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Optimizing dialysis dose by increasing blood flow rate in patients with reduced vascular-access flow rate
Optimizing Dialysis Dose by Increasing Blood Flow Rate in Patients With Reduced Vascular-Access Flow Rate Daxenos R.M. Hassell, MSc, Frank M. van der Sande, MD, PhD, Jeroen P. Kooman, MD, PhD, Jan P. Tordoir, MD, PhD, and Karel M.L. Leunissen, MD, PhD ● 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% ⴞ 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% ⴞ 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. INDEX WORDS: Dialysis dose; blood flow; vascular access.
A
DEQUATE HEMODIALYSIS is of main importance to reduce morbidity risk in intermittent maintenance hemodialysis patients.1,2 The parameter Kt/V is currently the most frequently used marker for dialysis adequacy.3,4 Varying selected parameters can modify delivered Kt/V, including dialysis duration, dialysis frequency, dialysate flow rate,5 and dialyzer blood flow rate. It is implied that vascular-access recirculation (AR) can attenuate delivered Kt/V.6-9 AR is the process by which returned dialyzed blood shortcircuits through the vascular-access conduit and reenters the dialyzer system through the arterial blood catheter, thus effectively bypassing the corporeal circulatory system. In the past, it was believed that AR was generally present and sig-
From the Departments of Nephrology and Surgery, University Hospital Maastricht, The Netherlands. Received March 2, 2001; accepted in revised form May 25, 2001. Address reprint requests to Frank M. van der Sande, MD, PhD, Department of Internal Medicine and Nephrology, University Hospital Maastricht, P Debeyelaan 25, PO Box 5800, 6202 AZ Maastricht, The Netherlands. E-mail: [email protected] © 2001 by the National Kidney Foundation, Inc. 0272-6386/01/3805-0004$35.00/0 doi:10.1053/ajkd.2001.28580 948
nificantly increased at greater dialyzer blood flow rates, especially in patients with suboptimal vascular-access sites.6,10 Recirculation values of 10% to 15% were experimentally measured at a dialyzer blood flow rate of 200 mL/min and increased to 20% to 30% at a dialyzer blood flow rate of 400 to 500 mL/min.6,10,11 Naturally, questions were raised concerning the efficacy of dialysis with a high dialyzer blood flow rate. However, with the advent of newer AR determination techniques, such as alternative methods of sample collection12 and the ultrasonic saline dilution method,13,14 it was shown that AR is less than generally assumed.12,15-18 However, from a theoretical point of view, it cannot be excluded that when prescribed blood flow rate approximates access flow rate, AR will occur that may impair dialysis efficacy. However, to date, the relation between blood flow and dialysis efficacy in patients with suboptimal graft function has not been studied. Nevertheless, this question is of clinical importance, partly because patients with low vascular-access flow rates were found to be at risk for receiving inadequate dialysis doses.19 In the current literature, there is insufficient analysis of the effect of dialyzer blood flow rates on AR and Kt/V in patients with suboptimal vascular access. The aim of the present study is
American Journal of Kidney Diseases, Vol 38, No 5 (November), 2001: pp 948-955
DIALYSIS DOSE AND BLOOD FLOW RATE IN SUBOPTIMAL VASCULAR ACCESS
to investigate the effect of different dialyzer blood flow rates on AR and Kt/V in patients with low vascular-access flow rates and those with normal vascular-access flow rates. PATIENTS AND METHODS
Patients All patients with impaired vascular-access flow rates (discussed next) in our dialysis center were included on the study, except for 1 patient with poor general health and 1 patient who refused to participate. Patients with normal vascular-access flow were randomly selected from our dialysis population. After obtaining informed consent for participation on the study, 21 patients (9 women) with a mean age of 68.1 ⫾ 11.4 (SD) years and mean time on hemodialysis therapy of 24.9 ⫾ 20.5 months were included. Renal disease was caused by hypertensive nephropathy (nine patients), reflux nephropathy (four patients), chronic glomerulonephritis (three patients), immunologic disease (three patients), diabetic nephropathy (one patient), and polycystic kidney disease (one patient). Residual renal function was calculated by the following formula: (UUN/SUN) ⫻ urine flow rate 共milliliters per minute兲 where UUN is urine urea nitrogen concentration and SUN is serum urea nitrogen concentration. Urine was collected in the first interdialytic period of the week. Fifteen patients had no discernable renal function, whereas the remaining six patients had an average remaining renal function of 2.72 ⫾ 3.12 mL/min (range, 0.6 to 8.4 mL/min). At the time of the study, 12 patients (5 women) had autogenous arteriovenous fistulae and 9 patients (4 women) had polytetrafluoroethylene arteriovenous grafts. Access sites were 17.8 ⫾ 14.7 months old, with the latest vascular diagnostic imaging performed 5.9 ⫾ 9.7 months previously. According to the revised National Kidney FoundationDialysis Outcomes Quality Initiative (DOQI), low vascularaccess flow rate is defined as a rate less than 600 mL/min, whereas normal vascular-access flow rate is defined as a rate greater than 600 mL/min for both fistulae and grafts.20,21 Eight patients (six fistulae; 4 women) had low vascularaccess flow rates, and 13 patients (six fistulae, 5 women) had normal vascular-access flow rates.
Study Protocol With each patient serving as his or her own control, measurements were performed during three standardized dialysis sessions differing in dialyzer blood flow rate: 200, 300, and 400 mL/min. Prescribed dialyzer blood flow rate remained constant during an entire dialysis session. Dialysate flow rate was 500 mL/min for all sessions. All patients underwent dialysis for the entire prescribed time, without exceeding the total time. Dialysis time was identical for each of the three sessions. Patients were treated for an average session time of 3.7 ⫾ 0.4 hours to achieve a double-pool Kt/V (dpKt/V) greater than 1.1. Treatment order was randomized. Patient dry weight was estimated by echography of the
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inferior caval vein.22,23 Ultrafiltration rate, which remained constant for a dialysis session, was prescribed using estimated dry weight and interdialytic and intradialytic weight gains. Dialysis was performed using a Gambro AK-100 or Gambro AK-200 module (Gambro, Lund, Sweden) and a polyamide-S membrane (Polyflux 8L; Gambro) or cellulose triacetate membrane (Sureflux 130Gga; Nissho Nipro, Zaventem, Belgium). The two dialyzers have similar clearance rates, and the same type was used for each of the three sessions. Fifteen-gauge fistula needles (F15AS; 1.8 ⫻ 25 mm; Gambro) were used for all patients. Similar cannulating procedures, interneedle distances, and needle directions were used for each of the three dialysis sessions. The internal diameter of the pump segment was 8 mm. Dialysate composition was as follows: sodium, 140 mmol/L; potassium, 2 mmol/L; acetate, 3 mmol/L; magnesium, 0.5 mmol/L; bicarbonate (individualized), 30 to 34 mmol/L; chloride, 108 mmol/L; and calcium, 1.5 mmol/L. Dialysate temperature, which remained constant for three sessions, was 36.2°C ⫾ 0.6°C.
