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Sunday, October 31, 2010

How Fistula Are Created

• A native arteriovenous fistula (AVF) is made by surgically linking an artery to a vein.
• This link is the anastomosis, and the site is marked by a scar.
• It takes 1-3 months for AVF to be strong enough to use large-gauge needles, so it is best to create one long before dialysis is needed.
• As soon as the surgery is done, rapid and strong arterial blood flow sent to the vein starts to enlarge the fistula vein and make it tougher. This is called arterialization. and we say the fistula is maturing.
• The most common type of native AVF links the radial artery and cephalic vein in the distal forearm (below the wrist and the elbow). This is called a radiocephalic fistula.
• The brachiocephalic (brachial artery and cephalic vein) fistula is used, and is the most common AVF of the upper arm.
• Other vessels that can be used are:
     • Basilic vein
     • Transposed basilic vein (the deep vein is brought closer to the surface of the skin and the vein is moved to the anterior [front] surface of the upper arm for easier needle insertion)
     • Transposed one of the brachial veins ( a pair of veins closely accompanying brachial artery and draining to the axillary vein)

Other vessels that can be used are:
     • Perforated vein in the antecubital fossa anastomosed to the brachial artery (perforating veins connect superficial and deep veins)
     • Ulnar artery .
     • Proximal radial artery
• While AVF is the best type of access, not every, patient can have one.
• The chosen veins must be healthy, straight, large enough to allow for large-gauge needles, and long enough to permit a number of needle sites.

• Patients must also be able to handle a 10% more increase in cardiac output (the amount of blood passing through the heart) to have a fistula.
     • A new access strains the heart, because arterial blood quickly short-circuits through tiny capillary blood vessels.
     • The heart must work harder due to rapid blood flow.
• Reasons why a patient may not be able to have an AVF:
     • Damage to veins due to intravenous drugs
     • Previous surgeries on the arteries and/or veins
     • Atherosclerosis: plaque or waxy cholesterol that blocks the vessels
     • Poor quality arteries due to peripheral vascular disease (PVD) or advanced diabetes
     • Only one working artery to bring blood to the hand
     • Damage to blood vessels due to intravenous drugs

Radiocephalic Fistula: Source: wikipedia.org

Fistula Procedure
• Vessel mapping to find the best vessels for a fistula
• Blood vessel sites are marked on the skin.
• An incision is made in the skin over the chosen vessel
• Then the vessels are sewn together There are four ways that arteries and veins can be joined to create AVF. Each has pros and cons:
     • The side-to-side (artery-side to vein-side.) anastomosis is easiest for a surgeon to do.
     • It is also the most likely to cause venous hypertension,
     • This is the problem in which the hand fills with fluid due to high pressure in the veins,
     • Sometimes the surgeon will do a side-to-side anastomosis, then tie off one or more of the vessels leading to the hand

• The side-to-end (artery-side to vein-end) anastomosis is preferred by many surgeons, even though it is hardest to do. This method gives high blood flows with few complications.
• The end-to-side (artery-end to vein-side) anastomosis has slightly lower blood flow rates than side-to-side.
• The end-to-end (artery-end to vein-end) anastomosis permits less blood flow through the access.

Source and for more information checkout http://www.mcs.anl.gov/uploads/cels/papers/P1410.pdf

• After the incision is closed, a thrill, or purring vibration, should be present over the new fistula.
• You should be able to hear a whooshing "bruit" with a • stethoscope along the course of the. vein.
• The bruit should be continuous and low-pitched. Both the thrill and the bruit help prove that the new fistula is patent (open).

Vascular Access: Makes Chronic Dialysis Possible

Image Source: barwonvascular.com.au

After completing this module, the learner will be able to:
• Describe the three main types of vascular access.
• Identify the predialysis assessments for all types of vascular access.
• Describe the methods of needle insertion forAVFs and grafts.
• Describe the predialysis assessment, accessing procedure, exit site care, and monitoring of catheters.
• Vascular access makes chronic hemodialysis possible because it allows the care team to "access" the
patient's blood.
• And access can be internal (inside the body) or external (outside the body).

