Vascular Access Recirculation Research Articles

Evaluation of partial pressure CO2 change in the dialyzer blood inlet during hemodialysis as a measure of vascular access recirculation.

Vascular access recirculation during hemodialysis is associated with reduced effectiveness and worse survival outcomes. To evaluate recirculation, an increase in pCO2 in the blood of the arterial line during hemodialysis (threshold of 4.5 mmHg) was proposed. The blood returning from the dialyzer in the venous line has significantly higher pCO2 , so in the presence of recirculation, pCO2 in the arterial blood line may increase (ΔpCO2 ) during hemodialysis sessions. The aim of our study was to evaluate ΔpCO2 as a diagnostic tool for vascular access recirculation in chronic hemodialysis patients. We evaluated vascular access recirculation with ΔpCO2 and compared it with the results of a urea recirculation test, which is the gold standard. ΔpCO2 was obtained from the difference in pCO2 in the arterial line at baseline (pCO2 T1) and after 5 min of hemodialysis (pCO2 T2). ∆pCO2 = pCO2 T2-pCO2 T1. In 70 hemodialysis patients (mean age: 70.52 ± 13.97 years; hemodialysis vintage of 41.36 ± 34.54, KT/V 1.4 ± 0.3), ∆pCO2 was 4 ± 4 mmHg, and urea recirculation was 7% ± 9%. Vascular access recirculation was identified using both methods in 17 of 70 patients, who showed a ∆pCO2 of 10 ± 5 mmHg and urea recirculation of 20% ± 9%; time in months of hemodialysis was the only difference between vascular access recirculation and non-vascular access recirculation patients (22 ± 19 vs. 46 ± 36, p: 0.05). In the non-vascular access recirculation group, the average ΔpCO2 was 1.9 ± 2 (p: 0.001), and the urea recirculation % was 2.8 ± 3 (p: 0.001). The ΔpCO2 correlated with the urea recirculation % (R: 0.728; p < 0.001). ΔpCO2 in the arterial blood line during hemodialysis is an effective and reliable diagnostic tool for identifying recirculation of the vascular access but not its magnitude. The ΔpCO2 test application is simple and economical and does not require special equipment.

Lessons for the clinical nephrologist: vascular access recirculation during continuous renal replacement therapy and regional citrate anticoagulation.

Lessons for the clinical nephrologist: vascular access recirculation during continuous renal replacement therapy and regional citrate anticoagulation.

Clearance Gap: Does It Really Matter for the Evaluation of Vascular Access Function?

Vascular access (VA) stenosis increases the risk of VA obstruction due to the gradual progress of intimal thickening. Therefore, we should try to detect VA stenosis early. However, we do not have a cutoff for when a difference between prescribed Kt/V and delivered Kt/V reflects a clinical issue. Thus, we have devised a new index, the 'clearance gap' (CL-Gap), which quantifies the difference between the effective clearance (eCL) of a hemodialysis (HD) patient and the theoretical clearance (tCL) of a dialyzer to detect a decrease in dialysis efficiency due to VA dysfunction. We propose a qualitative technique of analyzing dialysis (the CL-Gap method) concerning Kt/V by estimating the eCL based on the delivered Kt/V and the difference with the tCL based on the dialyzer. When VA recirculation and blood removal failure occurs, the eCL decreases, and it is expected that the CL-Gap increases. On the contrary, if uniform internal removal occurs, the eCL rises when we overestimate the delivered Kt/V and the CL-Gap is expected to decrease. However, we cannot judge whether a high CL-Gap indicates VA dysfunction immediately because it is necessary to consider not only VA dysfunction, but also the effect of other factors on the CL-Gap. Key Messages: We believe that it is important to think about VA function in a qualitative manner when managing the dose using the CL-Gap to achieve better dialysis treatment.

