6 hemodialysis medical equipment

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Lecture-6: Hemodialysis
Sherif H. El-Gohary , Phd
Assistant Professor, Biomedical Engineering
Sh.ElGohary@eng1.cu.edu.eg
Medical
Instrumentation
SBE 310

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Learning objectives
 Introduce Dialysis
 Dialysis Types
 Transport Mechanisms
 Working Principle and Block Diagram
 Dialysate Circuit
 Safety considerations

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Kidney Dialysis
• If one or both of a patient's kidneys fail (or fail
to operate to a sufficient level) then dialysis may
be used to regulate the concentration of urea
and solutes in the blood.
• Although this process may be clinically effective
it is often inconvenient for the patient so may be
used as only a short-term measure - until a
kidney transplant becomes possible.

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How to know dialysis is required
• The decision to initiate dialysis or hemofiltration in patients with
renal failure can depend on several factors, which can be divided into
acute or chronic indications.
• Acute indications for dialysis/hemofiltration:
• Hyperkalemia
• Metabolic acidosis
• Fluid overload (which usually manifests as pulmonary edema)
• Uremic Serositis complications, such as uremic pericarditis and
uremic encephalopathy
• And in patients without renal failure, acute poisoning with a
dialyzable drug, such as lithium, or aspirin

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How to know dialysis is required
Chronic indications for dialysis:
• Symptomatic renal failure
• Low glomerular filtration rate (GFR) (RRT often
recommended to commence at a GFR of less than 10-
15 mls/min/1.73m2)

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What is dialysis machine
Dialysis machine is a electro-mechanical-
hydraulics device which helps us to remove the
unwanted particles (Eg.- K+, Na+, CL-,
Calcium, Magnesium & etc.) from the patients
blood through hollow fiber artificial dialyzer.

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Principle of dialysis
• Dialysis works on the principles of the diffusion and
osmosis of solutes and fluid across a semi-permeable
membrane.
• Blood flows by one side of a semi-permeable
membrane, and a dialysate or fluid flows by the
opposite side. Smaller solutes and fluid pass through
the membrane. The blood flows in one direction and
the dialysate flows in the opposite.

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• The concentrations of undesired solutes (for example potassium,
calcium, and urea) are high in the blood, but low or absent in the
dialysis solution and constant replacement of the dialysate
ensures that the concentration of undesired solutes is kept low
on this side of the membrane.
• The dialysis solution has levels of minerals like potassium and
calcium that are similar to their natural concentration in healthy
blood. For another solute, bicarbonate, dialysis solution level is
set at a slightly higher level than in normal blood, to encourage
diffusion of bicarbonate into the blood, to neutralize the
metabolic acidosis that is often present in these patients.

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Types of dialysis
1. Hem filtration
2. Peritoneal dialysis
3. Hemo-dialysis

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Hem filtration
• Hemofiltration is a similar treatment to hemodialysis, but it
makes use of a different principle.
• The blood is pumped through a dialyzer or "hemofilter" as in
dialysis, but no dialysate is used.
• A pressure gradient is applied; as a result, water moves across the
very permeable membrane rapidly, facilitating the transport of
dissolved substances, importantly ones with large molecular
weights, which are cleared less well by hemodialysis.
• Salts and water lost from the blood during this process are
replaced with a "substitution fluid" that is infused into the
extracorporeal circuit during the treatment.
• Hemodiafiltration is a term used to describe several methods of
combining hemodialysis and hemofiltration in one process.

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Peritoneal dialysis

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Working principle of peritoneal dialysis
• In peritoneal dialysis, a sterile solution containing minerals
and glucose is run through a tube into the peritoneal
cavity, the abdominal body cavity around the intestine,
where the peritoneal membrane acts as a semipermeable
membrane.
• The dialysate is left there for a period of time to absorb
waste products, and then it is drained out through the tube
and discarded.
• This cycle or "exchange" is normally repeated 4-5 times
during the day, (sometimes more often overnight with an
automated system).

