the rise and fall of access blood flow surveillance in arteriovenous fistulas

The Rise and Fall of Access Blood Flow Surveillance inArteriovenous Fistulas

Nicola Tessitore,* Valeria Bedogna,* Giuseppe Verlato,† and Albino Poli†*Renal Unit,Department of Medicine,University Hospital,Verona, Italy, and †Department of Public Healthand Community Medicine,University of Verona,Verona, Italy

ABSTRACT

Vascular access blood flow (Qa) surveillance has beendescribed as a typical false paradigm, an example of hownew tests are sometimes adopted even without good-qualityevidence of their benefits. This may be true for grafts, butnot necessarily for arteriovenous fistulas. We reviewed theliterature on Qa surveillance in fistulas to see whether itcomplies with the World Health Organization’s criteria forscreening tests. Measuring Qa has a fairly good reproduc-ibility. Qa shows an excellent-to-good accuracy for stenosisbeing the only bedside screening test that achieves a veryhigh sensitivity while retaining a fair-to-good positive pre-dictive value for Qa thresholds of 600 ml/minute or higherassociated with a >25% drop in Qa, or findings suggestingstenosis on physical examination. The accuracy of Qa in

predicting thrombosis is hard to establish because of theheterogeneity of published studies, though a Qa of 300 ml/minute seems the most reliable cutoff. Qa surveillanceaffords a significant 2- to 3-fold reduction in the risk ofthrombosis by comparison with clinical monitoring alonewhen Qa criteria highly sensitive to stenosis are considered,regardless of the study design (randomized controlled trials,cohort studies with concurrent or historic controls). Usinghighly sensitive Qa screening criteria also halves the risk ofaccess loss, although this effect is not statistically signifi-cant. Our analysis strongly suggests that Qa surveillance isan effective method for screening mature fistulas, thoughfurther, appropriately designed studies are needed to fullyelucidate its benefits and cost effectiveness.

Access Blood Flow Surveillance

The vascular access has been seen as both the“lifeline” and the “Achilles’ heel” of hemodialysispatients (1), and assuring them a durable and well-functioning access is a priority issue. Access func-tion and patency can be threatened, however, bythe development of stenosis and its main unwantedconsequence, thrombosis.

Two seminal nonrandomized studies from the late1980s and early 1990s reported that measuringvenous pressure during dialysis and preemptiveangioplasty considerably reduced the rates ofthrombosis and access replacement (2,3), leading tothe conviction that vascular accesses should be rou-tinely checked for evidence of stenosis by means ofnoninvasive, device-based methods.

In 1995, came a breakthrough in vascular accesssurveillance with the introduction of a reversed-lineultrasound dilution method for measuring Qa duringthe hemodialysis session, affording quick, routine,

operator-independent measurements (4). Numerousother indicator dilution techniques have been used,based on different blood properties such as conduc-tivity (5–7), hematocrit (8–10), and temperature (11),and exploiting sensors incorporated in the dialysismachine, or techniques not requiring bloodlinesreversal, like the transcutaneous access flow (TQA)(12) and the variable flow Doppler (13,14). In addi-tion to visualizing stenosis, duplex ultrasound (DU)can also estimate directly Qa (15), but not duringdialysis, and it is also operator-dependent, relativelycostly and time-consuming, making it inconvenientas a routine screening procedure. All the above-men-tioned methods showed an excellent-to-good correla-tion with the gold standard ultrasound dilutiontechnique (16), with minimal differences in the meanQa values measured, although Qa measurementsobtained with different devices should not be com-pared directly, and only one technique should be usedin any given patient (17).All the vascular access guidelines (18–24) soon

adopted Qa as a mainstay screening tool for detect-ing stenoses requiring repair, and many(15,18,20,24) even considered it as the first-line tech-nique despite several drawbacks, i.e., it interruptsthe dialysis treatment and adds to staff workload,and it is costly and unsuitable for some types ofaccess (e.g., maturing fistulas (AVF), or matureAVF with noncommunicating branches or prevent-ing cannulation with two needles).

Address correspondence to: Dr Nicola Tessitore, EmodialisiBorgo Roma, Piazzale LA Scuro 10, 37134 Verona, Italy,Tel.: +39.045.8124652, Fax: +39.045.8124687, ore-mail: [email protected]

Seminars in Dialysis—Vol 27, No 2 (March–April) 2014pp. 108–118DOI: 10.1111/sdi.12187© 2014 Wiley Periodicals, Inc.

