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Nephrol Dial Transplant (2003) 18: 955–960 DOI: 10.1093/ndt/gfg075 Original Article Conﬁdence limits of arteriovenous ﬁstula ﬂow rate measured by the ‘on-line’ thermodilution technique Joe L. Ragg, John P. Treacy, Paul Snelling, Melinda Flack and Sonia Anderton Northern Territory Clinical School, Flinders University, Royal Darwin Hospital, Northern Territory, Australia Abstract Introduction Downloaded from http://ndt.oxfordjournals.org by on May 29, 2010 Background. A method is presented for estimating the conﬁdence limits (CLs), or accuracy, of the arterio- Major consensus guidelines have recommended the venous ﬁstula ﬂow rate measured at haemodialysis by measurement of the ﬂow of arteriovenous ﬁstulae as the the ‘on-line’ thermodilution technique. preferred means of monitoring their function, along Methods. This was by derivation of an expres- with physical examination [1,2]. Speciﬁcally, investiga- sion to estimate what variance a set of repeated mea- tion of ﬁstulae is recommended where ﬂow rate is below sures of ﬂow would yield, using values pertaining a particular threshold (500 and 600 mlumin in native to a single measure of ﬂow. (Laws of variance were and graft arteriovenous ﬁstulae, respectively), or where applied to the formula used to calculate ﬂow, to there is a fall in ﬂow rate (20–25% or more from baseline account for its variables’ values and measurement in either of these ﬁstulae). errors.) This enabled CLs of a single measure to be There is expectation that ‘on-line’ techniques of estimated. measuring ﬂow rate will supplant others in enabling Results. The variance estimated from a single measure ‘accurate and inexpensive repetitive measurements’ [1]. was compared with that actually observed upon imme- A number of reports have validated ﬂow rates measured diately taking a second measurement; differences in by such methods [3–9], and others have identiﬁed fac- 189 pairs were not signiﬁcantly different from zero tors that will compromise its accuracy [4]. However, no (Ps0.56). Applying the results demonstrated that study has aimed solely to describe conﬁdence limits measured ﬂow values of 430–570 mlumin typically had (CLs) of measured ﬂow, which are an easily understood associated 95% CLs that included 500 mlumin; there- and clinically applicable expression of measurement fore, true ﬂow could not be said to be either side of accuracy. Knowledge of the accuracy of on-line ﬂow 500 mlumin. The same was the case for 500–700 mlumin measurements is of clinical signiﬁcance, as interven- with regard to 600 mlumin. CLs widened considerably tion is recommended based on the value of these with the magnitude of ﬂow rate, limiting the accurate measurements. measurement of higher ﬂows and the detection of The aims of this study were to describe a method to falls in ﬂow. estimate the CL of access ﬂow rate as measured by the Conclusion. A method to estimate CLs of ﬂow rate ‘on-line’ thermodilution technique, and to apply this to clinical decision making when assessing arteriovenous measured by the thermodilution technique is presented ﬁstulae and grafts. and validated. Application demonstrates an accurate measurement of low ﬂow, but limitations at higher ﬂow and in detecting falls in ﬂow. Appreciating the magni- tude of such is critical to informed clinical decision Subjects and methods making when using ﬂow rate in an access surveillance Patients participating were all those attending a satellite programme. dialysis centre of a teaching hospital for stable chronic haemodialysis patients. Keywords: access ﬂow; accuracy; haemodialysis; thermodilution; vascular access Flow measurement technique Flow rate (Q) was measured by the ‘on-line’ thermodilution Correspondence and offprint requests to: Dr J. L. Ragg, 26 Learmonth technique, as described and validated by Schneditz et al. [8]. Street, Ballarat, Victoria 3350, Australia. This requires the substitution of the equation given for Email: joe.ragg@mh.org.au cardiopulmonary