry Ford Hospital, Detroit, Mich (A.B.). Received August 20, 1996; revision requested September 27; revision received October 15; accepted October 20. Address correspondence to K.L.S.

CARING for the hemodialysis patient is inseparable from the constant problem of maintaining vascular access patency. Although the autologous arteriovenous fistula (AVF) comes closest to the ideal access, growth of the United States End Stage Renal Disease program has been accompanied by decreasing use of the AVE in favor of prosthetic bridge grafts (PBGs) and the increasing use of permanent indwelling central catheters (1). The United States Renal Data System reported that PBGs were constructed almost twice as often as AVFs (52% vs 31%) in patients beginning hemodialysis (2). The dependence on PBGs is in part related to the advancing age of the hemodialysis population. Patients with peripheral arterial vascular disease or inadequate peripheral veins due to repeated venipuncture are less likely to be candidates for AVFs. PBGs have a shorter mean patency than AVEs (3). Substitution of PBGs for AWs has increased the cost to the health care system, with increased number of procedures needed to maintain access patency (43). Percutaneous balloon angioplasty of PBG stenoses provides only limited long-term patency (6-8). As a result, patients with PBGs undergo three to four times as many procedures to maintain equivalent secondary patency as do patients with AVFs (4,5,9,10). Even though a large percentage of vascular access problems can be managed in the outpatient setting, they still account for 21% of all hospitaliza- tions for chronic hemodialysis

patients and are a growing source of morbidity and cost (2,4,11-14). One strategy to reduce the dialysis access thrombosis rate in the future is to protect potential sites for AVF construction.

The increasing use of permanent indwelling central catheters (1,2) and the construction of exotic tertiary accesses (4) attest to the increasing difficulty of maintaining vascular access patency in an in- creasingly aging hemodialysis population. Loss of all vascular access sites from central vein stenosis or thrombosis is a feared and devas- tating complication if the patient is unable to receive peritoneal dialysis. Although still relatively uncom- mon, an average-sized hemodialysis unit of 120-200 patients might be confronted with this problem in a few patients each year.

Intimal hyperplasia a t the venous anastomosis is the most frequent cause of PBG failure. Until intimal hyperplasia can be controlled, any salvage procedure is likely to provide disappointing long-term results. The challenge of controlling intimal hyperplasia is a considerable one because the pro- portion of cells actively proliferating in the vein wall near the venous anastomosis is an order of magni- tude greater than in atherosclerotic arteries. Our understanding of this process continues to grow, but there is currently no mechanical nor phar- macologic means to prevent it (15,16).

Investigative effort has been aimed at prolonging the functional life of vascular accesses through earlier detection of stenotic lesions

164 Journal of Vascular and Interventional Radiology

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Table 1 Thrombosis Rate of Synthetic Bridge Grafts per Patient Dialysis Year

Study Group Thrombosis Rate

Duke Elevated pressure

with PTA 0.15 without PTA 1.4

Normal pressure 0.13 Jefferson

Population screened with pressure 0.19 Historic control population 0.58

Note.--PTA = percutaneous transluminal angioplasty. References 17 and 18.

and management of such lesions prior to thrombosis. Methods for monitoring accesses have focused on the hemodynamic manifestations of stenoses. There is evidence that such an approach is successful (Table 1). By one report, the baseline thrombosis rate in a chronic hemodialysis population is 0.58 thromboses per patient hemodialysis year (17). The introduction of a screening program to identify stenoses, and correction of such stenoses with angioplasty, reduces the thrombosis rate in the population to 0.19 thromboses per patient hemodialysis year. In another study, patients with significant stenosis detected by screening examination and treated with angioplasty have a thrombosis rate of 0.15 per patient dialysis year, similar to that in patients without stenosis (18). Patients likely to have stenoses based on a screening examination, who decline elective angioplasty, have a thrombosis rate of 1.4 per patient dialysis year. The purpose of this review is to discuss how such reductions in the thrombosis rate are achieved through monitoring and intervention. In addition to avoiding the detrimental effects of access thrombosis, this approach increases graft longevity.