Methods Dialysis dose was determined by using either a Biostat 1000 (Baxter Healthcare Corp, Deerfield, IL)24-26 or a Gambro DQM200 urea monitor.27 Both urea monitors are used together for clinical purposes on our dialysis faculty. The same monitor was used for the three dialysis sessions. Both monitors determine levels of intradialytic urea removed in the spent dialysate. These levels correlated predictably with blood urea concentrations. For the calculation of Kt/V, the DQM200 uses the second-generation Daugirdas formula.28 An equilibration correction formula29 is subsequently applied, converting the original single-pool Kt/V to a dpKt/V to account for urea rebound, thus shifting attention from dialyzer clearance to patient clearance. The Biostat 1000 also determines a dpKt/V based on two-exponential curve fit calculation.24 Actual dialyzer blood flow rate, vascular-access flow rate, and AR were calculated by means of a saline dilution technique using an HD01 Hemodialysis Monitor System (Transonic Systems Inc, Ithaca, NY).13,14,30-33 This system computes AR and vascular-access flow rate by monitoring ultrasound velocity changes after blood dilution with a saline bolus injected into the extracorporeal venous circulation. Actual dialyzer blood flow rate is measured directly. AR was measured in duplicate and vascular-access flow rates were measured in triplicate, both at least twice per session and an average of 3.0 ⫾ 0.8 times per session. With each measurement, actual dialyzer blood flow rate was recorded. The total time during which blood catheters were in reversed configuration for vascular-access flow rate measurement was equivalent for each of the three dialysis sessions. No flow or recirculation measurements were made in the first or last half hour of dialysis.
Statistical Analysis The significance of differences between samples was determined by a paired two-sample t-test for paired samples and an unpaired t-test for unrelated samples (Microsoft Excel 1998 [Microsoft Corp, Redmond, WA]; Macintosh
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[Apple, Cupertino, CA]). All P are two-sided and considered significant when less than 0.05. Data are given as mean ⫾ SD and, when necessary, with a range.
RESULTS
Predialysis weights for the three treatment sessions at dialyzer blood flow rates of 200, 300, and 400 mL/min for the low vascular-access flow rate group were 63.6 ⫾ 9.8, 63.7 ⫾ 9.6, and 63.8 ⫾ 10.0 kg, respectively (P ⫽ not significant [NS]). For the normal vascular-access flow rate group, respective weights were 68.7 ⫾ 10.1, 68.7 ⫾ 10.3, and 69.0 ⫾ 10.1 kg (P ⫽ NS). Interdialytic weight gains in the low vascularaccess flow rate group for dialyzer blood flow rates of 200, 300, and 400 mL/min session were 2.8 ⫾ 0.5, 2.4 ⫾ 0.6, and 2.5 ⫾ 0.5 kg (P ⫽ NS), whereas values for the normal vascular-access flow rate group were 2.2 ⫾ 0.6, 2.1 ⫾ 0.7, and 2.3 ⫾ 0.5 kg, respectively (P ⫽ NS). Access Blood Flow Rate Eight patients (six fistulae, 4 women) had low vascular-access flow rates, with a mean of 464 ⫾ 91 mL/min (range, 297 to 548 mL/min). The normal vascular-access flow rate group consisted of 13 patients (six fistulae, 5 women) with flow rates exceeding 600 mL/min (mean, 852 ⫾ 124 mL/min; range, 662 to 1,042 mL/min). Intersession correlation coefficients for vascular-access flow rates between sessions with dialyzer blood flow rates of 200 and 300 mL/min were r ⫽ 0.914 (P ⬍ 0. 05); 200 to 400 mL/min, r ⫽ 0.906 (P ⬍ 0.05); and 300 to 400 mL/min, r ⫽ 0.875 (P ⬍ 0.05). No patient would have been classified differently (ie, normal or low vascularaccess flow rate) according to variations in access flow rates between sessions. Circuit Blood Flow Rate In all cases, there was a discrepancy between prescribed and actual dialyzer blood flow rates achieved. In the low vascular-access flow rate group, actual dialyzer blood flow rates for prescribed rates of 200, 300, and 400 mL/min were 181 ⫾ 6, 266 ⫾ 7, and 344 ⫾ 13 mL/min, respectively. Rates in the normal vascular-access flow rate group were 184 ⫾ 6, 271 ⫾ 6, and 346 ⫾ 9 mL/min, respectively. Discrepancies between prescribed and actual dialyzer blood flow rate were significant in all cases (P ⬍ 0.05).
Circuit Pressures Mean venous circuit pressures in the low vascular-access flow rate group for dialyzer blood flow rates of 200, 300, and 400 mL/min were 73 ⫾ 28, 117 ⫾ 31, and 162 ⫾ 31 mm Hg, whereas arterial circuit pressures were –49 ⫾ 4, –108 ⫾ 45, and –146 ⫾ 27 mm Hg, respectively. Mean venous circuit pressures for dialyzer blood flow rates of 200, 300, and 400 mL/min in the normal vascular-access flow rate group were 76 ⫾ 17, 122 ⫾ 23, and 163 ⫾ 21 mm Hg, whereas arterial circuit pressures were –41 ⫾ 28, –95 ⫾ 29, and –140 ⫾ 24 mm Hg, respectively. In no case did the arterial or venous pressure exceed 220 mm Hg. Effect of Dialyzer Blood Flow Rate on AR There were nine sessions with AR (eight patients). Of those nine sessions, five instances of AR were observed in patients with low vascularaccess flow rates (0.9% ⫾ 0.6%; range, 0.3% to 1.3%), and four instances were observed in patients with normal vascular-access flow rates (1.2% ⫾ 1.1%; range, 0.5% to 2.8%). In the low vascular-access flow rate group, two patients had AR only at a dialyzer blood flow rate of 400 mL/min, one patient had AR only at 200 mL/min, and one patient had AR at 300 and 400 mL/min. In the normal vascular-access flow rate group, one patient had AR only at 300 mL/min, whereas three patients had AR only at 200 mL/min. Effect of Dialyzer Blood Flow Rate on Kt/V In the low vascular-access flow rate group (Table 1), dpKt/Vs at prescribed dialyzer blood flow rates of 200, 300, and 400 mL/min were 0.95 ⫾ 0.16, 1.17 ⫾ 0.16, and 1.36 ⫾ 0.19, respectively. In all patients, Kt/V increased when increasing the dialyzer blood flow rate from 200 to 300 mL/min. When increasing it from 300 to 400 mL/min, Kt/V increased in seven patients and remained the same in one patient. Vascularaccess flow rate in that patient was 402 mL/min. In the normal vascular-access flow rate group (Table 1), Kt/Vs at dialyzer blood flow rates of 200, 300, and 400 mL/min were 0.90 ⫾ 0.15, 1.07 ⫾ 0.16, and 1.17 ⫾ 0.21, respectively. Again, in all patients, Kt/V increased with a dialyzer blood flow rate increase from 200 to 300 mL/min. From 300 to 400 mL/min, Kt/V in-
DIALYSIS DOSE AND BLOOD FLOW RATE IN SUBOPTIMAL VASCULAR ACCESS Table 1.