PCO2 Reveals Arteriovenous Fistula Recirculation in Bicarbonate Hemodialysis (RecirCO2lation Test)

We propose arterial pCO₂ as test to discover vascular access recirculation (VAR) in bicarbonate hemodialysis (HD). We selected 30 HD patients with a ascertained well-functioning arteriovenous fistula (Control). In these patients, we artificially created VAR through the reversion of HD lines (Reversed). Results of the arterial gas analysis were collected at the start of HD (baseline) and after 5 min. At baseline, no differences of pH, pCO₂ and HCO₃ were found between the 2 groups. At 5 min, pCO₂ increased from 38.1 ± 3.3 to 47.2 ± 6.3 mm Hg (p < 0.0001) in Reversed, whereas no increase was found in Control (p = 0.052). Areas under curve of pCO₂-increase was 0.96 (0.91-1.00) and pCO₂ at 5 min 0.92 (0.85-0.98). pCO₂-increase >4.5 mm Hg showed sensitivity 86.7% and specificity 100% with positive predictive value (PPV) 100% and negative predictive value (NPV) 89%. A pCO₂ value above 43 mm Hg at 5 min showed sensitivity 80%, specificity 90%, PPV 89%, NPV 82%. pCO₂ increase >4.5 mm Hg and/or pCO₂ at 5 min >43 mm Hg may accurately detect VAR.

Evaluation of a New Method for Measuring Vascular Access Recirculation

Together with Nikkiso in Shizuoka, Japan, we developed a new method for measuring the rate of vascular access recirculation by the blood volume monitor. This measurement is performed via a method of dilution that employs a marker produced by rapid ultrafiltration using a dialysis machine. In this paper, we evaluate the reliability and safety of this machine, in vitro and in vivo. The safety of this method was evaluated by investigating hemolysis after rapid ultrafiltration. The measurement of free hemoglobin, potassium and haptoglobin in the circulating blood were performed before and after rapid ultrafiltration. No data was found to indicate hemolysis in vivo, detected by an increase in potassium or a decrease in haptoglobin. Evaluation of reliability in an experimental system was also performed on an in vitro recirculation system at a rate of 0, 10, 25, 50, and 70%. Almost all of the measured rates were within ± 10% of the theoretical rate. We performed 20 hemodialysis experiments with vascular access recirculation applying this monitor and simultaneous urea and creatinine dilution methods, which were the recommended standard measurements for vascular recirculation. In 53 measurements of standard vascular shunts with no postural change, differences of the results between the monitor and both dilution methods were only 4.0% and 3.2%, respectively. Regression analysis showed a significant and positive correlation between them (P < 0.0001). We conclude that this new method for measuring vascular access recirculation is applicable in terms of both accuracy and ease of operation.

Two-Operator Glucose Infusion Test (GIT2) for Vascular Access Recirculation Measurement during Hemodialysis

Vascular access recirculation rate (AR) monitoring is fundamental to guarantee treatment adequacy and to detect access failure early. We have evaluated the GIT2 test to measure AR unaffected by cardio-pulmonary recirculation (CPR), based on a short glucose infusion in place of the bolus and on a two-operator sampling, differently from the classical glucose infusion test (GIT). The GIT2 test is based on four steps: 1) basal (B) glucose arterial sample; 2) 10% glucose infusion for 1 min, by infusion pump at 600 ml/hr; (or 20% at 300 ml/hr); 3) simultaneous sampling at arterial (A) and venous (V) ports, after 35-40 sec from starting the infusion, taking care to avoid blood pump stop during the test; 4) AR=100*((A-B)/(V-B)). In vitro tests by dialysis on a 40 L tank containing a urea solution, with AR volumetrically simulated at 0, 5, 10, 20%, and in vivo comparison of GIT, GIT2 with stop-flow (SF) urea method. Our results have shown in vitro an almost perfect correspondence of SF urea method and a better reliability of GIT2 than GIT. The methylene-blue test has shown that a single color bolus in V reaches the A port after variable time, depending on blood flow and AR, while the continuous infusion determines a steady gradient after about 30". In vivo tests (n=24) show good correspondence between GIT2 (4.37 +/- 3.36) and SF (4.51 +/- 3.62), while GIT data (1.01 +/- 0.51) are significantly underestimated. In conclusion, our preliminary results have evidenced a good reliability of the new test, the continuous infusion causing a steady gradient in V and A that more precisely reflects the AR rate.

