Hemodialysis Lecture 3

Dialysate circuit

Main dialysate pump Dialysate main pump moves the treated water to the machine at a fixed flow rate. While working with a DC motor load, changing the input current of the motor is needed to obtain variable motor speed. It obtained by using a Bipolar Junction Transistor (BJT). 4

Motor speed control When the input voltage is changed, the current through the output device is changed, so it obtained a variable motor speed. To generate a variable voltage, a Pulse Width Modulation concept (PWM) is applied, which generates rectangular pulses with a variable pulse width and therefore, a variable DC value. 5

Preparation of dialysate Dialysate is made by: Mixing treated water with acetate concentrate at a ratio of 34:1 or 44:1 for acid concentrate and 20:1 or 25:1 for bicarbonate concentrate

Heater and temperature sensor The dialysis is normally done at the body temperature. The temperature of the dialysate is, therefore, monitored and controlled before it is supplied to the dialyzer. In all dialysate delivery systems, dialysate is kept in the range of 36 °C to 38 °C . 7

Temperature sensor Depends on thermistor resistance, the value of thermistor resistance changes according to temperature change. Changing in the resistance causes changes in the potential differences. Potential across the thermistor is digitized using an analog to digital converter. 8

Normal red blood cells (RBCs) begin to hemolyze at 42°C Overheated dialysate has been known to precipitate cardiac arrhythmias

Solid State Relay used for interfacing the heater (220V-5A) to the microprocessor chip (5V – 25mA) by two terminals for the load and more two for the control signal. Due to the value of temperature, the PC program will take the control decisions resulting in connecting or disconnecting the heater to keep the system temperature between 35 and 40 degrees.

Conductivity measurement Dialysate electrolyte levels must be kept within certain limits to keep patients safe. The dialysate proportioning system checks the total electrolyte level in dialysate by testing conductivity (how much electricity the fluid will conduct).

Conductivity is checked by placing a pair of electrodes in the dialysate. Voltage is applied to the electrodes, and the current is measured. The measurement gives the estimated total ion concentration of the dialysate.

Conductivity is usually checked at the point of mixing and again before the dialysate enters the dialyzer. Most dialysate delivery systems have internal, preset conductivity limits. When the dialysate concentration moves outside the preset safe limits, it triggers a conductivity monitoring circuit. The circuit stops the flow of dialysate to the dialyzer and shunts it to the drain. This is called bypass. Bypass keeps the wrong dialysate from reaching the patient.

The most common type of conductivity alarm is low conductivity The most common type of conductivity alarm is low conductivity. The most frequent cause is a lack of concentrate in one or both of the concentrate jugs. A high conductivity alarm is most often due to: 1- Poor water flow to the proportioning system. 2- Untreated incoming water. 3- Use of the wrong dialysate concentrate.

Flow rate Dialysate flow rate to the dialyzer is controlled by a flow pump. higher dialysate flow rates improve dialyzer efficiency, though little improvement occurs above 800 mL/min. Dialysate flow rates range from 0–1,000 mL/min. Some systems have flow meters that continuously display the dialysate flow rate on a gauge or a digital display. Others do not display flow rate at all.

Dialysate flow rate audible and visual alarms may be set off by: 1- Low water pressure. 2- Dialysate pump failure. 3- A blockage in the dialysate flow path. 4- A power failure.

Pressure regulation The hemodialysis machine uses a volumetric fluid balancing system. This type of system uses two chambers that fill and drain to control the volume of dialysate going to and coming from the dialyzer. This is known as volumetric control.

The balance chambers One of the main components of the volumetric control system is the balance chambers or balancing chambers. There are two identical chambers. Each chamber is divided in half by a flexible diaphragm. Each chamber half has an inlet and an outlet.

One side of each chamber is in the “to dialyzer,” or fresh dialysate flow path. The other side is in the “from dialyzer,” or used dialysate flow path. One chamber is filling with used dialysate, pushing fresh dialysate to the dialyzer. At the same time, the other chamber is filling with fresh dialysate, pushing the used dialysate to the drain.

One pump moves the proportioned dialysate to the balance chambers One pump moves the proportioned dialysate to the balance chambers. A second pump pulls dialysate from the dialyzer and pushes it to the balance chambers. This keeps a constant flow through the dialyzer. The volume of dialysate entering and exiting the dialyzer is the same, because the volume entering one side of the balance chamber displaces the same amount on the other side. So, the flow to and from the dialyzer is balanced.

Blood leak detector In a dialysis machine, a thin membrane separates the patient’s blood from the dialysate. Normally, the pressure of the blood side of the membrane is maintained at a much higher level than the pressure on the dialysate side. This necessary to minimize the time required for the dialysis procedure.

Besides this, in order to reduce the total time required for dialysis, the membrane area is made as large as possible. Therefore, these two conditions of a high pressure differential across a large fragile membrane may result in a leak in the membrane.

The blood leak detector consists of a light source and light sensor. Light Emitted Diode (LED) is placed on one side of the dialysate tube and a photo-resistance (Light Dependant Resistor (LDR)) is placed on the front side of the LED on the other side of the dialysate tube. 23

Blood leak causes increasing in the light absorbance causing change in the electric signal of the LDR.

The control procedure of the blood leak detector Analog voltage of the LDR is digitized using the A/D module and the result should give a constant number (in case of no color change). If the color is changed, the intensity of the LED light will change hence, the reading of the LDR will change. This leads to a change in the A/D result. 25