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Kt/V in All Patients Vascular Access Flow Rate (mL/min)
Vascular access flow rate (mL/min) Kt/V(200) Kt/V(300) Kt/V(400) Kt/V(300) ⫺ Kt/V(200) Kt/V(400) ⫺ Kt/V(300) Kt/V(400) ⫺ Kt/V(200)
Low
Normal
464 ⫾ 91 0.95 ⫾ 0.16 1.17 ⫾ 0.16 1.36 ⫾ 0.19 0.22 ⫾ 0.12* 0.19 ⫾ 0.12* 0.41 ⫾ 0.13*
852 ⫾ 124 0.90 ⫾ 0.15 1.07 ⫾ 0.16 1.17 ⫾ 0.21 0.17 ⫾ 0.10* 0.10 ⫾ 0.09* 0.27 ⫾ 0.11*
NOTE. Data expressed as mean ⫾ SD. Kt/V is the double-pool dialysis dose during blood flow rates of 200, 300, and 400 mL/min. Kt/V(300) ⫺ Kt/V(200), Kt/V(400) ⫺ Kt/V(300), and Kt/V(400) ⫺ Kt/V(200) are differences for patients in the low and normal vascular-access flow rate groups. Differences are given in dimensionless Kt/V units. *P ⬍ 0.05.
Fig 2. Kt/V in patients with normal vascular-access flow rates. dpKt/V is presented as a factor of prescribed dialyzer blood flow rate (Qb). Kt/V(200), Kt/ V(300), and Kt/V(400) are dialysis doses at dialyzer blood flow rates of 200, 300, and 400 mL/min. (●), Sessions with measured recirculation; (E), sessions with no measured recirculation.
creased in 10 patients and remained equal in 3 patients. Dialysis dose increases were significant between the three dialyzer blood flow rates for the low (Fig 1; Table 1) and normal vascular-access flow rate groups (Fig 2; Table 1; all P ⬍ 0.05). Mean arithmetical differences in Kt/V (Kt/ V300 – Kt/V200, Kt/V400 – Kt/V300, and Kt/V400 – Kt/V200) for the two vascular-access flow rate
groups are listed in Table 1. Kt/V differences did not vary significantly among the subgroups (P ⫽ NS); ie, with an increase in dialyzer blood flow rate, one group did not show a significantly greater augmentation in Kt/V than the other. The increase in Kt/V is seen in both arteriovenous fistulae (Fig 3; Table 2) and arteriovenous grafts (Fig 4; Table 2). When AR was found, there was
Fig 1. Kt/V in patients with low vascular-access flow rates. dpKt/V is presented as a factor of prescribed dialyzer blood flow rate (Qb). Kt/V(200), Kt/ V(300), and Kt/V(400) are dialysis doses at dialyzer blood flow rates of 200, 300, and 400 mL/min. (●), Sessions with measured recirculation; (E), sessions with no measured recirculation.
Fig 3. Kt/V in patients with arteriovenous fistulae. dpKt/V is presented as a factor of prescribed dialyzer blood flow rate (Qb). Kt/V(200), Kt/V(300), and Kt/ V(400) are dialysis doses at dialyzer blood flow rates of 200, 300, and 400 mL/min. (●), Sessions with measured recirculation; (E), sessions with no measured recirculation.
952 Table 2.
HASSELL ET AL Kt/V in Patients With Arteriovenous Fistulae and Grafts
Kt/V(300) ⫺ Kt/V(200) Kt/V(400) ⫺ Kt/V(300) Kt/V(400) ⫺ Kt/V(200)
Arteriovenous Fistulae
Arteriovenous Grafts
0.18 ⫾ 0.12* 0.17 ⫾ 0.11* 0.35 ⫾ 0.14*
0.20 ⫾ 0.08* 0.09 ⫾ 0.10* 0.30 ⫾ 0.14*
NOTE. Data expressed as mean ⫾ SD. Kt/V(300) ⫺ Kt/V(200), Kt/V(400) ⫺ Kt/V(300), and Kt/V(400) ⫺ Kt/ V(200) are differences for patients with arteriovenous fistulae and arteriovenous grafts. Differences are given in dimensionless Kt/V units. *P ⬍ 0.05.
no correlation between the magnitude of Kt/V change and amount of recirculation. Between the groups of patients with and without recirculation, there was no difference in the change in Kt/V as a result of dialyzer blood flow rate increases (P ⫽ NS). Comparing dialyzer blood flow rates of 200 and 400 mL/min, patients with AR had a Kt/V increase of 0.33 ⫾ 0.11, whereas patients without AR had an increase of 0.32 ⫾ 0.15. Effect of Dialyzer Blood Flow Rate on Dialysis Adequacy In the low vascular-access flow rate group, six of the eight patients received a dpKt/V less than 1.05 at a prescribed dialyzer blood flow rate of 200 mL/min (0.88 ⫾ 0.12; range, 0.70 to 1.03). At 300 mL/min, there were two patients (both dpKt/V, 1.00). At 400 mL/min, no patient had a dpKt/V less than 1.05. In the normal vascular-access flow rate group at a prescribed vascular-access flow rate of 200 mL/min, 12 of the 13 patients received dialysis doses less than 1.05 (0.87 ⫾ 0.09; range, 0.69 to 1.01). At 300 mL/min, 6 patients had doses less than 1.05 (0.94 ⫾ 0.1; range, 0.81 to 1.01), whereas at 400 mL/min, 4 patients had doses less than 1.05 (0.96 ⫾ 0.07; range, 0.86 to 1.00).
delivered dialysis dose, indicated by Kt/V. Moreover, in both groups, in no instance was there a decrease in Kt/V with increasing dialyzer blood flow rates. Of the 63 dialysis sessions, there were nine cases of AR in eight patients; five cases in the low vascular-access flow rate group and four cases in the normal vascular-access flow rate group. Measured AR was minimal, with values not exceeding 3%. This conforms to recent observations that AR is generally absent in wellfunctioning access sites.12,15 This study also shows that this concept holds true for access sites with low vascular-access flow rates in which vascularaccess flow and dialyzer blood flow rates approximate each other. Data show that high dialyzer blood flow rates failed to predictably increase AR in patients with AR; ie, when AR was measured, an increase in dialyzer blood flow rate did not increase recirculation in either of the two groups. Moreover, AR was found mainly in sites where the vascular-access flow rate exceeded dialyzer blood flow rate. The explanation for these findings is not completely clear. Because inadequate placement of fistula needles might result in some recirculation, special attention was given to cannulating procedures, interneedle distances, and needle directions. Possibly, small
DISCUSSION
In the present study, the effect of increasing dialyzer blood flow rates on AR and dialysis dose was assessed in patients with low and normal vascular-access flow rates. Results of the study show that an increase in prescribed dialyzer blood flow rate effected a significant increase in
Fig 4. Kt/V in patients with arteriovenous grafts. dpKt/V is presented as a factor of prescribed dialyzer blood flow rate (Qb). Kt/V(200), Kt/V(300), and Kt/ V(400) are dialysis doses at dialyzer blood flow rates of 200, 300, and 400 mL/min. (●), Sessions with measured recirculation; (E), sessions with no measured recirculation.
