Module 6 — Hemodialysis Devices and Equipment

Sources: Condensed study notes based on Core Curriculum for the Dialysis Technician, 7th ed. (MEI), plus free CDC/CMS/NIDDK resources.

Equipment overview — the treatment system as a safety chain

Study guide

HD equipment has two linked sides: the extracorporeal blood circuit and the dialysate-delivery circuit. Blood leaves through access, passes through tubing and the dialyzer, and returns; dialysate runs on the other side of the membrane.
The machine prepares and delivers dialysate, powers the blood and fluid pumps, controls UF, and watches pressure, air, blood leak, temperature, conductivity, and other conditions.
A dialyzer is the exchange device: hollow fibers separate blood from dialysate while allowing selected solutes and water to cross.
Safety is layered: correct setup, machine self-tests, alarms, visible line/access checks, and an attentive technician. No alarm can replace observation.

Why answers are correct

A treatment can be harmed by a wrong dialysate, incorrect tubing, an unrecognized leak, or a bypassed safety system even if the pump is running.

Limitations / traps

The machine does not independently verify every clinical decision or protect a patient from an unsecured needle. “No alarm” does not mean “no danger.”
Do not change blood flow, dialysate, temperature, UF, or anticoagulation outside the prescription and facility procedure.

Clinical pearl / safety

Before connection, confirm the prescribed dialyzer, concentrates, lines, sensors, clamps, and alarms; investigate anything that does not match the expected setup.

Exam cue

Trace the path: access → arterial line → pump → dialyzer → venous line → patient; dialysate travels separately across the membrane.

Dialyzer design, membrane area, and clearance

Study guide

Dialyzer fibers provide a large membrane surface. Blood travels inside the fibers while dialysate flows around them, usually in the opposite direction (countercurrent flow) to preserve a concentration gradient.
Clearance depends on membrane material and pore characteristics, effective surface area, blood and dialysate flow, solute size, temperature, time, and the direction of flow.
Small, water-soluble solutes generally clear most readily by diffusion. Larger or protein-bound molecules need suitable membrane characteristics and/or convection; no dialyzer removes every substance equally.
A larger surface area often increases potential clearance, but the dialyzer must still match the patient and prescription.
Convection moves water through pores and carries dissolved solute with it. High-flux membranes and adequate fluid movement support greater middle-molecule removal. A sieving coefficient describes how readily a solute follows water through the membrane.
Adsorption is attachment of molecules to the membrane surface. It may change biocompatibility and remove some substances, but a protein film can also reduce transport efficiency.

Why answers are correct

Countercurrent flow keeps blood exposed to dialysate with a favorable gradient along the fiber length. Increasing area or flow can improve opportunity for exchange, not guarantee a specific delivered dose.

Limitations / traps

Clearance is not the same as total removal from the body; redistribution from other fluid compartments continues during and after treatment.
“High-flux” is not synonymous with “better for every patient.” Membrane choice and treatment dose are prescribed.

Compare / contrast

Process	What moves?	Main driver	Device implication
Diffusion	Solutes	Concentration gradient	Fresh dialysate and countercurrent flow preserve the gradient
Convection	Water plus carried solute	Pressure-driven water movement	Requires suitable permeability and controlled UF/replacement
Adsorption	Molecules onto a surface	Surface interaction	Can help remove some solutes but may form a limiting film

Exam cue

Blood is inside the fibers; dialysate is outside; opposite directions support clearance.

Dialysate preparation and delivery

Study guide

Dialysate is made from treated water and prescribed concentrates. Acid concentrate supplies most electrolytes; bicarbonate concentrate supplies buffer. The machine mixes concentrates and water to the required proportions.
The intended solution supports removal of wastes while supplying selected electrolytes and buffer. Composition, temperature, flow, and conductivity must remain within the machine’s allowed range.
Conductivity is an electrical indication that the dialysate mixture is in the expected ionic/concentration range. It is a check on mixing, not a complete chemical analysis.
If conductivity, temperature, pH-related checks, proportioning, or another delivery condition is unsafe, the machine may enter bypass: dialysate is diverted from the dialyzer and the treatment is protected while the problem is addressed.
Dialysate temperature is controlled because overly warm fluid can injure blood cells and worsen patient risk; a cooler prescribed temperature may help selected patients tolerate treatment.
Dialysate flow (Qd/DFR) affects exposure of the membrane to fresh solution. A low-flow system may use a different treatment time or total volume than a conventional in-center machine.