ACCESS FLOW

Adequacy of dialysis as well as graft patency depend on access flow. For adequate dialysis, arterial access inflow must equal the prescribed dialysis blood pump

flow, which varies from 250 m u min (low-flux dialysis) to over 600 mumin (high-flux dialysis). Pub- lished reports for access blood flow vary from less than 300 mL1 min to over 3,000 mumin (19- 21). The risk of thrombosis depends on the type of access as well a s flow. Native AVFs maintain patency a t flows a s low as 200 mumin (22). These flows are too low for adequate dialysis and are detected clinically by an inabil- ity to withdraw blood a t the prescribed pump rate. Many studies assessing access flow and the risk of access thrombosis now suggest that a PBG with a flow rate below 600-800 mllmin is likely to have a venous (less frequently, an arterial) stenosis and is a t substan- tially increased risk for thrombosis (19,21-25). The likelihood of thrombosis within 6 months increases fourfold when access flow decreases below 600-800 mumin (21). Thus, PBGs can thrombose a t access flows more than adequate for dialysis with few clinical signs that the access is a t risk for thrombosis. Because flow varies inversely with intraaccess pressure, pressure measurements provide a means to detect changes in access flow. Elective angioplasty of significant lesions prevents thrombosis by increasing the flow (22).

SCREENING EXAMINATIONS FOR ACCESS STENOSIS

All screening examinations currently in use are aimed a t the early

detection of access-related stenoses. Thus, thromboses not related to flow-limiting stenoses will not be prevented by such an approach. In one study of accesses after thrombectomy, 20% had no significant stenosis (26). Etiologies for thrombosis in such cases include hyperco- agulable states, dehydration, hypo- tension, congestive heart failure, and graft compression.

A variety of techniques exist for early detection of access-related stenoses. Percent recirculation of dialyzed blood within the access is one such technique. I t reflects the amount of dialyzed blood that is re-withdrawn from the access for repeated dialysis without entering the central vascular pool. Retro- grade flow from the venous (return) needle to the arterial (withdrawal) needle during dialysis should not occur until access blood inflow is insufficient to meet the demands of the blood pump. In theory, an outflow stenosis that decreases the access flow below that demanded by the prescribed blood pump flow rate will increase the percent recirculation. In the past, the popularity of the test arose from its simplicity, relative ease, and low cost. However, the measurement of recirculation with use of urea-based methods is now known to be fraught with difficulty. Recent advances in measuring recirculation with use of on-line dilution methods have shown that recirculation is absent in the great majority of properly cannulated, well function- ing, a s well as dysfunctioning, accesses (27-32). Ironically, now that recirculation can be measured accurately, its role has diminished significantly. Recirculation appears quite late in the natural history of a failing PBG, with most failing grafts probably thrombosing before any recirculation occurs. Recircula- tion is valuable in screening native vein fistulas because most such accesses can maintain patency a t flows less than those needed for dialysis.

Ultrasound (US) with Doppler can help estimate stenosis severity based on focal acceleration of blood, as well as estimate volume flow

Sullivan and Besarab 165

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(21,33,34). It effectively predicts accesses at increased risk for thrombosis. However, Doppler US flow measurements are ~rohibi- tively expensive for routine screening assessments. Whereas some have suggested that there may be a cost benefit from US surveillance at 6-month intervals (35), others have found no such benefit (36). Some . ,

dialysis centers refer patients with a high probability of stenosis, as determined by low-cost screening methods, directly for angiography and balloon angioplasty without ever using US studies. Duplex Doppler can provide an important bridge between screening and intervention when the probability of stenosis as determined by screening methods is uncertain.

Physical examination has been analyzed as a screening test (37). Low cost and availability are its major assets. This technique appears quite good a t identifying grafts at low risk for thrombosis. However, it appears to be much less effective a t identifying grafts a t high risk for thrombosis.

Intraaccess pressure measurement is also an inexpensive and effective method of identifying access-related stenoses. The sim- plest measurement is the venous dialysis pressure. This is obtained from a pressure transducer within the dialysis machine while blood is being pumped. Because of flow through the dialysis machine, this number reflects the true intraaccess pressure, as well as the pump flow rate, the needle diameter, and the patient's hematocrit. It is the true intraaccess pressure that correlates with the degree of stenosis (38). Past attempts to estimate the true intraaccess pressure from the venous dialysis pressure with correction factors based on the needle

~ ~~ -

diameter, hematocrit, and pump flow rate lead to significant inaccu- racy (39). In addition, increased dialysis blood pump flow rates have diminished the degree to which the true intraaccess pressure influences the measured venous dialysis pressure. This problem can be overcome by decreasing the pump flow rate during dialysis while measuring the

venous dialysis pressure. Another simple maneuver that permits a direct measurement of the true intraaccess pressure is to stop flow and measure a static pressure, as is done routinely during percutaneous arterial interventions. This can be accomplished by the introduction of a stopcock and pressure transducer into the tubing used to return blood to the patient. Because pressure in the graft reflects the patient's systemic blood pressure, the systolic graft pressure is divided by the systemic systolic pressure measured from a blood pressure cuff, yielding a normalized ratio. Repeated measurements have demonstrated a high degree of correlation of interdi- alysis and intradialysis normalized pressure ratios in individual patients, despite fluctuations in systemic blood pressure (17).