DIALYSIS DOSE AND BLOOD FLOW RATE IN SUBOPTIMAL VASCULAR ACCESS
AR signals have to be interpreted as reflected background noise, especially given the low intensity of recirculation and apparent random distribution over the two subgroups. Data show that when additional distinct parameters were kept constant, an increase in prescribed dialyzer blood flow rate effected a significant increase in delivered dialysis dose in both the low and normal vascular-access flow rate groups. Moreover, in no instance did Kt/V decrease with increasing dialyzer blood flow rates. Increasing dialyzer blood flow rate from 200 to 300 mL/min resulted in a Kt/V increase in all patients. When increasing dialyzer blood flow rate from 300 to 400 mL/min, Kt/V remained unchanged in one patient in the low vascularaccess flow rate group and three patients in the normal vascular-access flow rate group. Therefore, if using a dialyzer blood flow rate increase from 300 to 400 mL/min for the purpose of Kt/V augmentation, Kt/V measurements for efficacy purposes would seem judicious. Dialysis adequacy in patients with negligible renal function is defined by the DOQI as a single-pool Kt/V of at least 1.2 per session thrice weekly,34,35 which equilibrates to a session dpKt/V of 1.0 at 4 hours.29 At a prescribed dialyzer blood flow rate of 200 mL/min, 18 of the 21 patients (6 patients, low vascular-access flow rates) received an inadequate session dialysis dose. Solely increasing dialyzer blood flow rate led to an improvement in delivered Kt/V. At 300 mL/min, the number of patients receiving inadequate dialysis was reduced to 8 patients (2 patients, low vascular-access flow rate group), and at 400 mL/min, the number was further reduced to 4 patients (0 patients, low vascularaccess flow rate group). This agrees with observations that a dialyzer blood flow rate that is too low can lead to impaired dialysis efficacy.19 Prescribed dialyzer blood flow rates were invariably greater than dialyzer blood flow rates obtained. The difference was more pronounced at greater dialyzer blood flow rates. This phenomenon previously was described36 and is at least partly caused by negative-pressure flattening of the tubing (congruent with a volume decrease) before entering the roller pump and subsequent reexpansion once clear of the pump. Moreover, the internal diameter of the pump segment and diameter of the needles also may contribute to
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this phenomenon.37,38 The effect is a reduction in actual dialyzer blood flow rate. A recent study showed that an actual dialyzer blood flow rate of 50 mL/min less than prescribed dialyzer blood flow rate contributes to inadequate dialysis treatment.39 One thus should consider this divergence between actual and prescribed dialyzer blood flow rate. A drawback of the present study is that only a few patients with very low vascular-access flow rates were included, whereas it is known that in some patients with arteriovenous fistulae, the fistula may be patent despite low access flow. However, there is no universal definition of a minimum acceptable flow rate in patients with arteriovenous fistulae. In our hospital, conforming with K/DOQI guidelines,21 patients with arteriovenous fistulae and arteriovenous grafts are referred for fistulographic examination when vascular-access flow rate is less than 600 mL/min or there is a decrease greater than 25% from baseline. Intervention is by means of percutaneous transluminal angioplasty or surgical intervention when a hemodynamically significant stenosis is present. We cannot exclude that in patients with very low access flow, severe AR may occur, which might have a negative effect on Kt/V. Nevertheless, in six of eight patients with arteriovenous fistulae in the low vascular-access flow rate group, vascular-access flow rate was 400 mL/min or less. In this situation, from a theoretical point of view, AR could have been expected at greater blood flow rates. A second possible drawback is the recent observation that access flow measurements by the Transonic may not be completely reproducible between different dialysis sessions because of the interference of mean arterial pressure on vascular-access flow rate.40 Nevertheless, the intersession correlation between the three different sessions was high in our study, and no patient would have been classified differently according to variations in vascular-access flow rates between the sessions. From this study, it is concluded that even in patients with low access flows, increasing the dialyzer blood flow rate leads to an increase in delivered Kt/V regardless of the vascular-access flow rate. Low access flow should in general not be a reason to reduce dialyzer blood flow rate.
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REFERENCES 1. Lowrie EG, Laird NM, Parker TF, Sargent JA: Effect of the hemodialysis prescription on patient morbidity: Report from the National Cooperative Dialysis Study. N Engl J Med 305:1176-1181, 1981 2. Bloembergen WE, Stannard DC, Port FK, Wolfe RA, Pugh JA, Jones CA, Greer JW, Golper TA, Held PJ: Relationship of dose of hemodialysis and cause-specific mortality. Kidney Int 50:557-565, 1996 3. Gotch FA, Sargent JA: A mechanistic analysis of the National Cooperative Dialysis Study (NCDS). Kidney Int 28:526-534, 1985 4. Held PJ, Port FK, Wolfe RA, Stannard DC, Carroll CE, Daugirdas JT, Bloembergen WE, Greer JW, Hakim RM: The dose of hemodialysis and patient mortality. Kidney Int 50:550-556, 1996 5. Hauk M, Kuhlmann MK, Riegel W, Kohler H: In vivo effects of dialysate flow rate on Kt/V in maintenance hemodialysis patients. Am J Kidney Dis 35:105-111, 2000 6. Sherman RA, Levy SS: Rate-related recirculation: The effect of altering blood flow on dialyzer recirculation. Am J Kidney Dis 17:170-173, 1991 7. Collins DM, Lambert MB, Middleton JP, Proctor RK, Davidson CJ, Newman GE, Schwab SJ: Fistula dysfunction: Effect on rapid hemodialysis. Kidney Int 41:1292-1296, 1992 8. Levy SS, Sherman RA, Nosher JL: Value of clinical screening for detection of asymptomatic hemodialysis vascular access stenoses. Angiology 43:421-424, 1992 9. Windus DW, Audrain J, Vanderson R, Jendrisak MD, Picus D, Delmez JA: Optimization of high-efficiency hemodialysis by detection and correction of fistula dysfunction. Kidney Int 38:337-341, 1990 10. Hasbargen JA, Bergstrom RJ: Variable blood pump flow rates and the effect on recirculation. Clin Nephrol 42:322-326, 1994 11. Hasbargen JA, Weaver DT, Hasbargen BJ: The effect of needle gauge on recirculation, venous pressure and bleeding from puncture sites. Clin Nephrol 44:322-324, 1995 12. Besarab A, Sherman R: The relationship of recirculation to access blood flow. Am J Kidney Dis 29:223-229, 1997 13. Depner TA, Krivitski NM, MacGibbon D: Hemodialysis access recirculation measured by ultrasound dilution. ASAIO J 41:M749-M753, 1995 14. Lindsay RM, Bradfield E, Rothera C, Kianfar C, Malek P, Blake PG: A comparison of methods for the measurement of hemodialysis access recirculation and access blood flow rate. ASAIO J 44:62-67, 1998 15. MacDonald JT, Sosa MA, Krivitski NM, Glidden D, Sands JJ: Identifying a new reality: Zero vascular access recirculation using ultrasound dilution. ANNA J 23:603608, 635, 1996 16. Sherman RA, Matera JJ, Novik L, Cody RP: Recirculation reassessed: The impact of blood flow rate and the low-flow method reevaluated. Am J Kidney Dis 23:846-848, 1994 17. Krisper P, Aschauer M, Tiesenhausen K, Leitner G, Holzer H, Schneditz D: Access recirculation in a native
