ObjectivesAfter 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 and monitoring systems.

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Module 4 cover photo credit:

Photo of dialyzers reprinted with permission from Jim Curtis, 2005.

IntroductionA dialyzer (see Figure 1) lets the patient’s blood interact with dialysate through asemipermeable membrane. Dialysate is a blendof treated water and chemicals; it removeswastes and fluid, and balances electrolytes. A delivery system supplies fresh dialysate and removes used dialysate.

Modern, high-tech delivery systems include ablood pump, an ultrafiltration pump, a dialysateconductivity monitor, alarms, and pressuregauges. Better membranes, safety monitors,and the use of computers have made dialysissafer.

These advances allow today’s staff to turn moreof their time to patients. Trained staff whoknow dialysis principles, equipment, andprocedures are the most vital monitors ofpatient safety.

This module covers hemodialysis devices,including dialyzers, dialysate, and deliverysystems. If you carefully follow your center’sprocedures and apply the principles reviewed in this module, you can master the use andmaintenance of each device and help deliversafe dialysis treatments.

DialyzersFUNCTIONS ANDCOMPONENTS

Healthy kidneys play a key role in one of thebody’s most complex tasks—keeping aconstant, stable setting for cell survival, evenwith changes in diet and fluids, exercise, and

health or illness. This stable environment iscalled homeostasis.

The dialyzer, dialysate, and delivery systemreplace some tasks that failed kidneys can nolonger do. Today’s dialysis treatment canremove wastes and excess fluid and help keepelectrolytes and pH (acid and base balance) at levels that sustain life.

Every dialyzer has a blood and a dialysatecompartment. The semipermeable membranekeeps the two compartments apart. Themembrane is housed in a plastic case, whichholds the dialyzer together and forms pathwaysfor blood and dialysate to flow in and out.1

During a treatment, the patient’s blood—withhigh levels of electrolytes, water, and wastes—flows through the blood compartment.Dialysate, a solution made of chemicals muchlike those in blood, flows through the dialysatecompartment on the other side of the membrane.1

DIALYZER CHARACTERISTICSMany aspects of a dialyzer can affect treatmenteffectiveness, comfort, and patient safety. Theseinclude biocompatibility (how much a membraneis compatible with the human body), membrane

391

Dialysate out

Blood flow frompatient

Blood back to

the patient

Dialysate in

Fibers

Figure 1:Dialyzer

surface area, molecular weight cutoff (the solutesize that can pass through the membrane),ultrafiltration coefficient, and clearance (therate of solute removal).

BiocompatibilityBiocompatible means not harmful to biologicalfunction.2 When blood touches a foreignsubstance, immune cells in the blood react todefend the body. This defense, which involvescomplement activation and other mechanisms,can vary from clotting, which prevents bloodloss, to severe allergic reactions.3

All materials used to make dialysis membranesreact to some degree with immune cells in theblood. These effects may be so subtle that thepatient does not notice them. They may causeminor symptoms during a treatment. Or, majorlife-threatening allergies (anaphylaxis) canoccur.3 It is vital to use a membrane the patientcan tolerate.

Biocompatibility of a membrane can be testedby checking the patient’s blood for certainproteins and chemicals. The body releasesthese when blood meets a foreign substance.Their levels suggest how biocompatible amembrane is with the patient’s blood.3

A membrane’s ability to adsorb (attract andhold) proteins into the fiber wall is key to itsbiocompatibility. Adsorbed proteins coat thesurface so blood does not touch the “foreign”membrane. This protein coating explains whyreprocessed (cleaned and reused) dialyzers can be more biocompatible than new ones(note: reprocessing dialyzers with bleach canstrip the protein coating off the membrane). In general, synthetic membranes are more

biocompatible than cellulose membranes.1

Synthetic fibers are hydrophobic (water-repelling); this makes them better able toadsorb blood proteins.4

Surface AreaSurface area is key to how well a dialyzer canremove solutes. If all other aspects are equal,dialyzers with more surface area can exposemore blood to dialysate. This means moresolutes can be removed from the blood. Totaldialyzer surface area can range from 0.5–2.4square meters.

Mass Transfer CoefficientMass transfer coefficient (KoA) is the ability ofa solute to pass through the pores of a dialyzer.The KoA, in theory, is the highest possibleclearance of a given dialyzer at infinite bloodand dialysate flow. The higher the KoA, themore permeable the dialyzer.5

Molecular Weight CutoffEach membrane has a molecular weight cutoffwhich determines the largest molecule that canpass through the membrane (see Figure 2).Molecular weight is measured in daltons (Da).It is the average weight of a molecule, expressedas the sum of the atomic weights of all theatoms in the molecule. Larger molecules havehigher molecular weights; smaller moleculeshave lower ones (see Table 1). Knowing therange of molecular weights that eachmembrane will allow through helps doctorschoose membranes that will remove certainmolecules from the blood. Dialyzers can be chosen with molecular weight cutoffsranging from 3,000 Da to more than 15,000 Da.3

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Ultrafiltration CoefficientsAnother key aspect of a dialyzer is how muchultrafiltration (UF) of water can occur across themembrane. UF is a way to remove excess waterfrom a patient during hemodialysis by applyingpressure. Hydraulic pressure applied to theblood or dialysate compartment forces wateracross the membrane. The dialysis machine canvary the hydraulic pressure to control theultrafiltration rate (UFR) and amount of waterremoved. High pressure in the bloodcompartment forces more fluid out of the bloodand into the dialysate.1 The pressure differenceacross the membrane (blood compartmentpressure minus dialysate compartmentpressure) is transmembrane pressure (TMP).1

Each dialyzer has a manufacturer’sultrafiltration coefficient (KUf). The KUf is the amount of fluid that will pass through themembrane in one hour, at a given pressure.5

The KUf helps the staff member predict howmuch fluid will be removed from the patientduring a treatment.

For example, a dialyzer with a KUf of 10 willremove 10 mL of fluid per hour for eachmillimeter of mercury (mmHg) of pressure.This is stated as mL/mmHg/hr. Let’s say adialyzer has a KUf of 10, and a TMP of 100mmHg. In this case, the patient would lose 1,000 mL of fluid per hour of dialysis (10 x 100 = 1,000).

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Molecule Molecular Weight (Da)

Albumin 66,000

Calcium (Ca++) 40

Creatinine 113

Nitric Oxide (NO3-) 62

Phosphorus (PO42-) 94.9

Urea 60

Water (H20) 18

Zinc (Zn2+) 65.3

Normal kidneyHigh-

fluxConventional

Urea

Crea

tinin

e

B 12

β 2m Albu

min

Figure 2:Molecular weight cutoffDrawing adapted with permission from Althin

Academy, Miami Lakes, FL

Molecular weight (daltons)

Sievingcoefficient*

0 100 1,000 10,000 20,000 50,000 60,000 100,0000

0.2

0.4

0.6

0.8

1.0

Table 1: Molecular Weights

*See page 94 for a definition of sieving coefficient.

ClearanceDialyzers vary in how well they remove solutesfrom the blood. The amount of blood that canbe cleared of a solute in a given period of time is called clearance (K).1 Clearance rates fordifferent molecules are given by the manufacturerfor certain blood and dialysate flow rates. TheAppendix on page 116 describes the formula for determining dialyzer solute clearance.

There are three main ways to remove solutesthat affect a dialyzer’s clearance: diffusion,convection, and adsorption.

