Princicles of Solute Kinetics during Therapeutic Apheresis

Abstract

Therapeutic apheresis (TA) is an extracorporeal depuration technique that, by separating plasma from blood cells, can remove pathogenic substances of high molecular weight (MW) such as auto-antibodies, immunocomplexes, light chains, etc. Solute kinetics in TA are based on the same pharmaco-kinetic principles used in hemodialysis (HD) patients, the main difference being the higher MW of the solutes of interest in AT. Since most high MW solutes have a prevalent intra-vascular distribution and/or are almost exclusively removed from the plasma volume (PV), during TA one can use the single-pool model derived Kt/V for prescribing the “AT dose”: the prescription of removing 1.2–1.4 times the patient's PV actually means a Kt/V of 1.2–1.4 for a solute whose V coincides with PV. However, for a more general modeling in TA, particularly between sessions, a multi-compartment model is required. By using a bi-compartmental model, Hanafusa has recently performed a series of computer simulations in order to analyze the impact of the various parameters on the solute kinetics in AT. In short, the main results of the above simulations are the following: the MW of the substance can help select the most suitable TA technique (the plasma exchange allowing the wider MW spectrum); a lower distribution volume allows for a more efficient removal; both the inter-compartment diffusion velocity and the half life (which is inversely related to the production rate) have an impact on the frequency of the treatment. Hanafusa also noted that the disease severity as well as the presence of bleeding or infection can impact not only on the dose but also on the selection of the AT modality and replacement fluids.