Why answers are correct

A conductivity alarm can indicate an incorrect concentrate connection or mixing problem. Bypass prevents questionable dialysate from continuing through the dialyzer; it does not mean the cause can be ignored.

Limitations / traps

Conductivity within range does not prove that every ion is correct or that the concentrates are connected correctly. Always verify labels, connections, and required independent tests.
Treated dialysate is not automatically sterile. Fluid intended for infusion into blood, such as replacement fluid in HDF, must meet sterility requirements.

Clinical pearl / safety

Stop and escalate a wrong concentrate, unexpected conductivity/temperature result, failed test, or unexplained bypass rather than forcing treatment out of bypass.

Exam cue

Bypass protects the patient from questionable dialysate; it is a safety state, not a nuisance to override.

Ultrafiltration and volumetric control

Study guide

UF removes water from blood by a pressure difference across the dialyzer membrane. The machine controls the prescribed UF goal and UF rate (UFR) while accounting for dialysate flow and fluid balance.
UF goal is the intended net fluid removal plus fluid expected to enter during treatment. UFR is the speed of removal; a larger goal or shorter treatment raises the rate.
Volumetric UF control uses balanced fluid measurement/pumping so the delivered fluid removal more closely matches the programmed amount, rather than relying only on pressure.
Plasma refill moves water from interstitial and intracellular spaces toward the vascular space. If removal exceeds refill, blood volume and pressure can fall.
Blood-volume monitoring (BVM) can show percent change in blood volume, hematocrit, and oxygen saturation during treatment. A rising hematocrit generally reflects less plasma water as UF proceeds.
BVM feedback may help the clinical team adjust the prescribed UF strategy for a patient’s refill capacity and reduce symptomatic hypotension; it does not determine target weight by itself.

Why answers are correct

The safe total UF goal is not enough information: the speed and the patient’s refill response matter. A patient can become hypotensive before the programmed goal is reached.

Limitations / traps

Target weight is not a technician-invented number, and BVM is not a substitute for BP, symptoms, weights, or clinical assessment.
A scale change is not always water; food, clothing, muscle, fat, and other real-weight changes can mislead UF planning.

Clinical pearl / safety

Watch for cramps, dizziness, nausea, headache, weakness, or falling BP. Report promptly; do not independently alter UF settings.

Exam cue

UF removes vascular water first; refill takes time. High UFR plus symptoms is a warning, not a routine alarm to silence.

Extracorporeal tubing, pumps, and anticoagulation

Study guide

The arterial (often red) line carries blood away from the patient. The blood pump draws it through the circuit and dialyzer. The venous (often blue) line returns blood to the patient.
The blood pump overcomes resistance from the access, needles/catheter, tubing, and dialyzer. Kinks, clamps, poor position, clotting, or a flow-limited access change pressures and delivered blood flow.
The venous drip chamber follows the dialyzer and precedes the venous line clamp. Air detection and pressure monitoring are commonly placed around this return segment.
The venous line clamp is a final protective barrier. On an adverse event, the machine should stop the blood pump and close the clamp so blood or air is not returned; verify its operation before treatment.
The heparin pump delivers anticoagulant through the designated heparin line to limit clotting in the circuit. Some systems use a machine-controlled line; others use a separate pump. Anticoagulation is prescribed and must be handled according to policy.
Pre-strung cassette/cartridge sets reduce setup complexity. They may include the dialyzer, lines, saline connections, and heparin pathway depending on the system.

Why answers are correct

A circuit can clot even when the pump is running if blood flow, anticoagulation, access function, or setup is inadequate. Conversely, more anticoagulant is not a technician-selected fix.

Limitations / traps

Red/arterial means away from the patient and blue/venous means toward the patient; color does not make an incorrectly connected line safe.
A line clamp can stop return but cannot replace frequent visual checks for blood loss.