DiffusionMost solutes are removed during dialysis by diffusion:movement of solutes across a semipermeablemembrane from an area of greater concentrationto an area of lesser concentration,1 until both sidesare equal. Diffusion is the best way to removesmall—low molecular weight—solutes. Thediffusion rate depends on blood and dialysateflow rates; membrane surface area and thickness;number of pores; solution temperature;membrane resistance; concentration gradient;and size, weight, and charge of the solutes.1

ConvectionWhen fluid crosses a semipermeable membrane,some solutes are pulled along with it. This iscalled convection, or solvent drag.6 Convectionis the best way to remove larger solutes.7

A sieving coefficient (see Figure 2 on page 93) is used to say how much solute is expected to be removed by convection. A sieving coefficientof 0.5 for a solute means that 50% of the solutewill pass through the membrane to the dialysateside. The other 50% will be adsorbed or

rejected by the membrane. Convectiveclearance depends on the molecular weightcutoff of the membrane, the membrane surfacearea, and the ultrafiltration rate (UFR).7

AdsorptionAdsorption, as you’ve learned, occurs whenmaterial sticks to the dialyzer membrane. Alldialyzers adsorb materials, usually smallproteins, to some extent. Hydrophobic syntheticmembranes adsorb more than cellulose membranes.

Adsorption in dialysis has pros and cons. It is useful because the adsorbed protein keepsthe membrane away from the blood, for betterbiocompatibility. But, adsorbed material canbuild up on the membrane and may preventsome diffusion and convection. Highlyadsorptive membranes may become lesseffective when they are reprocessed manytimes. Testing dialyzers for total cell volume(also called fiber bundle volume) may notreveal this problem.3 Total cell volume is anindirect measure of changes in solute transportfor hollow fiber dialyzers that are reused.8

A dialyzer’s adsorptive ability depends on themembrane material, surface area, and howmuch material has already adsorbed to themembrane.

DIALYZER DESIGNA hollow fiber dialyzer is a clear plastic cylinder thatholds thousands of fiber tubes almost as thin asstrands of hair. These fibers are held in place ateach end by polyurethane, clay-like “potting”material that holds the fibers open so blood canflow inside them.9 Hollow fiber dialyzers allowfor well-controlled, predictable UF.1

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During dialysis, blood enters the dialyzer at the top,flows through each fiber, and leaves at the bottom.Dialysate flows around the fibers in the oppositedirection, in a countercurrent flow (see Figure 3).

Because the fibers are rigid, there is nomembrane compliance (change in shape orvolume due to pressure). Instead, the fibershold almost the same amount of fluid at highpressures as they do at low pressures.Resistance to blood flow is low in hollow fiberdialyzers.1

MEMBRANESThe semipermeable membrane acts in someways like the vessel wall of a human nephron,because it is selective. Riddled withmicroscopic pores, the membrane allows onlycertain solutes and water to pass through.Large substances, such as protein and bloodcells, simply won’t fit through the small pores.1

There are other membrane factors that affectremoval of solutes and fluids during dialysis(see Figure 4). These include the membranematerial and characteristics of each dialyzer.Each of these will be discussed below.

Membrane MaterialsWhat the dialyzer membrane is made of canaffect diffusion and UF. The dialyzer materialcan also affect the efficiency of dialysis and thepatient’s comfort during treatment.

Cellulose membranesCellulose membranes are made from cotton-based material that is spun into hollow fibers.10

Dialyzers with cellulose membranes have thinfiber walls (8–15 microns).3 Solutes pass

795

Figure 3: Blood and dialysate flow:Countercurrent directions

Blood out

Blood in

Used dialysate and excess fluidfrom the patient’s body out to drain

Each blood-filled fiberis surrounded by

dialysate

Dialysate in Direction of blood flow

Direction of dialysate flow

Wall thickness

Internal diameter190 to 240µ

Surface areaNumber andsize of pores

Blood

Single, hollow fiber

Drawing adapted with permission from Althin Academy, Miami Lakes, FL

Figure 4:Membrane and fibercharacteristics affecting

solute transport

through them mainly by diffusion. Lowmolecular weight substances readily pass fromone side of the membrane to the other, withlittle regard to applied transmembranepressure. The size of molecules cleared by thesedialyzers is quite limited—about 3,000 Da.3

Removal of molecules in the larger molecularweight range, such as beta-2-microglobulin(β2m, 11,800 Da), is slower. Cellulose dialyzershave surface areas that range from 0.5–2.1meters. Larger cellulose membranes have invitro (tested in a laboratory) urea andcreatinine clearances that compare to syntheticdialyzers. Cellulose dialyzers are the leastbiocompatible, and cause the most complementactivation. This type of membrane is also leastable to remove solutes by adsorption.3

Modified cellulose membranesChanges have been made to improve the waycellulose membranes work. The hydroxylgroups (OH–) are removed and replaced withacetate (cellulose acetate), amino acids, orsynthetic molecules.10 Modified cellulosedialyzers have much thicker fiber walls, 22–40microns.3 They use convection, diffusion, andadsorption to remove solutes. Clearance ofsolutes, especially middle molecules, dependsmainly on UF rates. These dialyzers do a goodjob of removing solutes up to 15,000 Da, clearingβ2m to some extent. Biocompatibility of thesemembranes ranges from good to very good.The best of these are close to pure synthetics.3

Synthetic membranesSynthetic membranes are made from polymersthat are formed into hollow fibers.10 Thematerials used in synthetic membranes are:

polycarbonate, polyacrylonitrile (PAN),polysulfone (PSF), and polymethylmethacrylate(PMMA).5 These dialyzers have the thickest fiberwalls, 30–55 microns.3 Solutes are removed byconvection, diffusion, and adsorption. Clearanceof solutes, especially middle molecules, dependsmainly on UF rates. Synthetic membranes do agood job of removing solutes up to 15,000 Da,clearing β2m to some extent. Biocompatibilityof these membranes is very good. They arehighly adsorptive, so they can quickly keep theblood from touching the membrane.3

Measuring DialyzerEffectivenessA dialyzer’s effectiveness is checked by testingits clearance (K). Clearance is expressed as theamount of blood (in mL) that is completelycleared of a certain solute in one minute oftreatment, at a given blood flow rate (Qb) anddialysate flow rate (Qd).9

For example, a dialyzer has a stated ureaclearance of 250 mL/min at a Qb of 300 mL. Inone minute, 250 mL of blood would be clearedof urea by the dialyzer. If 300 mL of blood ispumped through the dialyzer in one minute,only 250 mL of blood will be cleared of urea.During dialysis, the patient’s blood passesthrough the dialyzer many times, so much ofthe urea in the blood can be removed.

The dialyzer’s surface area is fixed. So, either Qbor Qd must be increased to improve clearance.The 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.

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A higher Qd provides some increase in dialysisclearance; how much depends on dialyzer sizeand membrane permeability.1

DETERMINING DIALYZER CLEARANCE

Manufacturers test dialyzers in a lab (in vitro), usingwatery fluids that are thinner than blood. Whenmeasured during actual use on patients, a dialyzer’sreal clearance can differ from the manufacturer’sstated values by ±10–30%. The clearance of urea—a small molecular weight solute—is most oftenused to test the overall effectiveness of a dialyzer.

Clearance of a certain solute is checked by drawingsamples of blood going into and leaving thedialyzer. Once the solute concentration of theblood samples is tested, actual clearance can be calculated (see Appendix on page 116).