It must:
• Allow repeat access to the blood
• Handle blood flow rates that will ensure effective treatments
• Be made of materials that are not prone to causing reactions or infections

The three main types of access are:
• Fistula
• Graft.
• Catheters
• To create a fistula, a surgeon sews an artery and a vein together, most often in the arm.
• Arteries carry oxygen-rich blood from the heart and lungs to the rest of the body.
• The vessels selected for a fistula are large and have good flow, but are deep below the skin and hard to reach with needles.
• Veins bring blood back to the heart and lungs; they are easy to reach, but too small and too slow flowing for dialysis.
• Linking an artery and a vein is the best of both worlds.
• In 4-6 weeks, high-pressure blood flow from the artery thickens the vein wall and makes it dilate (enlarge) so large needles can be used.
• Because a fistula is below the skin and is the patient's own tissues, it is less prone to infection and clotting
than other types of access.
• A fistula can last for years - even decades - and research shows it is the best type of access now available.
• To create a graft, a surgeon links an artery and vein with a piece of artificial blood vessel.
• Like a fistula, a graft allows access to the large volume of blood needed for dialysis.
• Grafts are more prone to stenosis (narrowing of blood vessels), which can cause thrombosis (blood clots).

• Grafts are also more prone to infection than fistulas, and have a shorter useful lifespan (less than 5 years on average)

• Grafts are an option for patients who do not have blood vessels suited to create a fistula.
•  A catheter is a plastic, hollow tube placed in a deep central vein in the chest or leg.
• They allow short-term or long-term access to patient's blood.
• Deep central veins have a blood flow rate that allows adequate treatments.
• Catheters are made up of plastic that is foreign to the body, and they pass through the skin, creating a portal for bacteria.
• They are prone to stenosis, blood clots, and infection.
• Due to these problems, catheters must be replaced in the same or new vessels.
• Catheters are used for patients who:
     • Can't have a fistula or graft
     • Are waiting for a fistula or graft to be placed or to mature
     • Have acute kidney failure and may soon recover kidney function
     • Are waiting for a peritoneal dialysis catheter
     • Are waiting for a live donor kidney transplant
• Access is still the one greatest challenge to the success of dialysis.
• Access problem frustrate the care team and the patient.
• Trouble cannulating (putting needles into) fistula and grafts is a source of stress for both.
• Poor cannulation can lead to problems that may cause access failure.
• Access failure means loss of the dialysis lifeline.
• The NKF KDOQI and the Fistula First program are two ongoing efforts to improve access outcomes

Sunday, October 24, 2010

Stop Dialysis: Stopping Dialysis because Im Tried of It

I know some patients who are on dialysis for a period of time have cross their mind with regards to "Stopping their Dialysis" or ask themselves what if I would just stop going to the dialysis center, what would happen to me?

I have read somewhere not sure what site was it but it says
The expected duration of life after dialysis is stopped is one to two weeks.

I also read that death from stopping dialysis is painless. Hmmmnnn. I think it depends because I know you dont die from dialysis, you die from the complications of renal failure. Here is a good article about Treatment Options for End Stage Renal Failure or ESRD

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: 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: Hemodialysis Delivery System


PURPOSE: Hemodialysis Delivery System

• Most delivery systems monitor patient and machine safety parameters.
• These include blood flow, dialysate flow, dialysate temperature, conductivity, venous and arterial pressure, blood in dialysate leaks, patient blood pressure, etc.
• The delivery system is two major subsystems: the dialysate delivery system and the extracorporeal >» blood circuit
•  A delivery system is a machine that mixes and delivers dialysate, pumps blood through the dialyzer, and monitors various dialysis parameters to ensure a safe treatment.



Hemodialysis Devices: Dialysate

Sections of Dialysate


• Dialysate is a fluid that helps remove uremic wastes, such as urea and creatinine, and excess electrolytes, such as sodium and potassium.
• Dialysate can also replace needed substances, such as calcium and bicarbonate, which helps keep the body's pH balance.
• During a treatment, the patient's blood is on one side of the membrane, in the blood compartment, The dialysate is on the other side, in the dialysate compartment. Dialysate and blood never mix, unless the membrane breaks.
• Dialysate is prescribed to have a desired levels of solutes the patient needs and none of the ones that must be removed completely.
• The osmolality (solute particle concentration) of dialysate should closely match the blood to keep to much fluid from moving across the membrane.
• The concentration gradient created decide the diffusion rates of each solute across the membrane.
• Unwanted solutes leave the blood and move into the dialysate; desired solutes stay in the blood.
• Some solutes are added to dialysate in amounts that can cause them to enter the patient's blood. Most often, these are sodium, bicarbonate, and chloride.