Recently, this technique has been simplified to utilize the dialysis machine's transducer measured with the blood pump turned off, with a correction due to the difference in height between the access and pressure transducer (40). This is possible because of improved transducer accuracy in dialysis machines. Preliminary work with this simplified approach is promis- ing.

In native fistulas, blood entering the venous system can return through multiple collateral veins originating peripheral to a stenosis and can decrease the degree of pressure elevation despite the presence of a significant stenosis. Pres- sure measurements are of only modest value as a screening modal- ity in native fistulas.

I HEMODYNAMICS OF PBGs

An understanding of dialysis graft hemodynamics is necessary in order to use graft pressures to their best advantage, both for screening purposes and to guide interventions. In a synthetic bridge graft without any stenoses, there is a normal drop from arterial pressure to venous pressure. This is funda- mentally different from the arterial system, where equal pressure is

assumed at two nearby points in the absence of a significant stenosis. This difference makes interpreta- tion of Dressures in dialvsis access grafts &ore complex. Gowledge of the normal pressure profile between the arterial and venous end of the graft is necessary to make meaning- ful interpretations. Such information is available from both normal animal grafts and human grafts with no or low-grade stenoses. Normal canine femoral loop grafts lose approximately 50% of mean systemic pressure between the feeding artery and the arterial end of the graft. By the venous end, the pressure is approximately 25% that of mean systemic pressure (41). In humans, this same pressure profile is duplicated for stenoses less than 40% (26). In the midgraft, where the venous return needle is typi- cally placed, the pressure is approximately 40% that of systemic. Based on this information, the threshold for evaluating patients with dialysis access angiography is a normalized pressure ratio of 0.4, measured from the venous return needle.

For higher grade stenoses, the pressure profile changes (Fig 1). The shape of the pressure profile also depends on the location of the stenosis (Fig 2). For stenoses at the venous anastomosis, pressures are elevated over the entire length of the graft. For stenoses a t the arterial anastomosis, pressures are decreased, with the minimum being central venous pressure. Intragraft stenoses are associated with pressure elevations up to the point of stenosis. Most of the rise in pressure occurs between 40% and 70% stenosis by diameter. Central vein stenoses behave differently. In general, for a similar level of lumen compromise, these stenoses cause a lesser degree of graft pressure elevation. This is due in part to the development of collateral veins, which can decompress the vein peripheral to the stenosis. How- ever, such a flow pattern can cause ipsilateral extremity edema (42). Such physiology is not possible for intragraft or venous anastomotic stenoses, where the degree of


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The Effect of Stenosis Severity qn Graft Pressure

A AL MG VL CV Location

1.

The Effect of Hiah Grade Stenosis Location on Graft Pressures

A AL MG VL CV Location

Stenosis Location - Anastomosis

+ lntragraft - Central Vein

Anastomosis

Figures 1,2. (1) The effect of stenosis severity on graft pressure. With increasing stenosis severity, there is a rise in graft pressures. The PBGs in this analysis represent a typical group of failed accesses, with most having venous anastomotic stenoses. There is a significant difference in pressure ratios between groups 2 (21%-40%) and 3 (41%-60%). The goal of percutaneous interventions is to restore a normal pressure profile, as in groups 1 and 2. (A = artery, AL = arterial limb of graft, MG =

mid graft, VL = venous limb of graft, CV = central vein [reference 261). (2) The effect of high-grade stenosis location on graft pressures. The pressure ratio is influenced by the location of the stenosis. In general, measurement of pressure at a single point proximal (ie, on the arterial side) to a stenosis is adequate to evaluate its significance because pressures a t all points proximal to a stenosis are elevated (reference 26).

pressure elevation more closely correlates with the severity of stenosis.

UTILITY OF PRESSURES IN SCREENING FOR STENOSIS

A normalized systolic pressure ratio of 0.4 has both a high sensitivity and specificity in identifying stenoses of at least 50% in synthetic grafts, as well as excellent positive and negative predictive values (Table 2). Systolic pressures are used because the best correlation between stenosis severity and graft pressure occurs with systolic pressure, compared to mean or diastolic pressure (38). Neither the sensitivity nor the specificity of recirculation is as high. Thus, in synthetic bridge grafts, normalized pressures are better than recirculation in detecting significant stenoses. However, recirculation is a better method to screen for stenoses in native fistulas.