DialysatePURPOSE OF DIALYSATE

Dialysate is a fluid that helps remove uremicwastes, such as urea and creatinine, and excesselectrolytes, such as sodium and potassium,from the patient’s blood. Dialysate can alsoreplace needed substances, such as calcium andbicarbonate, which helps keep the body’s pH balance.

During a treatment, the patient’s blood is on oneside of the membrane, in the blood compartment.The dialysate is on the other side, in thedialysate compartment. Dialysate and bloodnever mix, unless the membrane breaks.

Dialysis patients’ blood has high concentrationsof waste products and excess water. Dialysate is

prescribed to have desired levels of solutes thepatient needs and none of the ones that must beremoved completely. The osmolality (soluteparticle concentration) of dialysate shouldclosely match the blood to keep too much fluid frommoving across the membrane. The concentrationgradients created decide the diffusion rates ofeach solute across the membrane. Unwantedsolutes leave the blood and move into thedialysate; desired solutes stay in the blood.1

Some solutes are added to dialysate in amounts thatcan cause them to enter the patient’s blood. Mostoften, these are sodium, bicarbonate, and chloride.

COMPOSITION OF DIALYSATE

The doctor prescribes the dialysate. Dialysatestarts out as two concentrated salt solutions:acid and bicarbonate (see Figure 5).

n The acid concentrate has precise amountsof sodium chloride, potassium chloride,magnesium chloride, calcium chloride,glucose, and acetic acid. The acetic acid is added to lower the dialysate’s pH.

n The bicarbonate concentrate has sodiumbicarbonate and in some cases, sodium chloride.

997

Figure 5:Dialysate contents

Bicarbonate bufferSodiumPotassium

Treated water

ChlorideGlucoseCalciumMagnesium

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The two concentrates are diluted with precise amountsof treated water to make the final dialysate. Theconcentrates come in three different formulations.

Because there are three formulations, care mustbe taken to match the right acid concentratewith the right bicarbonate concentrate. TheAssociation for the Advancement of MedicalInstrumentation (AAMI) has set standardsymbols to help match the concentrates. Thesesymbols are shown in the right column of Table 2,above.11 The companies listed in the first columnintroduced a certain formulation to the Americanmarket. Today, almost all of the hemodialysismachines can use any of the formulations.

When the concentrates are diluted with theprescribed amount of water, they will have theright concentration of electrolytes (particlesthat carry an electrical charge). Electrolytes arevital for cell function. Precise levels of sodiumand potassium are needed on each side of cellmembranes to allow nerve signals and other cellfunctions in the body. These levels are shown inTable 3. We will cover each substance briefly.

Sodium (Na+)Sodium is a major electrolyte of the body’s bloodplasma and interstitial (between the cells) fluid.In the body, sodium causes fluid to move across cellmembranes. In this way, fluid shifts betweenthe intracellular (inside the cells) space and the plasma and interstitial space. This fluidmovement includes the intravascular space (in the blood vessels). Fluid and solutes mustbe in the plasma to be removed by dialysis.

Normal sodium concentration in the blood isfrom 135–145 mEq/L. Sodium concentration in dialysate is most often kept in the samerange. Higher levels are sometimes used;1 if so, a careful patient assessment and a doctor’sprescription are needed. Dialysate deliverysystems can adjust the dialysate sodium levelduring a treatment. The dialysate sodium levelis changed according to a doctor’s prescription.This is called sodium modeling.

These systems can, for example, start atreatment at a high sodium concentration andthen slowly reduce it. This sodium change hasbeen shown to create more efficient fluid shiftsin the body, to remove fluid faster. Sodiummodeling also provides for better control ofblood pressure and fluid removal. This helps

Table 3: Typical Range of Substances in Dialysate

Substance Concentration in Dialysate

Sodium 135 to 145 mEq/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

Table 2: Concentrate Proportioning Ratios

Type/Style Parts Acid

PartsBicarb

PartsWater

PartsDialysate

Drake-Willock 1.00 1.83 34.00 36.83X

COBELaboratories 1.00 1.72 42.28 45X

Fresenius 1.00 1.225 32.775 35X

Table adapted with permission from AAMI

some patients tolerate more UF with fewercomplications. However, the use of sodiummodeling can increase thirst and body weight,and hypertension between dialysis treatments.12

Potassium (K+)Potassium is a major electrolyte of theintracellular fluid. The body keeps preciseamounts on both sides of cell membranes tosend nerve signals. Just enough potassium isadded to dialysate to bring the patient to anormal plasma potassium level: from 3.5–5.5mEq/L.13 Potassium in the dialysate rangesfrom 0–4 mEq/L, based on the patient’s needs.1

Magnesium (Mg++)Magnesium is vital to the nerves and muscles.It also triggers enzymes that are key tocarbohydrate use. Magnesium is found in the plasma at levels from 1.4–2.1 mEq/L.13

The magnesium range in dialysate is 0.5–1.0mEq/L.1

Calcium (Ca++)Calcium is found in the body in extracellular(outside the cells) and intracellular (inside thecells) fluid. It builds bones and teeth, helpsmuscles move, is needed for blood clotting, andhelps send nerve signals.13 The normal range ofcalcium in the plasma is 8.5–10.5 mg/dL(4.5–5.5 mEq/L); dialysate calcium is mostoften 2.5–3.5 mEq/L.1 Sometimes the lowerrange is used if patients take calcium-basedphosphate binders and/or calcitriol (activevitamin D), which can raise serum calciumlevels. Patients whose predialysis calcium levelsare quite high or low may have their dialysatecalcium changed by a doctor.

Chloride (Cl–)The concentration of chloride in dialysatedepends on the contents of chemicals such as sodium chloride, potassium chloride,magnesium chloride, and calcium chloride.Dialysate chloride ranges from 100–124 mEq/L.Normal plasma chloride levels are 98–111millimoles per liter (mM/L).14

Glucose (C6H12O6)Glucose may be added to dialysate to preventloss of serum glucose and to reduce catabolism(muscle breakdown). Adding glucose caloriescan help patients who are diabetic ormalnourished. Dialysate glucose levels mayrange from 0–250 mg/dL. The glucose indialysate can be two to three times higher thanin normal blood (70–105 mg/dL). This meansthat dialysate with glucose has an osmotic(water-pulling) effect that aids UF.

Bicarbonate (HCO3)Bicarbonate is a buffer—a substance that tends tomaintain a constant pH in a solution, even if anacid or base is added. Healthy kidneys keep thebody’s pH within the very tight limits that cellsneed to survive. The kidneys do this by makingand regulating bicarbonate. Bicarbonate isadded to dialysate to help maintain patients’ pH.

Bicarbonate is used by the body to neutralize acidsthat are formed when cells metabolize proteins andother foods used for fuel. People with chronickidney disease can’t excrete enough acids in theurine, so they are in a constant state of metabolicacidosis (i.e., having too much acid in the blood).

In dialysate, bicarbonate is used to replace thebody’s stores of buffer. Bicarbonate can reduce

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dialysis-related problems like hypotension, musclecramps, nausea, and fatigue after treatment.

HemodialysisDelivery Systems

PURPOSEA delivery system is a machine that mixesand delivers dialysate, pumps blood throughthe dialyzer, and monitors various dialysisparameters to ensure a safe treatment (seeFigure 6). Most delivery systems monitorpatient and machine safety parameters. Theseinclude blood flow, dialysate flow, dialysatetemperature, conductivity, venous and arterialpressure, blood in dialysate leaks, patient bloodpressure, etc. The delivery system is two major

subsystems: the dialysate delivery system andthe extracorporeal blood circuit. We’ll covereach one and its parts.