Clinical pearl / safety

Secure needles, tighten every connection, keep access visible, and look at the access, tubing, clothing, chair, and floor. A patient may not feel or notice rapid blood loss.

Exam cue

Arterial = out; venous = back. Heparin prevents circuit clotting; it does not correct a mechanical obstruction.

Pressure monitoring and alarm reasoning

Study guide

Pressure depends on blood flow and resistance and is meaningful only with its location and sign. Pre-pump arterial pressure is normally negative because the pump is pulling blood from the access. Post-pump/pre-dialyzer and post-dialyzer venous pressures are normally positive.
Common low arterial pressure causes include access inflow blockage, a kink/compression, poor needle position or infiltration, hypotension, vasoconstriction, a blood pump rate beyond access supply, or a poorly functioning catheter.
Less-negative-than-usual arterial pressure can suggest separation/leak or a changed monitoring condition; interpret it with the circuit and access in view.
High pre-dialyzer pressure can reflect increased resistance after the pump, such as a kink, clotting, a blocked dialyzer, or an abnormal pressure difference through the dialyzer.
Low venous pressure can occur with a separation, a drop in blood flow, or blockage before the monitoring site. High venous pressure can reflect downstream obstruction, a kink/clot, poor venous needle position or infiltration, catheter dysfunction, or access clotting.
Systems may use adjustable high/low limits or a preset range with a chosen midpoint. An out-of-range result can alarm, stop the pump, and clamp the venous line.

Why answers are correct

Pressure alarms are clues to resistance, disconnection, or flow change; they do not identify the cause alone. Compare the number and trend with the access, tubing, pump speed, and patient.

Limitations / traps

Never assume a high pressure always means “good flow,” or that a low pressure always means “less danger.” A venous needle dislodgement may not trigger the expected alarm if the needle/tubing creates misleading resistance.
Pressure monitors must be enabled and checked before treatment; alarm limits must not be widened merely to stop nuisance alarms.

Clinical pearl / safety

When an alarm occurs, protect the patient first, inspect the circuit/access, stop or clamp according to procedure, and escalate unresolved causes.

Exam cue

Negative before the pump; positive after it. Diagnose the location, direction, and trend before naming the cause.

Home HD and pre-strung systems

Study guide

Pre-strung tubing sets and cartridges are designed to make setup more consistent, especially for home HD. The user still needs training, correct loading, priming, self-tests, connection of fluid/waste lines, and alarm response.
NxStage: a portable home-HD system using a disposable pre-strung cartridge. Depending on the cartridge, a dialyzer may be included. Treatment programming includes total dialysate volume, total UF, UFR, blood flow, and dialysate flow. Its lower dialysate flow (roughly 100–300 mL/min in the module’s description) supports a much smaller water requirement than standard in-center HD; connected health tools can transmit treatment data.
Tablo: uses a pre-strung cartridge, built-in heparin line/pump, on-board RO water treatment, and dialysate made on demand. A patient can use a programmed key on another Tablo unit when traveling. It works more like conventional HD but has a maximum dialysate flow around 300 mL/min, so home treatments may be more frequent and/or longer.
Home systems still require air, pressure, blood-leak, and temperature safeguards. Training and prescribed settings—not portability—make the treatment safe.

Compare / contrast

Feature	NxStage	Tablo
Fluid approach	Small-volume single-pass dialysate; low DFR	On-board RO makes dialysate on demand
Setup	Disposable pre-strung cartridge; dialyzer may be included	Cartridge plus dialyzer; built-in heparin pathway
Notable use	Portable/travel-oriented home HD	Compact/travel-capable workflow with programmed key

Limitations / traps

“Home” does not mean unsupervised or risk-free. A self-test failure, air, kink, wrong connection, or unresolved alarm requires the trained response and clinical support.

Exam cue

Know the direction of each blood line and what is pre-strung; know that low-water designs still require full alarm and prescription discipline.

