Plasma kinetics of small molecules during and after hemofiltration: decrease in hemofiltration efficiency related to increase in ultrafiltration rate.

The influence of delay in mass transfer on the real efficiency of hemofiltration sessions (HF) was studied in 7 patients during HF at a moderate ultrafiltration rate (UF rate = 100 ml/min) and at a high UF rate (UF rate = 200 ml/min). Real efficiency was expressed as "effective clearance" (KE) and compared to plasma clearance (KP); KE/KP was calculated from the kinetics of small molecules during HF and stabilized rebound post HF. Rebound in urea and uric acid plasma levels stabilized by 90 min post HF; increase in the UF rate from 100 to 200 ml/min was responsible for a decrease in KE/KP of 4% for urea and 11% for uric acid. Plasma creatinine and phosphorus levels had not stabilized by 90 min post HF, and it was thus impossible to calculate effective clearance; on the other hand, the magnitude of the rebound phenomenon for these two molecules was considerably greater than for uric acid. The magnitude of the post HF rebound for creatinine and phosphorus might be associated with delayed diffusion from a bound form in the intracellular space.