How does fluid removal occur during the dialysis procedure?
The removal of water during the dialysis treatment is referred to as ultrafiltration. During a hemodialysis treatment, a transmembrane hydrostatic pressure gradient develops between the blood and dialysate compartments. The total pressure difference between these two compartments determines the rate of ultrafiltration. The removal of fluid during the hemodialysis session is increased by any of these factors:
• Higher transmembrane hydrostatic pressure: In most modern dialysis machines, the amount of fluid to be removed during the dialysis session is set on the dialysis machine, and the machine will automatically adjust the pressures to allow for the appropriate amount of fluid to be removed. A higher transmembrane pressure results in more fluid being removed per unit time.
• Higher ultrafiltration coefficient (K Uf ) of the dialysis membrane: The value of this coefficient is dependent on the dialyzer surface area, composition, thickness, and porosity.
• Longer duration of a dialysis session. Ultrafiltration will vary based on K uf and transmembrane pressure. This will result in a set amount of ultrafiltration per minute; by extending the number of minutes, one can get more ultrafiltration.