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Membranes used for hemodialysis
Contents:
1-Organic Cellulose (C6H10O5) membranes and its derivatives :
Cellulose is the most common type of dialyzer membrane. It is obtained from
treated wood products and cotton with heat and chemicals and formed into sheets
or extruded through dies as hollow fibers ;e.g. regenerated cellulose, cuprammonium cellulose (cuprophan), cuprammonium rayon, saponified cellulose
ester. Cellulose acetate and triacetate are other widely used cellulose
substances.
Nature:
The membrane is visualized as a thin sheet with
tiny pores. These pores are small enough to hold back blood cells and plasma
proteins, yet large enough to permit water molecules and many solutes to pass
through. Selective permeability is improved by making the membrane thinner,
increasing the numbers of channels between fibers , or increasing the diameter
of passages.
Positive and Negative features:
Hollow fibers have minimal compliance and permit
precise TMP calculation. They are inexpensive. Cellulose membranes however have
more incompatibility problems than do synthetic membranes specially cuprophan.
2-Synthetic membranes:
Synthetic membranes include
Polyacrylonitrile (AN ,PAN)
Polysulfone
Polycarbonte polymide
Polymethylmethacrylate (PMMA)
Ethylene-vinyl alcohol copolymer
Nature:
They are a thin , smooth luminal surface,
supported by a sponge-like wall structure. All have ultrafiltration coefficients
of 20 to 70 ml/hr/mmHg or more.
Positive features:
Middle molecule clearance is greater than cellulose membranes. B.
Microglobulin
is removed by adsorption to the membrane 100 mg /treatment. They have better
biocompatibility with blood than cellulose membrane.
Negative features:
Very expensive. Automated ultrfiltration control
is required because of very high water permeability Significant protein loss by
adsorption. Backfiltration from the dialysate and risk of bacterial or endotoxin
contamination, due to high hydraulic permeability
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Membrane biocompatibility:
Material
used in manufacture of dialysis membrane is associated with some degree of
blood-material interaction. Complement activation that occurs during dialysis
with cellulose membrane is instigated by: 1)Free hydroxyl radicals on the
membrane surface. 2) A cellulose material called Limulus-Amebocyte-Lysate-Reactive-Material
(LALRM). This activation leads to release of histamine, thromboxane, interleukin
1, and tumor necrosis factor as well as direct action on blood cells. The
clinical manifestation varies from minimal symptoms to severe anaphylactic
reaction. (See membrane reactions)
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Types of Dialyzers
(Artificial kidneys)
Dialyzers consists of a series of parallel flow paths designed to provide a
large surface contact area between blood and dialysate. The dialyzing membrane
has pores varying between 11-30 um. The membranes are protected by dialyzer
shell with 4 ports ,2 for blood and 2 for dialysate Dialyzers are broadly
classified as:
1-Coil dialyzer 2-Parallel Plate dialyzer 3-Hollow
Fiber dialyzer
Coil dialyzer :( are of historic
interest)
Basically consists of a flattened cellulose tubing wrapped as a coil and
through which patient’s blood flow during dialysis.The blood channels was long
to obtain the needed surface area, and resistance was high. UF was unpredictable
and blood leak were frequent.
Parallel Plate
dialyzer
Structure:
-Sheets of membranes are placed between supporting plates The plates have ridges
and grooves to support the membrane and allow flow of dialysate along it.
-Resistance to blood flow is low. The surface area vary from 0.25 to 1.5
msq.
Advantages:
-Blood volume is about 50-100 ml at 100 mmHg increases with high TMP (bulging
sheets)
-Heparin requirement usually low, minimal clotting in the blood compartment.
-Ulttrfiltration is reasonably predictable and controllable.
Disadvantages:
-Formation of local thrombi around inlet and outlet ports and corners due to
uneven blood flow at these parts. These may lead to bacterial growth and
endotoxin formation , therefore plates are not often reused.
Hollow Fiber dialyzer
Structure:
It consists of numerous hollow fibers (capillaries) through which the patient’s
blood flow. The hollow fibers are tiny its diameter ~150-250 um. The wall
thickness as little as 7 um. The number of fibers may be 20,000 or more
,depending upon length ,kind of membrane , and surface area of the dialyzer.
Hollow fiber is the most commonly used dialyzers
Advantages:
-It contained low blood volume in relation to surface area 60-90 ml (low priming
volume).
-Resistance to blood flow is low.
-Ultrafiltration can be precisely controlled,
-They are well adapted to reuse.
Disadvantages:
-Deaeration of the fiber predialysis is necessary to prevent air lock of the
fibers.
-Uneven blood distribution at the inflow header space leads to relative
stagnation centrally, with reduced perfusion and clotting of some center fibers
with the subsequent high residual blood volume and aggravation of anemia.
-More heparin is required for most of the patients.
-Adverse patient reaction due to residual toxic products of sterilizing agent.
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Data about the dialyzer includes the following information which guides the
dialysis personnel to select proper dialyzer for each patient :
1-Type of the dialyzer
:(see the previous tip)
2-Material of the membrane
:(see the previous tip)
3-Ultrafiltration Coefficient
,(KUF). (see the previous tip for
Ultrafitration)
The KUF is defined as the number of milliliters of fluid per hour that will be
transferred across the membrane per mmHg pressure gradient across the membrane
The KUF of most dialyzer ranges from 2 to 6 ml/hour. If KUF is low (or high) the
permeability to water is low (or high). In vivo KUF is often lower by 5-30% than
in vitro value The relationship between ultrafiltration, KUF and TMP is
expressed as:
Ultrafiltration rate (ml/hr) = KUF X TMP
4-Clearance
Usually
reported at blood flow rates of 200,300,and 400 ml/minute
Urea
:
A high efficiency dialyzer with thin, large
surface area, wide pores, good design will remove a higher percentage of waste
products. The efficiency of a dialyzer in removing urea can be described by a
constant referred to as Mass transfer urea coefficient -KoA
.This constant influence the relation between the blood flow rate to
the clearance. clearance .Dialyzers of low efficiency have in vitro KoA value of
less than 300 ; used for acute dialysis and small patient. Dialyzers of
moderate efficiency have in vitro KoA value of 300-600;higher efficiency
dialyzers have KoA more than 600-700.Once KoA of the dialyzer is known a
nomogram can be used to predict the blood water urea clearance (Kw) at given
blood (Qb) and dialysate (Qd) flow rate. The in vitro KoA is usually
overestimated ,therefore in vivo value should be estimated from another
nomogram.
Creatinine ,Vit.B12:
The dialyzer creatinine clearance
is ~ 80% of urea clearance. Vitamin B12 dialyzer clearance range is 30-60
ml/minute. It is used as indication for clearance of higher molecular weight.
Molecular weight of some nonionic substances in
mol wt:
BUN = 28 Urea = 60 Creatinine = 113 Albumin =
68000
Glucose =180 Vit B12 = 1355 Ethanol = 46 Methanol = 33
5-Surface area of the dialyzer:
Normally large surface area of the dialyze have high urea clearance. However
dialyzer design and the thinness of the membrane are quit important. Large
surface area is an undesirable feature when an unsubstituted cellulose membrane
is used. It increases the degree of complement activation.
6-The priming volume :
It is related to the membrane surface area; usually 60-90 ml in hollow fibers.
If this added to the priming volume of the blood lines(100-150 ml), the total
extracorporeal circuit will be 160-270 ml
7-Mode of dialyzer Sterilization
1-Exposure of the dialyzer to Ethylene Oxide gas
2-Gamma irradiation - becoming popular
3-Steam autoclaving - becoming popular
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