Hemodialysis Machine Hemodialysis Machine Basic Functions of Hemodialysis

Hemodialysis Machine

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.

Hemodialysis 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.

Hemodialysis Machine

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 R 3, therefore any temperature can be set.

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. 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.

Hemodialysis Machine

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.

Hemodialysis Machine

Hemodialysis Machine • The circuit for conductivity measuring system comprises of a 1. 5 k. Hz 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 k. Hz oscillator output. • Magnitude and phase from detector determine the direction and amount of deviation from pre-set value.

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.

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.

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.

Hemodialysis Machine

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 photoconductive 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.

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.

Hemodialysis Machine

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.

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.

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

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.

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.

ANALYSIS OF DIALYZER • Compartment diagram