After institution of a screening protocol, receiver operator curve analysis was performed to assess the optimum normalized pressure ratio threshold. As expected, there

Table 2 Diagnostic Accuracy of Normalized Pressure Ratio versus Recirculation in Detecting Stenoses 350%

Synthetic Graft Native AVF

Normalized Recircu- Normalized Recircu- Pressure* lationt Pressure* lation?

Sensitivity 91 29 48 7 1 Specificity 86 75 100 100 Positive predictive value 92 85 100 100 Negative predictive value 84 23 54 88

* Normalized pressure ratio 20.4. t Recirculation >15%. Reference 17.

is a trade-off between sensitivity and false-positive rate. A threshold of 0.4 produces a 14% false-positive rate and 91% sensitivity. Lowering the threshold to 0.3 produces a negligible increase in the sensitivity, but an increase in the false- positive rate to over 30%. Increas- ing the threshold to 0.5 reduces the false-positive rate minimally to 12%, but dramatically reduces the sensitivity to 65%. Thus, 0.4 is the best threshold for identifying synthetic access graft stenoses of at least 50%. But ultimately, the value

of such a program must be evalu- ated by its effect on access thrombosis rate and longevity.

EFFECT OF EARLY INTER- VENTION ON ACCESS THROMBOSIS RATE, ACCESS LONGEVITY, AND HOSPITAL DAYS FOR VASCULAR ACCESS PROBLEMS

Individual center statistics for thrombosis rate vary from 0.5 to more than one episode per year at


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Venous Limb Venous Limb Arterial Llmb Arterial Llmb + + +

0.50 +

\ \ Central vein Central Vein

3. 4.

Figures 3,4. (3) A schematic representation of a PBG with a significant venous anastomotic stenosis is depicted here. Typical systolic pressure ratios found with such a stenosis are displayed in the arterial and venous limbs of the access. (4) A normal access has pressure ratios less than or equal to those indicated in this diagram. The goal of percutaneous intervention is to achieve such pressure ratios.

risk. Three studies have demonstrated that detection of stenoses with pressure monitoring (17,18) or Doppler flow monitoring (21) followed by elective intervention reduces thrombosis rate. Schwab et a1 noted a two-thirds reduction in the thrombosis rate and a fourfold reduction in the access replacement rate (18). Our experience has been virtually identical. We measure static pressures three times per year, perform access angiography when normalized pressures rise above 0.4, perform angioplasty of hemodynamically significant stenoses of at least 50%. and use normalized pressure ratios as an endpoint for percutaneous intervention. A number of benefits are observed from this regimen.

First, it can reduce the thrombosis rate, thus reducing morbidity to the patient. During a historic control period when few angioplasty procedures were performed, the thrombosis rate was 0.58 per patient dialysis year. This same rate was duplicated during a prospective trial when patients were screened for elevated pressures, but angioplasty was performed on asymptomatic patients only for stenoses more than 70%. To significantly lower the thrombosis rate, it is necessary to perform angioplasty on stenoses greater than or equal to 50%, causing elevated graft pressures. This reduces the thrombosis rate by almost 70% to 0.19 episodes per patient dialysis year.

Another benefit that results from such a program is a reduction in the number of inpatient hospital days for vascular access problems.

Both days utilized due to thrombosis and days utilized for all vascular problems (thrombosis, replacement, revision, infection) decrease as the angioplasty rate increases above 1.5 procedures per patient year. In turn, an angioplasty rate greater than 1.5 per patient year is achieved only when the angioplasty criteria is lowered to dilate hemodynamically significant stenoses greater than or equal to 50% by diameter. During the historic control period and during a prospective trial using greater than 70% diameter stenosis (with elevated pressures) as the criteria for angioplasty in asymptomatic patients, the number of hospital days for vascular access problems averaged 4.2 per patient dialysis year. This is reduced to 1.7 dayslpatient year after angioplasty of asymptomatic stenoses of at least 50% associated with elevated graft pressures. Linear regression indicates a loss of slightly more than one episode of thrombosis and approximately five hospital days for each angioplasty performed with use of the criteria described (43). All patients with occluded grafts were managed as inpatients with surgical thrombecto- mies.