• The doctor prescribes the dialysate. Dialysate starts out as two concentrated salt solutions: acid and bicarbonate.
• The two concentrates are diluted with precise amounts of treated water to make the final dialysate. The concentrates come in three different formulations.
• Because there are three formulations, care must be taken to match the right acid concentrate with the right bicarbonate concentrate.

Acid Concentrate

Sodium chloride
Potassium chloride
Calcium chloride
Acetic acid (to lower thedialysate's pH)

Bicarbonate Concentrate
Sodium bicarbonate
In some cases, sodium chloride

• The Association for the Advancement of Medical Instrumentation (AAMI) has set standard symbols to match the concentrates.
• Today, almost all of the hemodialysis machines can use any of the formulations.
• When the concentrates are diluted with the prescribed amount of water, they will have the right concentration of electrolytes.
• Electrolytes are vital for cell function.

Substance ------------------------------ Concentration in Dialysate
Sodium ------------------------------ 135 to 145mEq/L
Potassium ---------------------------  0 to 4 mEq/L
Calcium -----------------------------  2.5 to 3.5 mEq/L
Magnesium -------------------------- 0.5 to 1.0 mEq/L
Chloride ----------------------------  100 to 124 mEq/L
Bicarbonate ------------------------  32 to 40 mEq/L
Glucose ----------------------------  0 to 250 mg/dL

Sodium (Na+)
• Is a major electrolyte of the body's blood plasma and interstitial (between the cells) fluid.
• Fluid and solutes must be in the plasma to be removed by dialysis.
• Normal sodium concentration in the blood is from 135-145mEq/L.
• Sodium concentration in the dialysate is most often kept in the same range.
• Dialysate delivery systems can adjust the dialysate sodium level during a treatment. The dialysate sodium level is changed according to a doctor's prescription. This is called sodium modeling.
     • It has been shown to create more efficient fluid shifts in the body, to remove fluid taster
     • Provides for better control of blood pressure and fluid removal
     • However, it can increase thirst and body weight, and hypertension between dialysis treatments

Potassium (K+)

• Is a major electrolyte of the intracellular fluid.
• The body keeps precise amounts on both sides of cell membranes to send nerve signals.
• Normal plasma potassium level is from 3.5 - 5.5 mEq/L.
• Potassium in the dialysate ranges from 0-4 mEq/L based on the patient's needs.

Magnesium (Mg++)
• Is vital to the nerves and muscles.
• It also triggers enzymes that are key to carbohydrate use.
• Normal plasma magnesium level is from 1.4-2.1 mEq/L.
• The magnesium range in dialysate is 0.5 - 1.0 mEq/L

Calcium (Ca++)
• Is Found in the body in extracellular (outside the cells) and intracellular (inside the cells) fluid.
• It builds bones and teeth, helps musjies move, is needed I'or blood clotting, and helps send nerve signals.
• The normal range of calcium in the plasma is 8.5 -10.5mg/dL(4.5-5.5mEq/L)
• Dialysate calcium is most often 2.5 - 3.5 mEq/L

Chloride (CI-)
• The concentration of chloride in dialysate depends on the contents of chemicals such as sodium chloride, potassium chloride, magnesium chloride, and calcium chloride.
• Normal plasma chloride levels are 98 - 111 millimoles per liter (mM/L)
• Dialysate chloride ranges from 100-124 mEq/L

Glucose (Cf,Hi206)
• Glucose may be added to dialysate to prevent loss of serum glucose and to reduce catabolism (muscle breakdown)
• Dialysate glucose levels may range from 0 - 250 mg/dL
• The glucose in dialysate can be two fo three times higher than in normal (70-105 mg/dL).
• This means that dialysate with glucose has an osmotic (water-pulling) effect that aids UF.

Bicarbonate (HCOa)
• Bicarbonate is a buffer - a substance that tends to maintain a constant pH in a solution, even if an acid or base is added.
• Healthy kidneys keep the body's pH within the very tight limits that cells need to survive. The kidneys do this by making and regulating bicarbonate.
• Bicarbonate is added to dialysate to help maintain , patients' pH.

Bicarbonate (HC03)

• Is used by the body to neutralize acids that are formed when cells metabolize proteins and other foods used for fuel.
• People with CK.D can't excrete enough acids in the urine, so they are in constant state of metabolic acidosis (i.e., having too much acid in the blood)
• in dialysate, bicarbonate is used to replace the body's stores of buffer.
• Can reduce dialysis-related problems like hypotension, muscle cramps, nausea, and fatigue after treatment.

