NDT Advance Access first published online on February 13, 2008
This version published online on February 26, 2008
Nephrology Dialysis Transplantation, doi:10.1093/ndt/gfm938
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Dosing intermittent haemodialysis in the intensive care unit patient with acute renal failure—estimation of urea removal and evidence for the regional blood flow model
1 Section of Dialysis and Extracorporeal Therapy, Department of Hypertension/Nephrology, OH, USA 2 Department of Renal Medicine, Newcastle-upon-Tyne Hospitals NHS Foundation Trust, UK 3 Department of Biostatistics and Epidemiology, Cleveland Clinic Foundation, OH 4 Department of Medicine, University of Illinois College of Medicine at Chicago, IL 5 Division of Nephrology, University of California Davis Medical Center, Sacramento, CA, USA
Correspondence and offprint requests to: Nigel S. Kanagasundaram, Department of Renal Medicine, Freeman Hospital, High Heaton, Newcastle-upon-Tyne NE7 7DN, UK. Tel: +44-0191-233-6161 ext 37149; Fax: +44-0191-2231233; E-mail: suren.kanagasundaram{at}nuth.nhs.uk
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Abstract |
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Background. Blood-side dosing methods may overestimate urea removal in comparison to dialysate-side measurements during intermittent HD (IHD) for acute renal failure (ARF). The present study sought to quantify this mass balance error (MBE) and explore potential explanatory factors.
Methods. Prospective, formal, blood-side urea kinetic modelling was performed in serial sessions (n = 42) in 18 intensive care unit ARF patients. Three blood-side estimates of urea removal were calculated and these were compared to urea removal derived from fractional dialysate sampling and use of an on-line urea monitor. We also examined urea rebound in these patients, as expressed by the intercompartmental urea clearance (Kc), and in a subset of patients examined the relation of Kc to cardiac output and systemic vascular resistance (SVR).
Results. The mean % MBE (MBE = blood – dialysate-estimated urea removal) was about 9% using conventional two-pool modelling based on a 60-min post-dialysis blood urea nitrogen (BUN) with or without the use of one or more intra-dialytic BUN values. The extent of MBE could not be explained by the clinical or dialytic variables that were measured. Part of the MBE error was due to overestimation of the intradialytic BUN profile, because model-independent profiling of intra-dialytic BUN values to compute urea removal reduced the MBE to 
6%. The log Kc was correlated with cardiac output and showed trends towards an inverse correlation with SVR.
Conclusions. Classical, two-pool, blood-side UKM produces a modest overestimate of urea removal in IHD for critically ill ARF patients. The source of this small, residual MBE is unknown. The amount of urea rebound, as reflected by Kc, varied among patients and associated with cardiac output and SVR, as predicted by the regional blood flow model.
Keywords: mass balance error; multi-compartment modelling; renal replacement therapy; urea kinetic modelling; vasopressors
Abbreviations: BUN, blood urea nitrogen • BUNeq, equilibrated BUN calculated using Runga–Kutta algorithm • DDQ, direct dialysis quantification • E, dialyzer extraction ratio • eKt/V, equilibrated Kt/V • eKt/ Vrate, equilibrated Kt/V calculated from rate equation • eKt/Vref, equilibrated Kt/V calculated using Runga–Kutta algorithm • fVDDQ, dialysate-side estimate of V using fractional DDQ • G, urea generation rate • IHD, intermittent haemodialysis • Kc, inter-compartmental mass transfer coefficient • Kd, dialyzer clearance • Kdcross, cross-dialyzer clearance • Kdcross 1–3, cross-dialyzer clearance at start, mid-point, end of session, respectively • KoA, mass transfer area coefficient for urea • Kr UN, residual renal clearance of urea nitrogen • MBE, mass balance error • Qb, extracoporeaal blood flow rate • Qd, dialysate flow rate • Quf, ultrafiltration rate • REMbl-dp, blood-side UN removal using two-pool curve fit to peri-dialysis BUNs • REMbl-intra, blood-side UN removal using two-pool curve fit to intradialysis BUNs • REMbl-emp, blood-side UN removal using empirical curve fit to intradialysis BUNs • REMdi, actual UN removed measured from fractional dialysate collection • REMdi-bio, actual UN removed measured by Biostat 1000 • SIRS, systemic inflammatory response syndrome • spKt/V, single-pool Kt/V • Td, treatment time • UKM, urea kinetic modelling • UN, urea nitrogen • V, urea distribution volume • Vdp, double-pool V • Vdprate, double-pool V based on rate equation • Vdpref, double-pool V calculated using Runga–Kutta algorithm • Vdp(wtadj), weight-adjusted value of estimated Vdprate (see text) • Vsp, single-pool V • Watson V, Watson anthropometric estimate of total body water
Received for publication: 11. 7.06
Accepted in revised form: 18.12.07
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