Kidney Replacement Therapy – The Basics

So what is kidney replacement therapy (KRT)?

As the name suggests, it is the artificial form of removal of wastes and excess fluids from the body as a proxy for the kidneys.

Who needs that?

Patients with acute kidney injury (AKI), as well as any patient with end stage kidney disease (ESKD) or Chronic Kidney Disease (CKD) – stage 5 that is not able to maintain the fluid and electrolyte balance and/or has uremia.  However, it is actually a bit more complicated than that. There has to be a clinical indication to start kidney replacement therapy. 

Wait…. what is this CKD and stages?

Let us take a few steps back and learn about kidneys…

Welcome to the world of kidneys

What does a Kidney do?

Very good. You got it all right.

CKD is a clinical syndrome characterized by a gradual loss of kidney function over time. It is defined as a presence of structural or functional damage for over 3 months. KDIGO defines it as decline in glomerular filtration rate (GFR) below 60 mL/min/1.73 m2 of BSA for over 3 months. GFR is calculated with creatinine levels. Please take a look at the table from NKF KDOQI guidelines.

KRT is utilized in acute as well as chronic settings. Chronically, when GFR falls below 15 mL/min, or when medical management to maintain all the kidney functions listed above fails, RRT comes in handy. 

Acutely, one needs KRT when kidneys are shut down due to insults like infection, inflammation, or toxin exposure or to remove accumulated toxins in the body. It is not that simple. Timing of initiation of KRT is challenging. There are a few RCTs taht studied early vs late initiation of KRT in the ICU but most of them did not show any benefit. More recently, the ELAIN study showed some benefit in early initiation of KRT, but AKIKI and IDEAL-ICU did not. RFN had two posts related to this topic. It is important to note that ELAIN was a single center study and consisted mostly of patients in surgical ICUs.

KRT can be intermittent or continuous. In intermittent, hemodialysis (HD) and peritoneal dialysis (PD) are the two most commonly used methods. Continuous RRT is often seen in ICU setting when patients are acutely ill or when BP is not high enough to support intermittent dialysis. This can be performed as continuous venovenous hemofiltration (CVVH), continuous venovenous hemodialysis (CVVHD), or CVVHDF (continuous venovenous hemodiafiltration). PD can be done continuously as well.

Mechanisms of KRT:

There are two mechanisms of RRT; diffusion and convection (image below). Dialysis depends on diffusion, whereas hemofiltration and hemodiafiltration depends on convection. The movement of solute particles across a semipermeable membrane is diffusion. In HD and CRRT, artificial membrane called dialyzer is used. In PD, patient’s own peritoneum functions as a semipermeable membrane. Dialysate or dialysis fluid, which is low in solutes but high in osmolality is utilized to draw water and solutes from blood.In convection, water is pushed through the membrane using hydrostatic pressure. Along with the water, this pressure drags toxins and waste molecules through the semipermeable membrane. 


Sai Sudha Mannemuddhu MD

Chief Fellow,

Division of Nephrology, Department of Pediatrics

University of Florida, Gainesville

Twitter ID: drM_sudha


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