Hemodialysis Devices: Dialyzers

Sections of Dialyzers


• The dialyzer, dialysate, and delivery system replaces some tasks that failed kidneys can no longer do.
• Every dialyzer has a blood and a dialysate compartment,
• The semi permeable membrane keeps the two compartments apart.
• The membrane is housed in a plastic case, which holds the dialyzer together and forms pathways for blood and dialysate to flow in and out.

Image Source: iomed.brown.edu
Many aspects of a dialyzer can affect treatment effectiveness, comfort, and patient safety. These includes:
• Biocompatibility (how much a membrane is compatible with the human body)
• Membrane surface area
• Molecular weight cutoff (the solute size that can pass through the membrane)
• Ultrafiltration coefficient
• .Clearance (the rate of solute .removal)

• Biocompatible means not harmful to biological function.
•All materials used to make dialysis membranes react to some degree with immune cells in the blood. It is vital to use a membrane the patient can tolerate.
• Biocompatibility of a membrane can be tested by checking the patient's blood for certain proteins and chemicals.
• A membrane's ability to adsorb (attract and hold) proteins into the fiber wall is key to its biocompatibility. Adsorbed proteins coat the surface so blood does not touch the "foreign" membrane.
• This protein coating explains why reprocessed (cleaned and reused) dialyzers can be more biocompatible than new ones.

Note: Reprocessing dialyzers with bleach can strip the protein coating off the membrane.
• Synthetic membranes are more biocompatible than
cellulose membranes.
• Synthetic fibers are hydrophobic (water repelling)
• These makes them better able to adsorb blood proteins,

Surface Area
• Surface area is key to how well a dialyzer can remove solutes.
• If all other aspects are equal, dialyzers with more surface area can expose more blood to dialysate. This means more solutes can be removed from the blood.
• Total dialyzer surface area can range from 0.5 - 2.4 square meters.

Mass Transfer Coefficient
• Mass transfer coefficient (KoA) is the ability of a solute to pass through the pores of a dialyzer.
• The KoA, in theory, is the highest possible clearance of a given dialyzer at infinite blood and dialysate flow.
• The higher the KoA, the more permeable the dialyzer.

Molecular Weight Cutoff
• Each membrane has a molecular weight cutoff determines the largest molecule that can pass through the membrane.
• It is measured in daltons (Da).
• It is the average weight of a molecule, expressed as the sum of the atomic weights of all the atoms in the molecule.

• Larger molecules have higher molecular weights; smaller molecules have lower ones.
• Dialyzers can be chosen with molecular weight cutoffs ranging from 3,000 to more then 15,000 Da.

Molecule Molecular Weight (Da)
Albumin ----------------------  66,000
Calcium (Ca++) --------------  40
Creatinine --------------------  113
Nitric Oxide (N03) -----------  62
Phosphorus (P042) ----------- 94.9
Urea -------------------------- 60
Water (H2O) ----------------- 18
Zinc (Zn2+) ------------------- 65.3

Image Source: lhsc.on.ca

Ultrafiltration Coefficients
• Another key aspect of a dialyzer is how much UF of water can occur across the membrane.
• UF is a way to remove excess water from a patient during hemodialysis by applying pressure.
• Hydraulic pressure applied to the blood or dialysate compartment forces water across the membrane.
• The dialysis machine can vary the hydraulic pressure to control the ultrafiltration rate (UFR) and amount of water removed.
• High pressure in the blood compartment forces more fluid out of the blood and into the dialysate.
• The pressure difference across the membrane (blood compartment pressure minus dialysate compartment pressure) is transmembrane pressure (TMP).
• Each dialyzer has a manufacturer's Kur.
• The Kuf is the amount of fluid that will pass through the membrane in one hour, at a given pressure.
• The Kuf helps the staff member predict how much fluid will be removed from the patient during a treatment.

Image Source: lhsc.on.ca

A dialyzer with a Kuf of 10 will remove 10 ml of fluid per hour for each mmHg of pressure.

Let's say a dialyzer has a Kuf of 10, and a TMP of 100 mmHg, the patient would lose 1,000 ml of fluid per hour of dialysis (10 x 100 = 1,000).

