Dialysis in a hospital: the machine, the water and the cycler

Draw the whole dialysis chain from the city water inlet to the patient, name the main assemblies of a haemodialysis machine, and say on which link a complaint most likely sits.

Three systems that only work together

A dialysis unit looks like a row of machines beside chairs or beds, but the machine is only the last link. Behind the wall there is a water treatment plant that turns ordinary tap water into water clean enough to be mixed into dialysis fluid. In the store room there are concentrates, the salts that turn that water into dialysis fluid. And somewhere in the same department, or at the patient's home, there may be peritoneal dialysis cyclers, which do a related job by a completely different route. As a service engineer you will be called to all of them, and many complaints that arrive as 'the machine alarms' start somewhere else in the chain.

Keep the purpose in mind, because every design decision follows from it. A patient whose kidneys have failed cannot remove waste products, excess salt and excess water. In haemodialysis, blood is taken out of the body, pumped through a dialyser (a filter made of thousands of hollow fibres), and returned. On the other side of the fibre walls flows dialysis fluid. Small molecules move across the membrane from where their concentration is high to where it is low, and water is drawn out of the blood by a small pressure difference. A typical patient spends several sessions a week, several hours each, with a large volume of dialysis fluid passing on the other side of a thin membrane from their blood. That volume and that closeness are why water quality and fluid composition matter so much: a contaminant that would be harmless in a glass of drinking water reaches the bloodstream in quantities no stomach would ever let through.

Write the chain down from memory before you continue. When the head nurse calls, your first task is to place the symptom on one link of it.

• City water supply and the plant room inlet
• Pre-treatment: sediment filters, softener, activated carbon
• Reverse osmosis (RO) unit, sometimes with a second pass or polishing stage
• Distribution loop that carries treated water to each machine outlet
• Concentrates: acid concentrate and bicarbonate, from canisters, cartridges or a central supply
• The haemodialysis machine: hydraulic module (dialysis fluid side) and blood module (blood side)
• The dialyser and blood lines (single-use disposables)
• The patient

The water treatment plant in one walk-through

Walk the plant in the direction the water flows. First come sediment filters that catch sand, rust and particles, because everything downstream is more expensive and clogs more easily. Then usually a softener: a resin that swaps calcium and magnesium for sodium, so that hardness does not scale the RO membranes. The softener regenerates with brine from a salt tank on a timer or a meter, and an empty salt tank is a classic cause of a slow decline in RO performance. Then activated carbon, which removes chlorine and chloramine used to disinfect city water. This stage is a patient safety stage, not only a membrane protection stage: chloramine passing into dialysis fluid damages red blood cells. That is why the chlorine check after carbon is one of the routine tests the unit must do, at a frequency set by its procedures and local rules.

The heart of the plant is reverse osmosis. A high-pressure pump pushes water against a semi-permeable membrane; most dissolved salts, bacteria and endotoxin fragments stay behind and go to drain with the reject stream, and the clean product water (permeate) passes through. Some plants add a second RO pass, ultraviolet lamps, ultrafilters or deionisation as a polishing stage. The product water then goes into a distribution loop, a ring of pipe that runs past every machine outlet and back to the plant, so that water keeps moving and does not stand in dead legs where bacteria form biofilm. Lesson 4 covers the plant in detail.

Why does the machine engineer need to know this? Because a machine fed with poor water may pass its own tests and still deliver a contaminated fluid, and because many machine alarms, such as low inlet pressure or unstable conductivity, are really water supply problems.

Concentrates and the colour code

The machine makes dialysis fluid by mixing treated water with two concentrates in fixed ratios. The acid concentrate carries sodium chloride, potassium, calcium, magnesium, a small amount of acid and often glucose. The bicarbonate concentrate provides the buffer. They are kept separate until the last moment because calcium and bicarbonate together precipitate as calcium carbonate, the same scale you find in a kettle. The concentrates may come in canisters, in dry bicarbonate cartridges that the machine dissolves itself, or from a central concentrate supply piped to each station.

Many manufacturers and units use a colour code for connectors and containers, commonly red for acid concentrate and blue for bicarbonate, and the service manual of at least one machine on the market explicitly instructs staff to respect it. Still, check the labels of the concentrates actually used on your site. The colour code exists because a wrong concentrate, a wrong mixing ratio or an exhausted canister changes the composition of the fluid the patient receives. Some hospitals also prepare concentrates themselves in mixing tanks; those units are equipment too, with pumps, floats and level controls, and they belong in your inventory.