Vascular Access Recirculation in Hemodialysis Patients With Two Noncuffed, Single‐lumen, Jugular Catheters in the Same Jugular Vein

Having achieved excellent results with single-lumen temporary hemodialysis catheters as a long-term vascular access in our center, in this study we examined the amount of vascular access recirculation in the case of using two single-lumen catheters in a single jugular vein. In 11 adult end-stage renal disease patients on chronic hemodialysis (HD), vascular access recirculation was studied using a thermodilution technique during dialysis at our center. The vascular access in all patients consisted of two pre-curved, 15 cm-long, 8-Fr catheters inserted in the right jugular vein. Recirculation measurements were performed with the blood flow set to 250 mL/min in all patients. In five patients, additional measurements were performed with the blood flow set to 200 mL/min. During 16 HD procedures, 55 recirculation measurements were taken. The rate of recirculation was <15% in 50 measurements, between 15% and 20% in four measurements, and >20% in one measurement. The average recirculation rate during a single procedure was <15% in 10 patients, and 16.2% in one patient. We did not find any significant differences in the recirculation rates measured either at higher or lower blood flow rates. The clinical importance of vascular access recirculation was noted in a single patient. Vascular access recirculation in hemodialysis patients with two single-lumen catheters in the same jugular vein is mostly acceptable. Some measures for preventing clinically important recirculation are also suggested.

Vascular access recirculation: setting a new detection method in the context of the overall utility of detection

Vascular access recirculation: setting a new detection method in the context of the overall utility of detection

Glucose infusion test (GIT) compared with the saline dilution technology in recirculation measurements

Glucose infusion test (GIT) is a new method to measure vascular access recirculation (R) based on basal glucose increase in the arterial blood line after a 20% glucose bolus (5 ml) into the venous chamber. We compared GIT with the ultrasound dilution method (HD01, Transonic Systems Inc., USA) in a circuit reproducing in vitro the phenomenon of R. We repeated the comparison in 162 chronic haemodialysis patients (133 fistulae, 17 central venous catheters, 12 prosthetic grafts). In vitro, we determined the timing for C2 sampling: QB 200 ml/min, C2 16-20 s; QB 300 ml/min, C2 13-17 s; QB 400 ml/min, C2 9-12 s. GIT showed no false positives nor false negatives (100% specificity and sensitivity) while HD01 did not recognize three cases with R=5% (91% sensitivity) and it yielded no false positive (100% specificity). The Bland-Altman analysis showed a bias of 0.2+/-1.3% and 1.3+/-2.9% for GIT and HD01, respectively. In vivo, only 16 out of 162 patients were found positive with both methods (GIT 13.5+/-13%; HD01 16.3+/-15%; P=NS) while three patients with minimal R (GIT 3.2%) were not recognized by HD01 although a low R peak was clearly evident and repeatable on the laptop plot. The Bland-Altman analysis showed an overall bias of 0.2+/-1.7% to the limits of agreement=-3.1 and 3.6% (n=162) and no correlation between the difference and the mean of positive tests. The pooled coefficient of variation of positive cases was 13.3 and 18.1% for GIT and HD01, respectively. Our in vitro study showed a good performance of GIT and its better sensitivity compared to HD01. These results were confirmed in vivo with only 3/162 discordant results due to a low R under the HD01 limit of detection (R=5%). In conclusion, the GIT proved to be a very accurate screening test for R, with a very low threshold of detection. In addition, it is simple, user-friendly and inexpensive.