• Dialyzers vary in how well they remove solutes from the blood.
• The amount of blood that can be cleared of a solute in a given period of time is called clearance (K).
• Clearance rates for different molecules are given by the manufacturer for certain blood and dialysate flow rates.
• There are three main ways to remove solutes that affect a dialyzer's clearance: diffusion, convection, and adsorption. Diffusion
• Most solutes are removed during dialysis by diffusion.
• Best way to remove small (low molecular weight) solutes.

Clearance: Diffusion
• The diffusion rate depends on:
• Blood and dialysate How rates
• Membrane surface area and thickness
• Number of pores
• Solution temperature
• Membrane resistance
• Concentration gradient .,:
• Size. weight and charge of the solutes

Clearance: Convection
• When fluid crosses a semipermeable membrane, some solutes are pulled along with it. This is called convection, or solvent drag
• Convection is the best way to remove larger solutes.
• A sieving coefficient is used to say how much solute is expected to be removed by convection.

Clearance: Convection
• A sieving coefficient of 0.5 for a solute means that 50% of the solute will pass through the membrane to the dialysate side. The other 50% will be adsorbed or rejected by the membrane.
• Convective clearance depends on:
• Molecular weight cutofft he membrane
• Membrane surface area
• Ultrafiltration rate (UFR)

Clearance: Adsorption
• Adsorption occurs when material sticks to the dialyzer membrane.
• All dialyzers adsorb materials, usually small proteins, to some extent.
• Hydrophobic synthetic membranes adsorb more than cellulose membranes
• Pros & Cons (in dialysis)
• It is useful because the adsorbed protein keeps the membrane away from the blood, for better compatibility.
• Bill adsorbed material can build up on the membrane and may prevent some diffusion and convection.
• Highly adsorptive membranes may become less effective when they are reprocessed many times.
• Testing dialyzers for total cell volume (also called fiber bundle volume) may not reveal this problem.

NOTE: If total cell volume is still high, dialyzer can still be used.

Clearance: Adsorption
• Total cell volume is an indirect measure of changes in solute transport for hollow fiber dialyzers that are reused.
• A dialyzer's adsorptive ability depends on:
     • Membrane material
     • Surface area .
     • How much material has already adsorbed to the membrane

• A hollow fiber dialyzer is a clear plastic cylinder that holds thousands of fiber tubes almost as thin as strands of hair.
• These fibers are held in place at each end by polyurethane, clay-like "potting" material that holds fibers open so blood can flow inside them.
• Hollow fiber dialyzers allow for well-controlled, predictable UF.
• Because the fibers are rigid, there is no membrane compliance (change in shape or volume due to pressure).
• Instead, the fibers hold almost the same amount of fluid at high pressures as they do at low pressures.
• Resistance to blood flow is low in hollow fiber dialyzers.

• The semipermeable membrane acts in some ways like the vessel wall of a human nephron, because it
is selective.
• Riddled with microscopic pores, the membrane allows only certain "solutes and water to pass through.
• Large substances such as protein and blood cells won't fit through the small pores.
• There are membrane factors that affect removal of solutes and fluids during dialysis.
• These include the membrane material and characteristics of each dialyzer.

Membrane Materials

• Can affect diffusion and UF as well as efficiency of dialysis and the patient's comfort during treatment. Cellulose membranes
• Are made form cotton-based material that is spun into hollow fibers.
• Dialyzers with cellulose membranes have thin fiber walls (8-15 microns).
• The size of molecules cleared by these dialyzers is quite limited - about 3,000 Da.
• Removal of molecules in the larger molecular weight range, such as betta-2-microglobulin (B2m 11,800) is slower.
• Cellulose dialyzers have surface areas that range from 0.5-2.1 meters.
• Larger cellulose membranes have in vitro (tested in a laboratory) urea and creatinine clearances that compare to synthetic dialyzers.
• Cellulose dialyzers are the least biocompatible, and cause the most complement activation.
• This type of membrane is also least able to remove solute by adsorption.

Modified cellulose membranes
• The hydroxyl group (OH-) are rempved and replaced with acetate (cellulose acetate), amino acids, or synthetic molecules.
• They have much thicker fiber walls, 22-40 microns.
• They use convection, diffusion and adsorption to remove solutes.
• Clearance of solutes, especially middle molecules, depends mainly on UF rates.
• These dialyzers do a good job of removing solutes up to 15,000 Da, clearing B2m to some extent.
• Biocompatibility of these membranes ranges from good to very good.
• The best of these are close to pure synthetics.

