Hemodialysis

Hemodialysis MachineHemodialysis Machine

• Basic Functions of Hemodialysis Machine
• - Mixes the dialysate.
• - Monitors the dialysate.
• - Pump the blood and controls administration of
anti-coagulants.
• - Monitors blood for presence of air.
• - Monitors ultra-filtration rate.
Dialysate Temperature Control and
Measurement
• Dialysis normally done at body temperature.
• Lower than body temperature, dialysis is less
efficient and blood has to be warmed before return
to patient.

• High temperature (>40o
C) will damage components
of blood.
• Thus, temperature of dialysate is monitored and
controlled before supplied to dialyzer.
• Temperature control system used to raise
temperature of dialysate to a required value.
• If temperature exceeds, safety cut-out will ensure
heater switched off.
• Required temperature varied from 36 to 42o
C.
• Two types of circuit for control of temperature :
• - Bi-metallic thermostat.
• - Electronic proportional controller.
• The latter use thermistor for sensing temperature
and triac for control of power to heater.

• Uni junction transistor off until capacitor charges to a point of
breakdown voltage.
• Then transistor conducts and capacitor discharged through
pulse transformer.
• Thus, triac gets a triggering pulse and switches on the
heaters.
• The triac switches off at the end of each half-cycle and
remains so until triggered once again.
• Thermistor has negative temperature coefficient.
• When temperature increase from set value, resistance of
thermistor decrease which will reduce charge rate of C.
• So frequency of charge and discharge reduces, less power
delivered to heaters which results in reduction in
temperature.
• Temperature can also be controlled by varying resistance R3,
therefore any temperature can be set.

• In micrprocessor-based hemodialysis machine, fluid
temperature is displayed on monitor and control circuitry will
control the heaters.
• Dual element heater assembly with 150 W and 300 W
element are used to heat up fluid and maintain it in operating
temperature.
• When temperature rises within 2.5o
C of preset temperature,
300 W heater off and only 150 W heater is used to maintain
the set temperature.
• Enabling of heaters also dependent upon the fluid flow rate.
• Microprocessor reads the flow pulses and determines if there
is adequate flow within the system.
• If flow inadequate heater elements disconnected.
• Flow is measured using flow-thru transducer.
• Sensor assembly includes light source and photo-transistor to
provide optical coupling with sensor.

• Flow-thru transducer produces precise number of
pulses per unit flow.
• Achieved by monitoring rotation of a disk which
contains light reflective white spots.
• Light pulses from rotating disk transmitted by
internal fiber optics.
• Pulses generated by flow transducer are amplified,
filtered and counted to determine flow rate.
Conductivity Measurement
• Conductivity of dialysate is monitored by
conducting cell to verify accuracy of proportioning.
• Result displayed as percentage deviation from the
standard.
• Composition of dialysate is checked by comparing
electrical conductivity of dialysate with standard
sample of dialysate.

• The circuit for conductivity measuring system comprises of a
1.5 kHz oscillator which drives a bridge circuit.
• One arm of bridge contains conductivity cell.
• Compensation thermistor placed in another arm of the
bridge.
• Thermistor is placed so that the circuit provide fast response
to changes of solution temperature.
• Without thermistor, change in temperature would affect
measurement.
• Enabling of heaters also dependent upon the fluid flow rate.
• After amplification, output from bridge capacitively coupled to
phase-sensitive detector.
• The phase is compared with the phase of 1.5 kHz oscillator
output.
• Magnitude and phase from detector determine the direction
and amount of deviation from pre-set value.

Dialysate Pressure Control and Measurement
• Negative pressure upon dialysate created by
effluent pump.
• Effluent pump is a fixed-flow, motor-driven gear
pump.
• Pressure between zero and maximum by
adjustment on machine panel.
• A relief valve limits maximum negative pressure
and minimizing risk of burst in dialyzer membrane.
• Pressure adjustment should not produce any
significant change in flow rate.
• Pressure measured by strain gauge transducer.
• Dialysate pressure is measured on one side of
membrane and venous pressure on the other side.
• Effective pressure across membrane is algebraic
sum of dialysate pressure and venous pressure.

• This effective pressure important in consideration
of filtration and weight control.
• If pressure goes beyond limit, effluent pump
switched off.
• Dialysate by-passed to drain by way of header tank
overflow and waste funnel.
Venous Pressure Measurement
• Measured at bubble trap.
• A tubing connects the trap to a strain gauge
transducer.
• If pressure beyond limit, power to blood pump will
be isolated and pump will not be used.
Bubble trap
• Air embolism is serious hazard in dialysis.
• Air may be sucked in due to inadequate flow in the
line in the pumped dialysis system.
• Bubble trap is equipped to diminish air embolism.

