Hemodialysis

HEMODIALYSIS MACHINE FOR STUDENT

1. Hemodialysis MachineHemodialysis Machine

2. 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.

3. Hemodialysis MachineHemodialysis Machine
•High temperature (>40
o
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 42
o
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.

4. Hemodialysis MachineHemodialysis Machine

5. Hemodialysis MachineHemodialysis Machine
•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.

6. Hemodialysis MachineHemodialysis Machine
•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 2.5
o
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.

7. Hemodialysis MachineHemodialysis Machine

8. Hemodialysis MachineHemodialysis Machine
•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.

9. Hemodialysis MachineHemodialysis Machine

10. Hemodialysis MachineHemodialysis Machine
•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.

11. Hemodialysis MachineHemodialysis Machine
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.

12. Hemodialysis MachineHemodialysis Machine
•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.

13. Hemodialysis MachineHemodialysis Machine
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.

14. Hemodialysis MachineHemodialysis Machine

15. Hemodialysis MachineHemodialysis Machine
•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 .

16. Hemodialysis MachineHemodialysis Machine
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 =

17. Hemodialysis MachineHemodialysis Machine

18. ANALYSIS OF DIALYZERANALYSIS OF DIALYZER
Clearance
•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.
•Q
B
and Q
D
are blood flow and dialysate respectively.
•C
Bi
, C
Bo
, C
Di
and C
Do
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 -=-=
DiBiCC
N
K
-
=

19. ANALYSIS OF DIALYZERANALYSIS OF 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

20. ANALYSIS OF DIALYZERANALYSIS OF DIALYZER
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 C
Di
=0 and C
Bo
=C
Bi
, blood dialysance will
equal to filtration K
B
=F
BoBi
QQF -=

21. ANALYSIS OF DIALYZERANALYSIS OF DIALYZER
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
=
-=
-
=

22. ANALYSIS OF DIALYZERANALYSIS OF DIALYZER
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 ==

23. ANALYSIS OF DIALYZERANALYSIS OF DIALYZER
•Compartment diagram