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Working principle of peritoneal dialysis
• Ultrafiltration occurs via osmosis; the dialysis solution used contains a high
concentration of glucose, and the resulting osmotic pressure causes fluid to
move from the blood into the dialysate.
• As a result, more fluid is drained than was instilled.
• Peritoneal dialysis is less efficient than hemodialysis, but because it is
carried out for a longer period of time the net effect in terms of removal
of waste products and of salt and water are similar to hemodialysis.
• Peritoneal dialysis is carried out at home by the patient and it requires
motivation.
• Although support is helpful, it is not essential. It does free patients from
the routine of having to go to a dialysis clinic on a fixed schedule multiple
times per week, and it can be done while travelling with a minimum of
specialized equipment.

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Hemodialysis
• Blood is removed from the body and circulated through
an extracorporeal fluid circuit (outside the body), then
returned to the patient.
• This circuit includes a hemodialyzer, which is where the
blood is cleaned.
• The hemodialyzer contains a selectively permeable
membrane, which is a filter that allows fluids and waste
(uremic toxins) to pass through, but prevents the
exchange of blood components, microorganisms and
the "skeletons" of dead microorganisms (endotoxins).

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• The fluid used to clean the blood (dialysate)
flows in the opposite direction to the blood on
the opposite side of the membrane
• While waste and extra fluid are removed from
the blood and end up in the dialysate by
controlling three processes:
Diffusion, ultrafiltration and osmosis.
Hemodialysis

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Diffusion
• Diffusion is the exchange of things dissolved in
fluid (solutes) across the membrane due to
differences in the amounts of the solutes on the
two sides (concentration gradient).
• By controlling the chemicals in the dialysate, the
dialysis machine controls this transfer of solutes
according to the doctor's prescription.

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Ultrafiltration
• Ultrafiltration is fluid flow through the
membrane, forced by a difference in
pressure on the two sides of the dialyzer
(pressure gradient).

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Transport Mechanisms
• Adsorption
–Molecular adherence to the surface or interior
of the membrane

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Hemodialysis Basic parts

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What is AV fistula
• An arteriovenous fistula is an abnormal connection or
passageway between an artery and a vein.
• To create a fistula, a vascular surgeon joins an artery and a
vein together through anastomosis.

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Parts of a Kidney Dialysis Machine
Dialysis Membrane (sometimes referred to as simply a
"dialyser)
• The "dialyser" part of a kidney dialysis machine consists
of a large surface area of cellulose acetate membrane
mechanically supported by a plastic structure.
• Blood is pumped past one side of this membrane while the
dialysate fluid passes on the other side. The membrane
may be folded-over many times so that the large area of
the membrane occupies a practical volume of space.

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Parts of a Kidney Dialysis Machine
Dialysate
The dialysate (solution) has the same solute concentrations
as those in ordinary plasma.
Therefore if the patient's blood plasma contains excess
concentrations of any solutes, these will move into the
dialysate, and if the blood plasma lacks the ideal
concentration of any solutes, these will move into the
patient's blood.
Conversely, the dialysate fluid does not contain any waste
products such as urea - so these substances in the patient's
blood move down the concentration gradient into the
dialysate.

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Parts of a Kidney Dialysis Machine
• Anticoagulant
Heparin is the usual anticoagulant that is added to
the patient's blood as it enters the dialysis machine
(in order to prevent the blood from clotting as it
passes through the machine).
• Preventing the blood from clotting should, in turn,
prevent any blood clots from blocking the filtration
surface of the system.
• However, heparin is not added during the final hour
of dialysis in order to enable the patient's blood
clotting activity to return to normal before he/she
leaves.

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Dialyzer Components–Fiber Bundle
•The fiber bundle comprises 7–17 x 103
semipermeable hollow fibers that allow
solute and fluid transfer between blood
and dialysate
•Typical fibers have internal diameter of
200 mm and wall thickness of 30– 40
mm. They provide 1.0–2.5 m2 of surface
area
•The fiber bundle is enclosed in an
outer housing that forms the dialysate
compartment
3
Fiber
bundle
Dialyzer
housing

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Dialyzer Components–Flow Paths
•Blood flows through the fiber
lumens. Typical clinical blood
flow rates are 200–450
ml/min
•Dialysate flows around the
external surface of the fibers.
Typical dialysate flow rates are
500–800 ml/min
•Blood and dialysate flow in
opposite directions (counter-
current flow) to maximize
diffusive solute transfer
5
Blood in
Blood out
Dialysate
out
Dialysate
in