108

PROGRESS IN DIALYSIS PRACTICE

Despite enthusiasm for this new technology, thevalue of Qa surveillance has always been highlycontroversial due to a shortage of good-quality evi-dence supporting its benefits. A group of expertsrecently said that Qa (and static venous pressure)measurements are only useful as an ancillary test tohelp confirm a clinical suspicion of stenosis oraccess dysfunction (25). It has also been stronglyargued that vascular access surveillance (as cur-rently practiced) is a false paradigm (26,27), since itdoes not satisfy all the criteria for an effectivescreening test: it is not particularly reproducible andaccurate in detecting stenosis or predicting throm-bosis, and there is no strong evidence from random-ized controlled trials (RCT) of its effectiveness inreducing thrombosis rates and prolonging accesssurvival. These considerations are based almostexclusively on data relating to grafts, however, andmay not apply to AVF, as implied by the phrase,“there is limited evidence that Qa surveillancereduces thrombosis without prolonging the life ofnative fistulae” (26).

The aim of the present study was to see whetherQa surveillance in AVF complies with the WorldHealth Organization criteria for screening tests (26–28) by reviewing the available literature on themethod’s reproducibility, sensitivity (SE, i.e, theability of a test to detect a diseased AVF) and posi-tive predictive value (PPV, i.e., the ability of a testto avoid unnecessary procedures) for identifying ste-nosis and incipient thrombosis, and its efficacy inreducing the thrombosis rate and prolonging accesssurvival. We also checked whether adopting differ-ent Qa criteria could influence outcomes becausestudies comparing Qa surveillance with other strate-gies have considered numerous different Qa thresh-olds for intervention in AVF.

Methods

Our analysis was conducted according to thecriteria adopted in two recent meta-analyses on sur-veillance in vascular accesses (29,30).

The results of studies comparing the effects of Qasurveillance with other screening strategies onthrombosis, access loss, and elective stenosis repairwere pooled using standard meta-analytical tech-niques. RCTs and cohort studies with concurrent(CT) and historical controls (HCT) were analyzedseparately. Pooled estimates of SE, PPV, and thearea under the ROC (receiver operator characteris-tics) curves (AUC) were also computed.

Statistical heterogeneity was quantified using theI2 statistic (31), which gives an approximation ofthe percentage of total variability caused bybetween-studies variability. When the result of thetest of heterogeneity was not significant (p > 0.050)and I2 was less than 30%, any significant heteroge-neity was ruled out, and a fixed-effects model wasused to assess the results, which were pooled using

the Mantel and Haenszel method. In all other cases,a random-effects model was used and the resultswere pooled using the DerSimonian and Lairdmethod (32). Meta-regression was used to assess dif-ferences in SE between the elements of clinical mon-itoring and the Qa thresholds used in studiescomparing Qa surveillance with clinical monitoring(33). Stata 12.0 software (StataCorp LP, CollegeStation, Texas, USA) was used for the statisticalanalysis.

Reproducibility of Qa Measurements

A major concern with Qa surveillance has alwaysbeen its low reproducibility since Qa measurementsvary considerably within and between dialysis ses-sions, due to the hemodynamic instability commonduring dialysis. Reproducibility was assessed usingthe coefficient of variation (CV) of two or threemeasurements taken during the same hemodialysissession (within-session CV) and during two differenthemodialysis sessions a few days or weeks apart(between-session CV). The mean weighted within-session CV was 5.5% for the ultrasound dilutionmethod (range 1.4–9.8) (6,8,10,11,14,16,17,34–37),9.4% for methods using hematocrit dilution (range6.9–17.4) (8–10,12,16,17), 8.5% for thermodilution(16), and 7.9% for conductivity (8). The pooledwithin- and between-session CV was assessed onlyfor the ultrasound dilution method and it was10.3% (range 7.9–14.2) (35–38). These figures showthat Qa measurements are fairly reproducible inAVF, accounting for up to 17% of the flow rate’svariability, and meaning that a >25% change in Qais distinguishable from background Qa variationswithin and between hemodialysis sessions.

Diagnostic Performance for Stenosis

A few studies assessed the overall performance ofQa measurement for diagnosing stenosis by consid-ering the area under the ROC curve: they haveshown an excellent discriminative ability for Qawith a pooled estimated AUC of 0.88 (95%CI:0.81–096; p < 0.0001, I2 73%) (35,37,39,40). Onesuch study (37) also reported that accuracy of Qameasurements depends heavily on the site of steno-sis, being very accurate for inflow stenoses locatedupstream or within the needling area (AUC 0.87[95%CI: 0.81–0.91]; p < 0.001), but having no dis-criminative capacity for outflow stenoses locateddownstream from the needling area (AUC 0.53[95%CI: 0.40–0.66]; p = 0.694), which can be detectedaccurately by physical examination (PE) andderived static venous pressure measurement instead(AUC 0.84[95%CI: 0.71–0.97] and 0.85[95%CI:0.72–0.98] respectively; p < 0.001).Table 1 shows the pooled estimates for SE and

PPV of the currently used Qa thresholds and criteria

BLOOD FLOW SURVEILLANCE IN ARTERIOVENOUS FISTULAS 109

for identifying stenosis, drawing a comparison withother screening tests used in clinical monitoring, i.e.,positive PE, inability to achieve the prescribed dialy-sis blood pump flow (dQb), elevated dynamic venouspressure (eVP), access recirculation (R) and a dropin dialysis dose (dKt/V), and DU.