Machine troubleshooting and air detection

Study guide

Air in the machine or bloodlines can cause a self-test failure. Recheck loose connections, kinks, incorrectly open/closed clamps, and priming; allow the machine to complete its tests rather than rushing it into a treatment mode.
Microbubbles can behave like small embolic/clot-like insults; larger air entry can be fatal. Air/foam detectors monitor the venous blood segment before blood returns to the patient.
Two detector approaches described are ultrasonic sensing, which detects changes in sound transmission, and photocell sensing, which detects changes in light transmission caused by a void in the blood column.
When air is detected, the machine should stop the blood pump and clamp the venous line. Alarm limits are generally manufacturer-set and may be calibrated only by qualified personnel.
Seat the drip chamber and tubing correctly in the sensor. The detector must remain in use during HD, with the venous clamp engaged with the monitored tubing segment.

Why answers are correct

The detector protects the return path, but it can only work if the tubing is seated correctly and the system is maintained. Setup errors can defeat a functioning sensor.

Limitations / traps

Never dismiss bubbles as harmless or bypass the detector. A visual “looks fine” check is not an acceptable substitute for the required sensor and tests.

Clinical pearl / safety

If air is suspected, stop/secure the return circuit per emergency procedure and summon the licensed team immediately; do not return blood through a compromised line.

Exam cue

Air detector + venous clamp + pump stop are a linked safety response.

Blood-volume monitoring (BVM)

Study guide

BVM follows intratreatment changes in blood volume and concentration. It can display percent blood-volume change, hematocrit, and oxygen saturation.
As UF removes plasma water, hematocrit may rise. A rapid or excessive fall in blood volume can precede hypotension because vascular refill cannot keep pace.
Crit-Line is an example of a BVM device. It uses optical measurement and can measure hematocrit and oxygen saturation; the sensor is clipped to the bloodline at the dialyzer’s arterial port.
BVM provides trend information for the clinical team. It should be interpreted with BP, symptoms, weights, UF goal/rate, and the patient’s overall condition.

Limitations / traps

A BVM trend does not prove that target weight is correct and does not replace patient assessment. Real weight change and sodium status can alter how easily fluid is removed.

Exam cue

Rising hematocrit during UF generally means plasma water is falling; use the trend to support—not replace—clinical assessment.

Waste management and “green” dialysis

Study guide

HD creates medical waste, water waste, and energy use. A treatment can generate roughly 1.5–8 kg of mostly plastic medical waste, some of it biohazardous because it contains blood.
Conventional single-pass HD may use 500 L or more of water per treatment, depending on dialysate flow. A large fraction of RO-treated water can be reject water that never contacts the patient.
Possible conservation strategies include recirculating or repurposing safe RO reject water, using lower prescribed dialysate flow when clinically appropriate, draining disposables after treatment where policy allows, and separating infectious, recyclable, and general waste.
Energy conservation includes efficient equipment, renewable energy where feasible, responsible supplies, maintaining equipment so it works efficiently, and selecting lighter/less wasteful disposables.
The module’s “4-R” framework: reduce waste, reuse items that did not contact blood, recycle when a suitable program exists, and repair equipment rather than replacing it unnecessarily.
Waste handling remains subordinate to infection control and facility policy. Blood-contaminated items are not ordinary recycling.

Why answers are correct

Water and energy savings are systems-level choices; they must not compromise dialysate quality, disinfection, sterility, or patient safety.

Limitations / traps

Never reuse a blood-contact item simply to reduce waste. Reject water is not automatically approved for every use; local water, infection-control, and environmental rules govern disposal/reuse.

Clinical pearl / safety

Follow the facility’s segregation and drainage procedure exactly; report a malfunctioning drain, RO system, or waste feature rather than improvising.

Exam cue

Reduce, reuse only when safe, recycle through an approved pathway, and repair—never trade infection control for “green” practice.

Conclusion — technician as the final monitor

Study guide

Machines test and monitor many variables, but the technician remains the essential human monitor. Alarms must be heard, interpreted, and acted on; equipment must be maintained and tested.
Safe delivery depends on attention to detail: correct dialysate and tubing, functional monitors, visible access, appropriate responses to alarms, and prompt escalation.

Why answers are correct

Delivery equipment is part of the treatment itself. A wrong connection or ignored warning can injure a patient even when the machine appears to be operating.