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Classification Of Dialyzer Membranes
7
•Dialyzer membranes can be classified based on their:
•Chemical composition
• Cellulosic versus synthetic
•Efficiency of small-solute (based on urea, 60 Da) removal
• Low-efficiency (urea KoA < 450 ml/min) versus high-efficiency
(urea KoA > 700 ml/min).
Most modern dialyzers are high-efficiency
• Conventional versus high-permeability
•Biocompatibility
• Biocompatible versus bioincompatible

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Cellulosic Membranes
•Cellulose was the first membrane material widely used for
hemodialysis. It is a polymer of cellobiose and occurs in natural
materials, such as cotton.
•Cellulose membranes are hydrogels and can be made very thin (6–15
mm dry thickness) while retaining good mechanical strength. They
allow high diffusive transport of small molecules (< 200 Da).
8 mm
8

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Synthetic Membranes
•These membranes were made of synthetic polymers
and were initially developed for hemodiafiltration
•There may be more than one type of polymer (i.e.,
alloy) in a given synthetic dialysis membrane that
may impart important characteristics (e.g., methallyl
sulfonate mixed with polyacrylonitrile and,
polyvinylpyrrolidone mixed with polysulfone)
•Synthetic membranes are thick (> 35 m)with cross-
sectional structures that were either homogeneous
(e.g., AN69®, Hospal) or asymmetric (e.g.,
polysulfone).
1
3

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Factors of Dialyzer Membranes
clearance of solute is dependent on the following:
–The molecule size of the solute
–The pore size of the semi-permeable
membrane
• The higher the ultrafiltration rate (UFR), the
greater the solute clearance.

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Membrane Properties–Diffusion Coefficient
1
4
•The primary mode for removal of small solutes (e.g., urea) by
hemodialysis is diffusion down the concentration gradient between
plasma water and dialysate.
• Transfer of small solutes (e.g., HCO3-) from dialyzate to plasma water
also occurs primarily by diffusion
•The rate of diffusion is a function of the thickness and porosity of
the membrane and the diffusivity of the solute in the membrane and
is expressed as the diffusion coefficient of the membrane for a given
solute
•The rate of diffusion is greatest for small molecules. The diffusivity of a
solute in a membrane decreases logarithmically as solute size increases
•The rate of diffusion also decreases as membrane thickness increases
and porosity decreases

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Membrane Properties–Diffusion Coefficient
15
1 2 3 4 5 6 7 8 9
DIFFUSIVITYx106
(cm2
/sec)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
UREA
CREATININE
URIC ACID GLUCOSE
VITAMIN B12
SOLUTE RADIUS x 108
(cm)
Diffusivity of solutes through a cellulose membrane
(adapted from Farrell PC, et al, J Biomed Mater Res
7:275-300, 1973)
Farrell PC, et al: Estimation of the permeability of cellulosic membranes from solute dimensions and diffusivities.
J Biomed Mater Res 7: 275-300, 1973

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Membrane Properties–Sieving Coefficient
1
6
•The primary mode for removal of large solutes by hemodialysis is
ultrafiltration, as water containing these solutes flows from plasma to dialysate
in response to a hydraulic pressure gradient
•The rate of ultrafiltration is a function of the water filtration rate, size of the
solute, and pore size of the membrane. The ability of a solute to pass
through the pores of a membrane is expressed as the sieving coefficient
of the membrane for a given solute
•A solute with a sieving coefficient of 1.0 passes freely through the
membrane, while the membrane is impermeable to a solute with a sieving
coefficient of 0
•Convection provides better removal of large solutes than diffusion because the
decrease in sieving coefficient with increasing solute size is less marked than the
decrease in diffusion coefficient

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Membrane Properties–Sieving Coefficient
SIEVING
COEFFICIENT
0.6
0.8
1.0
URE
A
1
7
GLUCOSE
SUCROS
E
VITAMIN
B12
0.4
0.2
0.0
1 2 3 4 5 6 7 8 9
SOLUTE RADIUS x 108
(cm)
Sieving coefficients for a cellulose membrane.
(adapted from Wendt RP, et al, J Memb Sci 5:23-
49, 1979)
Wendt RP, et al: Sieving properties of hemodialysis membranes. J Membr Sci 5: 23-49, 1979