recirculation (Equation 6 in Schneditz et al. # 2003 European Renal Association–European Dialysis and Transplant Association 956 J. L. Ragg et al. [8]) into the equation given for ﬂow rate (Equation 4 in variable’s measurement error). The UFR was kept as a Schneditz et al. [8]). The resulting formula consists of ﬁve constant value. Variables were re-allocated randomly to each component variables: other, and 10 000 Q values were generated from these. ð1{RxÞðQx{UFRÞ Table 1 demonstrates the results. First, the distributions Q~ n h io ð1Þ yielded were not normal in nature. However, their inverse 1{RxÞðQx{UFR Rx(1{ (RnððRxð1{RnÞQnÞ Þ) transformations approximated the normal (kurtosis near to 3, and skewness near to 0). Secondly, SDs of the distributions where Rn and Rx are the recirculation of dialysis lines in the were not constant (they increased with the median ﬂow rate usual and transposed positions, respectively; Qn and Qx are of the Q distribution, or vice versa for their inverse trans- the extracorporeal blood ﬂow rate, concurrent with the above formations). Figure 1 illustrates the distributions obtained Rn and Rx readings, respectively; and UFR is the ultraﬁltra- relevant to the patient with a ﬂow rate lying on the 50th tion rate (constant throughout the whole ﬂow measurement). percentile of all observed. The technique is invalid where there is recirculation in the ﬁstula. Therefore, where Rn values were )15% [10], measurements were excluded from analysis. Nonsensical read- Estimation of the conﬁdence limits of measured ﬂow rate ings (arbitrarily those )6.5 lumin or negative ﬂows) were also The 95% CLs of a single ﬂow (Q) measure can be calculated as excluded. These arise from very low recirculation measures being 1.96 standard measurement errors either side of a single with lines in the transposed position, raising the possibility measure, assuming that the distribution of repeated measures that the venous needle is not directly downstream of the is normal in nature. Table 1 demonstrates that such a arterial needle, e.g. in a collateral vein coming off upstream distribution is not normal in nature (see above), but that the of the arterial needle or in a separate vein all together. inverse transformation, invQ, with a corresponding SD of SD Downloaded from http://ndt.oxfordjournals.org by on May 29, 2010 Pairs of ﬂow readings were taken, as per the following invQ reasonably approximates normal (kurtosis near to 3, and approach. In the ﬁrst hour of dialysis, a pair of Rn readings skewness near to 0). The CLs calculated instead for invQ are was recorded, along with the concurrent Qn (as per machine valid. CLs on the scale of Q can then be calculated by: digital display). Lines were transposed and, similarly, a pair of 1 Rx and the concurrent Qx were recorded. The UFR (as per 95% CLs for Q~ ð2Þ machine digital display), constant throughout all measure- ½invQ+1:96|SD invQ ments, was recorded. Two ﬂow rates were calculated according As is also demonstrated in Table 1, SD invQ is not to Equation 1, using the ﬁrst value in each pair, and then constant. Speciﬁcally, it decreases with increasing magni- the second, along with the constant UFR. This was an tude of ﬂow rate. Therefore, an expression is required for approximation to taking two immediately sequential ﬂow substitution into Equation 2. The variance of the general readings (i.e. the need to transpose lines repeatedly, thereby function y, of the random variables x1, x2, x3. . . whose subjecting the patient to a small but theoretical risk of cross- variances are known, and where each x is independent of infection was avoided). the other, can be approximated by All measurements were performed using Fresenius 4008B dialysis machines ﬁtted with a blood thermodilution monitor varð yÞ~½y’ðx1Þ2 |varðx1Þz½y’ðx2Þ2 ð3Þ (BTM). Arterial needles faced the anastomoses and were as |varðx2Þz½y’ðx3Þ2 |varðx3Þz . . . near to it as possible. Vascular access was obtained with 14- gauge arterial and venous needles and Fresenius