Synthetic membranes
• Are made from polymers that are formed into hollow fibers.
• The materials used in synthetic membranes are:
     • Polycarbonate
     • Polyacrylonitrile(PAN)
     • Polysulfone (PSF)
     • Polymethylmethacrylate (PMMA)
• These dialyzers have the thickest fiber walls, 30-55 microns.
• Solutes are removed by convection, diffusion, adsorption.
• Clearance of solutes, especially middle molecules, depends mainly on UF rates.
• Do a good job of removing solutes up to 15,000 Da, clearing dim to some extent.
• Biocompatibility of these membranes are very good
• They are highly adsorptive, so they can quickly keep the blood from touching the membrane.

• A dialyzer's effectiveness is checked by testing its clearance.
• Clearance is expressed as the amount of blood (in mL) that is completely cleared ofc .'ertain solute in one minute of treatment, at a given blood flow rate (Qb) and dialysate flow rate (Qd).
• For example:
     • A dialyzer has a stated urea clearance of 250mL/mim at aQbof300mL.
     • In one minute, 250 ml of blood would be cleared of urea by the dialyzer
     • If 300mL of blood is pumped through the dialyzer in one minute, only 250 mL of blood will be cleared of urea.
• The dialyzer's surface area is fixed. So either Qb or Qd must be increased to increase clearance.
• Qb is always a factor that limits clearance, since there is a limit to how quickly blood can flow out of the patient's vascular access.

• Manufacturers test dialyzers in vitro, using watery fluids that are thinner than blood.
• When measured during actual use on patients, a dialyzer's real clearance can differ from the manufacturers stated values by ± 10 - 30%.
• The clearance of urea (a small molecular weight solute) is most often used to test the overall effectiveness of a dialyzer.

Hemodialysis Devices

After completing this module, the learner will be able to:

1. Identify the purpose and characteristics of dialyzers.
2. Describe the purpose and chemical composition of dialysate.
3. Describe dialysate preparation and the three monitoring functions of the dialysate delivery subsystem.
4. Describe the extracorporeal blood circuit functions anti monitoring systems.

• A dialyzer lets the patient's blood interact with dialysate through a semipermeable membrane.
Dialysate is a blend of treated water and chemicals; it removes wastes and fluid, and balances electrolytes.
• A delivery system supplies fresh dialysate and removes used dialysate.
• Modem high-tech delivery systems include a blood pump, an ultrafiltration pump, a dialysate conductivity monitor, alarms, and pressure gauges.
• Better membranes, safety monitors, and the use of computers have made dialysis safer.
• These advances allow today's staff to turn more of their time to patients. .
• Trained staff who know dialysis principles, equipment, and procedures are the most vital monitors of patient safely.

Tuesday, October 19, 2010

Applying Scientific Principles to Dialysis

Sections of Applying Scientific Principles to Dialysis

• Blood itself is a solution. Water is the solvent, and electrolyte, glucose, and many other substances are
the solutes. The principles of fluid dynamics, diffusion, UF, and osmosis apply to each dialysis treatment.
• To understand how dialysis removes fluid, you must know how fluids work inside the body.

Fluid in
• Beverages
• Intravenous fluids (if any)

Fluid Out
• Respiration
• Stool
• Perspiration and skin water loss
• Dialysate loss
• Residual urine

Intracellular compartments is fluid inside the cells
Interstitial compartments is fluid in between cells
Intravascular compartment is blood inside the vessels
• The body tries to keep equilibrium - to have the same level of osmotically-active solutes in all three compartments.
• During dialysis, only fluid from the intravascular compartment - the bloodstream – can be removed.