fistula in spite of a seemingly adequate access flow. Am J Kidney Dis 35:529-532, 2000 18. Lindsay RM: Assessment of access recirculation during haemodialysis. Curr Opin Nephrol Hypertens 6:570574, 1997 19. Bouchouareb D, Saveanu A, Bartoli JM, Olmer M: A new approach to evaluate vascular access in hemodialysis patients. Artif Organs 22:591-595, 1998 20. Schwab SJ, Oliver MJ, Suhocki P, Mccann R. Hemodialysis arteriovenous access: Detection of stenosis and response to treatment by vascular access blood flow. Kidney Int 59:358-362, 2001 21. National Kidney Foundation: K/DOQI Clinical Practice Guidelines for Vascular Access: Update 2000. Am J Kidney Dis 37:S137-S181, 2001 (suppl 1) 22. Cheriex EC, Leunissen KM, Janssen JH, Mooy JM, van Hooff JP: Echography of the inferior vena cava is a simple and reliable tool for estimation of ‘dry weight’ in haemodialysis patients. Nephrol Dial Transplant 4:563-568, 1989 23. Mandelbaum A, Ritz E: Vena cava diameter measurement for estimation of dry weight in haemodialysis patients. Nephrol Dial Transplant 11:24-27, 1996 24. Keshaviah PR, Ebben JP, Emerson PF: On-line monitoring of the delivery of the hemodialysis prescription. Pediatr Nephrol 9:S2-S8, 1995 (suppl 1) 25. Ronco C, Brendolan A, Crepaldi C, Frisone P, Ghiotto F, Zamboni S, Gastaldon F, La Greca G: On-line urea monitoring: A further step towards adequate dialysis prescription and delivery. Int J Artif Organs 18:534-543, 1995 26. Depner TA, Keshaviah PR, Ebben JP, Emerson PF, Collins AJ, Jindal KK, Nissenson AR, Lazarus JM, Pu K: Multicenter clinical validation of an on-line monitor of dialysis adequacy. J Am Soc Nephrol 7:464-471, 1996 27. Sternby J: Urea sensors—A world of possibilities. Adv Ren Replace Ther 6:265-272, 1999 28. Daugirdas JT: Second generation logarithmic estimates of single-pool variable volume Kt/V: An analysis of error. J Am Soc Nephrol 4:1205-1213, 1993 29. Daugirdas JT, Schneditz D: Overestimation of hemodialysis dose depends on dialysis efficiency by regional blood flow but not by conventional two pool urea kinetic analysis. ASAIO J 41:M719-M724, 1995 30. Krivitski NM: Novel method to measure access flow during hemodialysis by ultrasound velocity dilution technique. ASAIO J 41:M741-M745, 1995 31. Depner TA, Krivitski NM: Clinical measurement of blood flow in hemodialysis access fistulae and grafts by ultrasound dilution. ASAIO J 41:M745-M749, 1995 32. Krivitski NM: Theory and validation of access flow measurement by dilution technique during hemodialysis. Kidney Int 48:244-250, 1995 33. Bosman PJ, Boereboom FT, Bakker CJ, Mali WP, Eikelboom BC, Blankestijn PJ, Koomans HA: Access flow measurements in hemodialysis patients: In vivo validation of an ultrasound dilution technique. J Am Soc Nephrol 7:966-969, 1996 34. National Kidney Foundation: DOQI Clinical Practice Guidelines for Hemodialysis Adequacy. Am J Kidney Dis 30:S17-S21, 1997 (suppl 2) 35. National Kidney Foundation: K/DOQI Clinical Prac-
DIALYSIS DOSE AND BLOOD FLOW RATE IN SUBOPTIMAL VASCULAR ACCESS
tice Guidelines for Hemodialysis Adequacy: Update 2000. Am J Kidney Dis 37:S7-S64, 2001 (suppl 1) 36. Depner TA, Rizwan S, Stasi TA: Pressure effects on roller pump blood flow during hemodialysis. ASAIO Trans 36:M456-M459, 1990 37. Teruel JL, Fe´rnandez Lucas M, Marce´n R, Rodrı´guez JR, Lo´pez Sa´nchez J, Ricera M, Lian˜o F, Ortun˜o J: Differences between blood flow as indicated by the hemodialysis blood roller pump and blood flow measured by an ultrasonic sensor. Nephron 85:142-147, 2000 38. Kapoian T, Steward CA, Acevedo AR, Haddam DA,
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Sherman RA: Unrecognized fall in delivered BFR (Qb) and Kt/V from blood lines with 8 mm blood pump segment. J Am Soc Nephrol 9:1514A, 1998 (abstr) 39. Palevsky PM, Washington MS, Stevenson JA, Rohay JM, Dyer NJ, Lockett R, Perry SB: Barriers to the delivery of adequate hemodialysis in ESRD Network 4. Adv Ren Replace Ther 7:S11-S20, 2000 (suppl 1) 40. DeSoto DJ, Ram SJ, Faiyaz R, Birk CG, Paulson WD: Hemodynamic reproducibility during blood flow measurements of hemodialysis synthetic grafts. Am J Kidney Dis 37:790-796, 2001
A
DEQUATE HEMODIALYSIS is of main importance to reduce morbidity risk in intermittent maintenance hemodialysis patients.1,2 The parameter Kt/V is currently the most frequently used marker for dialysis adequacy.3,4 Varying selected parameters can modify delivered Kt/V, including dialysis duration, dialysis frequency, dialysate flow rate,5 and dialyzer blood flow rate. It is implied that vascular-access recirculation (AR) can attenuate delivered Kt/V.6-9 AR is the process by which returned dialyzed blood shortcircuits through the vascular-access conduit and reenters the dialyzer system through the arterial blood catheter, thus effectively bypassing the corporeal circulatory system. In the past, it was believed that AR was generally present and sig-
From the Departments of Nephrology and Surgery, University Hospital Maastricht, The Netherlands. Received March 2, 2001; accepted in revised form May 25, 2001. Address reprint requests to Frank M. van der Sande, MD, PhD, Department of Internal Medicine and Nephrology, University Hospital Maastricht, P Debeyelaan 25, PO Box 5800, 6202 AZ Maastricht, The Netherlands. E-mail: [email protected] © 2001 by the National Kidney Foundation, Inc. 0272-6386/01/3805-0004$35.00/0 doi:10.1053/ajkd.2001.28580 948
nificantly increased at greater dialyzer blood flow rates, especially in patients with suboptimal vascular-access sites.6,10 Recirculation values of 10% to 15% were experimentally measured at a dialyzer blood flow rate of 200 mL/min and increased to 20% to 30% at a dialyzer blood flow rate of 400 to 500 mL/min.6,10,11 Naturally, questions were raised concerning the efficacy of dialysis with a high dialyzer blood flow rate. However, with the advent of newer AR determination techniques, such as alternative methods of sample collection12 and the ultrasonic saline dilution method,13,14 it was shown that AR is less than generally assumed.12,15-18 However, from a theoretical point of view, it cannot be excluded that when prescribed blood flow rate approximates access flow rate, AR will occur that may impair dialysis efficacy. However, to date, the relation between blood flow and dialysis efficacy in patients with suboptimal graft function has not been studied. Nevertheless, this question is of clinical importance, partly because patients with low vascular-access flow rates were found to be at risk for receiving inadequate dialysis doses.19 In the current literature, there is insufficient analysis of the effect of dialyzer blood flow rates on AR and Kt/V in patients with suboptimal vascular access. The aim of the present study is
American Journal of Kidney Diseases, Vol 38, No 5 (November), 2001: pp 948-955
DIALYSIS DOSE AND BLOOD FLOW RATE IN SUBOPTIMAL VASCULAR ACCESS
to investigate the effect of different dialyzer blood flow rates on AR and Kt/V in patients with low vascular-access flow rates and those with normal vascular-access flow rates. PATIENTS AND METHODS
Patients All patients with impaired vascular-access flow rates (discussed next) in our dialysis center were included on the study, except for 1 patient with poor general health and 1 patient who refused to participate. Patients with normal vascular-access flow were randomly selected from our dialysis population. After obtaining informed consent for participation on the study, 21 patients (9 women) with a mean age of 68.1 ⫾ 11.4 (SD) years and mean time on hemodialysis therapy of 24.9 ⫾ 20.5 months were included. Renal disease was caused by hypertensive nephropathy (nine patients), reflux nephropathy (four patients), chronic glomerulonephritis (three patients), immunologic disease (three patients), diabetic nephropathy (one patient), and polycystic kidney disease (one patient). Residual renal function was calculated by the following formula: (UUN/SUN) ⫻ urine flow rate 共milliliters per minute兲 where UUN is urine urea nitrogen concentration and SUN is serum urea nitrogen concentration. Urine was collected in the first interdialytic period of the week. Fifteen patients had no discernable renal function, whereas the remaining six patients had an average remaining renal function of 2.72 ⫾ 3.12 mL/min (range, 0.6 to 8.4 mL/min). At the time of the study, 12 patients (5 women) had autogenous arteriovenous fistulae and 9 patients (4 women) had polytetrafluoroethylene arteriovenous grafts. Access sites were 17.8 ⫾ 14.7 months old, with the latest vascular diagnostic imaging performed 5.9 ⫾ 9.7 months previously. According to the revised National Kidney FoundationDialysis Outcomes Quality Initiative (DOQI), low vascularaccess flow rate is defined as a rate less than 600 mL/min, whereas normal vascular-access flow rate is defined as a rate greater than 600 mL/min for both fistulae and grafts.20,21 Eight patients (six fistulae; 4 women) had low vascularaccess flow rates, and 13 patients (six fistulae, 5 women) had normal vascular-access flow rates.
