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Thursday, October 21, 2010

Hemodialysis Devices: Extracorporeal Circuit

Sections of Extracorporeal Circuit

Image Source: universalkidneycenters.com

The extracorporenl circuit carries blood from the patient's access to the dialyzer and back to the access.

• It is the second major subsystem of the hemodialysis delivery system.
• It includes the:
     • arterial and venous blood tubing
     • blood pump
     • heparin pump
     • dialyzer
     • venous line clamp
     • blood flow monitors
     • pressure monitors rail-monitors

Components and Monitoring

Blood tubing
• During hemodialysis, blood from the patient's vascular access (arterial needle) flows to the dialyzer
• Blood flows back to the patient's access (venous needle) through blood tubing, or "lines".
• The inner diameter of the blood tubing is small.
• Only a small amount (about 100-250m) of blood is outside the patient's body at any time. 1:1
• There are two parts of the blood tubing: arterial and venous.
• The arterial segment is most often color-coded red; the venous segment is most often color-coded blue.
• Bloodlines are smooth on the inside to reduce clotting and air bubbles.

Parts of blood tubing :
• Patient connectors: A tip or Luer-Lok® connector, at the end of the arterial and venous blood tubing segments connects the tubing to the patient's needles or catheter ports.
• Dialyzer connectors: Luer-Lok connectors at the other end of the blood tubing segments connect the tubing to the dialyzer, The arterial blood tubing segment connects to the arterial end of the dialyzer. The venous blood tubing segment connects to the venous end of the dialyzer.
• Drip chamber/bubble trap: The drip chamber checks the arterial or venous pressure in the blood circuit. It uses a monitoring line with transducer protectors, and collects or "traps" any air that accidentally gets into the extracorporeal circuit. The drip chamber can also keep blood clots in the extracorporeal circuit from reaching the patient, by using a very fine mesh screen. This type of drip chamber is placed on the venous blood tubing segment, after the dialyzer and before the patient's access.
• Blood pump segment: The blood pump segment is a durable,pliable, larger diameter part of the arterial blood tubing. It is threaded through the blood pump roller.
• Heparin infusion line: During dialysis, heparin (a blood thinning drug) may be given to the patient through a very small diameter tube that extends out of the blood tubing. The heparin infusion line is most often placed on the arterial blood segment just before the dialyzer.
• Saline infusion line: This line allows saline to be given to the patient during dialysis. It is most often placed on the arterial blood tubing segment just before the blood pump, so saline can be pulled into the circuit. If the saline infusion line is not clamped correctly, too much fluid or air can enter the extracorporeal circuit.

Transducer protectors

• Is a mechanical device inside the machine that converts air pressure into an electronic signal.
• This signal is used to display venous pressure, arterial pressure, and TMP.
• Moisture would damage the transducer.
• Transducer protectors are a barrier between blood in the tube and the transducer in the machine.
• They connect to the machine's venous and/or arterial ports via a small tubing segment on top of the drip chamber.
• Transducer port lines have a small line clamp in the middle.
• The transducer protector connects to the end of these lines and is link between the machine and the blood tubing set (drip chambers).
• Transducer protectors use membranes with a nominal pore size of 0.2 microns that are hydrophobia when wetted, to keep fluid from passing through.
• If these filters get wet, they prevent air flow.
• Wetted or clamped transducer protectors cause pressure reading errors.
• A wet or clamped venous transducer protector will also cause TMP problems, since TMP is partly venous pressure.
• A loose or damaged transducer protector on a pre-pump arterial drip chamber port could allow air into the bloodline circuit.
• Change wet transducer protectors right away and inspect the machine side of the protector for contamination or wetting (Centers for Disease Control and Prevention.
• If a fluid breakthrough is found on the removed transducer protector, the machine's internal transducer protector (a back-up) must be inspected by a qualified technician.