Inside the haemodialysis machine: two modules

Every haemodialysis machine, whatever the brand, has two halves. The blood module, usually on the front, handles the patient's blood in single-use lines. The hydraulic module, inside the cabinet, makes and handles the dialysis fluid. The two meet only at the dialyser, where blood flows inside the fibres and dialysis fluid flows outside them in the opposite direction.

The blood module carries a peristaltic blood pump, which squeezes the blood line with rollers so the blood never touches the machine. It has pressure monitors on the arterial side (before the pump) and the venous side (on the return), usually connected to the blood lines through small transducer protectors with hydrophobic filters. Most machines also have a heparin pump, a small syringe driver that adds anticoagulant. On the return line there is a venous drip chamber, an air detector around or beside it, and a venous line clamp below it. The air detector and the clamp together form the last barrier between air in the circuit and the patient's vein.

The hydraulic module takes water from the outlet, reduces and stabilises its pressure, heats it, removes dissolved gas (degassing), mixes in the concentrates, checks the result for conductivity and temperature, and then delivers it to the dialyser through a balancing system that makes sure exactly as much fluid comes back as went in. The difference between the two, the fluid deliberately removed from the patient, is called ultrafiltration (UF). After the dialyser the used fluid passes a blood leak detector, which looks for red cells that would mean a torn membrane. The design of the balancing system differs between manufacturers: some use paired chambers separated by flexible membranes and switched by valves, others use different volumetric principles. The principle is the same: the machine must control the fluid removed from the patient precisely, because an error of a few hundred millilitres per hour adds up over a session to a dangerous fluid loss or overload.

Many machines also have a backup battery that keeps the blood side running and alarming for a short time if mains power fails, so that staff can return the blood to the patient by hand. Lesson 2 explains why that function is a safety function, not a convenience.

Peritoneal dialysis cyclers at overview level

Peritoneal dialysis (PD) uses the patient's own abdominal lining as the membrane. Sterile PD solution from bags is run into the abdomen through a permanent catheter, left to dwell, and drained. Done by hand during the day it is called continuous ambulatory PD; done at night by a machine, an automated cycler, it is automated PD. The cycler weighs or measures volumes, warms the solution on a heater plate, and controls fill, dwell and drain through a single-use cassette. No blood leaves the body, and there is no water plant, because the solution comes ready-made and sterile.

Cyclers are mostly in patients' homes, and they reach the engineer through the dialysis centre. Their typical alarms are about drainage, lines and air in the set, and the manufacturer's own troubleshooting guide for one widely used cycler lists mostly non-machine causes first: kinked or clamped lines, a closed transfer set, catheter position, fibrin, the cycler placed too high. The same guide warns that bypassing a low-drain alarm can leave too much fluid in the abdomen, a serious condition. For the engineer this means two things: check the simple causes before suspecting the machine, and never treat clinical alarm decisions as technical settings.

Where your work starts and ends

At Level 1 you inspect, clean, test, run the machine's own self-tests and disinfection programs, check calibration against reference instruments where the manufacturer's procedure allows, replace parts from the preventive maintenance kit, and read the error log. You check the water supply to the machine and work with the team that runs the water plant. You do not enter protected service levels without authorisation, you do not change factory constants or alarm limits, and you never disable or bridge a protective function. Water microbiology and chemistry are tested by a laboratory; you take part by collecting samples correctly when asked and acting on results. The fundamentals you will need from other courses are electrical safety (esafety), pressure and flow basics (pneumatics) and the fault-finding method; this course refers to them instead of repeating them.

On-site checklist

• Draw the chain: city water, pre-treatment, RO, loop, concentrates, machine, dialyser, patient.
• When several stations alarm at once, go to the water plant and concentrate supply first.
• Know which concentrates the unit uses and how they are colour-coded and connected.
• Identify the blood module and hydraulic module assemblies on each machine model in your care.
• Check whether each machine has a backup battery and when it was last tested.
• For PD cyclers, rule out lines, clamps, transfer set and cycler height before suspecting the machine.
• Write down what is outside your scope and who handles it: laboratory, manufacturer, water plant operator.

Check yourself