Study Protocol With each patient serving as his or her own control, measurements were performed during three standardized dialysis sessions differing in dialyzer blood flow rate: 200, 300, and 400 mL/min. Prescribed dialyzer blood flow rate remained constant during an entire dialysis session. Dialysate flow rate was 500 mL/min for all sessions. All patients underwent dialysis for the entire prescribed time, without exceeding the total time. Dialysis time was identical for each of the three sessions. Patients were treated for an average session time of 3.7 ⫾ 0.4 hours to achieve a double-pool Kt/V (dpKt/V) greater than 1.1. Treatment order was randomized. Patient dry weight was estimated by echography of the
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inferior caval vein.22,23 Ultrafiltration rate, which remained constant for a dialysis session, was prescribed using estimated dry weight and interdialytic and intradialytic weight gains. Dialysis was performed using a Gambro AK-100 or Gambro AK-200 module (Gambro, Lund, Sweden) and a polyamide-S membrane (Polyflux 8L; Gambro) or cellulose triacetate membrane (Sureflux 130Gga; Nissho Nipro, Zaventem, Belgium). The two dialyzers have similar clearance rates, and the same type was used for each of the three sessions. Fifteen-gauge fistula needles (F15AS; 1.8 ⫻ 25 mm; Gambro) were used for all patients. Similar cannulating procedures, interneedle distances, and needle directions were used for each of the three dialysis sessions. The internal diameter of the pump segment was 8 mm. Dialysate composition was as follows: sodium, 140 mmol/L; potassium, 2 mmol/L; acetate, 3 mmol/L; magnesium, 0.5 mmol/L; bicarbonate (individualized), 30 to 34 mmol/L; chloride, 108 mmol/L; and calcium, 1.5 mmol/L. Dialysate temperature, which remained constant for three sessions, was 36.2°C ⫾ 0.6°C.
Methods Dialysis dose was determined by using either a Biostat 1000 (Baxter Healthcare Corp, Deerfield, IL)24-26 or a Gambro DQM200 urea monitor.27 Both urea monitors are used together for clinical purposes on our dialysis faculty. The same monitor was used for the three dialysis sessions. Both monitors determine levels of intradialytic urea removed in the spent dialysate. These levels correlated predictably with blood urea concentrations. For the calculation of Kt/V, the DQM200 uses the second-generation Daugirdas formula.28 An equilibration correction formula29 is subsequently applied, converting the original single-pool Kt/V to a dpKt/V to account for urea rebound, thus shifting attention from dialyzer clearance to patient clearance. The Biostat 1000 also determines a dpKt/V based on two-exponential curve fit calculation.24 Actual dialyzer blood flow rate, vascular-access flow rate, and AR were calculated by means of a saline dilution technique using an HD01 Hemodialysis Monitor System (Transonic Systems Inc, Ithaca, NY).13,14,30-33 This system computes AR and vascular-access flow rate by monitoring ultrasound velocity changes after blood dilution with a saline bolus injected into the extracorporeal venous circulation. Actual dialyzer blood flow rate is measured directly. AR was measured in duplicate and vascular-access flow rates were measured in triplicate, both at least twice per session and an average of 3.0 ⫾ 0.8 times per session. With each measurement, actual dialyzer blood flow rate was recorded. The total time during which blood catheters were in reversed configuration for vascular-access flow rate measurement was equivalent for each of the three dialysis sessions. No flow or recirculation measurements were made in the first or last half hour of dialysis.
Statistical Analysis The significance of differences between samples was determined by a paired two-sample t-test for paired samples and an unpaired t-test for unrelated samples (Microsoft Excel 1998 [Microsoft Corp, Redmond, WA]; Macintosh
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HASSELL ET AL
[Apple, Cupertino, CA]). All P are two-sided and considered significant when less than 0.05. Data are given as mean ⫾ SD and, when necessary, with a range.
RESULTS
Predialysis weights for the three treatment sessions at dialyzer blood flow rates of 200, 300, and 400 mL/min for the low vascular-access flow rate group were 63.6 ⫾ 9.8, 63.7 ⫾ 9.6, and 63.8 ⫾ 10.0 kg, respectively (P ⫽ not significant [NS]). For the normal vascular-access flow rate group, respective weights were 68.7 ⫾ 10.1, 68.7 ⫾ 10.3, and 69.0 ⫾ 10.1 kg (P ⫽ NS). Interdialytic weight gains in the low vascularaccess flow rate group for dialyzer blood flow rates of 200, 300, and 400 mL/min session were 2.8 ⫾ 0.5, 2.4 ⫾ 0.6, and 2.5 ⫾ 0.5 kg (P ⫽ NS), whereas values for the normal vascular-access flow rate group were 2.2 ⫾ 0.6, 2.1 ⫾ 0.7, and 2.3 ⫾ 0.5 kg, respectively (P ⫽ NS). Access Blood Flow Rate Eight patients (six fistulae, 4 women) had low vascular-access flow rates, with a mean of 464 ⫾ 91 mL/min (range, 297 to 548 mL/min). The normal vascular-access flow rate group consisted of 13 patients (six fistulae, 5 women) with flow rates exceeding 600 mL/min (mean, 852 ⫾ 124 mL/min; range, 662 to 1,042 mL/min). Intersession correlation coefficients for vascular-access flow rates between sessions with dialyzer blood flow rates of 200 and 300 mL/min were r ⫽ 0.914 (P ⬍ 0. 05); 200 to 400 mL/min, r ⫽ 0.906 (P ⬍ 0.05); and 300 to 400 mL/min, r ⫽ 0.875 (P ⬍ 0.05). No patient would have been classified differently (ie, normal or low vascularaccess flow rate) according to variations in access flow rates between sessions. Circuit Blood Flow Rate In all cases, there was a discrepancy between prescribed and actual dialyzer blood flow rates achieved. In the low vascular-access flow rate group, actual dialyzer blood flow rates for prescribed rates of 200, 300, and 400 mL/min were 181 ⫾ 6, 266 ⫾ 7, and 344 ⫾ 13 mL/min, respectively. Rates in the normal vascular-access flow rate group were 184 ⫾ 6, 271 ⫾ 6, and 346 ⫾ 9 mL/min, respectively. Discrepancies between prescribed and actual dialyzer blood flow rate were significant in all cases (P ⬍ 0.05).