• Fluid dynamics create changes in pressure as blood is pumped out of the patient's body and through tubing and the dialyzer.
• Together, the tubing and the dialyzer are called the extracorporeal (outside of the body) circuit
• When the dialysis machine is switched on and treatment starts, the blood pump speeds the flow of blood from the patient.
• Blood passes through the needle - the first restriction in the circuit.
• Because the blood pump is pulling - rather than pushing - blood through this restriction, the pressure created is usually negative: less than zero.
• The amount of flow and restriction determine negative pressure, just as with positive pressure. As the flow or the restriction increases, the pressure will decrease. (the dialysis machine checks this pressure in the blood tubing before the pump [pre-pump arterial pressure]).
• As the blood pa5ses through the blood pump, it is pushed against the resistance of:
     • The tubing
     • The tiny hollow fibers in the dialyzer
     • The small opening of the (venous) blood return needle (or catheter)
• This resistance creates positive pressure inside the lines and dialyzer fibers. As blood passes through these resistance, the pressure changes.
• The highest positive pressure is measured in the arterial header, where blood enters the dialyzer fibers (post-pump arterial pressure).
• As blood moves through the fibers, resistance drops, so pressure drops too.
• The pressure measured after blood leaves the dialyzer (venous pressure) is the lowest positive pressure in the blood path.
• The average pressure of blood entering and leaving the dialyzer fibers is the true amount of force (positive hydraulic pressure) that aids UF of water out of the blood, through the membrane, and into the dialysate
• Dialysate flows through the dialyzer and around the hollow fibers in one direction.
• Blood flows in the opposite direction for countercurrent flow
• The machine can control the pressure differential between the blood and dialysate compartment as needed to reach the desired fluid removal.
• This pressure difference across the dialyzer membrane is called transmembrane pressure (TMP).


• The hollow fibers in the dialyzer are the semipermeable membrane.
• Blood passes through the insides of these tiny fibers (capillaries); dialysate surrounds them on the outside.
• Molecules of a certain size range pass back and forth between the blood and dialysate, always moving from an area of higher concentration to an area of lower concentration.
• Wastes in the patient's blood diffuse across the u membrane and into the dialysate.
• Used dialysate is sent to a drain and replaced with fresh dialysate, to maintain a high concentration gradient.
• This gradient allows as much waste as possible to be removed from the blood during each pass through the dialyzer.
• Electrolyte balance is also maintained with diffusion.
• It is vital to patient's health to keep the right level of electrolytes in the blood.
• To control the balance, electrolytes can be added to the dialysate.
• Electrolytes will move until the concentration is equal on both sides of the membrane.
• Keeping a constant low level of an electrolyte in the dialysate ensures that the excess is removed without allowing the levels in the blood to drop too low,
• As cleansed blood is returned to the patient, it slowly dilutes the rest of the blood.
• The drop in the concentration of solutes in the blood creates a gradient between the blood plasma (liquid
portion of blood) and the fluid in the cells and tissues.
• Because these cells have their own membranes, solutes - such as wastes and certain electrolytes -slowly pass out of the patient's cells and into the bloodstream. From there, they are dialyzed.
• This process allows some of the wastes from other body compartments to be cleared from the body by dialysis.
• This slow process of diffusion is why dialysis treatments require more than one pass of blood through the dialyzer to clear wastes from the blood
• The nephrologist factors in diffusion when prescribing a treatment. • He/she can choose a large or small dialyzer, based on the patient's body size, the length of the treatment needed, and size of molecules to be removed,


• UF requires pressure to force fluid through the membrane.
• The dialysis machine can create a hydraulic pressure difference, with higher pressure in the blood compartment than in the dialysate compartment.
• This TMP pushes excess water out of the blood and into the dialysate.
• A dialysis prescription calls for taking off enough fluid to bring a patient an estimated dry weight (EDW) by the end of the treatment.
• To figure out how much fluid you need to remove, just subtract a patient's EDW from the predialysis weight. Then add the amount of any fluids the patient will receive during the treatment.

• As water moves from the blood compartment to the dialysate compartment, molecules of dissolved solute are dragged along too (solvent drag).
• This process of solvent movement is called convection.
• The ease with which the solute is dragged along by the solvent is determined by the size of the solute molecule compared to the size of the membrane pores.
• Smaller solutes move easily, so the solution can sieve across the membrane without any change in concentration.
• Larger solutes move more slowly and the rate of convective transport is also slower.
• The convective transport of a solute depends on how porous (both size and number of holes) the membrane is.
• This measurement of porosity is known as the sieving coefficient (SC) of the membrane.

• Osmotic forces decide which way water will move from one body compartment to another.
• In hemodialysis, diffusion lowers the solute concentration in the blood.
• Higher solute concentration in the tissues and cells then pulls water out of the blood.
• Rapid drops in blood volume car occur, which causes drops in blood pressure And other symptoms.
• Often, sodium is added to the dialysate, so it diffuses into the blood.
• The higher blood sodium draws water from other body compartments into the blood, so it can be removed by UF.
• The sodium in the dialysate is then lowered towards the end of the dialysis treatment to pull the sodium back out of the bloodstream.