The I2 for Qa varies considerably in terms ofboth sensitivity and PPV, indicating ample inconsis-tency in published findings, probably due to the useof different definitions of stenosis, which in mostcases was a >50% reduction in vessel diameter, butsome studies (39,43,47,48) considered either a >50%reduction in vessel diameter or thrombosis within6 months.

The Qa thresholds recommended by the guidelines(ranging from Qa < 500 to Qa < 300 ml/minute)achieved quite a high PPV for stenosis (83–89%),higher than for eVP (51%) or dKt/V (38%), but simi-lar to the values achieved with higher Qa threshold(66–74%), the other clinical monitoring tools (76–95%), and DU (98%). The recommended Qa thresh-olds have a low sensitivity (SE 24–59%), which islower than that for Qa thresholds in the range of600–900 ml/minute (77–88%), or for DU (91%), orPE (75%).

Qa thresholds of 600–900 ml/minute have a highsensitivity (SE 77–88%), but similar to PE and DU.These Qa thresholds have the drawback, however,of a relatively low PPV, lower than that for DU,but similar to the PPV achieved with lower Qathresholds and all elements of clinical monitoring.

Unfortunately, few studies have tested the diag-nostic performance of a drop in Qa (dQa). Only

one considered this parameter alone, finding a SEof 80% and a PPV of 89% for a dQa > 25%, val-ues similar to those achieved with PE and Qathresholds in the range of 500–900 ml/minute. Com-bining a dQa > 25% with Qa thresholds in therange of 600–750 ml/minute improved the test’s sen-sitivity (92–95%) making it better than PE, andsimilar to a Qa < 900 ml/minute and DU. More-over, this combination retained a good PPV (79–86%), only lower than that for DU. Combining aQa < 500 ml/minute with a dQa > 25% achievesmuch the same performance as that of the singletests alone, or PE.Only one study assessed the diagnostic perfor-

mance of a combination of PE with Qa (37), a situ-ation mimicking the screening strategy thatcombines Qa surveillance with clinical monitoring,although it is unfortunate that the combination ofPE with changes in Qa was not assessed. The studyshowed that a strategy of performing PE first, fol-lowed by Qa measurement in case judged negativeon PE, and performing angiography in the presenceof a positive PE for stenosis “or” a Qa threshold of750 ml/minute or more improved sensitivity bycomparison with a positive PE finding alone(SE>90% vs 75%), with no loss in PPV (71–72%vs 76%). On the contrary, the “or” combination ofa positive PE with a Qa < 500 ml/minute performedno better than a positive PE (SE 81% vs. 75%;PPV 74% vs. 76%), and the same was true of the“and” combination of a positive PE with Qa (i.e.,the strategy that involves measuring Qa only afterPE has detected an access dysfunction, and prescrib-

TABLE 1. Performance in diagnosing stenosis

Screening criteria

Sensitivity Positive predictive value

N patients/Nstudies

Pooledestimate [95% CI] I2

N patients/Nstudies

Pooledestimate [95% CI] I2

Qa < 300 ml/minute (35, 37, 39) 586/3 24% [5–44] 92% 586/3 89% [80–97] 0Qa < 400 ml/minute (35, 37, 39) 586/3 39% [23–56] 84% 586/3 86% [79–93] 10Qa < 500 ml/minute (7, 35, 37, 39–46) 710/6 59% [45–73] 81% 1051/11 83% [75–92] 86%Qa < 600 ml/minute (35, 37, 39, 40) 645/4 77% [72–83] 0 645/4 74% [57–91] 90%Qa < 750 ml/minute (7, 35, 37, 39, 40) 668/5 84% [79–89] 0 668/5 69% [55–83] 83%Qa < 900 ml/minute (7, 35, 37, 39, 40) 668/5 88% [79–96] 71% 668/5 66% [52–80] 86%dQa > 25% (35) a 42/1 80% [56–94] – 42/1 89% [65–99] –Qa < 500 ml/minute “or” dQa>25% (35, 42) 42/1 80% [56–94] – 83/2 81% [68–94] 30%Qa < 600 ml/minute “or” dQa>25% (35) 42/1 95% [75–100] – 42/1 86% [65–97] –Qa < 750 ml/minute “or” dQa>25%(35, 37, 46–49)

