Department of Nephrology and Dialysis Research and Training Institute, Lapeyronie University Hospital, CHRU Montpellier, Montpellier, France
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Abstract |
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Methods. To explore this problem, we conducted a prospective 24-month trial comparing the flow performances and dialysis dose (Kt/Vdp) deliveries of both access options in a group of 42 haemodialysis patients during two study phases. During the first 12 months the patients completed a treatment period by means of permanent dual silicone catheters (DualKT). Then they were transferred to an AVA (40 native arterio-venous fistulas and two PTFE grafts) and monitored for an additional 12-month period. Assessments of flow adequacy and dialysis quantification were performed monthly.
Results. Dialysis adequacy was achieved in all cases. No patient had to be transferred prematurely to the AVA because of catheter failure. Three catheters had to be replaced due to bacteraemia in three patients. The mean effective blood flow rates achieved were 316±3.5 ml/min and 340±3.3 ml/min with DualKT and AVA, respectively, for a pre-set machine blood flow of 348±2.2 ml/min. Recirculation rates evaluated with the slow blood flow method were 8.6±0.6 and 12.1±0.8% for DualKT and AVA using mean values of the solute markers urea and creatinine. Due to the possibility of a comparison veno-venous vs arterio-venous blood circulation, a corrected arterio-venous access recirculation could be derived from the difference between the two, which was around 3%. The blood flow resistance of the DualKT was slightly higher than with AVA as indicated by venous pressure differences. Kt/Vdp delivered was 1.37±0.02 and 1.45±0.02 with DualKT and AVA access respectively. The loss of dialysis efficacy using catheters was estimated at 6%. However, in all cases Kt/Vdp values remained above the recommended values (Kt/Vdp 1.2). Protein nutritional state, as well as conventional clinical and biochemical markers of dialysis adequacy, remained in the optimal range.
Conclusion. Permanent venous catheters provide adequate haemodialysis on a long-term basis. Flow performances and dialysis doses are slightly reduced (56%) when compared with AVA. Regular assessment of dialysis performance is strongly recommended to assure dialysis adequacy. Lengthening dialysis time may represent a simple and efficient tool to compensate for reduced flow performances with catheter use.
Keywords: access adequacy; flow performances; haemodialysis adequacy; permanent catheter; recirculation; vascular access
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Introduction |
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The aim of this study was to evaluate flow performances of permanent catheters and their long-term impact on dialysis dose delivery as compared with native or graft arterio-venous accesses (AVA) in a group of ESRD patients on regular renal replacement therapy.
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Subjects and methods |
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Residual renal kidney function (RRF) was calculated every month based on mean urea and creatinine clearances obtained from a 48 h urea collection.
Study design
The study was a prospective, controlled, non-randomized trial comparing two consecutive periods of 12 months each. In the first phase (period A) HD was performed using permanent catheters (DualCath®, Medcomp, Harleyville, USA, and Hemotech, Ramonville, France). In the second study phase (period B) blood access was established via an AVF or a PTFE graft. Dialysis performance was evaluated every month without modifying the operational dialysis conditions and schedule of renal replacement treatment. The study compared the performance of the two different types of vascular access in the same group of patients. Due to the mode of patient enrolment, it was not possible to design a cross-over study for ethical reasons since dialysis is often launched as emergency procedure.
Vascular access
Vascular access for study phase A was obtained by placing a permanent catheter (DualKT) made of two independent silicone polymer catheters preferably into the right jugular vein (right vein, 40 patients; left vein, two patients) [7]. The length of the catheters was extemporaneously tailored to the chest size during placement, so right-side catheters were 2528 cm long and left-side catheters were 3032 cm long (the medial catheter serving as the arterial line was shorter). The tips of the catheters were located at the junction between the superior vena cava and the right atrium, and were spaced 23 cm apart. Catheters exited the skin 1012 cm below the clavicle on the chest wall. The external parts of the catheters were 68 cm long and ended in a luer lock connection facilitating the attachment of blood lines during HD sessions. AVA were created according to the forearm vascular network. Ultrasound vascular mapping (arterial and venous) followed by doppler examination was used to select the best approach for installing the AVA. Native AVF were attempted in all patients as a first option and were successfully constructed in 40 patients within 66±32 days after launching dialysis. PTFE graft accesses were created within 168±50 days as a secondary option in two ESRD patients after several failures at attempting to install a native AVF.