HCO3 increment in arterial line can reveal significant vascular access recirculation in high‐flux hemodialysis: A preliminary report

We report a new and simple way that can reveal the presence of vascular access recirculation (VAR) in patients undergoing hemodialysis (HD). Acid-base and blood gas parameters (pH, pO(2), pCO(2), and HCO(3)) were measured in blood samples drawn from an arterial fistula needle before the initiation of HD and from arterial and venous lines simultaneously 5 min later, in 31 patients (group A). Vascular access recirculation was measured using the glucose infusion test (GIT) immediately after the withdrawal of the 5-min samples. The same study was repeated in 30 patients in whom HD lines were reversed (group B). A comparison with baseline (predialysis) values of an analysis of the arterial line in group A at 5 min revealed that pCO(2) increased by 1.14+/-2.5 mmHg and HCO(3) by 0.6+/-0.6 mM/L (p<0.02 and p<0.00001, respectively). The corresponding pO(2) and pH values did not show significant differences. Glucose infusion test at 5 min (GITa) was -0.058+/-0.03%. A comparison with baseline (predialysis) values of an analysis of the arterial line in group B at 5 min revealed that pCO(2) increased by 7.7+/-3.5 mmHg and HCO(3) by 2.9+/-1.0 mM/L (p<0.000001 in each case). The pH level was significantly lower in comparison with baseline values (p<0.00001), while pO(2) did not show a significant difference. Glucose infusion test at 5 min (GITb) was 12.0+/-6.1% (p<0.000001 in comparison with GITa values). Clinically significant VAR was defined as HCO(3) increment >1.8 mM/L, based on the receiver-operating characteristics curve, which showed a threshold value of HCO(3) increment >1.8 mmol/L as a predictor of GIT recirculation. Five minutes after the initiation of high-flux HD with a 0 ultrafiltration rate, there is a small increment in arterial HCO(3) values relative to predialysis values. Clinically significant VAR is present when this increment is higher than 1.8 mM/L.

PRE-DIALYSIS APPLICATION OF THE GLUCOSE INFUSION TEST FOR RECIRCULATION DETECTION

Vascular access recirculation (R) is a well-known cause of decreased dialysis dose. In this paper a new easy protocol for R detection in pre-dialysis derived from the classic Glucose Infusion Test (GIT) is introduced. The pre-dialysis GIT (GIT-pre) is based on the glucose (5%, 10 ml) bolus injection directly into the venous needle and on a simultaneous withdrawal from the arterial needle. If the glucose value increases during the glucose bolus, R is present. This new protocol was validated on 29 chronic haemodialysis patients (20 AVFs, 7 CVCs, 2 PTFE grafts), comparing the glucose increase with the classic GIT during dialysis. Only one CVC had R with the blood lines in the normal position (deltaglu = 465 mg/dl with GIT-pre and a deltaglu = 186 mg/dl, R = 9.3% with classic GIT) while in the reverse blood line position, all CVCs showed a significant glucose increase (mean GIT-pre deltaglu = 195 mg/dl; mean GIT deltaglu = 140 mg/dl corresponding to a R = 8%). There were 5 AVFs with true R (correct blood lines position) clearly identified by both methods (mean values deltaglu = 316 mg/dl with GIT-pre and a deltaglu = 390 mg/dl, R = 19.5% with classic GIT). Preliminary results show good reliability of the new protocol in identifying VA R caused either by failing VA with stenosis or by reverse blood lines position. The GIT-pre is a simpler application of the classic GIT useful for testing new VA, new needle positions or CVC performance before starting dialysis. A simpler R test could increase the frequency of the measurements and consequently the power of R in early detection of VA problems.

Effect of cool temperature dialysate on the quality and patients' perception of haemodialysis.