115/2 92% [85–99] 0 326/4 79% [69–88] 74%

Positive PE “or” Qa < 500 ml/minute (37) b 119/1 76% [63–86] – 119/1 73% [63–86] –Positive PE “or” Qa < 600 ml/minute (37) b 119/1 86% [75–94] – 119/1 73% [62–84] –Positive PE “or” Qa < 750 ml/minute (37) b 119/1 90% [79–96] – 119/1 72% [61–82] –Positive PE “or” Qa < 900 ml/minute (37) b 119/1 98% [91–100] – 119/1 71% [61–82] –Positive PE “and” Qa <900 ml/minute (37) b 119/1 56% [42–69] – 119/1 78% [63–90] –Positive PE (37, 50–56) 1053/10 75% [68–81] 79% 1128/12 76% [67–85] 94%Inability to achieve the prescribed Qb (46, 57) 127/1 45% [31–58] – 184/2 91% [84–98] 0Recirculation (3, 35, 40, 42) 149/2 48% [5–92] 92% 165/4 95% [87–100] 10%Elevated dynamic VenousPressure (37, 43, 44)

280/3 35% [27–44] 0 295/3 51% [31–71] 74%

Drop in Kt/V (37b, 50) 93/1 16% [7–30] – 155/2 38% [27–50] 2%Duplex Ultrasound (58–63) 127/3 91% [85–97] 0 192/6 98% [94–100] 0

aunpublished, calculated from the database of the reference.bunpublished, calculated from the database of the reference for the presence of stenosis regardless of its location.

110 Tessitore et al.

ing angiography if both tests are positive) (1,25),even considering the ‘best’ Qa threshold of aQa < 900 ml/minute (SE 56%, PPV 78%).

In short, the available data show that all Qascreening criteria have a PPV similar to that of PEand lower than that of DU. The widely used Qathreshold of <500 ml/minute (alone or combinedwith a >25% drop in Qa or a PE positive for steno-sis) has much the same sensitivity as PE alone. Onthe other hand, Qa surveillance becomes more sensi-tive, with no loss of PPV when higher Qa thresholdsin the range of 600–750 ml/minute are combinedwith a drop in Qa > 25%, or Qa thresholds ranging750–900 ml/minute are associated with positive PEfindings.

The utility of highly sensitive screening methodsfor the purpose of an early detection and treatmentof stenosis nonetheless remains controversialbecause they add to the burden on resources(requiring unnecessary imaging procedures), whilethe benefits of such screening tests have yet to bedemonstrated, and they may even prove harmful(angioplasty for stable subclinical stenoses mayimpair access survival by prompting aggressiverestenoses (1,15,26)).

Diagnostic Performance for IncipientThrombosis

Few studies have focused on the performance ofQa for diagnosing incipient thrombosis, but twosuch studies indicated an excellent overall discrimi-native ability for Qa with a pooled estimated AUCof 0.95 (95%CI: 0.87–1.04; p < 0.0001, I2 56%)(35,64). Table 2 shows the pooled estimates for theSE and PPV of different Qa thresholds, dQa, R,dQb, and stenosis in predicting thrombosis.

The accuracy of Qa measurements in predictingthrombosis is hard to judge because the reportedsensitivity and PPV vary considerably among differ-ent studies (possibly reflecting differences in popula-tion characteristics and length of follow-up),although a Qa < 300 ml/minute seems to be thebest cutoff: it has a fairly good sensitivity (74%)combined with a consistently excellent PPV (89%),

higher than that of any other test. Our analysis jus-tifies the choice of a Qa < 300 ml/minute as a cutofffor prescribing further investigations by some of theguidelines (21,24) that aim to minimize the burdenof costly and potentially harmful unnecessary proce-dures by taking action only on a subset patients ata higher risk of thrombosis, even if this means thatsome accesses that might benefit from interventionare overlooked. Our findings also indicate that flowrate is a better predictor of any clotting than ana-tomical features: although stenosis underlies virtu-ally all thrombotic episodes, its very low PPVmeans that only a minority of stenotic AVF throm-bose, so taking action on finding stenosis wouldprompt a large number of unnecessary proceduresin accesses that would not have clotted.

Effect of Qa Surveillance on Thrombosis andAccess Loss

Numerous studies assessed the effect of Qa sur-veillance programs on access thrombosis in AVF, asoutlined in Table 3. The study design varied fromRCT (44,67–70) to cohort studies with concurrent(46,47) or historical controls (66,71–74). Some trialscompared Qa surveillance with clinical monitoring(66,68,69,71,74), others tested the effect of addingQa surveillance to clinical monitoring (44,46,47,67,70,72,73). Clinical monitoring for detecting andrepairing stenoses focused on several clinical param-eters of access dysfunction detectable during dialy-sis, mainly a positive PE (usually associated witheVP and/or dQb, and/or a dKt/V) (44,46,47,66,70,72–74), while some studies only considered adrop in Kt/V (68,69).Various criteria were adopted for Qa surveillance,

some very highly sensitive for stenosis (Qa<850 ml/minute, or Qa thresholds of 600–750 ml/minutecombined with a >25% dQa) achieving a sensitivityhigher than clinical monitoring (pooled SE 89%[95%CI: 83–95], I2 60% vs. 65%[95%CI: 55–76], I2

