Predilution hemofiltration: a new technology applied to an old therapy.

Postdilution hemofiltration (HF) as practised during the 80's is today associated with limitations of a medical, practical and economical nature. High blood flow rates are required to generate sufficient ultrafiltrate in order to achieve a clearance of small solutes comparable to hemodialysis within a reasonable time. High hematocrit and large body weight lead to extended treatment times. IV-quality solution is required in large volumes. This makes the use of bicarbonate difficult and has placed HF among the most expensive renal replacement therapies. These limitations can be resolved by performing HF in a predilution mode using an on-line prepared infusion solution. Diluting the blood before filtration increases the filtration fraction and the clearance of all solutes which are sieved by the membrane. Comparing pre- to postdilution at similar blood flow rates, the clearance may increase by 50% but three times as much infusion solution is required. To make predilution economically viable, the on-line preparation of the infusion solution is necessary, and this facilitates the use of bicarbonate. Compared to other dialysis therapies this new form of HF offers unequalled solute removal over a large molecular range.