Jan Sternby

Future Directions in Dialysis Quantification

The influence of dialysis prescription on outcome is well established, and currently the amount of dialysis prescribed is based on small molecular weight toxin removal as represented by the clearance of urea. The “normalized dose of dialysis” (Kt/Vurea) concept is well established. Most techniques for dialysis quantification require that blood samples be taken at the beginning and after the completion of dialysis. The postdialysis sample, however, gives cause for concern because of the “rebound phenomenon” due to nonuniform distribution of urea among body compartments. Blood samples give “indirect” measures of dialysis quantification. Thus direct urea concentration measurements in dialysate may be superior in urea kinetic modeling and these may be made “real time” during dialysis.

Hemodialysis Blood Access Flow Rates Can Be Estimated Accurately from On-Line Dialysate Urea Measurements and the Knowledge of Effective Dialyzer Urea Clearance

Measurement of blood flow rate (Qa) is used to monitor arteriovenous fistulas and grafts that are used for hemodialysis blood access. Most Qa measurements use indicator dilution techniques to measure the recirculation that is induced by the reversal of hemodialysis blood lines. R plus the dialysis circuit flow (Qb) allows the calculation of Qa. The principle of needle reversal also can be used with a dialysate urea monitor (e.g., DQM 200 [Gambro]) without injection of diluent; the effect of the reversal on urea concentration is observed. Access blood water flow rate (Qaw) in relation to the effective clearance (K) is found from the urea concentrations in the dialysate with needles in the normal (Cn) and reverse (Cr) positions: K/Qaw = (Cn - Cr)/Cr. Qa is calculated by adjusting Qaw for hematocrit and protein. For testing of this theoretical relationship, 20 patients who were dialyzed on Integra (Hospal) and Centrysystem 3 (Cobe) machines that were fitted with DQM 200 were studied. During each treatment, lines were reversed and Qa was measured by ultrasound velocity dilution (Transonic HD01 monitor); at the same time, Cn and Cr were measured by DQM 200 and K was calculated. K1 was determined from a predialysis blood urea concentration (Cb), initial dialysate urea concentration (Cd), dialysate flow rate (Qd), and the relationship K x Cb = Qd x Cd (K1). K was determined separately from a conductivity step method using Diascan (Hospal) attached to Integra machines only (K2). With the use of K1, 127 comparisons were made; a correlation existed (r = 0.916), although Bland-Altman analysis showed that the dialysate urea method gave a mean value 5.3% +/- 15.3 (+/-SD) higher than that of Transonic (P < 0.001). With the use of K2, there also was a correlation of (r = 0.944; n = 63), and Bland-Altman testing showed an NS difference of +3.5% between the dialysate urea and Transonic methods. Qa can be estimated from on-line dialysate urea measurements that are taken before and after line reversal together with knowledge of K.

Significance of Distribution Volume in Dialysis Quantification

The urea distribution volume is in several ways an important parameter in the treatment of end‐stage renal disease (ESRD) patients. It has a major impact on the relative dose of dialysis treatment as measured by Kt/V. It is also important in the assessment of fluid status, which has a direct influence on blood pressure. Nevertheless, urea distribution volume is usually not measured on a regular basis, probably because it has been perceived as a relatively complicated measurement. With the arrival of on‐line monitors for dialysate urea this situation has improved radically. In this article a number of volume determination methods are discussed, including some new methods based on on‐line dialysate urea monitoring, which are shown to perform well in comparison to reference methods.

Theoretical basis for and improvement of Daugirdas’ second generation formula for single-pool Kt/V

Purpose An empirically-derived equation to estimate hemodialysis treatment variable-volume single-pool Kt/V, where Kt/V = −ln(R-GFAC × t) + (4−3.5 × R) × UFV/W, was published in 1993 (1) and quickly became a standard tool for the estimation of dialysis dose. We aim to find a theoretical basis for this equation. Methods A mathematical derivation is used to find the connection between Kt/V and modeled urea concentrations. Results There is a theoretical basis for the empirical structure of the estimating equation, but the estimation of the effect of ultrafiltration on Kt/V can be improved. Finally, we show that the accuracy of the formula may be suboptimal for some extreme dialysis schedules and propose a new equation that is more robust across atypical dialysis prescriptions. Conclusions The currently used Kt/V estimating equation has a sound theoretical basis and an improved version is proposed that can maintain accuracy with a broader range of fluid removal.

The Measurement of Hemodialysis Access Blood Flow by a Conductivity Step Method

BACKGROUND AND OBJECTIVES Measurement of blood flow rate (Qa) is used to monitor dialysis access, AV fistulas, and grafts. Indicator dilution measurements of the recirculation (R) induced by reversal of hemodialysis blood lines are commonly used. This plus the dialysis circuit flow (Qb) allows calculation of Qa. R also changes the conductivity, which can be measured by a conductivity cell in the spent dialysate. The change in conductivity caused by line reversal should vary with Qa. A methodology for Qa measurement utilizing this conductivity step is proposed. This study compares conductivity step methodology against the reference method of ultrasound dilution (Qa-Trans). DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS This was an open diagnostic test study in a single academic hospital setting involving 15 hemodialysis-dependent patients. Each was studied over four hemodialysis treatments. During each treatment, two pairs of Qa measurements (conductivity step and Trans) were made. Pre- and postdialysis sodium levels were also measured. RESULTS Average Qa-conductivity step was 1040 ml/min. Average Qa-Trans was 1030 ml/min. The difference was NS. The data pairs showed mean difference of 1.3 +/- 17% (SD). The SD indicates a relatively large variation between data pairs. There was significant linear correlation between the Qa-conductivity step and Qa-Trans results (r = 0.91, P < 0.001). Serum sodium rose slightly but significantly over dialysis (P < 0.001). CONCLUSIONS Qa measurement by conductivity step may be an acceptable alternative to ultrasound dilution methodology. Care must be taken to prevent salt loading when the conductivity step is used.

Baxter Online Hemodiafiltration Systems

The interest in convection as a transport principle in haemodialysis grew in the 1970s. In 1980 Gambro launched the world’s first complete system for auto-controlled haemofiltration, AK-10 HFM 10-1, an important breakthrough in improving clearance of middle sized molecules. Bags with pharmacy-prepared infusion fluid were used and fluid balance was achieved by weighing devices. An experimentally modified HFM-10 system was found to safely deliver on-line prepared fluid for convective treatments.