DIALYSATE DELIVERY SYSTEMA dialysate delivery system controls theamounts of water and chemicals in dialysate,and checks its conductivity, temperature, pH,flow rate, and pressure. It also tests thedialysate for the presence of blood.

The Proportioning SystemIn a proportioning system, dialysate is made bymixing fresh concentrate with fixed amounts oftreated water. The mixing is controlled by theinternal mechanical and hydraulic design of thedelivery system. The exact amount of water andconcentrate is set by your center’s policies andprocedures (see Table 2, page 98).

Proportioning systems make dialysate in twoways. Both rely on a continuous supply of freshconcentrate and treated water:

n The first type of system mixes concentrateand water using fixed-ratio pumps. Fixed-ratio mixing uses diaphragm or piston pumpsto deliver set volumes of concentrate andwater to a mixing chamber.15

n The other type of proportioning system usesservo-controlled mechanisms: these haveconductivity control sensors that constantlycheck the dialysate’s total ion concentration.Electronic circuits compare the solution’s realconductivity to the prescribed level, and adjustthe proportioning to reach the prescribed value.15

Once mixed, dialysate is warmed andmonitored for conductivity, temperature,pressure, and flow rate (see Figure 7).

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100

Dialog+

Figure 6:Examples of hemodialysis

delivery systems

FRESENIUS

Drawing adapted with permission from B. Braun Medical Inc.and Fresenius Medical Care–North America, respectively

After dialysate leaves the dialyzer, it passes througha blood leak detector. Blood in the dialysate couldmean a tear in the membrane. So, blood leakdetectors are often treated as extracorporeal—outside the body—alarms, even though they checkthe dialysate. Used dialysate that has passed throughthe blood leak detector is discarded down a drain.

The Monitoring SystemUsing the wrong dialysate can make a dialysistreatment less effective. This mistake may evencause illness or death to a patient. Dialysate mustbe checked throughout each treatment to ensurethat it is the right concentration and temperature,and that it is flowing at the right rate. Some deliverysystems also check the dialysate pH continuously.

The following descriptions include generalinformation that suits most dialysis machines.To learn how to check alarms on the equipment youwill be using, see your center’s procedures manual.

ConductivityExcept for glucose, the chemicals in dialysateare all salts (electrolytes). Salts break apart in

water to form positive and negative chargedparticles called ions. Dialysate electrolyte levelsmust be kept within certain limits to keeppatients safe. The dialysate proportioningsystem checks the total electrolyte level indialysate by testing conductivity (how muchelectricity the fluid will conduct). Conductivityis checked by placing a pair of electrodes in thedialysate. Voltage is applied to the electrodes,and the current is measured. The measurementgives the estimated total ion concentration ofthe dialysate.15 A sensor cell may be usedinstead of the 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 firstconcentrate (most often acid) with water. Theother sensor measures the final dialysate afterthe second concentrate is added. Somemachines use conductivity sensors to make the dialysate itself. These have a second set ofsensors to check the mixtures, apart from theones that control the mixing. This multiple

13101

Single-patient proportioning systems like this one are most commonly used.

Openvalve

Closedvalve

Meter

Gauge

Pump

Heater

Flowmeter

Treated water

To drain

Ultrafiltrationpump

Blood leakdetector

Bypass

DialyzerFlowmeter

ThrottlevalveBlood

inBloodout

HeaterTemperature

Pressure

Dialysate

Deaerator Proportioningpump

Conductivitysensor & meter

Concentrate

Figure 7:Single-patient

dialysate delivery system

monitoring system, called redundant monitoring,is used so two sensors would have to fail beforea patient could be harmed.15

Conductivity is usually checked at the point of mixingand again before the dialysate enters the dialyzer.Depending on the equipment at your center,conductivity may be stated in micromhos/cm,millimhos/cm, microsiemens/cm, or millisiemens/cm.16 A millisiemens/cm is 1/1000 of asiemens/cm and a microsiemens/cm is1/1,000,000 of a siemens/cm. Siemens wasformerly called “mho” because conductance insiemens is the reciprocal of resistance in ohms.16

Most dialysate delivery systems have internal,preset conductivity limits. When the dialysateconcentration moves outside the preset safe limits,it triggers a conductivity monitoring circuit.

The circuit stops the flow of dialysate to thedialyzer and shunts it to the drain. This iscalled bypass. Bypass keeps the wrong dialysatefrom reaching the patient. The circuit also setsoff audible and visual alarms to alert the staff.The most common type of conductivity alarmis low conductivity. The most frequent cause isa lack of concentrate in one or both of theconcentrate jugs. A high conductivity alarm ismost often due to:15

n Poor water flow to the proportioning system

n Untreated incoming water

n Use of the wrong dialysate concentrate

Before each treatment, check the conductivityalarm to be sure it is working, and check themachine readings against an independentmeter. There must always be enough of bothconcentrates in the proportioning system tocomplete the whole treatment.15

TemperatureToo-hot dialysate can cause hemolysis (burstingof red blood cells). Too-cool dialysate is not lifethreatening, but it can make the patient cold andreduce diffusion so the treatment is less efficient.In all dialysate delivery systems, dialysate is keptin the range of 37°C to 38°C (98.6°F to 100.4°F).15

Water must be heated to a certain temperaturebefore mixing with the concentrates.

The method of warming the water depends onthe delivery system design. Some systems use aheat exchanger before the heater, to save energy.In these systems, used dialysate transfers its heatto the incoming cold water, warming it before it enters the heater. Most systems use a heatercontrolled by a thermistor, a type of thermostat.

To check dialysate temperature, a separatetemperature monitor is placed in the dialysatepath before the dialyzer. This monitor’s limitsare preset, and it works independently of theheater control thermistor. Many alarm systemshave a low setting, which should not be below33°C (91°F). (With some delivery systems, the patient is the only “monitor” of lowtemperature.) The high limit should be set at no higher than 41°C (105°F).15 If thetemperature is too hot or cold, a circuit sets offaudible and visual alarms.15 The circuit alsotriggers bypass to shunt dialysate to a drain.15

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

Flow rateDialysate flow rate to the dialyzer is controlledby a flow pump. Some delivery systems have apreset flow rate; others let the flow vary as the

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doctor prescribes. In general, higher dialysateflow rates improve dialyzer efficiency, thoughlittle improvement occurs above 800 mL/min.

Dialysate flow rates range from 0–1,000 mL/min.Some systems have flow meters that continuouslydisplay the dialysate flow rate on a gauge or a digitaldisplay. Others do not display flow rate at all.

Dialysate flow rate audible and visual alarms may be set off by:

n Low water pressure

n Dialysate pump failure

n A blockage in the dialysate flow path

n A power failure

A high/low conductivity, high/low pH, hightemperature, or in some cases blood leak alarm,can trigger the delivery system to switch intobypass mode.15

Check the delivery system before each treatmentto be sure that the bypass mode works properlyfor all dialysate alarm conditions.

Blood leak detectorDialyzer membranes are fragile and can tear,letting blood and dialysate mix. If this occurs, thepatient could have major blood loss and/or theblood could be contaminated by the nonsteriledialysate. A blood leak detector (see Figure 8) is used to check for blood in the used dialysate.The detector can sense very small amounts ofblood, less than can be seen with the naked eye.

