Hemodialysis Devices: Dialyzers

Thursday, October 21, 2010

Hemodialysis Devices: Dialyzers

Article by a Renal Nurse

Sections of Dialyzers

FUNCTIONS AND COMPONENTS

• The dialyzer, dialysate, and delivery system replaces some tasks that failed kidneys can no longer do.
• Every dialyzer has a blood and a dialysate compartment,
• The semi permeable membrane keeps the two compartments apart.
• The membrane is housed in a plastic case, which holds the dialyzer together and forms pathways for blood and dialysate to flow in and out.

DIALYZER CHARACTERISTICS

Image Source: iomed.brown.edu

Many aspects of a dialyzer can affect treatment effectiveness, comfort, and patient safety. These includes:

• Biocompatibility (how much a membrane is compatible with the human body)
• Membrane surface area
• Molecular weight cutoff (the solute size that can pass through the membrane)
• Ultrafiltration coefficient
• .Clearance (the rate of solute .removal)

Biocompatibitily
• Biocompatible means not harmful to biological function.
•All materials used to make dialysis membranes react to some degree with immune cells in the blood. It is vital to use a membrane the patient can tolerate.
• Biocompatibility of a membrane can be tested by checking the patient's blood for certain proteins and chemicals.
• A membrane's ability to adsorb (attract and hold) proteins into the fiber wall is key to its biocompatibility. Adsorbed proteins coat the surface so blood does not touch the "foreign" membrane.
• This protein coating explains why reprocessed (cleaned and reused) dialyzers can be more biocompatible than new ones.

Note: Reprocessing dialyzers with bleach can strip the protein coating off the membrane.
• Synthetic membranes are more biocompatible than
cellulose membranes.
• Synthetic fibers are hydrophobic (water repelling)
• These makes them better able to adsorb blood proteins,

Surface Area
• Surface area is key to how well a dialyzer can remove solutes.
• If all other aspects are equal, dialyzers with more surface area can expose more blood to dialysate. This means more solutes can be removed from the blood.
• Total dialyzer surface area can range from 0.5 - 2.4 square meters.

Mass Transfer Coefficient
• Mass transfer coefficient (KoA) is the ability of a solute to pass through the pores of a dialyzer.
• The KoA, in theory, is the highest possible clearance of a given dialyzer at infinite blood and dialysate flow.
• The higher the KoA, the more permeable the dialyzer.

Molecular Weight Cutoff
• Each membrane has a molecular weight cutoff determines the largest molecule that can pass through the membrane.
• It is measured in daltons (Da).
• It is the average weight of a molecule, expressed as the sum of the atomic weights of all the atoms in the molecule.

• Larger molecules have higher molecular weights; smaller molecules have lower ones.
• Dialyzers can be chosen with molecular weight cutoffs ranging from 3,000 to more then 15,000 Da.

Molecule Molecular Weight (Da)
Albumin ---------------------- 66,000
Calcium (Ca++) -------------- 40
Creatinine -------------------- 113
Nitric Oxide (N03) ----------- 62
Phosphorus (P042) ----------- 94.9
Urea -------------------------- 60
Water (H2O) ----------------- 18
Zinc (Zn2+) ------------------- 65.3

Image Source: lhsc.on.ca

Ultrafiltration Coefficients
• Another key aspect of a dialyzer is how much UF of water can occur across the membrane.
• UF is a way to remove excess water from a patient during hemodialysis by applying pressure.
• Hydraulic pressure applied to the blood or dialysate compartment forces water across the membrane.
• The dialysis machine can vary the hydraulic pressure to control the ultrafiltration rate (UFR) and amount of water removed.
• High pressure in the blood compartment forces more fluid out of the blood and into the dialysate.
• The pressure difference across the membrane (blood compartment pressure minus dialysate compartment pressure) is transmembrane pressure (TMP).
• Each dialyzer has a manufacturer's Kur.
• The Kuf is the amount of fluid that will pass through the membrane in one hour, at a given pressure.
• The Kuf helps the staff member predict how much fluid will be removed from the patient during a treatment.

Image Source: lhsc.on.ca

A dialyzer with a Kuf of 10 will remove 10 ml of fluid per hour for each mmHg of pressure.

Let's say a dialyzer has a Kuf of 10, and a TMP of 100 mmHg, the patient would lose 1,000 ml of fluid per hour of dialysis (10 x 100 = 1,000).

Clearance
• Dialyzers vary in how well they remove solutes from the blood.
• The amount of blood that can be cleared of a solute in a given period of time is called clearance (K).
• Clearance rates for different molecules are given by the manufacturer for certain blood and dialysate flow rates.
• There are three main ways to remove solutes that affect a dialyzer's clearance: diffusion, convection, and adsorption. Diffusion
• Most solutes are removed during dialysis by diffusion.
• Best way to remove small (low molecular weight) solutes.

Clearance: Diffusion
• The diffusion rate depends on:
• Blood and dialysate How rates
• Membrane surface area and thickness
• Number of pores
• Solution temperature
• Membrane resistance
• Concentration gradient .,:
• Size. weight and charge of the solutes

Clearance: Convection
• When fluid crosses a semipermeable membrane, some solutes are pulled along with it. This is called convection, or solvent drag
• Convection is the best way to remove larger solutes.
• A sieving coefficient is used to say how much solute is expected to be removed by convection.