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Membrane Properties–Hydrophobicity
1
8
•The nature of the polymers used in a membrane determines the
membrane’s tendency to repel water, referred to as hydrophobicity.
•In general, cellulose membranes are less hydrophobic while many
synthetic polymer membranes are more hydrophobic
•Hydrophobic surfaces adsorb serum proteins; adsorption can
contribute significantly to low–molecular-weight protein removal
by some membranes
•Proteins may adsorb to both the planar surface of the membrane
and the inner surface of its pores. The adsorbed protein may reduce
diffusive and convective removal of other solutes by effectively
reducing the pore size of the membrane

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Overall Solute Removal
1
9
•In practice, solute removal occurs through a
combination of diffusion, convection, and adsorption.
•The relative contributions of the three mechanisms
depend on the solute, the membrane, the geometry of
the dialyzer, and operating conditions (blood and
dialysate flow rates and ultrafiltration rate)

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Diffusive Solute Removal
20
•Solute removal by diffusion is limited not only by the
membrane, but also by boundary layers that form at the blood-
membrane interface and the dialysate-membrane interface
•The overall resistance to diffusive solute removal is the sum of
the resistances associated with the membrane and these two
boundary layers. The reciprocal of the overall resistance is the
mass transfer coefficient (Ko).
•The diffusive capacity is usually expressed as KoA for a given
dialyzer and a given solute, where A is the membrane surface
area of the dialyzer
•KoA is generally considered to be largely independent of the
blood and dialysate flow rates.

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Clearance
•Diffusive solute removal by a dialyzer is usually described in
terms of clearance (K), which is defined as the volume of
blood completely cleared of a given solute per unit time, or
•where QBi and QBo are the blood flow rates; CBi and CBo are
the solute concentrations at the inlet and outlet of the
dialyzer, respectively
•Unlike KoA, clearance is dependent on both the blood and
dialysate flow rates. For this reason, it is not a good means
of characterizing the innate dialyzer performance
K 
QBiCBi  QBoCBo
CBi
2
1

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Clearance and KoA
•Clearance is related to KoA by the Michael’s equation:
•where Z is the ratio of the blood flow rate to the dialysate flow rate.
•The Michael’s equation can be used to predict clearance
using KoA values provided by dialyzer manufacturers,
blood flow rate, and dialysate flow rate.

2
2




 


 Z 
Q
exp
1
Q
 exp
K  Q
 B 
KOA1 Z
B 
KOA1 Z
B

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Ultrafiltration Coefficient
2
5
•The ultrafiltration coefficient (kUF) is a measure of the water permeability
of a membrane and is usually expressed in mL/hr/mm Hg
• For example, if a dialysis machine generated a transmembrane pressure
(TMP) of 200 mm Hg, a dialyzer with a kUF of 12 ml/hr/mm Hg would
produce an ultrafiltration rate of 12 ml/hr/mm Hg x 200 mm Hg = 2.4 L/hr
•The FDA defines a high-flux dialyzer as one with a kUF e 12 ml/hr/mm Hg
and a low-flux dialyzer as one with a kUF < 12 ml/hr/mm Hg
•The kUF is practically never a limiting factor for fluid removal. The
ultrafiltration rate is almost invariability limited by the patient’s
tolerance of the rate of fluid removal

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Dialysate Circuit

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Introduction
4
• The process of hemodialysis pumps the patients’ blood against
dialysate that may be generated by the dialysis machine or at a
central location
•Dialysis machines are essentially composed of pumps, monitors,
and alarms that allow safe proportioning of dialysate
•Knowledge of the components of a dialysate circuit
are important for patient safety and care
•It is important for each nephrologist to become familiar
with his/her dialysis machine for patient safety
•The blood circuit will not be reviewed here

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Dialysate Circuit Outline
3
The main components of the dialysate circuit include:
•Deaeration (is the removal of air molecules (usually meaning oxygen) from another
gas or liquid)
•Dialysate proportioning and conductivity
•Dialysate formulation
•Monitors, alarms, and conductivity
•Ultrafiltration: Volumetric and flow-sensor control
•Dialysate disinfection and rinsing
•Emergencies

