What is the method to determine potassium removal during a hemodialysis (HD) session?

From the Guidelines

Potassium removal during a hemodialysis (HD) session can be determined by measuring the conductivity clearance across the dialyzer membrane.

Methods for Measuring Potassium Removal

• Measuring conductivity (or ionic) clearance across the dialyzer membrane is a method that does not require consumables or blood sampling and can be used with each dialysis treatment to predict the delivered Kt/V in real time before the treatment is finished 1.
• This method is based on the assumption that changes in dialysate conductivity are caused by transmembrane movement of small electrolytes, mostly sodium, that behave like urea.
• To calculate Kt/V, time on dialysis and V must be determined accurately, and multiple ionic clearance measurements must be performed throughout the treatment to avoid errors from changes in clearance during dialysis 1.
• Alternatively, small-solute clearance can also be measured directly across the dialyzer from changes in dialysate outflow conductivity in response to pulsed changes in the dialysate inflow concentration, and this method requires a correction for cardiopulmonary recirculation and an independent measure of V 1.

Considerations for Measuring Potassium Removal

• The dose of hemodialysis for an individual patient should not be based solely on the minimum acceptable amount of urea removed in a single dialysis session, as a patient may still be inadequately dialyzed in terms of potassium removal, correction of acidosis, or failure to render the patient a sufficient protein/caloric intake to prevent malnutrition 1.
• The target spKt/V dose remains 1.4 volumes per dialysis, minimum dose 1.2, for thrice-weekly HD in patients with low residual native kidney clearance (Kru < 2 mL/min) 1.

From the Research

Method to Determine Potassium Removal

The method to determine potassium removal during a hemodialysis (HD) session can be understood through various studies that have investigated potassium kinetics and removal during HD.

• Potassium is removed mainly by diffusion during dialysis, with an average removal of 70-150 mmol per session 2.
• The removal of potassium can be calculated using a pseudo one-compartment model, which takes into account the dependence of serum potassium concentration on time during HD treatments and postdialysis 3, 4.
• The model can estimate potassium mobilization clearance (K(M)) and predialysis central distribution volume (V(pre)), which are important parameters in determining potassium removal 4.
• The diffusive and convective parts of electrolyte removal, as well as the intra- and extracellular space components, can also be calculated to understand potassium balance during HD 5.
• Dialytic potassium removal can be measured by dialysate collection, and total body potassium (TBK) can be measured by whole-body counting of 40K to understand the relationship between dialytic potassium removal and total body potassium 6.

Factors Influencing Potassium Removal

Several factors can influence potassium removal during HD, including:

• Dialysate potassium concentration: higher concentrations can increase potassium removal 2.
• Presence of glucose-free dialysate: can increase potassium removal 2.
• Sodium profiling: can affect potassium removal 2.
• Hyperkalemia: can increase potassium removal 2.
• Predialysis body weight and serum potassium concentration: can affect potassium mobilization clearance and predialysis central distribution volume 4.

Calculating Potassium Removal

The calculation of potassium removal can be done using various methods, including:

• Measuring dialysate collection 6.
• Using a pseudo one-compartment model to estimate potassium mobilization clearance and predialysis central distribution volume 3, 4.
• Calculating the diffusive and convective parts of electrolyte removal, as well as the intra- and extracellular space components 5.