The effects of cool dialysate on the urea reduction ratio (URR) in high efficiency haemodialysis have not been completely studied. After reviewing the literature, it appeared that patients' perceptions of cool dialysis have not been studied. Since patients' perception have an impact on patient satisfaction, this motivated the authors to research this area of practice. This study was designed to determine whether a high URR and haemodynamic stability could be achieved by using cool dialysate in two groups of patients. The first group of five patients were known to have hypotension episodes during dialysis, and the second group of five patients were documented as having stable blood pressure (BP) during and after dialysis, after excluding vascular access recirculation and any other problems. Each patient was dialysed for three sessions using cool dialysate (35 degrees C) followed by another three sessions using a standard dialysate temperature (36.5 degrees C). All other dialysis session parameters were maintained. The results show that the dialysate cooling resulted in an increased ultrafiltration in the low BP group (P = 0.05). Cool dialysis had neither an adverse nor a beneficial effect on urea removal in the two groups (P = NS). The mean arterial pressure post- and intra-dialysis was significantly higher in dialysis with cool dialysate in the low BP group (P < 0.01 and P < 0.007, respectively). The mean arterial pressure in the stable BP group remained unchanged when cool dialysate was used (P = NS). The intra-dialytic pulse rates in the low and stable BP groups were similar. A total of seven episodes of symptomatic hypotension were observed in the low BP group, but none in the stable BP group (P < 0.0001). Patients' perceptions about cool dialysate were measured by a questionnaire which showed that 80% of them felt more energetic after dialysis and requested to be always dialysed with cool dialysate. Cool dialysate improves tolerance for dialysis in hypotensive patients and helps increase ultrafiltration while maintaining haemodynamic stability during and after dialysis. Patients' perceptions were positive, as most of the selected sample felt more energetic and generally well during and after dialysis, and this had a positive impact on their activities of daily living.

A screening test for vascular access recirculation?

A screening test for vascular access recirculation?

Optimizing dialysis dose by increasing blood flow rate in patients with reduced vascular-access flow rate

Dialysis efficacy indexed by Kt/V can generally be augmented by increasing the dialyzer blood flow rate. However, increasing the dialyzer blood flow rate may lead to vascular-access recirculation (AR) in patients with a compromised vascular-access flow rate. This can have an attenuating effect on dialysis efficacy. The aim of the present study is to investigate the effect of dialyzer blood flow rates of 200, 300, and 400 mL/min on AR and Kt/V in 8 patients with low (<600 mL/min) and 13 patients with normal (>600 mL/min) vascular-access flow rates. AR and vascular-access flow rate were determined using an ultrasound saline dilution technique, and session-delivered Kt/V was computed using an on-line dialysate urea monitor. AR was minor and only observed in 4 patients in the low vascular-access flow rate group (0.9% ± 0.6%) at dialyzer blood flow rates of 200 mL/min (1 patient), 300 mL/min (2 patients), and 400 mL/min (3 patients) and 4 patients in the normal vascular-access flow rate group (1.2% ± 1.1%) at dialyzer blood flow rates of 200 mL/min (3 patients) and 300 mL/min (1 patient). Kt/V increased with increasing dialyzer blood flow rates in both groups, and in individual cases, there was no decrease in Kt/V at greater dialyzer blood flow rates in either group. Also in those patients with minor AR, Kt/V increased at greater dialyzer blood flow rates, except in 1 patient in the low-flow group, in whom Kt/V remained unchanged at a change in dialyzer blood flow rate from 300 to 400 mL/min, whereas AR increased. From this study, it is concluded that even in patients with low access flow, increasing dialyzer blood flow rate in general leads to an increase in delivered Kt/V regardless of vascular access flow rate. © 2001 by the National Kidney Foundation, Inc.

GIT (Glucose Infusion Test): Polycentric Evaluation of a New Test for Vascular Access Recirculation

