How a dialysis machine works

Learn about the major components of a hemodialysis machine in two categories—the blood circuit and the dialysate circuit.

Daphne H. Knicely, MD MEHP
Daphne H. Knicely, MD MEHP
20th Jan 2020 • 4m read

Hemodialysis machines are all a bit different, but they have many similar components. In this video, from our Dialysis Essentials course, we'll look at the major components of a hemodialysis machine in two categories—the blood circuit and the dialysate circuit.

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Video transcript

hemodialysis machines are all a little bit different, but they have many similar components. We'll look at the major components of a haemodialysis machine in two categories, the blood circuit and the dialysate circuit. First, let's look at the blood circuit. The blood circuit for the hemodialysis machine is divided into the inflow or arterial blood line, which carries blood from the vascular access in the patient to the machine in the outflow or venous blood line, which carries blood from the machine back to the vascular access and the patient.

These two segments are divided by the dialyzer. Within the inflow bloodline, there's a blood pump dividing the inflow bloodline into the pre pump in post pump. Let's start by looking at the inflow bloodline pre pump. Along the segment there's a port through which you can collect blood samples assailing used to prime the dialyzer circuit infrareds back at the end of dialysis in an arterial pressure monitor which reads the negative pressure along this portion of the circuit.

If the connection is broken along this portion, air can enter the blood line. bubbles of air can be trapped in the fibers of the dialyzer, leading to reduce dialysis efficiency and clogging of the circuit. If enough air gets into the circuit undetected, it can even enter the bloodstream causing an air embolism.

Usually, the pressure monitor is configured to stop the blood pump if there is a disconnection and loss of negative pressure that exceeds the set limits. Typically, there will be alarms and visual warnings If this occurs, the blood pump is a spring loaded roller pump that completely occludes the blood tubing and works like milking a straw along its length.

The roller squeezes the tubing as it rolls over, then releases the tubing to allow refilling the inflow bloodline post pump is the segment between The blood pump in the dialyzer. This segment of the blood circuit contains support for heparin infusion. Most patients need a bolus of heparin at the beginning of dialysis and or throughout to keep the circuit running without clotting.

Though some people can be dialyzed. Without anticoagulation. In some hemodialysis machines, there might be a pressure monitor here as well. The outflow or venous blood line is located after the dialyzer there is a venous strip chamber or air trap that allows removal of any accumulated air from the line of venous pressure monitor and an air detector.

The venous air trap and detector are very important for patient safety. The chamber traps any air that may have entered the blood line before the blood is returned to the patient. Any detected increase in air triggers an alarm. If this happens the haemodialysis machine will start working. The inflow pressure valve is usually combined with the outflow or venous pressure monitor, to estimate the average pressure in the system during dialysis pressure cutoff limits are set.

So if there is a sudden rise in pressure due to an impending clot, or sudden lowering and pressure due to a disconnection, the blood pump will turn off. Now let's focus on the dialysate circuit.

In general, dialysis machines mix concentrated electrolyte solutions or powders with purified water to make the final dialysis solution or dalisay that will be delivered to the dialysis filter. dialysis solutions must be delivered at the proper temperature of 35 to 38 degrees Celsius, and excess air must be removed from the solution before us.

There are several monitors and alarms in the dialysate circuit for temperature and electrical conductivity. If the proportioning system excessively dilutes or concentrates a dialysis solution, it can lead to electrolyte disturbances and or hemolysis. The constant of the dialysis solution will be reflected by its electrical conductivity.

If either the dialysis solution conductivity or temperature is found to be out of limits, a bypass valve is activated to divert the dialysis solution around the dialyzer directly to the drain and not to the patient. The blood leak detectors located in the dialysis solution outflow line if this detector senses blood, as occurs when a leak develops through the dialyzer membrane, an alarm is activated.