Renal replacement therapy

Last updated: Tuesday, December 03, 2024

Renal replacement therapy (RRT) is indicated when renal function is so poor that the kidneys are barely operational. RRT is used in the management of acute kidney injury (AKI) to remove toxins, excess fluid and to correct biochemical disturbances. It also forms part of ongoing regular care in patients with end-stage chronic renal failure where kidneys have ceased to function permanently.
There are four main types of RRT in common use:

  • Haemodialysis (HD)
  • Peritoneal dialysis (PD)
  • Haemofiltration (HF)
  • Haemodiafiltration (HDF)
In HD, blood is presented to one side of a membrane, and a dialysis solution is presented to the other side. This encourages toxins to leave the patient’s blood and enter the solution by diffusion. Fluid is removed by a process called ‘ultrafiltration’. In PD this membrane is the patient’s peritoneum. There are two kinds of PD. Continuous Ambulatory Peritoneal Dialysis (CAPD) involves regularly instilling fluid into the abdomen during the day, and later draining it. Automated Peritoneal Dialysis (APD) is the same basic process except that a machine delivers and then drains the fluid at night while sleeping.

In HF, blood is presented to one side of a membrane, but there is no solution on the other side: pressure is used to drive water and solutes through the membrane by convection and this 'filtrate' is then removed. HDF is a hybrid of HD and HF.

HD is performed intermittently (e.g. three times a week) and usually as an outpatient. HF and HDF are used typically in intensive care patients where they run continuously.

 A detailed discussion of the differences between techniques is beyond this tutorial, but the diagram below compares the techniques.



NHS Choices has information for patients about dialysis, and you can also see a detailed diagram of a typical method by which haemodialysis is organised.

Drug removal by RRT

Factors affecting the removal of a drug from the blood by RRT include:

  • Protein binding – highly protein bound drugs are not generally removed by RRT.

  • Molecular weight – very large molecules are less likely to be removed than smaller ones.

  • Water solubility – dialysis solutions are aqueous so water-soluble drugs enter them preferentially. Lipid-soluble drugs tend to have larger volumes of distribution and so concentrations in plasma are comparatively small.

  • Flow rate, and the chemistry and surface area of the membrane.

An outpatient attends his regular haemodialysis session
© Crown copyright 2017
    
Dose adjustment to allow for RRT is only necessary for drugs that require dose adjustment because of the presence of renal failure. But given the major differences between the forms of RRT, it is vital to know which type a patient is receiving before offering any advice! In general the following basic rules apply:

  • No RRT is as effective as the normal kidney – so doses used will never be larger than those recommended in normal renal function.

  • Drugs which are cleared by the kidneys are usually dialysed, and vice versa, although there are some anomalies.

  • If there are no specific guidelines on how to dose a drug in a particular RRT system then for HD and PD use a dose as would be used for a GFR of less than 10mL/min (i.e. severe failure). In practice, although it is recognised that haemofiltration is worse at drug clearance than haemodiafiltration, intensive care units tend to dose patients on either as if they have a GFR of about 15-25 mL/min, i.e. as for moderate renal impairment. There is no need to calculate a patient's CrCl from their serum creatinine to guide drug dosing if they are receiving RRT.

  • Always aim to give drugs after any session of HD otherwise the drug could be removed before it has time to act fully. For haemofiltration or haemodiafiltration, since both are continuous processes, there is no need to schedule doses around RRT sessions.