Circuit Pressures Mean venous circuit pressures in the low vascular-access flow rate group for dialyzer blood flow rates of 200, 300, and 400 mL/min were 73 ⫾ 28, 117 ⫾ 31, and 162 ⫾ 31 mm Hg, whereas arterial circuit pressures were –49 ⫾ 4, –108 ⫾ 45, and –146 ⫾ 27 mm Hg, respectively. Mean venous circuit pressures for dialyzer blood flow rates of 200, 300, and 400 mL/min in the normal vascular-access flow rate group were 76 ⫾ 17, 122 ⫾ 23, and 163 ⫾ 21 mm Hg, whereas arterial circuit pressures were –41 ⫾ 28, –95 ⫾ 29, and –140 ⫾ 24 mm Hg, respectively. In no case did the arterial or venous pressure exceed 220 mm Hg. Effect of Dialyzer Blood Flow Rate on AR There were nine sessions with AR (eight patients). Of those nine sessions, five instances of AR were observed in patients with low vascularaccess flow rates (0.9% ⫾ 0.6%; range, 0.3% to 1.3%), and four instances were observed in patients with normal vascular-access flow rates (1.2% ⫾ 1.1%; range, 0.5% to 2.8%). In the low vascular-access flow rate group, two patients had AR only at a dialyzer blood flow rate of 400 mL/min, one patient had AR only at 200 mL/min, and one patient had AR at 300 and 400 mL/min. In the normal vascular-access flow rate group, one patient had AR only at 300 mL/min, whereas three patients had AR only at 200 mL/min. Effect of Dialyzer Blood Flow Rate on Kt/V In the low vascular-access flow rate group (Table 1), dpKt/Vs at prescribed dialyzer blood flow rates of 200, 300, and 400 mL/min were 0.95 ⫾ 0.16, 1.17 ⫾ 0.16, and 1.36 ⫾ 0.19, respectively. In all patients, Kt/V increased when increasing the dialyzer blood flow rate from 200 to 300 mL/min. When increasing it from 300 to 400 mL/min, Kt/V increased in seven patients and remained the same in one patient. Vascularaccess flow rate in that patient was 402 mL/min. In the normal vascular-access flow rate group (Table 1), Kt/Vs at dialyzer blood flow rates of 200, 300, and 400 mL/min were 0.90 ⫾ 0.15, 1.07 ⫾ 0.16, and 1.17 ⫾ 0.21, respectively. Again, in all patients, Kt/V increased with a dialyzer blood flow rate increase from 200 to 300 mL/min. From 300 to 400 mL/min, Kt/V in-
DIALYSIS DOSE AND BLOOD FLOW RATE IN SUBOPTIMAL VASCULAR ACCESS Table 1.
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Kt/V in All Patients Vascular Access Flow Rate (mL/min)
Vascular access flow rate (mL/min) Kt/V(200) Kt/V(300) Kt/V(400) Kt/V(300) ⫺ Kt/V(200) Kt/V(400) ⫺ Kt/V(300) Kt/V(400) ⫺ Kt/V(200)
Low
Normal
464 ⫾ 91 0.95 ⫾ 0.16 1.17 ⫾ 0.16 1.36 ⫾ 0.19 0.22 ⫾ 0.12* 0.19 ⫾ 0.12* 0.41 ⫾ 0.13*
852 ⫾ 124 0.90 ⫾ 0.15 1.07 ⫾ 0.16 1.17 ⫾ 0.21 0.17 ⫾ 0.10* 0.10 ⫾ 0.09* 0.27 ⫾ 0.11*
NOTE. Data expressed as mean ⫾ SD. Kt/V is the double-pool dialysis dose during blood flow rates of 200, 300, and 400 mL/min. Kt/V(300) ⫺ Kt/V(200), Kt/V(400) ⫺ Kt/V(300), and Kt/V(400) ⫺ Kt/V(200) are differences for patients in the low and normal vascular-access flow rate groups. Differences are given in dimensionless Kt/V units. *P ⬍ 0.05.
Fig 2. Kt/V in patients with normal vascular-access flow rates. dpKt/V is presented as a factor of prescribed dialyzer blood flow rate (Qb). Kt/V(200), Kt/ V(300), and Kt/V(400) are dialysis doses at dialyzer blood flow rates of 200, 300, and 400 mL/min. (●), Sessions with measured recirculation; (E), sessions with no measured recirculation.
creased in 10 patients and remained equal in 3 patients. Dialysis dose increases were significant between the three dialyzer blood flow rates for the low (Fig 1; Table 1) and normal vascular-access flow rate groups (Fig 2; Table 1; all P ⬍ 0.05). Mean arithmetical differences in Kt/V (Kt/ V300 – Kt/V200, Kt/V400 – Kt/V300, and Kt/V400 – Kt/V200) for the two vascular-access flow rate
groups are listed in Table 1. Kt/V differences did not vary significantly among the subgroups (P ⫽ NS); ie, with an increase in dialyzer blood flow rate, one group did not show a significantly greater augmentation in Kt/V than the other. The increase in Kt/V is seen in both arteriovenous fistulae (Fig 3; Table 2) and arteriovenous grafts (Fig 4; Table 2). When AR was found, there was
Fig 1. Kt/V in patients with low vascular-access flow rates. dpKt/V is presented as a factor of prescribed dialyzer blood flow rate (Qb). Kt/V(200), Kt/ V(300), and Kt/V(400) are dialysis doses at dialyzer blood flow rates of 200, 300, and 400 mL/min. (●), Sessions with measured recirculation; (E), sessions with no measured recirculation.
Fig 3. Kt/V in patients with arteriovenous fistulae. dpKt/V is presented as a factor of prescribed dialyzer blood flow rate (Qb). Kt/V(200), Kt/V(300), and Kt/ V(400) are dialysis doses at dialyzer blood flow rates of 200, 300, and 400 mL/min. (●), Sessions with measured recirculation; (E), sessions with no measured recirculation.
