1. Hemodialysis Machine

2. Hemodialysis 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 Machine
High temperature (>40oC) 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 42oC.
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 Machine

5. Hemodialysis 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 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 2.5oC 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 Machine

8. Hemodialysis 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 Machine

10. Hemodialysis 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 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 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 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 Machine

15. Hemodialysis 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 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.

17. Hemodialysis Machine

18. ANALYSIS 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.
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.

19. ANALYSIS 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.

20. ANALYSIS 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 CDi=0 and CBo=CBi, blood dialysance will equal to filtration KB=F

21. ANALYSIS 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.

22. ANALYSIS 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.

23. ANALYSIS OF DIALYZER
Compartment diagram