blood lines. where: y9(x1): is the partial derivative of y with respect to x1, etc.; var (x1) is the variance of x1, etc.; and the expression is calculated at the mean of each x. Measurement error of recirculation and extracorporeal Analogously, the function invQ is of the variables Rn, Rx, blood ﬂow rate Qn and Qx whose variances (measurement error) are known, each of which is independent of the others. Values can be Recirculation as measured by the BTM has been validated substituted accordingly to yield an expression for the variance elsewhere [11]. In this study, measurement accuracy of of the invQ function (and therefore its standard deviation, SD recirculation was assumed to be reﬂected by the immediate invQ). A qualiﬁcation is that the expression is not calculated at reproducibility of the measure, and this was quantiﬁed by the mean or true values of each variable (see Appendix). For considering the SDs of small sets (pairs) of recirculation simplicity, the measurement error of UFR is not discussed, measures. The pairs of recirculation measures were recorded as per the ﬂow measurement technique above, and the measurement error of recirculation was taken as the median Table 1. Summary of the distributions representing repeated mea- of all SDs calculated. sures of ﬂow rate (computer simulation) for those patients whose For the purposes of this study, a measurement error of 4% ﬂow rate lay on the 10th, 30th, 50th, 70th and 90th percentiles of all those measured (with the inverse transformation also shown) coefﬁcient of variation for extracorporeal blood ﬂow rate was assumed (see Discussion) [12], and this was assumed to Percentile Q (mlumin) SD Skewness Kurtosis be unaffected by access ﬂow rate. 10th 438 40 0.40 3.28 Computer simulation of repeated measures of ﬂow rate 30th 708 72 0.47 3.56 50th 906 112 0.68 4.03 A computer simulation of repeating ﬂow measurements 70th 1192 178 0.95 5.42 10 000 times, in those patients whose ﬁstula ﬂow rate lay on 90th 1862 419 1.66 10.42 the 10th, 30th, 50th, 70th and 90th percentiles of those 10th Inv 0.0023 0.00021 0.14 2.97 measured, was undertaken. This was by noting the value of the 30th Inv 0.0014 0.00014 0.17 3.11 variables used to calculate ﬂow at the particular percentile, 50th Inv 0.0011 0.00013 0.09 2.99 and generating sets of 10 000 such variable values (the mean 70th Inv 0.0008 0.00012 0.10 3.13 90th Inv 0.0005 0.00010 0.07 3.07 being the originally noted value, with spread equal to that CL limits of AV ﬁstula ﬂow rate measured by ‘on-line’ thermodilution 957 Fig. 1. The distribution of repeated ﬂow rate measurements obtained by computer simulation, relevant to the patient with the ﬂow rate of 906 mlumin (which lay on the 50th percentile of all ﬂow rates observed): (A) the usual and (B) with inverse transformation. and could be made zero by setting UFR to zero during ﬂow 248 mlumin (IQR 232–277) and UFR 760 mluh (IQR measurements. 555–900), and the calculated ﬂow was 919 mlumin (IQR Finally, the measurement error of recirculation can be 657–1254). Downloaded from http://ndt.oxfordjournals.org by on May 29, 2010 lessened by taking two immediately sequential measures and averaging these (lessened to a new value of ‘measurement Correlation of repeated ﬂow measures error’ud2, the standard error of a mean of two values). Herein, CLs for ﬂow rate, where ﬂow rate is calculated using such There was good correlation between repeated measures values, are referred to as CL2, whereas in the instance of using of ﬂow rate (Ts0.68, P-0.001) although the just single measures of recirculation, they will be referred to as repeatability lessened considerably with increasing CL1. magnitude of ﬂow rate. This is illustrated in Figure 2. Ethics Measurement error of recirculation and extracorporeal Ethical approval was granted by the Top End Human Research Ethics Committee. blood ﬂow rate The magnitude of difference within Rn pairs was not Statistical analysis related to its averaged value (Ts0.03, Ps0.55). It was related for Rx pairs (Ts0.14, Ps0.006), but to a Values were expressed as median (IQR, interquartile range). relatively small real extent only (Table 2). The median Correlation of continuous variables was tested by Kendall’s SD of all Rn pairs was 1.48, and of all Rx pairs was rank correlation coefﬁcient (T). The null hypothesis that 1.41. differences between matched pairs is zero was tested by the Sign test. Statistical analysis was carried out using STATA 7.0 statistical software. Signiﬁcance was considered as Conﬁdence limit of a measured ﬂow rate P-0.05. Equation 3 estimates the variance that repeated mea- sures of invQ would have by using variables pertaining Results to a single measure of ﬂow. This is to enable the calculation of conﬁdence limits of a single measure of There were 56 patients studied (and 56 ﬁstulae). They ﬂow according to Equation 2. To test this, variables and were of age 51 years (IQR 46–59) and 35 were female. their measurement error relevant to the calculation All ﬁstulae were native, although two were interposi- of the ﬁrst Q in each of the 189 pairs were substituted. tion vein grafts. Arteriovenous anastomoses were at the anatomical snuffbox in 21, the other distal quarter radial artery in 16, the proximal three-quarters radial artery in one, and the brachial artery in 18. There were 197 pairs of measures taken. Six were excluded from analysis because of nonsensical ﬂow readings (see Methods above). A further two were also excluded from analysis because of clearly discrepant or ‘outlier’ Rx pair measurements. These Rx readings were 41.0 and 15.4%, and 52.1 and 14.1%; the cause for these was unclear. Thus 189 pairs of readings were analysed, at three (IQR 2–5) measures per patient. The summary statistics for variables measured to calculate ﬂow, were Rn 7.7% (IQR 6.0–9.7), Rx 25.4% (IQR 20.9–30.9), Qn 275 mlumin (IQR 240–283), Qx Fig. 2. Correlation of immediately sequential ﬂow measurements. 958 J. L. Ragg et al. Table 2. The standard deviation of a pair of Rx measurements Average n Median SD -20 43 1.34 20–30 94 1.41 30–40 37 1.41 40–50 11 2.12 )50 4 2.05 The median values of all those calculated are tabled in groups according to the average value of the Rx pair. Table 3. Conﬁdence limits of the measured ﬂow rates that lay on a selected percentile of all measured Percentile Flow (mlumin) CL1 (mlumin)a CL2 (mlumin)a Fig. 3. Observed ﬂow rates up to 2500 mlumin, and their estimated 5 306 265–361 272–349 CL2 limits. 10 438 373–532 384–510 Downloaded from http://ndt.oxfordjournals.org by on May 29, 2010 20 599 504–739 521–707 30 709 593–883 614–839 Table 4. Flow rate example (Q) and values 20% less, and the ﬂow 40 808 659–1044 686–982 rates whose estimated CL2 values lay immediately next to these 50 908 738–1179 770–1107 values 60 1016 815–1348 854–1254 70 1192 938–1636 988–1503 Q (CL2) Q Q less 20% Q (CL2) 80 1375 1066–1938 1129–1760 90 1857 1352–2964 1453–2572 95 2475 1588–5613 1747–4242 696 (600–829) 600 480 405 (356–471) 1093 (906–1378) 900 720 614 (532–727) a 1621 (1193–2529) 1200 960 762 (644–945) CLs estimated according to Equation 2, where either single measures of Rn and Rx (CL1), or two measures each of Rn and 1955 (1492–2835) 1500 1200 946 (776–1211) Rx were taken and averaged (CL2), before substitution to calculate 2591 (1876–4187) 1800 1440 1084 (867–1446) the access ﬂow rate by Equation 1. The differences between the variance estimated by values, and their values minus 20%, with the measured Equation 3 and that actually observed in all 189 pairs ﬂow rates whose estimated upper or lower CL2 limits were not signiﬁcantly different from zero (Ps0.56). lay immediately nearest these values. It demonstrates, for example, that falls from a measured 696 to 405 mlumin, or 2591 to 1084 mlumin are required to Examples of practical application indicate a true 20% fall in ﬂow rate. There was considerable widening of CLs of the measured access ﬂow rate, or a lesser reproducibility of measurement, with increasing access ﬂow rate. This Discussion is demonstrated