All patients were treated in the same dialysis facility with the same hygienic preparation. The dialysis schedule was three sessions per week lasting 34 h. Therapy modes were as follows: conventional HD in three patients, high flux haemodialysis in seven patients and high flux on-line haemodiafiltration (HDF) in 32 patients. Accompanying treatment modalities, such as the type of dialyser, dialysis duration, prescribed blood flow and anticoagulation, were not changed during the study period.
Vascular access handling was identical for all patients. DualKT were connected according to strict hygiene conditions, including using a mask, gloves, sterile gown, and drape. Catheter antithrombotic lock was routinely performed using standard heparin (1.52 ml per catheter, which is 750010000 IU/ml, according to the length of catheter) (Choay®, Paris, France). Between dialysis sessions, catheters were wrapped in gauze and protected by a plastic dressing (Opsite®). Microbial catheter colonization was checked for once every month by culturing the tip clot. In case of a positive result an appropriate antibiotic therapy consisting of vancomycin (125 mg) or gentamycin (80 mg) was applied systemically.
All AVA were punctured with 15 gauge needles in strict aseptic conditions following the routine protocol used in our unit.
HD was performed with 4008H-S machines (Fresenius, Bad Homburg, Germany). Ultrapure bicarbonate dialysate (sterile and pyrogen-free) obtained from cold-sterilization through ultrafilters was utilized in all patients. The dialysate flow was set to between 600 and 700 ml/min, with blood flow at 300400 ml/min. HDF was performed in 32 patients with a blood flow set between 350 and 400 ml/min, an effluent dialysate flow of 700800 ml/min and infusate flow of 100 ml/min.
Haemodialyser use was as follows (single use): low flux polysulphone (F8HPS, Fresenius) in three patients, high flux polysulphone (HF80S, Fresenius) in 37 patients and polyacrylonitrile-AN69 (Filtral 12, Hospal, Lyon, France) in two patients.
The antithrombotic treatment schedule was optimized for each patient in order to prevent extracorporeal blood clotting. Standard heparinization (Choay®) consisting of an initial i.v. bolus and a maintenance dose by means of a continuous i.v. infusion was performed in 30 patients. Low molecular weight heparin (Fragmin®) was used in 12 patients with haemorrhagic risk.
Vascular access performance and haemodialysis dose
Vascular access performances and dialysis dose deliveries were evaluated midweek on the first week of every month. Effective blood flow, total recirculation, dialysate flow, instantaneous clearances (both on dialysate- and blood-side) for urea and creatinine were calculated during the first hour of dialysis.
Total recirculation was determined by the slow blood flow method. Briefly, arterial (CA1) and venous (CV) blood samples were taken simultaneously at the beginning of the measuring cycle. The blood pump speed was reduced to 50 ml/min for 2 min and a second arterial blood sample (CA2) was taken. The calculation is based on urea and creatinine as solute markers [8]. This method has recently been disputed by several authors but has been shown to be convenient, reproducible and valid when performed with expertise compared with more advanced methods [9,10].
Effective blood flow in the extracorporeal circuit was evaluated by the transit bubble time method on the arterial line in a race-track segment (1 m of calibrated blood tubing). Validation of this traditional method has recently been confirmed by us compared with an ultrasonic method (Transonics®, NY, USA) (unpublished data).
Dialysate flow was calculated by collecting the effluent dialysate in a graduated glass cylinder over a 2 min period. Blood and dialysate clearances for urea and creatinine were measured 60 min after the start of dialysis by simultaneously drawing an arterial and a venous blood sample, and an effluent dialysate sample, at zero ultrafiltration rate.
Urea and creatinine kinetic modelling (UKM and CKM, respectively) was performed over a complete dialysis cycle (pre-session 1 to pre-session 2 of HD). Clearances derived from three blood samples (Cpre/post the first session and Cpre the next session) were used as input values for the corrected variable double pool volume model, as well as body weight (BWpre/post and BWpre), dialysis time (tHD), interdialytic time (tinter) and residual kidney function. Effective body clearances for urea, dialysis dose (Kt/Vdp), creatinine index, equivalent to creatinine generation rate, a surrogate of lean body mass [11], and normalized protein catabolic rate (nPCR) were calculated using appropriate formula.
Calculations and statistics
Recirculation (Rec) was calculated as the average of:
| (|<|(|>|1|<|)|>|) |
Effective blood flow (eQb) was calculated according to Gotch [12], such that
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The dialysis dose obtained from a single-pool variable volume model (Kt/Vsp), was corrected for a two-pool effect to give Kt/Vdp, according to the formula established by Daugirdas and Schneditz [13]:
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Venous pressure recorded at different blood flow was used as a surrogate for extracorporeal flow resistance.