93%, p = 0.011) (46,47,67–69,72,74), others with asensitivity similar to clinical monitoring (Qa thresh-olds of 450–500 ml/minute and/or dQa cutoffs inthe range of 15–25%: pooled SE 64%[95%CI:

TABLE 2. Performance in predicting incipient thrombosis

Screening criteria

Sensitivity Positive predictive value

N patients/N studiesPooled estimate

[95% CI] I2N patients/N

studiesPooled estimate

[95% CI] I2

Qa < 300 ml/minute (35, 64) 118/2 74% [34–100] 73% 118/2 89% [70–100] 0Qa < 400 ml/minute (35, 64) 433/2 64% [0–100] 98% 433/2 57% [40–73] 0Qa < 500 ml/minute (35, 64, 66) 1461/3 51% [0–100] 99% 1461/3 46% [2–89] 94%Qa < 700–800 ml/minute (64–66) 1764/3 28% [9–48] 81% 1764/3 41% [9–73] 96%dQa > 25% (35) 30/1 67% [9–99] – 30/1 25% [4–65] –Recirculation (35) 64/1 75% [48–94] – 64/1 45% [23–68] –Inability to achieve theprescribed Qb (35)

65/1 58% [28–85] – 65/1 58% [28–84] -

Stenosis (35, 66) 84/2 100% [88–100] 0 84/2 26% [14–38] 0


TABLE

3.Characteristics

ofthestudies

Reference,publication

year

Study

design

Study

strategy

Clinicalmonitoring

Qasurveillance

No

Screeningcriteria

Meanfollow-up

(months)

No

Screeningcriteria

Meanfollow-up

(months)

Sands,(67)1999

RCT

DU

vs.DU

+Qa

26

DU

every6months

642

Qa<600ml/minute

ordQa>25%

(monthly

Qameasurement)

6

Tessitore,(69)

2004

RCT

Kt/V

vs.Qa

31*

Dropin

Kt/V

29

36*

Qa<750ml/minute

ordQa>25%

(3-m

onthly

Qameasurement)

34

Polkinghorne,

(44)

2006

RCT

CM

vs.CM

+Qa

68

Abnorm

alPE/inabilityto

achieve

theprescribed

Qb/highVP/

dropin

Kt/V

17 median

69

Qa<500ml/minute

ordQa>20%

(monthly

Qameasurement)

17 median

Scaffaro,(70)

2009

RCT

CM

vs.CM

+Qa

58

Abnorm

alPE/highVPandAP

12

53

Qa<500ml/minute

(3-m

onthly

Qameasurement)

12

Tessitore,(68)

2003

qRCT

Kt/V

vs.Qa

14*

Dropin

Kt/V

15

18

aQa<850ml/minute

(3-m

onthly

Qameasurement)

19

Roca-Tey,(47)

2004

CT

CM

vs.CM

+Qa

94

Abnorm

alPE/highVPandAP

12

50

Qa<700ml/minute

ordQa>20%

(4-m

onthly

Qameasurement)

12

Tessitore,(46)

2008

CT

CM

vs.CM

+Qa

97

Abnorm

alPE/inabilityto

achievethe

prescribed

Qb/difficultcannulation

25

62

Qa<750ml/minute

ordQa>20%

(4-m

onthly

Qameasurement)

29

McC

arley,(71)

2001

HCT

dVPvs.Qa

41

HighVPatQb200ml/minute

23

43

dQa>25%

(monthly

Qameasurement)

10

Lok,(66)

2003

HCT

CM

vs.Qa

189

Abnorm

alPE/inabilityto

achieve

theprescribed

Qb/highVP/

dropin

Kt/V

12

241

Qa<500ml/minute

ordQa>15%

(2-m

onthly

Qameasurement)

12

Branger,(72)

2004

HCT

CM

vs.CM

+Qa

68

Abnorm

alPE

24

58

Qa<600ml/minute

ordQa>25%

(monthly

Qameasurement)

24

Shaihin,(73)

2005

HCT

CM

vs.CM

+Qa

102

Abnorm

alPE/highVPandAP

R/dropin

Kt/V

21

68

Qa≤450ml/minute

ordQa>25%

(monthly

Qameasurement)

23

Wijnen,(74)

2006

HCT

CM

vs.Qa

63

Abnorm

alPE/highVPandAP

36

60

Qa<600ml/minute

ordQa>20%

(3-m

onthly

Qameasurement)

36

RCT

=randomized

controlled

trial;qRCT

=quasi-randomized

controlled

trial;CT

=cohort

trialwithconcurrentcontrols;HC

=cohort

trialwithhistoricalcontrols;DU

=duplexultrasound;

CM

=clinicalmonitoring;Kt/V

=dialysisdose;PE

=physicalexamination;VP=dynamic

dialysisvenouspressure;AP=dynamic

dialysisarterialpressure;R

=accessrecirculation;Qb=dialy-

sisbloodpumpflow;Qa=accessbloodflow;dQa=dropin

Qa.

aOnly

patients

whohadQameasuredatenrollmentare

included.