in Table 3, which gives ﬂow rates lying on various percentiles of all those measured, along The main ﬁndings from this study were the increasing with their estimated CL1 and CL2 values. This is also width of the CLs with higher ﬂow rates, as shown in demonstrated in Figure 2, which shows graphically the Table 3, consistent with there being a much lesser immediate reproducibility of access ﬂow rate measure- repeatability of measure in patients with higher ﬁstula ments, and Figure 3 which demonstrates graphically ﬂow rates, as shown in Figure 2. As demonstrated, it is all ﬂow rate measurements with estimated CL2 values. both the value of, and the measurement error of, the The observed ﬂow rates with estimated upper or component variables of the equation to calculate ﬂow lower CLs (CL2) nearest 500 mlumin were 427 mlumin rate, along with the mathematical properties of the (372–499) and 559 mlumin (491–649), respectively, which equation used to calculate ﬂow rate, that ultimately indicates that a measured ﬂow rate of ;430– determine the ‘measurement error’ or repeatability of 570 mlumin will indeterminately classify the true access the calculated ﬂow rate. Whilst measurement errors of ﬂow rate to be either side of a 500 mlumin threshold. the component variables of the equation to calculate Similarly, for a 600 mlumin threshold, ﬂow rates with ﬂow rate are constant across access ﬂow rates, estimated CL2 limits of 529 mlumin (459–625) and ‘measurement error’ of the calculated ﬂow rates is not. 709 mlumin (614–839) indicate that a measured ﬂow rate It increases with access ﬂow rate in a somewhat of ;500–700 mlumin will indeterminately classify the exponential fashion. It is this, rather than a physiolo- true ﬂow rate to be either side of a 600 mlumin threshold. gical phenomenon, that explains the increasing ‘mea- Very large falls in measured ﬂow rate need to occur to surement error’ of calculated access ﬂow rate with determine a true 20% fall in ﬂow rate, particularly falls access ﬂow rate by the ‘thermodilution technique’. Such from the higher ﬂow rates. Table 4 gives arbitrary Q a pattern has been noted previously, but not quantiﬁed; CL limits of AV ﬁstula ﬂow rate measured by ‘on-line’ thermodilution 959 Schneditz et al. [8] noted that paired readings obtained blood ﬂow rates have been shown not to be perfectly by the on-line thermodilution method had ‘good accurate. There is limited literature pertaining exclu- correlation’ only to ﬂows of 1.5 lumin. sively to the Fresenius machines. Sands et al. [12] Also of note is the eccentric placement of the CLs compared extracorporeal ﬂow rate displayed on the about the Q value. This is consistent with the skew Fresenius 2008H machine with readings obtained by distribution that results from repeated measures of Q, an ‘on-line’ sensor of extracorporeal blood ﬂow as demonstrated in Table 1 and Figure 1. (Transonic2 haemodialysis monitor). The ﬂow rate There is some consensus that the optimal access ﬂow displayed slightly overestimated ﬂow in comparison rate thresholds indicating impending native and graft with the on-line sensor, with a measurement error ﬁstulae dysfunction are 500 and 600 mlumin, respectively coefﬁcient of variation of ;5–6%. The measurement [2]. This study indicates that measured ﬂow rates of error may have been overestimated, as there must be at 430–570 mlumin and 500–700 mlumin indeterminately least some measurement error associated with the classify access ﬂow rate either side of these thresholds. Transonic2 haemodialysis monitor against which it Therefore, to justify ﬁstulae intervention upon obtaining was compared. Furthermore, intra-patient measure- ment errors were not reported. Conceivably, these could ﬂow measures within these ranges, even greater import- be less than the values reported, which were for all ance needs to be placed on corroborating clinical and patients’ measures combined. surveillance method evidence of impending ﬁstula The reproducibility of ﬂow measurement by the dysfunction. thermodilution