Protein catabolic rate was evaluated from the interdialytic urea generation rate obtained from UKM and converted to nPCR using the relationship established by Borah and Gotch [14].
Time averaged concentrations (TAC) for urea and creatinine were calculated as
| (|<|(|>|4|<|)|>|) |
Results were expressed as mean±SE. Statistical differences between the data sets for DualKT and AVA were evaluated by the non-parametric paired Wilcoxon test. Linear regression analysis was performed on paired blood flow data. Statistical significance was accepted for a P-value <0.05.
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Results |
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One AVF had to be revised by means of percutaneous angioplasty while two PTFE grafts with pre-existing occlusive stenosis had to be fixed surgically during study period B.
The favourable results observed in our dialysis population are mainly due to the strict hygiene policy implemented for catheter handling in our unit.
Overall results
Dialysis adequacy determined by conventional clinical data and biochemistry was evaluated for all patients. The mean results for the two study periods, catheters vs AVA, are presented in Table 1.
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Effective blood flow
The eQb delivered during the dialysis vs time remained quite stable over time for both types of vascular access. The eQb achieved with DualKT and AVA vs the setQb is displayed in Figure 1.
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For a setQb of 350 ml/min, a reduction in the effective flow by 35 ml/min (9.7%) with catheters, and by 10 ml/min (2.7%) with AVF, was observed. The blood flow variation over time averaged 14 and 15% with DualKT and AVA, respectively. Therefore, considering that the average dialysis session lasted 220 min, catheter use reduced the amount of blood cleared by 7 l, as compared with 2 l using AVA.
Flow resistance
An estimation of the flow resistance for both access types was derived from the venous pressures values recorded at a setQb of 350 ml/min on the dialysis monitor. A comparison for DualKT and AVA is presented in Figure 2. As shown DualKT exhibits a higher resistance to extracorporeal flow used in the same operational conditions.
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Recirculation vs time
Mean monthly Rec vs time for catheters and AVA is illustrated in Figure 3. As shown, values remain relatively constant over time in both types of vascular access. Interestingly, Rec values were always higher with AVA than with DualKT.
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Dialysis dose vs time
Dialysis dose was evaluated in this study using the fractional body urea clearance, Kt/Vsp. The double-pool equivalent, Kt/Vdp, was obtained by correcting for intracorporeal disequilibrium and rebound [15]. As shown in Figure 4, Kt/Vdp delivered with either type of vascular access remained stable, averaging 1.4 over the 2-year period. However, it is clear that the Kt/Vdp delivered with DualKT is 56% lower than that achieved with AVA. Nevertheless, the dialysis dose remained above that recommended in the DOQI guidelines (Kt/V
1.2) in all cases.
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Residual renal function
Monthly assessment of the RRF was based on a calculation using the mean of the measured urea and creatinine clearances during a 48 h urea collection. During phase A, the mean RRF was 0.6±1.4 ml/min and 23.6% patients still had a residual clearance, while during phase B, the mean RRF was 0.4±1.2 ml/min and only 22.6% patients kept a residual clearance. The difference between the two study periods was significant (P<0.05, paired t-test). Note that the RRF value was not included in the calculation of the dialysis dose (Kt/V).
Protein nutritional status and protein catabolic rate
The nutritional status in all of our ESRD patients was maintained. Serum albumin measured by laser nephelometry improved significantly during the AVA period, which could have been due to the overall improvement of the nutritional status of the patients after the first year. The creatinine index, a marker of lean body mass and nitrogen reserve, was near normal and no significant change (21±0.4 vs 19.7±0.5 mg/kg/24 h) was observed over the study period. The nPCR, evaluated from the urea generation rate based on UKM, did not exhibit significant changes over the 2-year observation time. The mean value of nPCR averaged 1.05 g/kg/24 h, which can be considered normal for stable patients equivalent to their protein nutritional intake.
No significant changes were observed in other markers of dialysis adequacy such as body weight, mean arterial pressure, correction of anaemia, control of acidosis or control of the phosphoruscalcium equilibrium.
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Discussion |
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Performances of DualKT in this study were shown to be slightly inferior to those achieved by means of AVA. The effective blood flow with venous catheters for instance was about 1015% lower than with AVA. For the same setQb of 348±2.2 ml/min, the effective flows achieved with DualKT and AVA were 316±3.5 and 340±3.3 ml/min, respectively. From the mean duration of dialysis sessions (220 min) it was calculated that the amount of blood cleared during a session was 97.7% (74.8 l) with AVA, and 90.8% (69.5 l) with catheters of the expected volume (76.6 l). Accordingly, one can estimate that the effective blood volumes cleared per session are 10 and 3% lower than expected with DualKT and AVA, respectively. These findings are in good agreement with the fact that silent reduced blood flow rate is one of the main determinants of reduced urea clearance and delivered dialysis dose [16]. Therefore, to assure an adequate treatment, it appears highly desirable to use the effective blood flow rate derived from correcting the nominal blood flow for pressure effects or by measuring the extracorporeal flow with adequate devices [17].