Introduction. Vascular access recirculation (AR), which is often unacknowledged, remains an important cause of inadequate dialytic dose. The glucose infusion test (GIT) is a new method for detecting and quantifying AR. This paper reports on a polycentric evaluation of the new test and a comparison with the classical Urea-test (UT). Methods. GIT protocol comprises withdrawal from the arterial port (sample A), injection into the venous drip chamber of 1 g glucose in 4 seconds, withdrawal from the arterial port (sample B) continuously from 13 to 17 seconds. Glucose is determined on A and B by a reflectance photometer. If B = A then there is no recirculation. If B exceeds A by at least 20 mg/dl there is recirculation. AR quantification: AR% = (B-A) / 20. GIT was performed on 623 patients from eleven dialysis centers to screen the patients for AR. Subsequently, GIT and Urea-test (UT) were compared in 189 paired tests. The reproducibility of GIT and UT was studied in 28 paired tests performed in sequence. Results. The screening test by GIT was positive in 68 cases (11 %). The majority of positivities was found in central venous catheters (CVC, 27/50 cases, 54 %), whereas only 7 % of artero-venous fistulas (AVF) were positive. In the CVC group, Tesio catheters were more frequently positive compared to Dual Lumen Catheters (64 % vs. 29 %). The comparison GIT - UT showed that results matched in 162 tests (79 negative and 83 positive both by GIT and UT), showing that on the grounds of UT, GIT has high sensitivity and specificity. In 27 tests GIT was positive, but UT negative. This disagreement is due to the different minimal limit of detection, 1 % for GIT and 5% for UT. The reproducibility was greater with GIT than with UT with a lower D% (respectively -0.6 +/- 2.5 and -0.4 +/- 6.1 %, p<0.001) and a lower coefficient of variation (17 vs 33 %). Conclusions. The screening of 623 patients by GIT confirmed that AR in AVF is normally absent, whereas an un-expectedly high frequency of moderate AR in CVC was found. The GIT-UT comparison showed that the new test is simple and immediate, and gives results with higher accuracy, sensitivity and reproducibility than UT.

Glucose infusion test: A new screening test for vascular access recirculation

Vascular access recirculation is an important cause of diminished dialysis efficiency. We propose a new screening test based on glucose infusion as a tracer for recirculation. The glucose infusion test (GIT) protocol comprises a basal blood sample (A) from the arterial port, a 5 mL bolus of 20% glucose into the venous chamber (time 0), followed by a second sample (B) in four seconds (from 13 to 17 s with QB 300 mL/min) from the same port. The blood glucose level is determined at the bedside on A and B with a reflectance photometer (CV 1.8%). Interpretation of the test is straightforward: If B = A, there is no recirculation, whereas if B > A, recirculation can be calculated from the regression equation: 0.046 x (B - A) + 0.07, obtained from in vitro tests reproducing artificial recirculation at 0, 5, and 10%. To validate this new method in vivo, we compared GIT and the urea test on 39 hemodialysis patients, obtaining a good correlation (r = 0.93). The two tests were considered positive (recirculation present) when the lower 95% confidence intervals were more than zero. Our patients were divided into two groups: those with (22 out of 39, mean recirculation 11.8%) or without recirculation (17 out of 39, mean 0.06%). The urea test did not recognize 7 out of 22 patients because they had a small recirculation below the urea test limit of detection. GIT was more sensitive (detection limit 0.3%), simpler, and immediate in showing the results than the urea test. It is an accurate and low-cost technique for screening and follow-up of vascular access in a dialysis unit.

Measurement of Vascular Access Recirculation Unaffected by Cardiopulmonary Recirculation: Evaluation of an Ultrasound Method

Recently developed devices provide detection of access recirculation (AR) and cardiopulmonary recirculation (CPR) by optical, thermal, conducimetrical, and ultrasound methods (USM). We evaluated the last one both in vitro reproducing AR by a bypass pump and in vivo. In vitro, the USM sensitivity was about 5%. In vivo, the USM was compared with the traditional urea method (UM) in 69 patients. 8.7% of the cases resulted positive by both UM and USM. One case was USM positive and UM negative. The UM sensitivity threshold was 6–10%. The accuracy (in vitro) and the repeatability (in vivo) of the USM were satisfactory. USM clearly distinguished AR from CPR. In conclusion, AR determination by USM, avoiding misleading interferences with CPR, is a rapid, easy, and noninvasive method to routinely exclude a potential cause of reduced dialytic efficiency.

Impaired delivery of hemodialysis prescriptions: an analysis of causes and an approach to evaluation.