technique has been expressed by Downloaded from http://ndt.oxfordjournals.org by on May 29, 2010 There is consensus that a fall in access ﬂow rate of Schneditz et al. [9] as the mean difference between 20–25% or more is indicative of impending ﬁstula pairs of readings. This was 26"298 mlumin (ns52). dysfunction [2]. For practical purposes, this study indi- However, this summary statistic does not account cates that the thermodilution technique of measuring speciﬁcally for the lesser reproducibility at higher ﬂow access ﬂow rate has limited sensitivity in detecting such rates. Nor was it clear if in the calculation of ﬂow, the falls, particularly as mentioned in the Results, from the recirculation measures were a single measure or the higher access ﬂow rates. average of a pair. In the same paper, ﬂow rate estimated While ﬂow measurement increasingly is recom- by the ultrasound dilution technique gave a slightly mended as the preferred method of surveillance of better reproducibility of 27"212 mlumin. Our value, arteriovenous ﬁstulae function, there is limited litera- calculated in the same way, using single recirculation ture pertaining to the measurement error of recircula- measurements to calculate ﬂow was less reproducible at tion, extracorporeal blood ﬂow rate and ﬁstula ﬂow 10"453 mlumin. rate measured by the thermodilution technique and, to Also of relevance to the ultrasound dilution tech- some extent, by other methods. nique, an in vivo validation [3] expressed reproducibility Regarding recirculation by the thermodilution tech- of ﬂow rate as the coefﬁcient of variation (SDumean) of nique, Schneditz et al. [9] have expressed reproduci- 46 sets of ﬁve consecutive measures, and this was bility as the difference between paired readings. This 13.4"6.4%. There was no signiﬁcant difference across was À0.4"1.84% (ns52) for Rn readings and À0.29" three brackets of ﬂow magnitude. In this study, with 3.26% (ns54) for Rx readings. Our values, calculated single measures of variables to calculate ﬂow rate, this by the same method, were less reproducible at 0.00" was less at 16.2"14.6%. As expected, the reproduci- 3.26% and 0.00"3.46%, respectively. Wang et al. [10] bility was better at lower ﬂow rates. Across three equal expressed the reproducibility as the relative deviation centile brackets of ﬂow rate, these were 12.2"9.9, from the mean of two consecutive measures, (R1–R2) 16.1"11.6 and 20.0"19.1%. This would suggest that 3 100u(mean of R1 and R2), and for Rn and Rx readings the accuracy of measurement by the thermodilution combined this was 2.5"11.5% (ns220). Our value, technique is comparable with that by the ultrasound calculated in the same way, was less reproducible at dilution technique. This is so particularly at the more À1.1"33.3%. critical lower ﬂow rates, and where two recirculation Another independent dialysis unit reported a similar measures are taken and averaged before entering into measurement error using a similar machine and BTM the equation to calculate ﬂow rate (see the improvement monitor: a median SD of 1.63 (ns97) and 1.34 (ns97) in CL2 over CL1 (Table 3). for Rn and Rx pairs of readings, respectively (D. Bolsch, Hartley Dialysis Center Adelaide, Australia, personal communication). Conclusion Relevant to the ultrasound dilution technique of measuring recirculation, Depner et al. [14] described This study presents and validates a method to estimate the reproducibility of Rx measures by the Pearson conﬁdence limits of ﬂow rate measured by the thermo- correlation coefﬁcient and the mean absolute error of dilution technique. Application demonstrates reason- paired readings as 0.98, 3.9"2.8%, which was similar ably accurate measurement of low ﬂow, but limitations to that calculated for this study (0.92, 2.6"2.4). at higher ﬂow and in detecting falls in ﬂow, and Regarding the measurement error of extracorporeal awareness of this is critical to informed clinical decision blood ﬂow rate, the Fresenius 4008B machine gives a making where using ﬂow rate in access surveillance digital readout of Qn and Qx. Displayed extracorporeal programmes. 