Urea rebound and delivered Kt/V determination with a continuous urea sensor

Abstract

Background. The recent introduction of urea sensors for dialysis monitoring has made possible new approaches to urea kinetic modelling. In this study we show how the equilibrated postdialysis urea concentration (C(eq)) and Kt/V corrected for double-pool urea kinetics (Kt/V(dp) can be accurately determined using an on-line sensor providing a continuous measure of blood water urea. A modification of the Smye constant volume double-pool theory led to the following equations for C(eq) and Kt/V(dp) where C(pre) is the blood concentration measured at the start of dialysis, t is the length of the dialysis session (in min) and S(ex) is the constant slope of the blood urea logarithm concentration decline following development of the intercompartmental urea concentration gradient in the first 30-60 min of dialysis. Methods. These equations were tested in 11 patients undergoing 165-240 min of paired filtration dialysis with continuous monitoring of blood urea concentration. C(pre) was determined as the plateau concentration during a preliminary period of 15-20 min of slow isolated ultrafiltration. S(ex) was accurately determined from linear regression applied to the urea sensor data from the 80-min point to the end of dialysis. Results. C(eq) and Kt/V(dp) determined from the above equations compared closely to values determined from 25-40 min of urea rebound monitoring with the urea sensor: 10.6 ± 3.0 versus 10.8 ± 2.7 mmol/l (mean ± SD) for C(eq) and 1.21 ± 0.24 versus 1.18 ± 0.20 for Kt/V(dp), compared to single-pool values of Kt/V = 1.34 ± 0.23. Conclusion. This technique may be readily programmed into on-line urea monitors to provide current and extrapolated values of C(eq) and Kt/V(dp) from about the first hour of dialysis.