52–76], I2 79%, p = 0.889 vs. clinical monitoring)(44,66,70,71,73).

All the studies reporting on elective interventionrates (44,46,66,70–73) showed that Qa surveillanceled to a greater recourse to elective stenosis repair,with a pooled estimate of the relative risk (RR) of2.05 (95%CI: 1.22–3.43) (p < 0.0001, I2 83%).

Figure 1 shows the results of a meta-analysis onthe five RCTs that assessed the effect of Qa surveil-lance on the risk of thrombosis. Overall, Qa surveil-lance significantly reduced this risk (RR 0.57[95%CI: 0.36–0.90]; p = 0.015, I2 15%). But when thestudies were analyzed separately, based on the Qacriteria adopted for stenosis detection and repair,the reduction in the risk of thrombosis remainedsignificant only in the three studies with a sensitivityfor stenosis higher than that for clinical monitoring(RR 0.35[95%CI: 0.18–0.69]; p < 0.001, I2 0%)(Fig. 1a), not in the other two (RR 0.88[95%CI:0.47–1.66]; p = ns, I2 4%) (Fig. 1b).

Figure 2 shows the results of a meta-analysis ontwo CT, both of which adopted highly sensitive Qacriteria for stenosis: Qa surveillance significantlyreduced the risk of thrombosis (RR 0.33[95%CI:0.15–0.71]; p = 0.005, I2 0%).

When the results of the five studies involving his-torical controls were considered together (Fig. 3),Qa surveillance significantly reduced the risk of

thrombosis (RR 0.63[95% CI: 0.44–0.89]; p = 0.010,I2 0%), but this effect was driven by the two studiesbased on criteria with a sensitivity for stenosishigher than that for clinical monitoring (RR 0.50[95%CI: 0.2–0.86]; p = 0.013, I2 0%) (Fig. 3a),while the risk was not reduced by Qa surveillance inthe other three studies (RR 0.73[95%CI: 0.46–1.17];p = 0.195, I2 8%) (Fig. 3b).To sum up, our analysis suggests that the choice

of Qa criteria for detecting stenosis in AVF is a cru-cial determinant of the success of Qa surveillance inreducing the rate of thrombosis, regardless of studydesign: adopting highly sensitive criteria for identi-fying stenosis for elective repair consistently pro-vided a significant, 2- to 3-fold lower risk ofthrombosis than clinical monitoring, while less sen-sitive Qa criteria were unable to do so (I2 0–4%).The usefulness of highly sensitive stenosis screeningprograms is also supported by the findings of aninterim report on a RCT conducted at our institu-tion (75). This trial was designed to compare theoutcome of elective repair for subclinical stenosis inAVF with a Qa > 500 ml/minute, with the strategyrecommended by the K/DOQI guidelines (15) ofrepairing stenosis only in the event of clinicallyevident access dysfunction or a Qa < 400 ml/min-ute. It involved AVF in which stenoses were identi-fied by highly sensitive screening criteria, i.e., a

A

B

Fig. 1. Meta-analysis of the effect of Qa surveillance on AVF thrombosis in RCTs. (A) Results of studies adopting Qa criteria favor-

ing a high sensitivity for stenosis, more sensitive than clinical monitoring. (B) Results of studies adopting less sensitive Qa criteria, with

a sensitivity similar to clinical monitoring. The vertical line indicates no treatment effect; squares and horizontal lines, point estimates

and associated 95% confidence intervals (95% CI) for each study; diamonds, pooled relative risk; RR, relative risk; weight, the percent

contribution of each study to the pooled relative risk; I2, the statistical heterogeneity between the studies as a percentage of the total

variability.