960 J. L. Ragg et al. Appendix 3. Bosman PJ, Boereboom FT, Bakker CJ et al. Access ﬂow measurements in hemodialysis patients: in vivo validation of an ultrasound dilution technique. J Am Soc Nephrol 1996; 7: 966–969 The true mean or true value of each of the variables 4. Krivitski NM. Theory and validation of access ﬂow measure- used to calculate ﬂow are not known, and therefore ment by dilution technique during hemodialysis. Kidney Int 1995; conditions for applying Equation 3 are not fully met. 48: 244–250 The effect on the accuracy of the calculated CLs was 5. Lindsay RM, Blake PG, Malek P et al. Hemodialysis access investigated as follows. blood ﬂow rates can be measured by a differential conductivity technique and are predictive of access clotting. Am J Kidney Dis The true ﬂow rate should in 95% of instances be 1997; 30: 475–482 contained within the 95% CLs. 6. Lindsay RM, Bradﬁeld E, Rothera C et al. A comparison of Consider again the aforementioned computer simula- methods for the measurement of hemodialysis access recircula- tion, of taking 10 000 consecutive ﬂow rates in the tion and access blood ﬂow rate. ASAIO J 1998; 44: 62–67 patients whose ﬂow rate lay at the 10th, 30th, 50th, 70th 7. Lindsay RM, Blake PG, Malek P et al. Accuracy and precision and 90th percentiles of all ﬂow rates measured. A of access recirculation measurements by the hemodynamic random sample of 1000 from each was taken. The CL recirculation monitor. Am J Kidney Dis 1998; 31: 242–249 for each of these ﬂow rates was calculated according to 8. Schneditz D, Fan Z, Kaufman A, Levin NW. Measurement of Equation 2. The ‘true’ ﬂow rate (median ﬂow rate of the access ﬂow during hemodialysis using the constant infusion approach. ASAIO J 1998; 44: 74–81 overall distribution) was outside the relevant CL in 202 9. Schneditz D, Wang E, Levin NW. Validation of haemodialysis of the 5000 (4.0%) instances. This would validate the recirculation and access blood ﬂow measured by thermodilution. calculation of the CLs according to Equations 2 and 3 Downloaded from http://ndt.oxfordjournals.org by on May 29, 2010 Nephrol Dial Transplant 1999; 14: 376–383 as being reasonable despite the conditions for applying 10. Wang E, Schneditz D, Kaufman AM, Levin NW. Sensitivity and Equation 3 not being fully met. speciﬁcity of the thermodilution technique in detection of access recirculation. Nephron 2000; 85: 134–141 11. Kaufman A, Kramer M, Godmere RO. Hemodialysis access Acknowledgements. The authors would like to thank the patients recirculation (R) measured by blood temperature monitoring participating in this study, and the Fresenius company for supply (BTM): a new technique [abstract]. J Am Soc Nephrol 1991; of blood thermodilution monitor (BTM) during the trial period. 2: 232. 12. Sands J, Glidden D, Jacavage W, Jones B. Difference between References delivered and prescribed blood ﬂow in hemodialysis. ASAIO J 1996; 42: M717–M719 1. National Kidney Foundation. KuDOQI Clinical Practice Guidelines for Vascular Access, 2000. Am J Kidney Dis 2001; 13. Armitage P, Berry G. Statistical Methods in Medical Research. 37: S137–S181 Blackwell Science, Oxford, UK, 1994: 90–92 2. Ethier JH, Lindsay RM, Barre PE et al. Clinical practice 14. Depner TA, Krivitski NM, MacGibbon D. Hemodialysis access guidelines for vascular access. Canadian Society of Nephrology. recirculation measured by ultrasound dilution. ASAIO J 1995; J Am Soc Nephrol 1999; 10 [Suppl 13]: S297–S305 41: M749–M753 Received for publication: 4.3.02 Accepted in revised form: 6.1.03

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arteriovenous fistula, blood flow, vascular access, hemodialysis patients, level of evidence, lower extremity, target lesion, risk factors, confidence limits, flow rate, risk patients, collateral circulation, arteriovenous fistulas, intermittent claudication, acc aha guidelines

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