Scientific Principles Used in Dialysis

Sections of Scientific Principles Used in Dialysis


• A solution is a mixture of a solvent and a solute.
• The solvent is a fluid.
• The solute is any substance that can be dissolved into the solvent.
• Example: Salt water - water is the solvent and salt is the solute.
• Dialysate is the solution that is used during dialysis.
• Water is the solvent.
• The solutes are electrolytes (e.g., potassium, calcium, sodium, magnesium, and chloride ions) and glucose (sugar)
• Electrolytes levels in the dialysate closely match the levels in human blood.
• This reduces the loss of these electrolytes out of the blood and into the dialysate during dialysis.

• A semipermeable membrane is a type of thin, flexible filter - a barrier that allows only particles smaller than  a certain size to pass through it.
• In dialysis, the semipermeable membrane's holes allow small molecules, such as water and urea, to pass through easily.
• The small size of the pores keeps larger molecules and blood cells from passing through the membrane.

• Diffusion is the process by which atoms, molecules, and/or other particles move from an area where they are in high concentration to an area where they are in concentration.
• Diffusion can occur in solids, gases, or liquids, such as blood.
• In dialysis, diffusion occurs across an artificial semipermeable membrane

Source: advancedrenaleducation.com

Factors Affecting Diffusion: the Nature of the Solution

1. Concentration Gradient: How concentrated is the fluid on each side of the membrane?
     • Solutes can move through a membrane in either direction, but always toward the area of lesser concentration..
     • As the concentration gradient increases, solute movement increases too.
     • Diffusion stops when the concentrations on both sides of the membrane are equal.
     • Concentration gradient allow dialysate to remove wastes from a patient's blood and to balance electrolytes in the blood with electrolytes in the dialysate.

2. Molecular weight of the solutes: How large are the dissolved particles?
     • Smaller particles (urea and salts) diffuse more easily and quickly than larger ones (such as RBcs, WBCs, albumin, platelets, viruses and bacteria.

3. Temperature: How warm is the fluid?
     • Molecules move faster at higher temperatures, so warmer fluids allow faster diffusion.
     • Dialysis temperature is controlled during dialysis forpatient safety, comfort and faster diffusion.

Factors Affecting Diffusion: the Nature of the Membrane
1. Membrane permeability: How plentiful and large are the pores?
     • More pores allows faster diffusion
     • Larger pores allow larger molecules to pass through

2. Surface area of the membrane: How big is the membrane?
     • Surface area is the amount of membrane in direct contact with the blood and dialysate
     • Larger surface areas allow more diffusion

3. Flow geometry: How do the fluids flow?

• In dialysis, blood flows one way while dialysate flows the opposite way.
• This countercurrent flow of blood to dialysate speeds up diffusion, because with this arrangement, a high concentration gradient between the blood and dialysate cm be maintained throughout the length of the dialyzer

A concurrent flow -would occur if blood and dialysate moved in the same direction

Only molecules smaller than the pores will
pass through. Source: http://www.fmqai.com/library/attachment-library/4scientificprinciples.pdf

• Osmosis is movement of a sol vent through a semipermeable membrane from an area of lower solute concentration toward an area of higher solute concentration.
• The difference in concentration is called an osmotic pressure gradient.
•  Osmotic pressure can be overcome by hydraulic pressure using a pump, gravity, or other means.
Hydraulic pressure is pressure created naturally (such as from gravity) or artificially (such as a pump).
• Hydraulic pressure affects the amount of water thai is removed from the patient during dialysis.

Source: advancedrenaleducation.com

• Filtration - is the movement of fluid through a filter as the result of hydraulic pressure.
• Ultrafiltration-water removal from blood due to a pressure, gradient across a membrane.

Source: toltecint.com

• The transfer of heat and solutes by physical circulation or movement of the parts of a liquid or gas.
• Also called 'solute drag'


• A fluid is a liquid or gas that changes shape at a steady rate when acted upon by a force.
• The field of "dynamics" addresses the motion and equilibrium of systems.
• Fluid dynamics applies to dialysis, because it describes how two fluids (blood and dialysate) are pumped through tubing.
• Several factors affect the movement, or flow, of fluid through tubing:
     • Flow rate - the amount of fluid that flows through the tubing in a given period of time (e.g., 10ml/min)
     •  Flow velocity -is the speed at which the fluid moves through a given length of tubing.
     • Velocity is based on the rate of flow and the area of a cross section of the tube.
• If the flow rate is held constant but the cross section of the tube is reduced by half, the flow velocity will double.

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