Hemodialysis Devices: Dialyzers

Many dialyzer charachteristics can influence dialysis effectiveness and treatment comfort and safety for pts. These include: biocompatibility, membrane surface area, molecular weight cutoff, ultrafiltration coefficient, and clearance

The similarity to the human body When blood is exposed to a foreign material, immune cells in the blood react to defend the body. These defenses can vary from clotting, which prevents blood loss, to severe allergic reactions

React to some degree with immune cells in the blood. Clinical effects may be so subtle that they go unnoticed, may cause minimal symptoms during dialysis, or may cause major life threatening allergic reactions (anaphylaxis) It is important to use a membrane that the patient can tolerate

The relative biocompatibility of a dialyzer membrane can be measured by testing the pts blood for specific proteins and chemicals. These substances are released in the body when blood encounters a foreign substance and their levels indicate the degree of the dialyzers biocompatibility with the pts blood

A membranes ability to absorb proteins into the fiber wall is an important factor in determining the biocompatibility. Absorbed proteins coat the inside of the membrane. They effectively isolate the blood so that it is no longer exposed to the foreign substance of membrane material

This protein coating explains why reprocessed dialyzers can be more biocompatible than new ones. In general, synthetic membranes are more biocompatible than cellulose membranes because their hydrophob nature makes them more likely to absorb blood proteins.

Water repelling

Surface area is important in determining a dialyzers potential to remove solutes. With all other characteristics being equal, with more surface area, more blood can be exposed to dialysate, and more solutes can be removed from the blood. Total dialyzer surface area can range from 0.5 to 2.2 square meters

Each membrane has a specific molecular weight cutoff which determines the larges molecule that can pass through the membrane. Physician can then determine which molecules they want to remove by choosing the right dialyzer

Is the average wieght of a molecule expressesd as the sum of the atomic weights of all the atoms in a molecule Measured in daltons

This is a way to remove excess water from a pt during dialysis. Hydraulic pressure in the blood or dialysate compartment forces water across the membrane. The dialysis machine can vary the hydraulic pressure to control the rate of ultrafiltration and amount of water removed. High pressure in the blood compartment forces more fluid out of the blood and into the dialysate

Each dialyzer has a manufacturer's specified ultrafiltration coefficient (KUF). This helps the tech predict how much fluid will be removed from the pt during a dialysis treatment. The KUF is the amount of fluid that will pass through the membrane in one hour, at a given pressure.

A dialyzer with a KUF of 10 will remove 10mL of fluid per hour for every millimeter of mercury (mmHg) of pressure. This is stated as mL/mmHg/hr. Therefore, with a KUF of 10 and a pressure difference across the membrane of 100mmHg. a pt would loose 1,000mL of fluid per hour of dialysis (10x100=1000)

Blood compartment pressure minus dialysate compartment pressure is TMP or transmembrane pressure

Dialyzers vary in their ability to remove solutes from the blood. The amount of a specific solute that is removed from a pts blood during dialysis is referred to as clearance.