Qa < 900 ml/minute and/or a PE positive for steno-sis and/or high static dialysis venous pressure; theseAVF were randomized either to elective stenosisrepair or to intervention according to the guidelinesallowing to compare the effects of two screeningstrategies with a different sensitivity for stenosis(98% for the elective stenosis repair arm vs. 39%for intervention triggered by a Qa<400 ml/minute).We found that electively correcting subclinical ste-nosis in AVF with a still high Qa led to a significant3-fold lower risk of thrombosis, with an incidencerate ratio (IRR) of 0.37 (95%CI: 0.12–0.97)

(p = 0.033) by comparison with the approach rec-ommended in the guidelines.Only a few published studies have also addressed

the role of Qa surveillance on access longevity(Fig. 4). A meta-analysis on the two RCTs (toppanel) showed a statistically insignificant halving ofthe risk of access loss by comparison with interven-tion triggered by a declining dialysis dose (RR 0.41[95%CI: 0.15–1.12]; p = 0.081, I2 0%). The sameresult emerged from a controlled trial, but not froma study involving historical controls (bottom panel).It should be noted, however, that the studies report-

Fig. 2. Meta-analysis of the effect of Qa surveillance on AVF thrombosis in controlled trials. The vertical line indicates no treatment

effect; squares and horizontal lines, point estimates and associated 95% confidence intervals (95% CI) for each study; diamonds, pooled

relative risk; RR, relative risk; weight, the percent contribution of each study to the pooled relative risk; I2, the statistical heterogeneity

between the studies as a percentage of the total variability.

A

B

Fig. 3. Meta-analysis of the effect of Qa surveillance on AVF thrombosis in observational studies with historical controls. (A) Results

of studies adopting Qa criteria favoring a high sensitivity for stenosis, more sensitive than clinical monitoring. (B) Results of studies

adopting less sensitive Qa criteria, with a sensitivity similar to clinical monitoring. The vertical line indicates no treatment effect; squares

and horizontal lines, point estimates and associated 95% confidence intervals (95% CI) for each study; diamonds, pooled relative risk;

RR, relative risk; weight, the percent contribution of each study to the pooled relative risk; I2, the statistical heterogeneity between the

studies as a percentage of the total variability.


ing a benefit of Qa surveillance (albeit statisticallyinsignificant) (46,68,69) came from the same institu-tion as the one adopting highly sensitive stenosisscreening criteria, suggesting that this approachmight also prolong access patency. This impressionis also supported by our previously mentioned RCT(75) showing that elective repair of subclinical ste-nosis in AVF with a Qa > 500 ml/minute affords asignificant 3-fold lower risk of access loss by com-parison with taking action only once the stenosishas become hemodynamically significant as recom-mended by the current guidelines (IRR 0.36[95%CI:0.09–0.99; p = 0.041).

These findings on the benefits of early stenosisdetection also suggest that the current techniquesfor elective stenosis repair (angioplasty and surgery)are effective in AVF, and that any concerns aboutan early and aggressive treatment of stenosis beingharmful (due to angioplasty triggering an aggressiverestenosis) are probably excessive in this type ofaccess.

Cost effectiveness of Qa Surveillance

Very few cohort studies—one with concurrent(46) and two with historical controls (71,74)—haveconsidered the cost effectiveness of Qa surveillance:two of them (46,71) showed a significant reductionand the other (74) found no change in the directaccess-related costs by comparison with clinicalmonitoring. It was impossible to calculate a pooledestimate from these studies because not all of themprovided a measure of variability. It is also difficultto generalize from their findings because of thegreat variability in the rates of access thrombosisand loss, and in the costs of imaging and interven-

tion procedures at different institutions. An eco-nomic simulation using the best data available inthe literature (76) indicated that Qa surveillanceincurs a modest increase in cost by comparison withno monitoring, giving the impression that Qascreening may be good value for money. No differ-ence emerged in the cost of Qa surveillance, what-ever the threshold used (Qa < 500 ml/minute vs.Qa<750 ml/minute). This finding was consistentwith the results of our previously described RCT(75) showing no difference in cost between the twoQa screening strategies. At variance with theassumption made by Tonelli et al. (76), however, ahigher Qa threshold is economically more attractivebecause it is associated with a mean additional costof just € 282 and € 321 per thrombosis and accessloss avoided, respectively, figures that comparefavorably with a mean € 3500 for thrombectomyand € 3800 for the placement of a new access at ourinstitution, where surgery is performed as an inpa-tient procedure.Thus, while the available data support the cost

effectiveness of Qa surveillance, they do not allowfor any final conclusions to be drawn because theyare based on too few studies, with weaknesses intheir study design, and differences in their outcomesand the reported unit costs. Further prospectivecost-benefit analyses are needed, comparing Qa sur-veillance with clinical monitoring.