Heparin Pump
• Usually of the plastic syringe type.
• Pump driven by stepper motor and drive screw
mechanism.
• This drives the plunger of the syringe into its barrel
which produces the pumping action.
• Stepper motor speed determined by computer
based on heparin flow rate.
• Speed of stepper motor monitored using optical
encoder.
Blood Leak Detector
• Blood leakage across dialyzer membrane can be
detected by using photo-electric transducer.
• Leak detector examines light absorption of
dialysate at 560 nm i.e. absorption wavelength of
haemoglobin.

• A chopped light system with AC amplifiers is
employed.
• Chopping achieved by driving LED with square
wave of current.
• Compensation thermistor placed in another arm of
the bridge.
• The light is detected with cadmium sulphide photo-
conductive cell.
• Absolute value circuit provides signal whose peak
value is proportional to the received 560 nm light.
• The peak value is compared to a reference voltage
which is pre-set.
• Maximum setting detects blood leaks at rate of 65
mg/l of dialysate.
• If blood leak is detected, the effluent pump
switched off automatically .

Ultrafiltrate Monitor
• Used to monitor amount of fluid removed from the patient.
• Also control the rate at which fluid is removed.
• Ultrafiltration rate calculated by CPU in hemodialysis
machine.
• The load cell and associated electronics are used to monitor
weight changes of fluid in reservoir.
• The load cell utilizes a strain gauge that produces a
differential resistance proportional to the applied force.
• The differential input connected to instrumentation
amplifier which gives gain.
• Weight signal in DC is changed to a proportional frequency.
• Pulses corresponding to the weight are then counted and
given to the microprocessor.
)(
)(
hourstimeTreatment
litresrequiredremovalfluidTotal
RateationUltrafiltr =

ANALYSISOF DIALYZERANALYSISOF DIALYZERClearance
• Complete removal of a solute from blood during a single
pass defines the dialyzer clearance for that solute as equal
to dialyzer blood flow.
• Under condition of steady-state dialysis, mass conservation
requirement is expressed as
• N is overall solute transfer rate between blood and
dialysate.
• QB and QD are blood flow and dialysate respectively.
• CBi, CBo, CDi and CDo are solution concentrations C in blood,B, or
dialysate, D, at the inlet, i, or the outlet, o of the machine.
• Dialyzer clearance is defined as mass transfer rate N divided
by concentration gradient prevailing at the inlet of dialyzer.
)()( DiDoDBoBiB CCQCCQN −=−=
DiBi CC
N
K
−
=

ANALYSISOF DIALYZERANALYSISOF DIALYZER
• K is clearance.
• Mass transfer rate also means the amount of
solute from the blood per unit time, which in turn
equals to the amount of solute accepted in
dialysate per unit time.
• There are two expressions for dialysance
• Maximal achievable clearance at any combination
of blood and dialysate flow rate without reference
to solute concentration.
DiBi
DiDoD
D
DiBi
BoBiB
B
CC
CCQ
K
CC
CCQ
K
−
−
=
−
−
=
)(
)(
DB
DB
QQ
QQ
K
+
×
=max

Filtration
• Ultrafiltration is defined as difference between
blood flow entering the dialyzer and blood flow
leaving the dialyzer.
• Ultrafiltration can be enhanced by increasing
resistance to blood flow at dialyzer outlet.
• Blood compartment pressure will be raised by
subjecting dialysate to a negative pressure.
• When CDi=0 and CBo=CBi, blood dialysance will
equal to filtration KB=F
BoBi QQF −=

Example
• A dialyzer has urea clearance 160 ml/min. Urea
concentration at blood inlet is 0.2 mg/ml while
urea concentration at dialysate inlet is 0 mg/ml.
Calculate urea transfer rate, N.
min/32
)02.0(160
mg
N
CC
N
K
DiBi
=
−=
−
=

Example
• Concentration of urea in blood outlet and dialysate outlet
are 0.08 mg/ml and 0.06 mg/ml respectively. Given blood
flow rate is 200 ml/min and dialysate flow rate is 600
ml/min. Calculate urea transfer rate. Assume dialysate at
inlet has zero concentration of urea.
min/36)06.0(600 mgN ==

• Compartment diagram

Hemodialysis

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