952 Table 2.
HASSELL ET AL Kt/V in Patients With Arteriovenous Fistulae and Grafts
Kt/V(300) ⫺ Kt/V(200) Kt/V(400) ⫺ Kt/V(300) Kt/V(400) ⫺ Kt/V(200)
Arteriovenous Fistulae
Arteriovenous Grafts
0.18 ⫾ 0.12* 0.17 ⫾ 0.11* 0.35 ⫾ 0.14*
0.20 ⫾ 0.08* 0.09 ⫾ 0.10* 0.30 ⫾ 0.14*
NOTE. Data expressed as mean ⫾ SD. Kt/V(300) ⫺ Kt/V(200), Kt/V(400) ⫺ Kt/V(300), and Kt/V(400) ⫺ Kt/ V(200) are differences for patients with arteriovenous fistulae and arteriovenous grafts. Differences are given in dimensionless Kt/V units. *P ⬍ 0.05.
no correlation between the magnitude of Kt/V change and amount of recirculation. Between the groups of patients with and without recirculation, there was no difference in the change in Kt/V as a result of dialyzer blood flow rate increases (P ⫽ NS). Comparing dialyzer blood flow rates of 200 and 400 mL/min, patients with AR had a Kt/V increase of 0.33 ⫾ 0.11, whereas patients without AR had an increase of 0.32 ⫾ 0.15. Effect of Dialyzer Blood Flow Rate on Dialysis Adequacy In the low vascular-access flow rate group, six of the eight patients received a dpKt/V less than 1.05 at a prescribed dialyzer blood flow rate of 200 mL/min (0.88 ⫾ 0.12; range, 0.70 to 1.03). At 300 mL/min, there were two patients (both dpKt/V, 1.00). At 400 mL/min, no patient had a dpKt/V less than 1.05. In the normal vascular-access flow rate group at a prescribed vascular-access flow rate of 200 mL/min, 12 of the 13 patients received dialysis doses less than 1.05 (0.87 ⫾ 0.09; range, 0.69 to 1.01). At 300 mL/min, 6 patients had doses less than 1.05 (0.94 ⫾ 0.1; range, 0.81 to 1.01), whereas at 400 mL/min, 4 patients had doses less than 1.05 (0.96 ⫾ 0.07; range, 0.86 to 1.00).
delivered dialysis dose, indicated by Kt/V. Moreover, in both groups, in no instance was there a decrease in Kt/V with increasing dialyzer blood flow rates. Of the 63 dialysis sessions, there were nine cases of AR in eight patients; five cases in the low vascular-access flow rate group and four cases in the normal vascular-access flow rate group. Measured AR was minimal, with values not exceeding 3%. This conforms to recent observations that AR is generally absent in wellfunctioning access sites.12,15 This study also shows that this concept holds true for access sites with low vascular-access flow rates in which vascularaccess flow and dialyzer blood flow rates approximate each other. Data show that high dialyzer blood flow rates failed to predictably increase AR in patients with AR; ie, when AR was measured, an increase in dialyzer blood flow rate did not increase recirculation in either of the two groups. Moreover, AR was found mainly in sites where the vascular-access flow rate exceeded dialyzer blood flow rate. The explanation for these findings is not completely clear. Because inadequate placement of fistula needles might result in some recirculation, special attention was given to cannulating procedures, interneedle distances, and needle directions. Possibly, small
DISCUSSION
In the present study, the effect of increasing dialyzer blood flow rates on AR and dialysis dose was assessed in patients with low and normal vascular-access flow rates. Results of the study show that an increase in prescribed dialyzer blood flow rate effected a significant increase in
Fig 4. Kt/V in patients with arteriovenous grafts. dpKt/V is presented as a factor of prescribed dialyzer blood flow rate (Qb). Kt/V(200), Kt/V(300), and Kt/ V(400) are dialysis doses at dialyzer blood flow rates of 200, 300, and 400 mL/min. (●), Sessions with measured recirculation; (E), sessions with no measured recirculation.
DIALYSIS DOSE AND BLOOD FLOW RATE IN SUBOPTIMAL VASCULAR ACCESS
AR signals have to be interpreted as reflected background noise, especially given the low intensity of recirculation and apparent random distribution over the two subgroups. Data show that when additional distinct parameters were kept constant, an increase in prescribed dialyzer blood flow rate effected a significant increase in delivered dialysis dose in both the low and normal vascular-access flow rate groups. Moreover, in no instance did Kt/V decrease with increasing dialyzer blood flow rates. Increasing dialyzer blood flow rate from 200 to 300 mL/min resulted in a Kt/V increase in all patients. When increasing dialyzer blood flow rate from 300 to 400 mL/min, Kt/V remained unchanged in one patient in the low vascularaccess flow rate group and three patients in the normal vascular-access flow rate group. Therefore, if using a dialyzer blood flow rate increase from 300 to 400 mL/min for the purpose of Kt/V augmentation, Kt/V measurements for efficacy purposes would seem judicious. Dialysis adequacy in patients with negligible renal function is defined by the DOQI as a single-pool Kt/V of at least 1.2 per session thrice weekly,34,35 which equilibrates to a session dpKt/V of 1.0 at 4 hours.29 At a prescribed dialyzer blood flow rate of 200 mL/min, 18 of the 21 patients (6 patients, low vascular-access flow rates) received an inadequate session dialysis dose. Solely increasing dialyzer blood flow rate led to an improvement in delivered Kt/V. At 300 mL/min, the number of patients receiving inadequate dialysis was reduced to 8 patients (2 patients, low vascular-access flow rate group), and at 400 mL/min, the number was further reduced to 4 patients (0 patients, low vascularaccess flow rate group). This agrees with observations that a dialyzer blood flow rate that is too low can lead to impaired dialysis efficacy.19 Prescribed dialyzer blood flow rates were invariably greater than dialyzer blood flow rates obtained. The difference was more pronounced at greater dialyzer blood flow rates. This phenomenon previously was described36 and is at least partly caused by negative-pressure flattening of the tubing (congruent with a volume decrease) before entering the roller pump and subsequent reexpansion once clear of the pump. Moreover, the internal diameter of the pump segment and diameter of the needles also may contribute to
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this phenomenon.37,38 The effect is a reduction in actual dialyzer blood flow rate. A recent study showed that an actual dialyzer blood flow rate of 50 mL/min less than prescribed dialyzer blood flow rate contributes to inadequate dialysis treatment.39 One thus should consider this divergence between actual and prescribed dialyzer blood flow rate. A drawback of the present study is that only a few patients with very low vascular-access flow rates were included, whereas it is known that in some patients with arteriovenous fistulae, the fistula may be patent despite low access flow. However, there is no universal definition of a minimum acceptable flow rate in patients with arteriovenous fistulae. In our hospital, conforming with K/DOQI guidelines,21 patients with arteriovenous fistulae and arteriovenous grafts are referred for fistulographic examination when vascular-access flow rate is less than 600 mL/min or there is a decrease greater than 25% from baseline. Intervention is by means of percutaneous transluminal angioplasty or surgical intervention when a hemodynamically significant stenosis is present. We cannot exclude that in patients with very low access flow, severe AR may occur, which might have a negative effect on Kt/V. Nevertheless, in six of eight patients with arteriovenous fistulae in the low vascular-access flow rate group, vascular-access flow rate was 400 mL/min or less. In this situation, from a theoretical point of view, AR could have been expected at greater blood flow rates. A second possible drawback is the recent observation that access flow measurements by the Transonic may not be completely reproducible between different dialysis sessions because of the interference of mean arterial pressure on vascular-access flow rate.40 Nevertheless, the intersession correlation between the three different sessions was high in our study, and no patient would have been classified differently according to variations in vascular-access flow rates between the sessions. From this study, it is concluded that even in patients with low access flows, increasing the dialyzer blood flow rate leads to an increase in delivered Kt/V regardless of the vascular-access flow rate. Low access flow should in general not be a reason to reduce dialyzer blood flow rate.
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