Hemodialysis Kinetics 101

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Transmembrane Pressure

June 16, 2017 itg

CLINICAL PEARL:

In modern dialyzers with volumetric control of ultrafiltration, TMP’s primary role is to help monitor filter function:
↓ A drop in TMP could be due to a leak or filter rupture
↑ A rise in TMP could be due to filter clotting

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TMP

Transmembrane pressure(mm Hg) is the hydrostatic pressure gradient which allows for ultrafiltration or convection across a dialyzer membrane i.e. pulls water from the blood to the dialysate compartment. This is achieved by applying a negative suction pressure at the dialysate compartment relative to the positive hydrostatic pressure in the blood compartment.

For a given TMP, the ultrafiltration rate (UF) will depend on the Kuf (or the leakiness) of the dialyzer membrane. High flux dialyzers need less TMP to achieve same UF.

With older low flux dialyzers, TMP was manually calculated to achieve a certain level of UF.

In modern high flux dialyzers, ultrafiltration is volumetrically controlled. This is because a small change in TMP with a high Kuf dialyzer membrane, can lead to significant changes in UF. Therefore, imprecise TMP calculations can cause massive fluid shifts and hypotension when using high flux dialyzers.

Although it is no longer used to control ultrafiltration during hemodialysis, TMP still has value for monitoring dialyzer filter function.

The 'key' to KT/V

Basics

Kt/Vurea

Clinical Cases

Namrata Krishnan, MD

Yale University School of Medicine
Veterans Affairs Medical Center
2017