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Dialysis Machine Dialysate Circuitry
5
•Once pure product water has been generated, bicarbonate and
acid solutions are mixed with water to form dialysate solution
•Mixing or proportioning may be done by the individual
machine or centrally in a dialysis unit
•Several components of proportioning ensure safe dialysate that
is monitored by a series of alarms, pumps, and monitors
•Fluid ultrafiltration occurs by volumetric or flow
sensor controllers
•Disinfection prevents bacterial overgrowth

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Block diagram of dialysis

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The Dialysis Circuit
8

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Degassing Dialysate Water
9
•Treated water inflows into the dialysis machine
and passes through a heat exchanger prior to
entering the heater
•Heating the treated water assists in degassing the
cold water
•Water is heated to body temperature (33˚–39˚ C)
by stainless steel heating elements
•Temperature is monitored downstream by a special
temperature monitoring device

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Deaeration
10
•Water heated to physiologic temperatures is
subjected to negative pressure to remove any air
•Air in the water can interfere with dialysate flow and
cause “air trapping”
•Negative pressure is maintained by a closed loop composed
of a pump, constricting valve, air trap, and vent
•Heating treated water to 85C followed by cooling
prior to proportioning can also de-gas purified
water

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Deaeration
11

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The Dialysis Circuit
8

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Dialysate Proportioning
1
2
•Proportioning assures proper mixing of heated and
treated water to produce the appropriate dialysate solution
•Proportioning pumps mix premade fresh dialysate acid
(A) and bicarbonate (B) solution
•Acid solutions contain acid/chloride salts including
sodium, potassium, calcium, magnesium, and acetate
• Bicarbonate solutions are made fresh, since pre-
prepared bicarbonate can release CO2 and encourage
bacterial growth

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Dialysate Proportioning
1
3
•Dialysate solutions are passed through a small filter prior
to and after formation
•Potential problems include:
•Incorrect bicarbonate or acid concentrate
•Inadequate dialysate mixing
•Clogged filters
•Device alarms disarmed by the operator
•Precipitation of calcium or bicarbonate salts

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Dialysate Formulation
1
4
Electrolyte Concentration
Sodium 134–145 meq/L
Potassium 0–4 meq/L
Calcium 0.0–3.5 meq/L
(2.25 standard)
Magnesium 0.5–1.0 meq/L
Chloride 100–124 meq/L
Bicarbonate 32–40 meq/L
Glucose 0–250 mg/dL

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Dialysate Modeling
1
5
•Sodium
•Sodium modeling can be used to maintain hemodynamic
stability during ultrafiltration.
•However, some controversy exists regarding its use due
to the increased incidence of patient thirst, which may
lead to more intradialytic weight gain and fluid
retention
•Sodium modeling programs are available on dialysis
machines and allow alteration of sodium
concentration over time

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Dialysate Monitoring
1
6
•pH
•The recommended pH range is 6.8–7.6. Not all machines
have a monitor, but dialysate pH should be monitored each
session
•Temperature
•A heat sensor monitors dialysate temperature near the dialyzer
and provides a short feedback loop for changes. Temperature
should be between 35˚– 42 ˚ C
• Low temperatures can cause shivering
• High temperatures can cause protein denaturing or hemolysis
(destruction of red blood cells)

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Dialysate Monitoring
•Conductivity
•Conductivity is the amount of electrical current conducted
through a dialysate and reflects electrolyte concentration
•A constant voltage is applied across two electrodes 1 cm apart in
the dialysate flow. If the concentration of electrolytes changes,
the voltage will change
•Conductivity should be between 12–16mS/cm (millisiemens per
centimeter). The greater the number of ions, the greater the
conductivity of the dialysate
•Conductivity can be affected by temperature, or concentration
of acid to base
•Alarms will stop dialysate flow if conductivity is out of limits

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Conductivity
18

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The Dialysis Circuit
8

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Alarms—Conductivity
1
9
•Conductivity alarms can occur in the following:
An empty concentrate jug
Change in electrolyte concentration of dialysate
Abnormal water inlet pressure
Water leaks or puddles beneath the mixing chamber
Concentration line connector unplugged
•The conductivity settings should never be adjusted
while the patient is on the dialysis machine

