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Thursday, October 21, 2010

Hemodialysis Devices: Dialysate Delivery System

Sections of Dialysate Delivery System

Image Source: lhsc.on.ca

• Controls the amounts of water and chemicals in dialysate, and checks its conductivity, temperature, pH, flow rate, and pressure.
• It also tests the dialysate for the presence of blood. The Proportioning System
• In a proportioning system, dialysate is made by mixing fresh concentrate with fixed amounts of treated water.

The Proportioning System
• The mixing is controlled by the internal mechanical and hydraulic design of the delivery system.
• The exact amount of water and concentrate is set by your center's policies and procedures. (See table on concentrate proportioning ratios)
• Proportioning system make dialysate in two ways. Both rely on a continuous supply of fresh concentrate and watel to a mixing chamber.
• The first type of system mixes concentrate and water using fixed-ratio pumps. Fixed-ratio mixing uses diaphragm or piston pumps to deliver jet volumes of concentrate and water to a mixing chamber.
• The other type of proportioning system uses servo-controlled mechanisms: these have conductivity control sensors that constantly check the dialysate's total ion concentration.
• Once mixed, dialysate is warmed and monitored for conductivity, temperature, pressure, and flow rate.
• After dialysate leaves the dialyzer, it passes through a blood leak detector. Blood in the dialysate could mean a tear in the membrane. So blood leak detectors are often treated as extracorporeal - outside the body - alarms, even though they check the dialysate.
• Used dialysate that has passed through the blood leak detetector is discarded down a drain

The Monitoring System
• Using the wrong dialysate can make a dialysis treatment less effective.
• This mistake may even cause illness or death to a patient.
• Dialysate must be checked throughout each treatment to ensure that it is the right concentration and temperature, and that it is flowing at the right rate.

• Is how much electricity the fluid will conduct
• Except for glucose, the chemicals in dialysate are all salts (electrolytes). Salts break apart in water to form positive and negative charged particles called ions.
• The dialysate proportioning system checks the total electrolyte level in dialysate by testing conductivity.
• 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.
• A sensor cell may be used instead of electrodes.
• Most hemodialysis delivery systems have two or more independent conductivity monitors - with separate sensors and monitoring circuits.
• One sensor measures the mixture of the first concentrate (most often acid) with water.
• The other sensor measures the final dialysate after the second concentrate is added.
• Some machines use conductivity sensors to make the dialysate itself
• These have a second set of sensors to check the mixtures, apart from the ones that control the mixing.
• This multiple monitoring system, called redundant monitoring, is used so two sensors would have to fail before a patient could be harmed.
• Conductivity is checked at the point of mixing and again before the dialysate enters the dialyzer.
• Maybe stated in micromhos/cm, millimhos/cm, microsiemens/cm, or millisiemens/cm.
• 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 :^nd shunts it to the drain. This is called bypass.
• Bypass keeps the wrong dialysate from reaching the patient.
• The circuit also sets off audible and visual alarms to alert the staff.
• 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:
• Poor water flow to the proportioning system.
• Untreated incoming water
• Use of the wrong dialysate concentrate
• There must be enough of both concentrates in the proportioning system to complete the whole treatment.

Before its treatment, check the conductivity alarm to be sure it is working, and check the machine readings against an independent meter.

• Too-hot dialysate can cause hemolysis (bursting of red blood cells.
• Too-cool dialysate is not life threatening, but it can make the patient cold and reduce diffusion so treatment is less efficient.
• In all dialysate delivery systems, dialysate is kept in the range of 37°C to 38°C (98.6°F to 104°F)
• Water must be heated to a certain temperature before mixing with the concentrates. •
• The method of warming the water depends on the delivery system design.
• Some systems use a heat exchanger before the heater, to save energy. These systems, used dialysate transfers its heat to the incoming cold water, warming it before it it enters the heater.
• Most systems use a heater controlled by a thermistor, a type of thermostat.
• To check dialysate temperature, a separate temperature monitor is placed in the dialysate path before the dialyzer. This monitor's limits are preset, and it works independently of the heater control thermistor.
• Many alarm systems have a low setting, which should notbebelow33°C(91°F).
• With some delivery systems, the patient is the only "monitor" of low temperature.
• The high limit should be set at no higher than 41°C (105°F).
• If the temperature is too hot or cold, a circuit sets off audible and visual alarms.
• The circuit also triggers bypass to shant dialysate to a drain.