Clearance: Convection
• A sieving coefficient of 0.5 for a solute means that 50% of the solute will pass through the membrane to the dialysate side. The other 50% will be adsorbed or rejected by the membrane.
• Convective clearance depends on:
• Molecular weight cutofft he membrane
• Membrane surface area
• Ultrafiltration rate (UFR)

Clearance: Adsorption
• Adsorption occurs when material sticks to the dialyzer membrane.
• All dialyzers adsorb materials, usually small proteins, to some extent.
• Hydrophobic synthetic membranes adsorb more than cellulose membranes
• Pros & Cons (in dialysis)
• It is useful because the adsorbed protein keeps the membrane away from the blood, for better compatibility.
• Bill adsorbed material can build up on the membrane and may prevent some diffusion and convection.
• Highly adsorptive membranes may become less effective when they are reprocessed many times.
• Testing dialyzers for total cell volume (also called fiber bundle volume) may not reveal this problem.

NOTE: If total cell volume is still high, dialyzer can still be used.

Clearance: Adsorption
• Total cell volume is an indirect measure of changes in solute transport for hollow fiber dialyzers that are reused.
• A dialyzer's adsorptive ability depends on:
     • Membrane material
     • Surface area .
     • How much material has already adsorbed to the membrane

DIALYZER DESIGN
• A hollow fiber dialyzer is a clear plastic cylinder that holds thousands of fiber tubes almost as thin as strands of hair.
• These fibers are held in place at each end by polyurethane, clay-like "potting" material that holds fibers open so blood can flow inside them.
• Hollow fiber dialyzers allow for well-controlled, predictable UF.
• Because the fibers are rigid, there is no membrane compliance (change in shape or volume due to pressure).
• Instead, the fibers hold almost the same amount of fluid at high pressures as they do at low pressures.
• Resistance to blood flow is low in hollow fiber dialyzers.

MEMBRANES
• The semipermeable membrane acts in some ways like the vessel wall of a human nephron, because it
is selective.
• Riddled with microscopic pores, the membrane allows only certain "solutes and water to pass through.
• Large substances such as protein and blood cells won't fit through the small pores.
• There are membrane factors that affect removal of solutes and fluids during dialysis.
• These include the membrane material and characteristics of each dialyzer.

Membrane Materials

• Can affect diffusion and UF as well as efficiency of dialysis and the patient's comfort during treatment. Cellulose membranes
• Are made form cotton-based material that is spun into hollow fibers.
• Dialyzers with cellulose membranes have thin fiber walls (8-15 microns).
• The size of molecules cleared by these dialyzers is quite limited - about 3,000 Da.
• Removal of molecules in the larger molecular weight range, such as betta-2-microglobulin (B2m 11,800) is slower.
• Cellulose dialyzers have surface areas that range from 0.5-2.1 meters.
• Larger cellulose membranes have in vitro (tested in a laboratory) urea and creatinine clearances that compare to synthetic dialyzers.
• Cellulose dialyzers are the least biocompatible, and cause the most complement activation.
• This type of membrane is also least able to remove solute by adsorption.

Modified cellulose membranes
• The hydroxyl group (OH-) are rempved and replaced with acetate (cellulose acetate), amino acids, or synthetic molecules.
• They have much thicker fiber walls, 22-40 microns.
• They use convection, diffusion and adsorption to remove solutes.
• Clearance of solutes, especially middle molecules, depends mainly on UF rates.
• These dialyzers do a good job of removing solutes up to 15,000 Da, clearing B2m to some extent.
• Biocompatibility of these membranes ranges from good to very good.
• The best of these are close to pure synthetics.

Synthetic membranes
• Are made from polymers that are formed into hollow fibers.
• The materials used in synthetic membranes are:
     • Polycarbonate
     • Polyacrylonitrile(PAN)
     • Polysulfone (PSF)
     • Polymethylmethacrylate (PMMA)
• These dialyzers have the thickest fiber walls, 30-55 microns.
• Solutes are removed by convection, diffusion, adsorption.
• Clearance of solutes, especially middle molecules, depends mainly on UF rates.
• Do a good job of removing solutes up to 15,000 Da, clearing dim to some extent.
• Biocompatibility of these membranes are very good
• They are highly adsorptive, so they can quickly keep the blood from touching the membrane.

MEASURING DIALYZER EFFECTIVENESS
• A dialyzer's effectiveness is checked by testing its clearance.
• Clearance is expressed as the amount of blood (in mL) that is completely cleared ofc .'ertain solute in one minute of treatment, at a given blood flow rate (Qb) and dialysate flow rate (Qd).
• For example:
     • A dialyzer has a stated urea clearance of 250mL/mim at aQbof300mL.
     • In one minute, 250 ml of blood would be cleared of urea by the dialyzer
     • If 300mL of blood is pumped through the dialyzer in one minute, only 250 mL of blood will be cleared of urea.
• The dialyzer's surface area is fixed. So either Qb or Qd must be increased to increase clearance.
• Qb is always a factor that limits clearance, since there is a limit to how quickly blood can flow out of the patient's vascular access.

DETERMINING DIALYZER CLEARANCE
• Manufacturers test dialyzers in vitro, using watery fluids that are thinner than blood.
• When measured during actual use on patients, a dialyzer's real clearance can differ from the manufacturers stated values by ± 10 - 30%.
• The clearance of urea (a small molecular weight solute) is most often used to test the overall effectiveness of a dialyzer.