The blood leak detector shines a beam of lightthrough the used dialysate and onto a photocellor photoresistor. Normally, dialysate is clear, sothe light can pass through. But even a tinyamount of blood will break the light beam. The

detector will sense such a break, triggeringaudible and visual alarms.16

When a blood leak alarm occurs, the blood pumpstops and the venous line clamps to preventfurther blood loss.15 In some systems, a bypassmode shunts dialysate to the drain. Thisreduces negative pressure and keeps blood frombeing drawn through the tear into the dialysate.

A Hemastix® (strip that reacts to blood) should beused to check the extent of the leak. The test mustbe taken where the dialysate leaves the dialyzer:15

n If blood or pink color can be seen in thedialysate path, there is a major leak.

n Clear dialysate and a positive Hemastix testsuggest a minor leak.

n Clear dialysate and a negative Hemastix testmean a false alarm.

Depending on your center’s procedures for ablood leak, you stop the treatment withoutreturning the patient’s blood. This keepspossibly contaminated blood from reaching the patient, where it could cause an infection.

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Figure 8:Photoelectric blood leak detector

If the light beam is interrupted by blood, an alarm will sound, and the blood pump will stop.

Dialysate out Dialysate in

Light bulb

Photoelectric cell

The blood leak detector’s basic sensitivity isusually preset by the manufacturer. Adjustmentscan be made within this limited range.17

pHpH is a measure of how acidic or alkaline(basic) a solution is. The pH of a solution isbased on the number of acid ions (hydroniumions) or alkali (base) ions (hydroxyl ions) itcontains. A solution with:

n An equal number of acid and base ions isneutral and has a pH value of 7.0.

n More acid ions is acidic and the pH valuewill be less than 7.0.

n More base ions is alkaline and the pH will be greater than 7.0.

Bleach (sodium hypochlorite) is alkaline, with a pHof 11.0. White vinegar is an acid, with a pH of 2.9.The pH of blood is normally from 7.35–7.45; a weakbase. Dialysate must have a pH close to bloodso it does not change the blood pH. In general,the range of dialysate pH is from 7.0–7.4.

Some delivery systems monitor pHcontinuously throughout the treatment.Dialysate pH affects the patient’s blood pH.Whether or not the delivery system has a pHmonitor, at the start of each treatment, anexternal test must be done to ensure that thedialysate pH is in a safe range. The mostaccurate pH measure uses a pH electrode,which puts out a small voltage when placed in a solution. The voltage is read by a detectioncircuit that converts the signal into a pH valueand displays it. Test strips coated with achemical that changes color based on pH areanother way to measure pH. For any method,you must have known test fluids on hand that

can be used to check that the meter or strips are still accurate.

Ultrafiltration Control UltrafiltrationRemoving excess fluid is another key part of anadequate treatment. Fluid removal is achievedthrough ultrafiltration (UF) (the movement offluid across the dialyzer membrane in responseto positive and negative pressures). UF occursduring the treatment when the pressure on theblood side of the dialyzer membrane is morepositive than the pressure on the dialysate side.This pushes fluid in the blood across the membraneinto the dialysate compartment where it is thenexpelled in the drain. The difference betweenthese pressures (the pressure gradient) is thetransmembrane pressure, or TMP.

TMP and dialysate pressureThe TMP determines how much fluid from theblood is forced across the membrane. In thepast, dialysis machines used a manual system ofsetting the TMP or a negative dialysate pressurefor achieving fluid removal. A technician ornurse needed to determine the total fluid lossrequired for the patient and then calculate thehourly UFR. Mathematical equations were usedto find the TMP needed for the KUf of the dialyzerbeing used (see Module 6: Hemodialysis Proceduresand Complications for TMP calculation). Dependingon the type of machine, the technician or nursewould set the TMP or a negative dialysatepressure to achieve the calculated TMP. Withtoday’s volumetric dialysis, TMP is calculatedand set for you. All you need to enter is thedesired fluid removal amount (in mL) and thetreatment time. Fluid removal accuracy of the

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older systems was not nearly as precise as today’sUF control systems due to variables including:

n The KUf values reported by dialyzer companiesare usually in vitro values. In practice, the invivo KUf is often somewhat lower (5%–30%).18

n Clotting of the dialyzer fibers reduces the KUfby reducing the surface area of the membrane.

n Increasing or decreasing the blood pumpspeed changes the venous pressure.

n An increase or decrease in the dialysate flow, or a kink or blockage in the dialysatelines changes the dialysate pressure.

These conditions have no effect on fluidremoval accuracy with UF control machines.

UF control systemsUF control is the means by which the dialysismachine removes fluid from the patient andaccurately measures it. The amount of fluidremoved in a specific period of time is theultrafiltration rate (UFR).19 Most dialysismachines use a volumetric fluid balancingsystem (see Figure 9). This type of system usestwo chambers that fill and drain to control thevolume of dialysate going to and coming fromthe dialyzer. This is known as volumetriccontrol. Another type of machine uses sensorsin the fluid path to and from the dialyzer tocontrol and monitor the flow rate of thedialysate. This is known as flow control.

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Figure 9:Volumetric UF control system

Drain

Dialyzer

Bloodleakdetector

Airseparationchamber

UF pumpDrain

Flow pump 2Flow pump 1

Proportioneddialysate

Useddialysateis pushedout to thedrain

Freshdialysate is

pushed out tothe dialyzer

Deaerationpump

Conductivity celland temperature

sensor

Balancechambers

Flexiblemembrane

DialysateUsed dialysateAirUnused path this cycle

Valve-open

Valve-closed

Volumetric UF ControlOne of the main components of the volumetricUF control system is the balance chambers orbalancing chambers.20 There are two identicalchambers. Each chamber is divided in half by a flexible diaphragm. Each chamber half has an inlet and an outlet. One side of eachchamber is in the “to dialyzer,” or fresh dialysateflow path. The other side is in the “fromdialyzer,” or used dialysate flow path. There arevalves on each inlet and outlet. These valvesopen and close so that as fluid enters on oneside of the chamber, it pushes on the diaphragmand forces fluid out on the other side. Thetiming of the valves opening and closing issynchronized. One chamber is filling with useddialysate, pushing fresh dialysate to the dialyzer.At the same time, the other chamber is fillingwith fresh dialysate, pushing the used dialysateto the drain.

One pump moves the proportioned dialysate to the balance chambers. A second pump pullsdialysate from the dialyzer and pushes it to thebalance chambers. This keeps a constant flowthrough the dialyzer. The movement ofdialysate to and from the dialyzer takes place ina closed loop. The volume of dialysate enteringand exiting the dialyzer is the same, because the volume entering one side of the balancechamber displaces the same amount on theother side. So, the flow to and from the dialyzeris balanced.

Another main component in the system is theultrafiltration pump (UF pump) or the fluidremoval pump. Its function is to remove fluidfrom the closed loop. This results in fluidremoval from the patient through the dialyzer

membrane. Most UF pumps are diaphragm orpiston type, and are most often placed in theused dialysate flow path. The pumps work witha stroking movement that removes a small,fixed amount of fluid on each stroke (about 1ccor less). The removal of fluid from the closedloop creates a negative pressure in the loop.Therefore, pressure is negative in the dialysatecompartment of the dialyzer, relative to theblood compartment pressure. This creates thepressure gradient that is needed for UF. Whenthe UF pump is off, there is no pressuredifference between the blood and dialysate—and no fluid is removed.