In this study Rec was evaluated by the slow blood flow method. Due to this methodology, the Rec values discussed refer to total recirculation including both vascular access and cardiopulmonary bypass phenomenon. The recirculation of DualKT (8.6±0.6%) was significantly lower as compared with AVA (12.1±0.8%), and did not alter over time, as shown in Figure 3. To interpret our findings correctly, certain conditions need to be taken into account. Firstly, the validity of our study is kept intact since patients served as their own controls during the AVA period. Secondly, veno-venous extracorporeal circulation excludes cardiopulmonary recirculation. Thirdly, a Rec value determined by the slow blood flow method delivers a figure for total Rec. Accordingly, we therefore estimated true intra-access Rec for AVA as the difference between AVA and catheter values. Based on this rationale, AVA Rec averaged 3% in our protocol, a value close to reported data determined by a more sensitive ultrasound method [18].
Blood flow resistance calculated from venous pressure and blood flow ratio was significantly higher within catheters than AVA. One can estimate from this simple index that the DualKT implemented a 13% higher resistance to flow than AVA. Interestingly, the resistance as illustrated by the venous pressure profile did not change significantly over time either for AVA or for DualKT (Figure 2). This observation confirms that the flow resistance of catheters can remain unchanged for 12 months and endoluminal fibrin sheat and/or biofilm formation was unlikely.
Dialysis dose delivery in ESRD patients was objectively assessed by monthly evaluation of the body urea Kt/Vdp index. As shown in this study, body Kt/Vdp was significantly reduced with permanent catheter use. Indeed, the loss of efficacy associated with DualKT averaged 6% overall in the study. Such a reduction is not striking when a highly efficient programme is regularly provided and overall dialysis efficiency is secured by a quality assurance programme, as in our centre. No patient had to be transferred prematurely to AVA because of inadequate dialysis. Loss of the dialysis dose may become critical when short and/or low efficiency dialysis schedules are prescribed.
In this respect, it is also remarkable that the protein nutritional status (albumin, nPCR and creatinine index) was adequately maintained in our study population.
Although it was not the primary objective, it is worth mentioning that catheter-related morbidity (infection, thrombosis, dysfunction) remained quite low compared with reported incidences [19]. Disregarding the eight patients who were excluded from the study for different reasons, only three patients developed bacteriaemia related to endoluminal catheter colonization. Those notable results were achieved by a continuous quality care improvement policy developed by our nursing staff in conjunction with physicians and microbiologists [20]. In this respect, it is important to recognize the role of early detection and treatment of microbial catheter colonization.
Performances of permanent catheters for HD are largely dependent on factors including catheter type, surgical practice, placement, catheter handling and catheter care. The flow adequacy of catheters may change silently with time, leading to under-dialysis [21]. This study is in agreement with studies reporting that two independent catheters are more likely to be able to provide a high, consistent flow rate, reduced recirculation rate and low resistance profiles [22,23]. This, brought together with short-term dialysis concepts, was the basic prompt that led to the DualKT concept. Indeed, the results reported in this investigation, based on a double catheter, must be interpreted with caution and one should not expect similar results for all permanent venous catheter devices. It is advisable to periodically evaluate any permanent catheter for functional adequacy, like flow performances, recirculation and internal flow resistance. Early detection of catheter dysfunction is essential to prevent shortages in the dialysis doses delivered.
In conclusion, permanent venous catheters are increasingly accepted as a valuable alternative vascular access in patients without AVA at dialysis entry, and when AVA is not feasible or suitable, or is contraindicated, even for prolonged periods of time. A permanent catheter may be an immediately available bridging solution prior to an AVF. Fistula creation should be done early in the course of the ESRD, as recommended by DOQI and other guidelines.
Permanent catheter use is associated with a 6% reduction in the dialysis dose. Such a loss of dialysis efficacy has no deleterious consequences when an efficient treatment programme is applied. However, it may become critical with borderline dialysis prescriptions, requiring careful attention to prevent poor patient outcomes and to maintain dialysis quality.
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Acknowledgments |
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Notes |
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References |
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