Our approach

Currently at our center (where access to DU islimited) we use a highly-sensitive bedside stenosisscreening program, based on recording abnormal-ities during each dialysis session, performing PE

Fig. 4. Top panel: Meta-analysis of the effect of Qa surveillance on access loss in RCTs. Bottom panel: Results of the controlled trial

and the observational study with historical controls. The vertical line indicates no treatment effect; squares and horizontal lines, point

estimates and associated 95% confidence intervals (95% CI) for each study; diamonds, pooled relative risk; RR, relative risk; weight, the

percent contribution of each study to the pooled relative risk; I2, the statistical heterogeneity between the studies as a percentage of the

total variability.


and measuring Qa (by the ultrasound dilution tech-nique) and derived static venous pressure ratio(VAPR) every 2 to 4 months, and tailoring screen-ing to the site of the AVF anastomosis (77). Accessimaging (by angiography or DU) is prompted byany persistent dialysis-related abnormalities in allAVF. If no access dysfunction is detected duringdialysis, lower-forearm AVFs (i.e., those with thearteriovenous anastomosis located in the lower-thirdof the forearm) are screened by measuring Qa andusing a Qa threshold <700 ml/min as an indicationto imaging (SE for inflow stenosis 90%[95%CI:66–98]); more proximally located AVFs are screenedusing PE as the initial tool and measuring Qa andVAPR only in AVFs found negative on PE: indica-tion for imaging are the combination of a positivePE with a Qa<900 ml/min (SE for inflow stenosis96%[95%CI:82–100]) or a VAPR>0.5 (SE for out-flow stenosis 93%[95%CI:68–100]). This strategyenables to detect and locate stenosis in a single dia-lysis session with a sensitivity >90% and it has alsothe advantage of containing the surveillance-relatedworkload and cost, its tradeoff being a 12–47% ofunnecessary imaging procedures. Soon after thecompletion of our previously-mentioned RCT (75),we have decided to repair electively all >50%stenoses detected by our screening program, usingangioplasty or surgery, the choice of interventionbeing made case by case at discretion of anddepending on the availability of the radiologist andthe vascular surgeon with the view to correctingstenosis without any major reduction in the venouscapital available for puncture.

In theory, bearing in mind that the prevalence ofstenosis is approximately 50% (35,37,40) and non–stenotic AVF very seldom clot (15), this strategyshould result in thrombosis and access loss rates aslow as 4 % (95% CI:1.5–9) and 3 % (95%CI:1–8)per year at risk, respectively (75).

Conclusions

The available literature shows that Qa surveil-lance in AVF complies with the majority of theWorld Health Organization’s criteria for a screen-ing test (28), and indicates that it is an effectiveAVF screening strategy, although its usefulnessremains to be firmly established because studies todate on its performance in diagnosing stenosis andincipient thrombosis, and its efficacy in improvingclinically relevant outcomes are limited in number,of moderate-to-poor quality (29,30), and very het-erogeneous in terms of pooled estimates of theirresults.

Qa measurements have a good reproducibility,proving the best available bedside screening tool forstenosis. Measuring Qa is the only way to achieve avery high sensitivity by combining Qa thresholds of600–750 ml/minute with a drop in Qa > 25%, or aPE positive for stenosis with a Qa < 900 ml/minute

(SE 92–98%, similar to the SE of DU), coupledwith a fairly good PPV (PPV 71–86%, similar tothat of clinical monitoring, but lower than that ofDU).A Qa threshold of 300 ml/minute affords the

highest PPV for incipient thrombosis (89%, higherthan the surrogate measures of low flow or steno-sis). Despite this finding suggesting that actionshould be taken on AVFs with low flow, studiescomparing Qa surveillance with clinical monitoringconsistently showed that it assures a significant 2-to 3-fold reduction in the risk of thrombosis onlywhen higher Qa thresholds highly sensitive to steno-sis are adopted. This strategy also coincided with atrend toward a better access patency.The few published studies on the topic also sug-

gest that Qa surveillance can be cost effective, orinduce only a modest increase in cost, and can beconsidered good value for money.Our analysis suggests that, if a Qa surveillance

program is adopted, then high Qa thresholds shouldbe used, bearing in mind the moderate overall qual-ity of evidence of a benefit and the downside ofunnecessary imaging procedures in 16–32% ofcases. On the other hand, deciding not to offer Qasurveillance is still a reasonable option for the timebeing (providing that routine clinical monitoring isperformed), if the 2- to 3-fold lower access throm-bosis rate and the uncertain benefits on the survivalof vascular accesses with yearly thrombosis rates aslow as 4–17% on clinical monitoring alone(7,44,46,47,55,66,71–73) do not warrant the extracost and workload associated with surveillance.Clearly, a scientifically sound, multicenter ran-

domized controlled study comparing Qa surveillancewith clinical monitoring in a sufficiently large sam-ple is eagerly awaited to establish the value of Qasurveillance in AVF (26,29,30), but this is unlikelyto happen because the dialysis community seems tohave lost interest in research on vascular access sur-veillance, which has been virtually abandoned inrecent years.

Disclosure

The authors have no conflict of interest todeclare.

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the rise and fall of access blood flow surveillance in arteriovenous fistulas

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