As the pump removes fluid from the closed loop,the same amount is replaced, moving across thedialyzer membrane into the loop. This allowsthe machine to precisely remove the rightamount of fluid from the patient. You or thenurse will determine the total amount of fluidthat should be removed and use the machinecontrols to enter it along with the duration ofthe treatment in hours. The machine’scomputer will calculate the UFR, which decidesthe rate at which the UF pump will run.

Other important components in the system areused to perform control, monitoring, and safetyfunctions. Pressure sensors serve functionssuch as controlling pump speeds, preventingoverpressurization, calculating TMP, and detectingleaks within the system. Air separation chambersremove any air coming out of the dialyzer. (Thiswill occur when priming a new [dry] dialyzer.)Any air in the system could result in incorrectfluid removal. The air separation chambermaintains a level of fluid while releasing air out the top that is routed to the drain.

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Flow ControlAnother type of UF system is the flow controlsystem (see Figure 10). This system has flowsensors on the inlet and the outlet side of thedialyzer to control dialysate flow through thedialyzer. Inlet and outlet flow pumps are set sothe flow measured at the inlet and outlet flowsensors is equal. This flow balance is the key tothe system’s accuracy and ensures that the onlyfluid removed from the patient is that which isremoved by the UF pump.19

This UF control system uses a postdialyzer UFpump that removes fluid at the UFR calculatedby the machine’s computer. The speed of thepump is equal to the UFR . It is determined by the time needed to fill a small chamber of a known volume (UF burette). There are high-and low-level sensors in the UF burette that

signal a valve to open and close. When the UFburette becomes full, the valve opens and apump empties it by forcing air into the top andthe contents to the drain. When it has emptied,the valve closes and the UF burette begins to fillagain.19 This occurs repeatedly throughout thetreatment. The outlet flow sensor does notmeasure the fluid removed by the UF pump.Flow control UF systems may also use pressuresensors and air separation chambers in thesame way that volumetric systems do.

EXTRACORPOREAL CIRCUITThe extracorporeal circuit carries blood fromthe patient’s access to the dialyzer and back tothe access. It is the second major subsystem of the hemodialysis delivery system. Theextracorporeal circuit includes the arterial and

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Figure 10:Flow control UF system

Drain 2

Postdialyzerflow sensorPredialyzer

flow sensorBubbletrap

Drain

Bypass

High sensorUF buretteLow sensor

From airpump

UFpump

Flow pump 1

Dialyzer

Blood out Blood in

Flowpump2

BLD

DialysateUsed dialysateAir

Pressure sensor

Valve direction of flow (black segments = flow direction)

venous blood tubing, blood pump, heparinpump, dialyzer, venous line clamp, blood flowmonitors, pressure monitors, and air monitors(see Figure 11). We will cover each part next.

Components and MonitoringBlood tubingDuring hemodialysis, blood from the patient’svascular access (arterial needle) flows to thedialyzer. Blood flows back to the patient’s access(venous needle) through blood tubing, or“lines” (see Figure 12). The inner diameter of the blood tubing is small. Only a smallamount—about 100–250 mL—of blood isoutside the patient’s body at any time.

There are two parts of the blood tubing: arterialand venous. The arterial segment is most often

color-coded red; the venous segment is mostoften color-coded blue. Bloodlines are smoothon the inside to reduce clotting and air bubbles.Many manufacturers offer custom-made bloodtubing for various types of equipment and patients.

Each set of blood tubing has different, specializedparts. The order in which those parts areinstalled in the delivery system varies with thesystem’s design, prescribed treatment, and themonitoring desired. Parts of blood tubing include:

n Patient connectors: A tip, or Luer-Lok®connector, at the end of the arterial andvenous blood tubing segments connects thetubing to the patient’s needles or catheter ports.

n Dialyzer connectors: Luer-Lok connectorsat the other end of the blood tubingsegments connect the tubing to the dialyzer.The arterial blood tubing segment connectsto the arterial end of the dialyzer. Thevenous blood tubing segment connects tothe venous end of the dialyzer.

n Drip chamber/bubble trap: The dripchamber checks arterial or venous pressurein the blood circuit. It uses a monitoring line with transducer protectors (see page109), and collects or “traps” any air thataccidentally gets into the extracorporealcircuit. The drip chamber can also keep bloodclots in the extracorporeal circuit fromreaching the patient, by using a very fine meshscreen. This type of drip chamber is placedon the venous blood tubing segment, afterthe dialyzer and before the patient’s access.

n Blood pump segment: The blood pumpsegment is a durable, pliable, larger diameterpart of the arterial blood tubing. It isthreaded through the blood pump roller.

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Figure 11:Extracorporeal circuit

Used dialysate back tomachine and out to drain

Blood pump (Qb)

Arterial sample port (CBi)

Arterial pressure(pre-pump)

Drip chamber

Drip chamber

To patient

From patient

Venous pressure

Venous sample port (CBo)

Fresh dialysate from machine

n Heparin infusion line: During dialysis,heparin (a blood thinning drug) may begiven to the patient through a very smalldiameter tube that extends out of the bloodtubing. The heparin infusion line is mostoften placed on the arterial blood tubingsegment just before the dialyzer.

n Saline infusion line: This line allows saline tobe given to the patient during dialysis. It is mostoften placed on the arterial blood tubing segmentjust before the blood pump, so saline can bepulled into the circuit. If the saline infusionline is not clamped correctly, too much fluidor air can enter the extracorporeal circuit.

Transducer protectorsA transducer is a mechanical device inside themachine that converts air pressure into anelectronic signal. This signal is used to displayvenous pressure, arterial pressure, and TMP.Moisture would damage the transducer.Transducer protectors are a barrier betweenblood in the tubing and the transducer in themachine. They connect to the machine’s venousand/or arterial ports via a small tubing segmenton top of the drip chamber. Transducer portlines have a small line clamp in the middle. Thetransducer protector connects to the end ofthese lines and is the link between the machineand the blood tubing set (drip chambers).

Transducer protectors use membranes with anominal pore size of 0.2 microns21 that arehydrophobic when wetted, to keep fluid frompassing through. If these filters get wet, theyprevent air flow.21 Wetted or clampedtransducer protectors cause pressure readingerrors. A wet or clamped venous transducerprotector will also cause TMP problems, since

TMP is partly venous pressure. A loose ordamaged transducer protector on a pre-pumparterial drip chamber port could allow air intothe bloodline circuit.

In May, 1999, the federal Food and DrugAdministration (FDA) put out a safety alert on cross-contamination from wet transducerprotectors.22 In April 2001, the Centers forDisease Control and Prevention (CDC) maderecommendations. These require centers tochange wet transducer protectors right awayand inspect the machine side of the protectorfor contamination or wetting. If a fluid

21109

Figure 12:Circulation in human body and extracorporeal circuit

Bloodpump

Dialyzer

Lines

Access

Kidneys

HeartArteriesand veins

During dialysis, the blood pumpacts like the human heart,pushing blood through the lines(vessels). The dialyzer, orartificial kidney, cleans theblood, which is then returned to the patient’s own circulation.

breakthrough is found on the removedtransducer protector, the machine’s internaltransducer protector (a back-up) must beinspected by a qualified technician. You may berequired to disinfect the machine’s transducerprotector port and replace the internal transducerprotector before the next treatment can begin.23

Blood pump/blood flow rateThe blood pump (see Figure 13) moves bloodfrom the patient’s arterial needle through theblood tubing, to the dialyzer, and then back tothe patient through the venous needle. Mostoften, the type of blood pump used is a rollerpump. This pump uses a motor that turns aroller head. Speed of the roller head determinesblood flow rate, which is set by the staff person.