Blood pump / blood flow rate

• By changing the roller speed, blood flow through the extracorporeal circuit can be set according to the prescription.
• Blood flow rates can be varied between 0 mL/min and 600mL/min
• Some machines count blood pump turns and calculate the number of liters processed in a treatment.
• Knowing the number of liters prescribed to be processed allows calculation of the blood flow rate:
divide liters processed by minutes of treatment.
• This value can be used as a quality assurance tool; it should equal the blood flow rate shown on the machine.
• For an effective treatment, the blood flow rate must be accurate and reflect the doctor's prescription.
• Pump occlusion is the amount of space between the rollers and the pump housing.
• The blood pump rollers must press against the blood pump segment hard enough to pull and push the blood
through the extracorporeal circuit.
• If the rollers are too tight, the blood pump segment may crack or RBC may be destroyed.
• If the rollers are too loose, blood may escape out the back of the segment, reducing blood flow below the prescribed level. . .
• Modern rollers use springs to create occlusion, so the pump segment must be inserted properly.
• Pulling down the ends of the pump segment in the housing will compress the springs and cause the wrong occlusion. This will also reduce blood flow by decreasing the amount of pump segment in the blood pump.
• Pump occlusion must be checked periodically and adjusted per the manufacturer's instructions. It should also be checked when the tubing size or manufacturer changes
• In case of emergency, all blood pumps have a way to allow hand cranking.

• Most often, the pump will have a handle, either with the pump head or one that can be inserted into the pump, which can be used to crank the pump.
• The pump head should be hand cranked just fast enough to keep the venous pressure at the pre-alarm level.

Extracorporeal Pressure Monitor
• Pressure in the extracorporeal circuit depends on blood flow rate and resistance to the flow.
• Resistance occurs in nearly every part of the extracorporeal circuit:
     • Access needles or catheters
     • Blood tubing
     • Dialyzer

• The blood pump is used to overcome this resistance.
• Pressures are displayed in millimeters of mercury (mmHg) on a gauge, meter, or screen.
• Depending on the equipment, pressure can be read at several sites.
• Extracorporeal pressure monitoring is needed to calculate TMP and ensure patient safety.
• In some systems, pressure monitors have upper and lower limits that can be set. In others, they have a preset range within which staff can choose a midpoint.
• When pressure exceeds the high or low setting, the system will trigger audible and visual alarms, stop the pump, and clamp the venous line.

You must check the extracorporeal blood pressure alarm to ensure that it works properly before each treatment.

• A pre-pump or post-pump drip chamber may be placed on the arterial bloodline.
• A monitoring line or pressure gauge connection at each drip chamber is used to check arterial and/or venous pressure in the extracorporeal circuit.
• The pressure described below may be monitors, depending on the dialysis delivery system used:
Arterial pressure is pressure from the patient's access to the blood pump. It is also called pre-pump pressure. When a blood pump is used with a fistula or graft, arterial pressure will usually be less than zero, or negative. Resistance from the vascular access and the pulling of the blood pump creates this negative pressure.
Predialyzer pressure is pressure between the blood pump and the dialyzer, also called post-pump pressure, predialyzer pressure, or post-pump arterial pressure.
Predialyzer pressure is pressure between the blood pump and the dialyzer, also called post-pump pressure, predialyzer pressure, or post-pump arterial pressure. Pressure in this segment of the blood tubing is greater than zero, or positive: Predialyzer pressure is monitored to detect clotting in the dialyzer. Suspect clotting if there is a large pressure differential on each side of the dialyzer.
• Venous pressure is pressure from the monitoring site to the venous return. This pressure is often called postdialyzer pressure. Pressure in this segment is positive.

Air detectors
• Air/foam detectors continuously check the blood in the venous tubing segment for air and foam.
• The system may check for air at the venous drip chamber or at the blood tubing just below it.
• Air detectors are ultrasonic devices that check for changes in a sound wave sent through a cross-section of the blood path.
• Sound travels faster than air than liquid.
• Therefore, any air in the blood will raise the speed at which the sound wave passes through the blood, setting off an alarm.
• An air detector's alarm sensitivity limits are most often preset by the manufacture, but can be calibrated by qualified technicians
• When the air detector senses air, it will trigger audible and visual alarms, stop the blood pump, and clamp the venous blood tubing to keep air from getting into the patient's bloodstream.
• The air detector must always be used during the dialysis treatment and venous line clamps engaged with the

You must check the air detector to be sure it is working properly before each treatment, following the manufucturer’s instructions.