Vocabulary

Uncommon words from this chapter. Plain definitions + a short dialysis-related example.

extracorporeal: The blood path that runs outside the body through tubing and the dialyzer.; Example. The extracorporeal circuit must stay visible because a leak can empty blood quickly.
clearance: How effectively the dialyzer removes a given solute over time.; Example. Clearance is not the same as total removal from the whole body.
countercurrent: Blood and dialysate flowing in opposite directions.; Example. Countercurrent flow preserves the concentration gradient along the fiber.
conductivity: An electrical check that the mixed dialysate is in the expected salt range.; Example. A conductivity alarm can mean a wrong concentrate or mixing problem.
bypass: A safety state that sends questionable dialysate away from the dialyzer.; Example. Bypass protects the patient; it is not an alarm to override.
volumetric: Controlled by measuring fluid volume, not only by guessing from pressure.; Example. Volumetric UF control aims to remove the programmed amount of water.
plasma refill: Water moving from tissues back into blood as UF removes vascular water.; Example. If UF exceeds plasma refill, blood pressure can fall before the goal is reached.
hematocrit: The fraction of blood made of red cells.; Example. A rising hematocrit during UF often means plasma water is falling.
anticoagulation: Medicine or method used to limit clotting in the circuit.; Example. Anticoagulation is prescribed; more heparin is not a technician-selected fix.
heparin: A common anticoagulant delivered into the extracorporeal circuit.; Example. Heparin helps prevent circuit clots but does not open a kinked line.
infiltration: Needle injury that lets blood leak into tissue instead of staying in the vessel.; Example. Poor needle position can cause infiltration and a pressure alarm.
proportioning: The machine's mixing of water and concentrates in the right amounts.; Example. Unsafe proportioning can put the machine into bypass.
high-flux: A more open membrane that lets larger molecules and more water move.; Example. High-flux is prescribed; it is not automatically better for every patient.
sieving coefficient: A number that estimates how readily a solute follows water through the membrane.; Example. The sieving coefficient describes convective passage of a solute.
ultrasonic: Using sound waves to sense a change, such as air in a bloodline.; Example. An ultrasonic air detector watches the venous return before blood goes back.
photocell: A light sensor that can detect a gap or foam in the blood column.; Example. A photocell air detector looks for a change in light through the tubing.
embolic: Related to a bubble or clot that can block a vessel.; Example. Microbubbles can act like small embolic insults; large air can be fatal.
biohazardous: Waste that can spread infection because it contains blood or body fluid.; Example. Used bloodlines are biohazardous and are not ordinary recycling.
reject water: RO water that did not pass through as product water and never contacts the patient.; Example. Reject water may be reused only if local rules allow; it is not automatically clean.
cartridge: A pre-strung disposable set that holds lines and often the dialyzer pathway.; Example. Home systems such as NxStage use a pre-strung cartridge to simplify setup.

Must-know

Blood path: access → arterial line → pump → dialyzer → venous line → patient; dialysate stays on the opposite side of the membrane.
Countercurrent flow and membrane surface area support clearance; diffusion clears solutes, convection carries solute with water, and adsorption attaches molecules to a surface.
Conductivity/temperature and other delivery checks protect against incorrect dialysate; bypass is a protective state, not an alarm to override.
UF removes water by pressure. UFR must be compatible with plasma refill; high UFR can cause hypotension and organ injury.
Red/arterial is away from the patient; blue/venous returns blood. Heparin limits circuit clotting but does not fix mechanical obstruction.
Pre-pump arterial pressure is generally negative; post-pump and venous pressures are generally positive. Interpret alarms by location, direction, trend, and circuit inspection.
Venous line clamp, air detector, and pump-stop response protect against air and blood loss; verify them before treatment.
NxStage and Tablo use pre-strung/cartridge-oriented home-HD workflows but still require trained setup, tests, and alarm response.
BVM trends blood-volume change, hematocrit, and oxygen saturation; it supplements—not replaces—BP, symptoms, weights, and clinical judgment.
Module 6 ends before displayed page 381, where Module 7 (Vascular Access) begins.