Urea kinetics during hemodialysis measured by microdialysis--a novel technique.

A microdialysis technique has been developed for estimation of concentrations of low molecular size compounds in the interstitial fluid in vivo. With this technique urea kinetics in the interstitial fluid and plasma were studied in ten patients during and after hemodialysis. There was a close correspondence between urea measurements in plasma and interstitium during hemodialysis. Urea rebound occurred in plasma during two hours after dialysis (15.8 +/- 6.5% in the first hour and 11.8 +/- 5.9% in the second hour). The urea rebound in the interstitium was delayed about 60 minutes after that of plasma (2.8 +/- 8% and 14.1 +/- 7.8% in the first and second hours, respectively) and continued for up to four hours after dialysis. The relationship between plasma urea rebound and the efficiency of hemodialysis and ultrafiltration volume was studied in 17 patients. Results showed a close relation between the fractional urea removal during dialysis and the plasma urea rebound. The contribution of de novo urea genesis to the rebound was estimated from the interdialytic increase in plasma urea concentrations, and was 17 to 24% of the plasma urea rebound during two hours postdialysis. The initial plasma urea rebound could in part result from urea influx to plasma from the enterohepatic recirculation of urea nitrogen. Plasma urea rebound should be taken into account for determination of the amount of dialysis delivered during hemodialysis.