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      Effects of Postdialysis Urea Rebound on the Quantification of Pediatric Hemodialysis

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          Urea rebound (UR) causes single pool urea kinetic modeling (UKM), which is based on end-dialysis urea instead of its equilibrated value (Ceq), to erroneously quantify hemodialysis (HD) treatment. We estimated the impact of postdialysis UR on the results of formal variable volume single pool (VVSP) UKM [Kt/V, urea distribution volume (V), urea generation rate (G), normalized protein catabolic rate (nPCR), and urea reduction ratio (URR)] in children on chronic HD. Thirty-eight standard pediatric HD sessions in 15 stable patients (9 female, 6 male) aged 14.5 ± (SD) 3.28 years were investigated. The HD sessions lasted 3.75 ± 0.43 h. The single pool urea clearance was 4.84 ± 1.25 ml/min/kg. All HD sessions were evaluated by VVSP and URR (%) with postdialysis urea taken at the end of HD and with Ceq taken 60 min after the end of HD, incorporating double pool effects and representing true double pool values. The anthropometric V was calculated by Cheek and Mellits formulae for children. VVSP significantly overestimated Kt/V by 0.26 ± 0.18 U (1.68 ± 0.36 vs. 1.42 ± 0.30, p < 0.0001), i.e., 19.05 ± 13.07%, G/V (0.20 ± 0.04 vs. 0.18 ± 0.04, p < 0.0001), nPCR (1.26 ± 0.23 vs. 1.18 ± 0.22 g/kg/day, p < 0.0001), and URR (73.92 ± 6.49 vs. 69.22 ± 7.06, p < 0.0001). VVSP significantly underestimated kinetic V in comparison to anthropometric V (18.74 ± 4.04 vs. 20.76 ± 4.43 liters or expressed as V/body weight: 58 ± 8 vs. 65 ± 9%, p < 0.05), while double pool kinetic V was more accurate (21.45 ± 4.34 liters, V/body weight: 64 ± 6%, p > 0.05). We conclude that UR has a significant effect on all results of UKM even after standard pediatric HD, and the degree of this efffect is documented. We suggest an increase of the minimum required prescribed single pool Kt/V in children and reduction of any delivered single pool Kt/V by approxiamtely 0.26 Kt/V U. Overestimation of nPCR by approximately 0.08 g/kg/day and underestimation of V by 8.5% should be kept in mind.

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          Is Post-Dialysis Urea Rebound Significant with Long Slow Hemodialysis?

           G. Jean,  C Chazot,  B Charra (1998)
          Background: According to previous studies, postdialysis urea rebound (PDUR) is achieved within 30–90 min, leading to an overestimation of Kt/V of between 15 and 40% in 3- to 5-hour dialysis. The purpose of the study was to assess the impact of PDUR on the urea reduction ratio (URR), Kt/V and normal protein catabolic rate (nPCR) with long 8-hour slow hemodialysis. Methods: This study was performed in 18 patients (13 males/5 females), 62.5 ± 11.7 years of age, hemodialyzed for 3–265 months. Initial nephropathies were: 3 diabetes; 2 polycystic kidney disease; 3 interstitial nephritis; 2 nephrosclerosis; 3 chronic glomerulonephritis, and 5 undetermined. Residual renal function was negligible. The dialysis sessions were performed using 1- to 1.8-m 2 cellulosic dialyzers during 8 h, 3 times a week. Blood flow was 220 ml/min, dialysate flow 500 ml/min, acetate or bicarbonate buffer was used. Serial measurements of the urea concentration were obtained before dialysis, immediately after dialysis (low flow at t = 0), and at 5, 10, 20, 30, 40, 60, 90 and 120 min, and before the next session. The low-flow method was used to evaluate the access recirculation, second-generation Daugirdas formulas for Kt/V, and Watson formulas for total body water volume estimation. The difference between the expected urea generation (UG) and urea measured after dialysis (global PDUR) defines net PDUR (n-PDUR). Results: The n-PDUR usually became stable after 58 ± 25 (30–90) min. Its mean value was 17 ± 10% of the 30-second low-flow postdialysis urea (3.9 ± 2 mmol/l). This small postdialysis urea value and the importance of UG in comparison with shorter dialysis justify the use of n-PDUR. Ignoring n-PDUR would lead to a significant 4% overestimation (p < 0.001) of the URR (79 ± 7 vs. 76 ± 8%), 12% of Kt/V (1.9 ± 0.4 to 1.7 ± 0.38) and 4% of the nPCR (1.1 ± 0.3 to 1.05 ± 0.3). n-PDUR correlated negatively with postdialysis urea (r = 0.45 p = 0.05), positively with URR (r = 0.31 p = 0.01) and Kt/V (r = 0.3 p = 0.03) but not with K, and negatively with the urea distribution volume (r = 0.33 p = 0.05). Mean total recirculation, ultrafiltration rate, predialysis urea levels and urea clearance did not correlate with n-PDUR. Conclusion: We found a significant PDUR in long-slow hemodialysis after a mean of 1 h after dialysis. This PDUR has a less important impact upon dialysis delivery estimation than short 3- to 5-hour hemodialysis, especially for the lower Kt/V or URR ranges. This is explained by the low-flux, high-efficiency, and long-term dialysis. Its inter-individual variability incites us to calculate PDUR on an individual basis.
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            Paediatric haemodialysis: estimation of treatment efficiency in the presence of urea rebound

             S. W. Smye,  J. EVANS,  E Will (1992)

              Author and article information

              S. Karger AG
              February 2000
              28 January 2000
              : 84
              : 2
              : 124-129
              aUniversity Children’s Hospital, Belgrade, and bDepartment of Statistics, Economic Faculty, University of Belgrade, Serbia
              45559 Nephron 2000;84:124–129
              © 2000 S. Karger AG, Basel

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              Figures: 3, Tables: 1, References: 29, Pages: 6
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