Ultrafiltration

The principle of ultrafiltration was first described in 1907 by Heinrich Jakob Bechhold (van Treeck 2019). With regard to the medical use of ultrafiltration, the Swede Niels Alwall (1904 - 1986) succeeded in 1947 in developing a dialyser that not only removed uremic toxins but also dewatered by controlled ultrafiltration. Such devices were in use in Europe until the 1960s (Hepp 2017).

Ultrafiltration is a physical "screening principle".

Liquids (or also gases) are separated from each other by a semi-permeable membrane. The pores of the membranes used in ultrafiltration are usually between 10 nm - 30 nm in size and thus permeable for low-molecular substances up to a molecular weight of 25,000 Dalton.

As soon as there is a pressure difference between the liquid solutions (or gases) separated by the membrane, these can penetrate the membrane and with them the molecules contained in them (Günthert 2004).

In the field of medicine, the principle of ultrafiltration is used, for example, in dialysis.

By increasing the pressure in the blood (or by reducing the pressure in the dialysis fluid), the fluid stored in the body, including the particles it contains such as uremic toxins, drug residues, etc., can diffuse through the membrane into the isotonic or isoionic dialysate fluid (Kuhlmann 2015)

The concentration gradient between blood and dialysate is maintained by machine (Kuhlmann 2015).

The uremic toxins are removed by convection, in which the substances contained in the filtered volume are entrained by the suction of the liquid flow (so-called "solvent drag" [Jacobi 2012]).

However, this is only of secondary importance. Ultrafiltration in dialysis is primarily used to remove plasma water (Kuhlmann 2015)

The amount of ultrafiltration can be adjusted on the dialyzer, whereby the amount of residual urine remaining determines the amount of ultrafiltration: the more residual urine, the lower the amount of ultrafiltration can be adjusted and vice versa (Geberth 2011).

Use of ultrafiltration

As ultrafiltration is an invasive technique for removing fluids (Kasper 2015), it is used in renal replacement therapy in the context of chronic renal insufficiency or intoxication with ultrafiltration substances, e.g.:

• Hemofiltration (HF): In HF - unlike hemodialysis - the dialysis fluid is not passed through a dialyzer. Instead, a haemofiltration solution is injected intravenously and removed by the haemofilter with the help of ultrafiltration. In addition, urinary substances are removed from the blood by convection (Kuhlmann 2015).
• In peritoneal dialysis (PD), see also APD: Here the peritoneum is used as a semi-permeable membrane through which liquids and urinary substances can be removed from the body via the dialysate with the help of ultrafiltration (Kuhlmann 2015).
• Peritoneal equilibration test(PET): PET measures the ultrafiltration properties of the peritoneum, among other things. The high-percent glucose used in the test stimulates ultrafiltration via aquaporin channels (Bruck 2017).
• for intoxications: A kidney replacement treatment is also used for extracorporeal toxin elimination in intoxications with dialysable or ultrafiltratable toxic substances. These substances include various alcohols, barbiturates, bromides, carbamazepine, caffeine, lithium, methotrexate, salicylates, theophylline, valproic acid, etc. (Kuhlmann 2015 / Herold 2020)
• in acute heart failure: Ultrafiltration during dialysis was recently investigated in a study (Cardiorenal Rescue Study = CARRESS-HF) also in patients with acute decompensated heart failure and deteriorating kidney values. One group received pharmacological treatment, the other group received dialysis. It was found that the level of mortality, the length of stay in hospital and the weight loss achieved were the same in both groups. However, in the dialyzed group, creatinine continued to rise, leading to kidney failure and bleeding complications. Ultrafiltration cannot therefore be recommended as a primary strategy in acute heart failure (Kasper 2015).

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