Serial kinetic modeling is commonly used in hemodialysis to assess the adequacy of dialysis. A variety of problems lead to declining Kt/V in previously stable patients. These include noncompliance, vascular access recirculation, and dialyzer dysfunction. The purpose of this study was to find the relative frequencies of these problems in a group of patients undergoing routine hemodialysis. Simultaneous urea kinetic modeling and access recirculation were tested during 3 consecutive months. The baseline Kt/V was defined as the average of each patient's Kt/V values obtained during the previous 4 mo. A clinically important fall in Kt/V was defined as a decline of > or =0.2 if the baseline Kt/V was > or =1.2, or a decline of > or =0.1 if the baseline Kt/V was <1.2. Ninety-three of 375 (25%) sessions met the criteria for a significant decline in urea kinetic modeling. The baseline Kt/V in this group was 1.33 +/- 0.20 (mean +/- SEM) and declined to 1.02 +/- 0.18 in the abnormal month (P < 0.05). In 42% of instances with a decline of Kt/V, reduced blood processing due to a lower blood flow or shorter time than prescribed was responsible. Recirculation of >12% was found in 25% of sessions with a decrease in Kt/V. These patients most often had access dysfunction or reversed needles. The remaining one-third of patients with decreases in Kt/V had no problem identified, and subsequent monthly kinetic modeling results returned to baseline. These results suggest that analysis of falling urea kinetic modeling results should include a careful review of the dialysis record for reductions in prescribed time or blood flow rates followed by vascular access testing. If these evaluations are unrevealing, urea kinetic modeling results usually return to baseline in the next month.

The relationship of recirculation to access blood flow

The relationship between vascular access recirculation and access blood flow has been obscured by measurement techniques that overestimate recirculation. We measured brachial artery blood flow (a surrogate for access blood flow) using Doppler ultrasound and access recirculation using a two-needle slow/stop flow method and the standard peripheral vein method in 77 chronic hemodialysis patients. These patients had 25 native arteriovenous fistulae and 52 polytetrafluoroethylene grafts. Access recirculation by the slow/stop flow method was uniformly absent in these patients unless their access blood flow rate was less than the dialyzer blood flow rate. The peripheral vein method averaged 10.7% in these patients; many values were considerably higher. We conclude that access recirculation is a function of access blood flow and does not occur (in the absence of or reversed needle cannulation) unless access blood flow is markedly impaired.

Effect of access recirculation on the modeled urea distribution volume

The effect of vascular access recirculation (AR) on the modeled urea distribution volume (V) is not straightforward. When blood is sampled properly so that it is not admixed with recirculated blood, AR will cause an unexplained increase in V in cases in which AR is present throughout the dialysis session (when AR is limited to the terminal portion of a dialysis session it will cause little or no change in the modeled V). On the other hand, when blood is sampled from the arterial line after simply stopping the pump, postdialysis blood urea nitrogen (BUN) represents arterial line BUN and does not always reflect the BUN in the patient's blood. Under these conditions, when AR is present throughout the dialysis session the modeled V usually shows an unexplained decrease, but V may be unchanged, depending on the urea reduction ratio (URR). We performed a mathematical analysis to predict when V would be decreased and when it would be unchanged when the postdialysis BUN is contaminated with admixed blood. The analysis revealed that when AR is present uniformly throughout the dialysis session, the modeled V should be underestimated when the URR is ≤0.70. When the URR is greater than 0.70, even severe degrees of AR may not be reflected by a change in V. When AR is limited to the terminal part of the dialysis session or when AR increases during the dialysis session, and when V is based on admixed postdialysis blood, underestimation of V will occur in almost all circumstances. In a cross-sectional comparison of modeled to anthropometric volume ratios in eight patients with severe AR and in 11 controls, and in sequential modeling studies in a single patient in whom severe AR developed gradually over time, the volume ratio was low in most, but not all instances when modeled V was based on an admixed postdialysis BUN sample.