The blood pump segment of the blood tubing isthreaded between the rollers and the pumphead. The rollers turn, blocking the tubing andpushing blood out of the segment. After theroller has passed, the segment resumes itsshape and blood is drawn in to refill the pumpsegment. In this way, blood is pulled into andpushed out of the segment at the same time.

By changing the roller speed, blood flowthrough the extracorporeal circuit can be set

according to the prescription. Blood flow rates canbe varied between 0 mL/min and 600 mL/min.

Some machines count blood pump turns andcalculate the number of liters processed in atreatment. Knowing the number of liters prescribedto be processed allows calculation of the bloodflow rate: divide liters processed by minutes oftreatment. This value can be used as a qualityassurance tool; it should equal the blood flow rateshown on the machine. For an effective treatment,the blood flow rate must be accurate and reflectthe doctor’s prescription. The staff member mustverify that the rate on the readout is correct.

Pump occlusion is the amount of space betweenthe rollers and the pump housing. The bloodpump rollers must press against the bloodpump segment hard enough to pull and pushthe blood through the extracorporeal circuit. If the rollers are too tight, the blood pumpsegment may crack or red blood cells 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 insertedproperly. Pulling down the ends of the pumpsegment in the housing will compress the

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110

The blood pump segment of the tubing is stronger and thicker to withstand the pressure of the pump.

Figure 13:Blood pump

Step one – Blood enters the pump Step two – Blood is pushed along by the rollers

Step three – A constant blood flow rate is created

Housing

Roller

Blood pumpsegment

springs and cause the wrong occlusion. Thiswill also reduce blood flow by decreasing theamount of pump segment in the blood pump.

It has to be realized that the blood flow measured bythe number of pump revolutions assumes a stableblood volume pushed with each revolution. A high negative pressure flattens the tubing inthe roller pump and decreases its volume. Theblood flow indicated by the dialysis blood rollerpump is always greater than the delivered bloodflow, and this difference is in turn conditionedby the negative pressure induced by the bloodroller pump in the arterial bloodline

Pump occlusion must be checked periodically andadjusted per the manufacturer’s instructions.The occlusion should also be checked when thetubing size or manufacturer changes.

In case of emergency, all blood pumps have a wayto allow hand cranking. Most often, the pumpwill have a handle, either with the pump head orone that can be inserted into the pump, whichcan be used to crank the pump.24 The pump headshould be hand cranked just fast enough tokeep the venous pressure at the pre-alarm level.

Extracorporeal pressure monitorsPressure in the extracorporeal circuit depends onblood flow rate and resistance to the flow. Resistanceoccurs in nearly every part of the extracorporealcircuit: access needles or catheters, bloodtubing, and the dialyzer. The blood pump isused to overcome this resistance. Pressures aredisplayed in millimeters of mercury (mmHg)on a gauge, meter, or screen. Depending on theequipment, pressure can be read at several sites.

Extracorporeal pressure monitoring is needed tocalculate TMP and ensure patient safety. In some

systems, pressure monitors have upper and lowerlimits that can be set. In others, they have a presetrange within which staff can choose a midpoint.

When pressure exceeds the high or low setting, thesystem will trigger audible and visual alarms, stopthe blood pump, and clamp the venous line. Youmust check the extracorporeal blood pressurealarm to ensure that it works properly beforeeach treatment.

A pre-pump or post-pump drip chamber may beplaced on the arterial bloodline. A monitoringline or pressure gauge connection at each dripchamber is used to check arterial and/or venouspressure in the extracorporeal circuit (see Figure 14).

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–

+

+

+

Figure 14:Pressure monitoring devices onarterial and venous bloodlines

The extracorporeal circuit has gauges to measure venous and arterial pressures.

Line clamp

Bloodpump

Air detector

Arterialbloodline

Arterial pressuregauge (negative,

pre-pump pressure)

Arterial pressuregauge (positive,post-pump pressure)

Venousbloodline

Arterial(pre-pump)pressure

Vascularaccess

Venouspressure gauge

(positive,postdialyzer

pressure)

The pressures described below may bemonitored, depending on the dialysate deliverysystem used:

n Arterial pressure is pressure from the patient’saccess to the blood pump. It is also called pre-pump pressure. When a blood pump is used witha fistula or graft, arterial pressure will usuallybe less than zero, or negative. Resistance fromthe vascular access and the pulling of theblood pump creates this negative pressure.

n Predialyzer pressure is pressure betweenthe blood pump and the dialyzer, also called

post-pump pressure, predialyzer pressure, orpost-pump arterial pressure. Pressure inthis segment of the blood tubing is greaterthan zero, or positive. Predialyzer pressure ismonitored to detect clotting in the dialyzer.Suspect clotting if there is a large pressuredifferential on each side of the dialyzer.

n Venous pressure is pressure from themonitoring site to the venous return. This pressure is often called postdialyzerpressure. Pressure in this segment ispositive.

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LOW ALARM HIGH ALARM

Arterial pressure (negative) (pre-pump)

• Blockage of arterial blood flow from the vascularaccess

• Compression or kinking of the arterial bloodline•Wrong position or infiltration of the arterial needle• Blood pump set at a rate higher than the vascularaccess can supply

• Hypotension• Vasoconstriction (tightening of the patient’s bloodvessels)

• Poorly working central catheter

• A bloodline separation (if the upper limit is setbelow zero)

• A leak between the patient and the monitoring site• A decrease in the blood pump speed• Infusion of saline or medications

Predialyzer pressure (positive) (post-pump)

• A bloodline separation or leak between themonitoring point and the dialyzer, or at the needle

• Occlusion in the blood tubing between the bloodpump segment and the monitoring site

• A kink in the blood tubing anywhere from thepatient to the monitoring site

• Poor blood flow or drop in blood flow rate

• A clotted dialyzer• Poor placement or infiltration of the venous needleor catheter

• A rise in the blood flow rate• A kink in the blood tubing from the dialyzer back tothe monitoring site

Venous pressure (positive)

• Separation of blood tubing from the venous needleor catheter

• Drop in blood flow rate• Blockage in the blood tubing before the monitoringsite

• A severely clotted dialyzer

• A blockage in the blood tubing between themonitoring site and the venous needle

• Poor position or infiltration of the venous needle• Poorly working central catheter• Clotting access

Table 4: Pressure Alarm Triggers

Common causes of high and low pressure alarmsfor arterial pressure, predialyzer pressure, andvenous pressure are shown in Table 4.25

Air detectorsAir can cause death if it gets into the patient’sbloodstream. Air/foam detectors continuouslycheck the blood in the venous tubing segmentfor air and foam (see Figure 15). The systemmay check for air at the venous drip chamber or at the blood tubing just below it.

Air detectors are ultrasonic devices that checkfor changes in a sound wave sent through across-section of the blood path. Sound travelsfaster through air than liquid. Therefore, any airin the blood will raise the speed at which thesound wave passes through the blood, settingoff an alarm. An air detector’s alarm sensitivitylimits are most often preset by the manufacturer,but can be calibrated by qualified technicians.

When the air detector senses air, it will triggeraudible and visual alarms, stop the blood pump,and clamp the venous blood tubing to keep airfrom getting into the patient’s bloodstream.

You must check the air detector to be sure it is working properly before each treatment,following the manufacturer’s instructions.The air detector must always be used during the dialysis treatment and venous line clampsengaged with the tubing segment.