Before each dialysis treatment, check the dialysate temperature alarm to ensure that it is working properly.

Flow rate
• Dialysate flow rate to the dialyzer is controlled by a flow pump.
• Some delivery systems have a preset flow rate; let the flow vary as the doctor prescribes.
• Higher dialysate flow rates improves 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:
     • Low water pressure
     • Dialysate pump failure
     • A blockage in the dialysate flow path
     • A power failure
• A high/low conductivity, high/low pH, high temperature, or in some cases blood leak alarm, can trigger the delivery system to switch into bypass mode.

Check the delivery system before each treatment to be sure that the bypass mode works properly for all dialysate alarm conditions

Blood leak detector
• Is used to check for blood in the used dialysate.
• It can sense very small amounts of blood, less than can be seen with the naked eye.
• The blood leak detector shines a bean of light through the used dialysate and onto the photocell or photoresistor.
• Normally, dialysate is clear, so the light can pass • through. But even a tiny amount of blood will break the light beam. The detector will sense such a break, triggering audible and visual alarms.
• When a blood leak alarm occurs, the blood pump stops and the venous line clamps to prevent further blood loss.
• In some systems, a bypass mode shunt dialysate to the drain.
• This reduces negative pressure and keeps blood from being drawn through the tear into the dialysate
• A Hemastix® (strip that reacts to blood) should be used to check the extent of the leak. The test must be taken where the dialysate leaves the dialyzer:
• If blood or pink color can be seen in the dialysate path, there is a major leak.
• Clear dialysate and a positive Hemastix test suggest a minor leak
• Clear dialysate and a negative Hemastix test mean a false alarm
• Depending on your center's procedures for a blood leak, you stop the treatment without returning the patient's blood.
• This keeps possibly contaminated blood from reaching the patient, where it could cause an infection.
• The blood leak detector's basic sensitivity is usually preset by the manufacturer. Adjustments can be made within this limited range.

• pH is a measure of how acidic or alkaline (basic) a
solution is.
• The pH of a solution is based on the number of acid ions (hydronium ions) or alkali (base) ions (hydroxyl ions) it contains.
• A solution with:
     • An equal number of acid and base ions is neutral and has a pH value of 7.0
     • More acid ions is acidic and the pH value will be less than 7.0
     • More base ions is alkaline and the pH will be greater than 7.0
• Bleach (sodium hypochlorite) is alkaline with a pH of 11.0. •
• White vinegar is an acid, with a pH of 2.9.
• The pH of blood is normally from 7.25-7.45; a weak base.
• Dialysate must have a pH close to blood so it does not change the blood pH.
• In general, the range of dialysate pH is from 7.0-7.4
• Whether or not the delivery system has a pH monitor, at the start of each treatment, an external test must be done to ensure that the dialysate pH is in a safe range.
• The most accurate pH measure uses a pH electrode, which puts out a small voltage when placed in a solution.
• The voltage is read by a detection circuit that converts the signal into a pH value and displays it.
• Test strips coated with a chemical that changes color based on pH are another way to measure it.

Ultrafiltration Control: Ultrafiltration
• Occurs during the treatment when the pressure on the blood side of the dialyzer membrane is more positive than the pressure on the dialysate side.
• This pushes fluid in the blood across the membrane into , the dialysate compartment where it is then expelled in the drain. The difference between these pressures is the TMP.