Heparin system
• When the patient's blood touches the artificial materials of the lines and dialyzer, it tends to clot.
• Heparin, an anti-clotting drug, or anticoagulant, is used to prevent clotting in the extracorporeal blood circuit.
• Some centers give heparin intermittently (on and off) during dialysis; a prescribed amount is injected into the arterial bloodline at prescribed times.
• Also, heparin can be given by bolus (the full prescribed amount is given all at once just before the treatment).
• Other centers give heparin by continuous infusion (a prescribed rate throughout the treatment).
• A syringe filled with heparin, a heparin infusion line, and an infusion pump are used and the pump slowly injects the heparin into the extracorporeal circuit.
• For most patients, heparin is stopped before the end of the treatment so blood clotting can go back to normal.
• A continuous infusion heparin pump has four parts:
     1. A syringe holder
     2. A piston to drive the plunger of the syringe
     3. An electric motor to drive the plunger forward and infuse heparin from the syringe
     4. A way to set the prescribed infusion

• Heparin pumps have variable speeds that can be set to the physician's prescription.
• Heparin is infused into the heparin line on the arterial blood tubing before the dialyzer.
• Most heparin lines are placed after the blood pump segment.
• This helps avoid negative pressure at the part of the blood circuit that could otherwise draw air into the extracorporeal circuit through the heparin line

• Can be used for acute, home, and chronic dialysis treatments.
• It needs no water treatment system.
• It does not contain water and concentrate proportioning pumps.
• Instead, premixed chemicals are added to 6 L of tap water.
• The water and chemicals are cycled through a sorbent regenerative cartridge to purify the dialysate.
• Then the dialysate is collected in a disposable bag in the device and circulated to the dialyzer.
• Used dialysate is then cycled through the cartridge, where it is chemically converted back into fresh dialysate and returned to the storage bag.
• The sorbent cartridge also removes all calcium, magnesium, and potassium from the used dialysate, since their concentrations were altered by passage through the dialyzer.
• These electrolytes are added back into the regenerated dialysate in the prescribed amounts by an infusion system.
• The patient's ultrafiltrate is also converted into dialysate by passage through the cartridge.
• Each increase in the total volume of dialysate is a direct reflection of total UF and is continuously displayed.
• The sorbent cartridge has four chemical layers.
• Besides regenerating dialysate, the layers serve as a water treatment system; they purify the 6 L of dialysate made with tap water.
• The sorbent cartridge also serves as a continuous dialysate disinfection system, keeping bacteria and endotoxin levels below 1 cfu mL and 0.5 EU/mL, respectively.
• Depending on which cartridge is used, the system can do short (3-5 hour) treatments at dialysate flow rates up to 400 mL/min, or long, slow (5-8 hour) treatments at dialysate flows between 200-300 mL/min.
• Since using the sorbent cartridge means no continuous water source, floor drain, or water treatment system are needed, sorbent systems can be used anywhere that an electrical outlet (or suitable generator) is present.

• The delivery system plays a key role in monitoring dialysis.
• During each treatment, the machinery checks almost every aspect of the patient's care except one: you.
• The dialysis staff is the most important monitor of all to keep patient safe.
• Alarms are of no use if someone forgets to turn them on or to check them against an independent meter.
• The patient can be in great danger if a staff hooks up the wrong dialysate to the machine
• It is vital to recall that dialyzers and delivery systems are not just machines, and dialysate is not just salty water.
• They are precise parts of a medical treatment that can help patients with kidney failure lead full and active lives.
• Your attention to detail and skill at finding and troubleshooting problems will make all the difference in patient's outcomes.
• Your job is to help them by staying alert at all times and learning all you can about equipment and procedures. Checkout Guidelines for Dialysis Care for more information.

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

Hemodialysis Devices: Extracorporeal Circuit - Related Hemodialysis Article


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