Heparin systemWhen the patient’s blood touches the artificialmaterials of the lines and dialyzer, it tends to clot. Heparin, an anti-clotting drug, oranticoagulant, is used to prevent clotting in the extracorporeal blood circuit.

Some centers give heparin intermittently (on andoff) during dialysis; a prescribed amount is injectedinto the arterial bloodline at prescribed times. Also,heparin can be given by bolus (the full prescribedamount is given all at once just before the treatment.)

Other centers give heparin by continuousinfusion (a prescribed rate throughout thetreatment.) A syringe filled with heparin, aheparin infusion line (see Figure 16), and aninfusion pump are used and the pump slowlyinjects heparin into the extracorporeal circuit.

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Figure 15:Air detector

Figure 16:Heparin infusion line

#1 #2

Sensor device

Sensor deviceLine clamp

Line clamp

#2 uses a photocellto monitor change in light transmission

#1 uses an ultrasonic sensordevice to monitor change in sound transmission

To dialyzer

Heparin infusion line

Heparin pump

For most patients, heparin is stopped before theend of the treatment so blood clotting can goback to normal. A continuous infusion heparinpump has four parts:

1. A syringe holder

2. A piston to drive the plunger of the syringe

3. An electric motor to drive the plungerforward and infuse heparin from the syringe

4. A way to set the prescribed infusion rate

Heparin pumps have variable speeds that can be set to the physician’s prescription.

Heparin is infused into the heparin line on thearterial blood tubing before the dialyzer. Mostheparin lines are placed after the blood pumpsegment. This helps avoid negative pressure atthe part of the blood circuit that couldotherwise draw air into the extracorporealcircuit through the heparin line.

SORBENT DIALYSISSorbent dialysis can be used for acute, home,and chronic dialysis treatments. A sorbentdialysis system (see Figure 17) needs no watertreatment system. It does not contain water andconcentrate proportioning pumps. Instead,premixed chemicals are added to 6 L of tapwater. The water and chemicals are cycledthrough a sorbent regenerative cartridge topurify the dialysate. Then the dialysate is

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114

RenalSolutions

Figure 17:Allient sorbent dialysis systemDrawing adapted with permission from RenalSolutions, Inc.

HISORB +

SORB +

FLOW

SORB + and HISORB + Cartridge Detail

Used dialysate

Activated carbon & purification layer

Urease layer

Zirconium phosphate layer

Zirconium oxide& zirconium

carbonate layer

Cartridge effluent composition: NaAc, NaHCO3, NaCl, CO2, H2O

BINDS: RELEASES:• Phosphate• Fluoride• Heavy metals

• Ammonium• Calcium• Magnesium• Potassium• Metals• Other cations

• Sodium (less)• Hydrogen

• Acetate• Bicarbonate (more)• Sodium

• Nothing (converts urea)

• Ammonium carbonate

• Heavy metals• Oxidants• Chloramines• Creatinine• Uric acid• Other organics• Middle molecules

• Nothing

collected in a disposable bag in the device and circulated to the dialyzer.

Used dialysate is then cycled through thecartridge, where it is chemically converted backinto fresh dialysate and returned to the storagebag. The sorbent cartridge also removes allcalcium, magnesium, and potassium from theused dialysate, since their concentrations werealtered by passage through the dialyzer. Theseelectrolytes are added back into the regenerateddialysate in the prescribed amounts by aninfusion system.26 The patient’s ultrafiltrate isalso converted into dialysate by passagethrough the cartridge. Each increase in the totalvolume of dialysate is a direct reflection of totalUF and is continuously displayed.

The sorbent cartridge has four chemical layers.Besides regenerating dialysate, the layers serveas a water treatment system; they purify the 6 Lof dialysate made with tap water. The sorbentcartridge also serves as a continuous dialysatedisinfection system, keeping bacteria andendotoxin levels below 1 cfu/mL and 0.5EU/mL, respectively.

Depending on which cartridge is used, thesystem can do short (3–5 hour) treatments atdialysate flow rates up to 400 mL/min, or long,slow (5–8 hour) treatments at dialysate flowsbetween 200–300 mL/min. Since using thesorbent cartridge means no continuous watersource, floor drain, or water treatment systemare needed, sorbent systems can be usedanywhere that an electrical outlet (or suitablegenerator) is present.26

ConclusionAs you have read in this module, the deliverysystem plays a key role in monitoring dialysis.During each treatment, the machinery checksalmost every aspect of the patient’s care exceptone: you.

The dialysis staff person is the most importantmonitor of all to keep patients safe. Alarms areof no use if someone forgets to turn them on orto check them against an independent meter.

The patient can be in great danger if a staffperson hooks up the wrong dialysate to themachine. It is vital to recall that dialyzers anddelivery systems are not just machines, anddialysate is not just salty water. They are preciseparts of a medical treatment that can helppatients with kidney failure lead full and activelives. Your attention to detail and skill at findingand troubleshooting problems will make all thedifference in patients’ outcomes. Your job is tohelp them by staying alert at all times and bylearning all you can about the equipment andprocedures at your center.

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116

AppendixFORMULA FOR DIALYZER SOLUTE CLEARANCE5

Where:K is clearance

CBi (concentration at the blood inlet) is the concentration of “x” solute in the blood entering the dialyzer(arterial sample)

CBo (concentration at the blood outlet) is the concentration of “x” solute in the blood leaving the dialyzer(venous sample)

Qb is blood flow rate in mL/min

For example, imagine that you wanted to calculate the clearance of urea in a patient for whom:

CBi = 82 mg/dL (arterial BUN sample)

CBo = 8 mg/dL (venous BUN sample)

Qb = 350 mL/min

Step 1:

Step 2:

Step 3: 0.9 x 350 = 316 mL/min

Therefore, during each minute of dialysis, 316 mL of this patient’s blood has been cleared of urea.

CBi – CBo

CBix QbK =

(82–8)

82x 350

74

82x 350

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References1. Salai PB: Hemodialysis, in Parker J (ed): Contemporary Nephrology Nursing. Pitman, NJ,

American Nephrology Nurses’ Association, 1998, pp 527-574.

2. Dorland WAN: Dorland’s Illustrated Medical Dictionary. Philadelphia, PA, W.B. Saunders Co.,1988, p 207.

3. Curtis J: Splitting fibers: understanding how dialyzer differences can impact adequacy. Nephrol News Issues 5:36-39, 2001.

4. Cheung AK: Adsorption of unactivated complement proteins by hemodialysis membranes. Am J Kidney Dis 14(6):472-477, 1989 (abstract).

5. Mujais SK: Technical and functional considerations in choosing dialyzers, in Nissenson AR,Fine RN (eds): Dialysis Therapy (3rd ed). Philadelphia, PA, Hanley & Belfus, Inc., 2002, pp 99-109.

6. Ahmad S: Brief history and general principles of dialysis, in: Manual of Clinical Dialysis.London, UK, Current Medicine Group Ltd., 1999, pp 1-3.

7. Collier LM, Curtis J, Evans M, Glynn CF, Hawbecker L: Dialyzers, in Curtis J, Varughese P (eds):Dialysis Technology: A Manual for Dialysis Technicians (3rd ed). Dayton, OH, NationalAssociation of Nephrology Technicians/Technologists, 2003, pp 42-43.

8. National Kidney Foundation. KDOQI clinical practice guidelines and clinical practicerecommendations for 2006 updates: hemodialysis adequacy, peritoneal dialysis adequacy,and vascular access. Am J Kidney Dis 48(1 Suppl 1):S1-S322, 2006.

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