Ultrafiltration Control: TMP and dialysale pressure
• It determines how much fluid from the blood is forced across the membrane.
• In the past, dialysis machines used a .manual system of setting the TMP or a negative dialysate pressure for achieving fluid removal.
• With today's volumetric dialysis, TMP is calculated and set for you. All you need to enter is the desired fluid removal (in mL) and the treatment time.
• Fluid removal accuracy of the older systems was not nearly as precise as today's UF control systems due to variables including:
•  The Kuf values reported by dialyzer companies are usually in vitro values. In practice, the in vivo Kur is often somewhat lower (5% - 30%)
• Clotting of the dialyzer fibers reduces the Kur by reducing the surface area of the membrane
• Increasing or decreasing the blood pump speed changes the venous pressure
• An increase or decrease in the dialysate flow, or a kink or blockage in the dialysate lines changes the dialysate pressure.
• These conditions have no effect on fluid removal accuracy with UF control machines.

Ultrafiltration Control: UF control systems
• UF control is the means by which the dialysis machine removes fluid from the patient and accurately measures it. .
• The amount of fluid removed in a specific period of time is the Ultrafiltration rate (UFR).
• Most dialysis machines use a volumetric fluid balancing system. (see next slide)
• 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.

• Another type of machine uses sensors in the fluid path to and from the dialyzer to control and monitor the flow ; of the dialysate. This is known as, flow control.

Volumetric UF Control
• One of the main components of the volumetric UF 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

1. One of the main components of the volumetric UF 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
• There are valves on each inlet and outlet
• These valves open and close so that as fluid enters on one side of the chamber, it pushes on the diaphragm and forces fluid out on the other side.
• The timing of the valves opening and closing is synchronized.
• 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.
• A second pump pulls dialysate from the dialyzer and pushes it to the balance chambers.
• This keeps a constant flow through the dialyzer.
• The movement of dialysate to and from the dialyzer takes place in a closed loop.
• The volume of fluid 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.

Volumetric UF Control
2. Another main component in the system ultrafiltration pump (UF pump) or the fluid removal pump.
• Its function is to remove fluid from the closed loop.
• This results in fluid removal from the patient through the dialyzer membrane.
• Most UF pumps are diaphragm or piston type, and are most often placed in the used dialysate flow path.
• The pumps work with a stroking movement that removes a small, fixed amount of fluid on each stroke (about 1 cc or less)
• The removal of fluid from the closed loop creates a negative pressure in the loop.
• Therefore, pressure is negative in the dialysate compartment of the dialyzer, relative to the blood compartment pressure.
• This creates the pressure gradient thai is needed for UF.
• When the UF pump is off, there is no pressure difference between the blood and dialysate - and no fluid is removed.

3. Other important components in the same are used to perform control, monitoring, and safety functions.
• Pressure sensors serve functions such as controlling pump speeds, preventing overpressurization, calculating TMP, and detecting leaks within the system.
• Air separation chambers remove any air coming out of the dialyzer. (This will occur when priming a new (dry) dialyzer.
• Any air in the system could result in incorrect fluid removal.
• The air separation chamber maintains a level of fluid while releasing air out the top that is routed to the drain.

Flow Control
• Another type of UF system is the flow control system.
• This system has flow sensors on the inlet and the outlet side of the diayzer to control dialysate flow through the dialyzer.
• Inlet and outlet flow pumps are set so the flow measured at the inlet and outlet flow sensors is equal.
• This flow balance is the key to the system's accuracy and ensures that the only fluid removed from the patient is that which is removed by the UF pump.
• This UF control system uses a postdialyzer UF pump that removes fluid at the UFR calculated by the machine's computer.
• The speed of the pump is equal to the UFR.
• It is determined by the time needed to fill a small chamber of a known volume (UF burette).
• Flow control UF systems may also use pressure sensors and air separation chambers in the same way that Volumetric systems do.

If you have trouble understand the above information, kindly checkout first Hemodialysis Delivery System and Hemodialysis Devices.

Hemodialysis Devices: Dialysate Delivery System - Related Hemodialysis Article


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