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Alarms—Temperature and Pressure Monitors
20
•Temperature Monitor
• A malfunctioning heating element can cause abnormal dialysate temperatures
• Cool temperatures (<35˚C) will result in shivering
• Warm temperature (>42 ˚C) can cause protein denaturing or hemolysis (>45 ˚C)
•Pressure Monitor
• The pressure range is –400 to +350 mmHg with an accuracy of ± 10%
• Alarm limits are set at ± 10% of the pressure setting
• Pressure in the dialysate compartment should not exceed that in the blood compartment
or there is an increased risk of blood contamination by unsterile dialysate secondary to
dialyzer membrane rupture and back filtration
• Ultrafiltration (UF) is controlled by transmembrane pressure (TMP)
• TMP = PBO – PDO

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Blood Leak Monitor
2
1
•Blood should not cross the blood/dialysate membrane
• Leakage of blood into the dialysate circuit is detected
by the blood leak monitor, which is usually located
downstream from dialyzer
•Infrared or photoelectric cells detect decreases in light
from source
•Red blood cells scatter light and trigger alarm, which
deactivates the blood pump

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Blood Leak Monitor
22

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Volumetric-based Ultrafiltration
2
3
•Ultrafiltration is the process of removing fluid from
the patient in a controlled fashion, during which
volume is accurately measured
•Most dialysis machines use volumetric-based
control, which uses a balancing chamber(s)
composed of 2 compartments separated by a
flexible membrane
•One side of the membrane allows fresh dialysate in,
while the other allows spent or used dialysate out

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Volumetric-based Ultrafiltration
2
4
• Valves are connected on the inlet and outlet and
allows fluid to enter one side of the chamber,
which pushes an equal amount of fluid out of the
other side of the chamber
•One chamber fills with used dialysate and pushes
fresh dialysate to the dialyzer, while the other
chamber is filling with fresh dialysate and pushes
used dialysate to the drain
•One pump moves proportioned dialysis to the balance
chambers; a second pump pulls dialysate from the dialyzer and
pushes it to the balance chambers

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Ultrafiltration Pump
2
5
•The UF pump or the fluid removal pump removes
fluid from the closed loop, which results in fluid
removal from the dialyzer membrane
•Most UF pumps are piston type and placed in the
used dialysate flow path by negative pressure
•When the UF pump is off, there is no pressure
gradient between the blood and dialysate and no
fluid is removed from the patient

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Volumetric UF Control
26

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Ultrafiltration: Flow Control
2
7
•Flow-control UF has flow sensors on the inlet
and outlet side of the dialyzer that allow
control of dialysate flow
•A post-dialyzer UF pump removes fluid at an UF
rate calculated by the dialysis machine

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Ultrafiltration: Flow Control
28

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Flow Sensor UF Controller
29

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Volumetric UF Controller
30

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Dialysate Disinfection and Rinsing
3
1
•Dialysis machines should be disinfected according to
the manufacturer’s recommendations, usually daily
•The dialysate circuit should be exposed to disinfectant
•Reused bicarbonate/acid containers should be disinfected
between use
•Disinfectants and rinse solutions include:
•Formaldehyde
•Hypochlorite (bleach)
•Peracetic acid

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Dialysate Disinfection and Rinsing
3
1
•Machines should be rinsed between chemicals and before
a dialysis session
•Dead space is needed between dialysate effluent line and
drain
•Some dialysis machines incorporate a bacterial and
endotoxin-retentive ultrafilter that prevents bacterial
contamination. This is termed “ultrapure dialysate”

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Emergencies—Clinical
3
2
•Dialysis machine proportioning problems can
result in severe serum electrolyte abnormalities.
Some of these emergencies include:
•High or low serum sodium, potassium, calcium or
magnesium
•High or low plasma osmolarity due to hyper- or
hypo-osmolar dialysate

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Emergencies—Clinical
3
2
•Clinical emergencies can occur if significant levels of contaminants are
in the dialysate
• Copper or cupraphane may be released from heating element or dialyzer
and can cause severe hemolysis
• Chloramines and nitrates can cause severe hemolysis
• Fluoride can cause severe pruritis, nausea, and ventricular tachycardia or
fatal ventricular fibrillation
• Aluminum can cause bone disease, anemia, and fatal progressive
neurologic deterioration commonly known as dialysis encephalopathy
syndrome
• Lead, copper, zinc, and aluminum can leach from metal pipes and cause
anemia

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