A final benefit is the increase in the mean usable graft life-span as a result of this prospective monitoring and intervention protocol. During the historic baseline period, mean synthetic graft age was approximately 2 years. After institution of the program described here, mean graft age increased to 3 years. At the beginning of our studies, vascular accesses

less than 1 year constituted more than 50% of all accesses. By the end, this fraction of "young accesses" approached the patient turnover rate from death, transfer, or transplantation of 30%. We believe that interruption of the thrombosis, thrombectomy, re- thrombosis cycle by prospective detection of accesses a t risk for thrombosis followed by elective angioplasty produced the 1-year increase in the mean age of accesses of our patients. This occurred even as the fraction of native accesses constructed decreased.

UTILITY OF PRESSURES IN ASSESSING PERCUTANEOUS INTERVENTIONS

Pressure assessment can provide valuable information when planning percutaneous interventions in dialysis access grafts in three ways: (a ) determination of an endpoint for intervention; (b) selection of patients for intervention; and (c) detection of stenoses. After intervention, pressures can play a role in the decision to perform further angioplasty or use adjunctive techniques, such as stent placement or atherectomy. The goal is to achieve a pressure ratio of less than 0.5 and 0.33 in the arterial and venous limbs of the graft, respectively (Figs 3,4). Systemic pressure for these calculations is determined from a blood pressure cuff on the contralateral arm. In the case of stenoses of moderate severity at angiography, pressures can help


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determine the hemodynamic significance of these stenoses, and there- fore the need for intervention. Occa- sionally, routine angiographic views will fail to reveal a hemodynamically significant stenosis (Fig 5). The search can be assisted by measurement of pressures. An abnor- mal pressure can prompt additional

Figure 5. Venous anastomotic stenosis difficult to detect a t angiography. (a) This patient with a loop graft in the right thigh presented with elevated pressures. In the venous limb, the pressure was 85/50 mm Hg while the systemic blood pressure was 159192 mm Hg, yielding an elevated systolic ratio of 0.53. An angiogram in the anteroposterior projection did not reveal a venous anastomotic stenosis, which was expected based on the pressure ratio. (b) Because of the elevated pressure, angiography in the 25" right anterior oblique projection was performed to further assess the venous anastomosis. Narrowing of the contrast material column was seen in the region of the anastomosis (arrow), although this could represent a jet of contrast material entering the common femoral vein. (c) A 40" right anterior oblique view was similarly inconclusive. (d) A shallow left anterior oblique projection superim- posed the anastomosis on the common femoral vein and did not contribute to the assessment. A pullback pressure was obtained, indicating a 63 mm Hg gradient across the venous anastomosis (representing a pressure ratio gradient of 0.43). Thus, there was a hemodynamically significant venous anastomotic stenosis. After angioplasty of the venous anastomosis, the venous limb pressure dropped to 19/15 mm Hg with a systemic blood pressure of 155173 mm Hg (ratio 0.12). The arterial anastomosis was normal.

views and reveal a hemodynamically significant stenosis not appar- ent on all projections.

It can be difficult to determine the significance of central vein stenoses with angiography alone. Graft pressures permit an assessment of the risk of access thrombosis in the presence of an outflow

vein stenosis. Patients with vein stenoses and normal graft pressures are probably not a t increased risk for access thrombosis, although the risk of thrombosis of the vein stenosis must be considered when select- ing patients for intervention. Pro- gression of stenosis severity will generally be detected prior to vein


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thrombosis with regular screening access pressure measurements.

- - I CONCLUSION

Many benefits accrue from early intervention on dialysis access- related stenoses. These include increased graft life (17), a reduction i n temporary dialysis catheters with their a t tendant risk of vein stenoses (44), a reduction in missed dialysis sessions due to access thrombosis, a n d improved patient quality of life by replacing emergent declotting with elective angioplasty. This approach increases t h e number of angioplasties performed, b u t th is is offset by a reduction in inpatient hospital days (43). The key to achieving these benefits is identifi- cation of grafts at risk for thrombosis with a screening program and early intervention. Several screening protocols have yielded success (17,18,21,45). The most cost-effective strategies to detect stenoses a n d correct them remain to be defined. Multicenter controlled studies a r e needed to establish t h e long-term effectiveness a n d relative costlbenefit of monitoring techniques a n d interventions. Despite t h e fact that some questions remain unanswered, our current understanding would argue for t h e use of a screening program combined with early intervention. Nephrologists a n d interventional radiologists should work cooperatively to develop such a program i n their own institutions, a n d incorporate it into t h e routine care of dialysis patients.

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Hemodynamic Screening and Early Percutaneous Intervention Reduce Hemodialysis Access Thrombosis and Increase Graft Longevity