SKGK – Session 22 – Topic: Hemodialysis (Answers) Date: 27 April 2017

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Hemodialysis machine: is a machine that filters a patient’s blood to remove excess water and waste products when the kidneys are damaged, dysfunctional, or missing. The dialysis machine itself can be thought of as an artificial kidney.

🌹 Mixes the dialysate.

🌹 Monitors the dialysate.

🌹 Pump the blood and controls administration of anti-coagulants.

🌹 Monitors blood for presence of air.

🌹 Monitors ultra-filtration rate.

3⃣ Working principles:

The principle of hemodialysis is the same as other methods of dialysis; it involves diffusion of solutes across a semipermeable membrane. Hemodialysis utilizes counter current flow, where the dialysate is flowing in the opposite direction to blood flow in the extracorporeal circui. Counter-current flow maintains the concentration gradient across the membrane at a maximum and increases the efficiency of the dialysis.

Fluid removal (ultrafiltration) is achieved by altering the hydrostatic pressure of the dialysate compartment, causing free water and some dissolved solutes to move across the membrane along a created pressure gradient.

The dialysis solution that is used may be a sterilized solution of mineral ions. Urea and other waste products, potassium, andphosphate diffuse into the dialysis solution. However, concentrations of sodium and chloride are similar to those of normalplasma to prevent loss. Sodium bicarbonate is added in a higher concentration than plasma to correct blood acidity. A small amount of glucose is also commonly used.

The basic components of the machine:
Roller pump for blood, Dialyzer, Blood leakage detector, Dialysate pressure gauge, Dialysate pump, Syringe, Dialysate flow meter, Conductivity Cell, Venous pressure gauge, and Heater.

Dialysate Temperature Control and Measurement
Dialysis is normally done at body temperature. Lower than body temperature, dialysis is less efficient and blood has to be warmed before return to patient. High temperature (>40oC) will damage components of blood.

Thus, temperature of dialysate is monitored and controlled before supplied to dialyzer.

Temperature control system used to raise temperature of dialysate to a required value.

Two types of circuit for control of temperature : Bi-metallic thermostats.  Electronic proportional controller. The latter use thermistor for sensing temperature and triac for control of power to heater.

Dialysate Pressure Control and Measurement

Negative pressure upon dialysate created by effluent pump.

Effluent pump is a fixed-flow, motor-driven gear pump.

Pressure between zero and maximum by adjustment on machine panel.

A relief valve limits maximum negative pressure and minimizing risk of burst in dialyzer membrane.

Pressure adjustment should not produce any significant change in flow rate.

Pressure measured by strain gauge transducer.

Dialysate pressure is measured on one side of membrane and venous pressure on the other side.

Effective pressure across membrane is algebraic sum of dialysate pressure and venous pressure.

🌹 Conductivity Measurement
Conductivity of dialysate is monitored by conducting cell to verify accuracy of proportioning.

Result displayed as percentage deviation from the standard. Composition of dialysate is checked by comparing electrical conductivity of dialysate with standard sample of dialysate.

Blood Leak Detector
Blood leakage across dialyzer membrane can be detected by using photo-electric transducer.  Leak detector examines light absorption of dialysate at 560 nm i.e. absorption wavelength of haemoglobin.

Ultrafiltrate Monitor
It’s used to monitor amount of fluid removed from the patient.
Also controls the rate at which fluid is removed.

From Maintenance perspectives
Problem: Low access pressure alarm
The most probable cause:

🌹 The vas cath is kinked

🌹 The line is kinked

🌹 The vas cath is sucking against a vessel wall

🌹 The vas cath has become occluded with clot

Check all necessary circuits and patient.

By Keneni Benti
ME in BM


The dialysis machine mixes and monitors the dialysate. Dialysate is the fluid that helps remove the unwanted waste products from your blood. It also helps get your electrolytes and minerals to their proper levels in your body. The machine also monitors the flow of your blood while it is outside of your body.

Bundela Avanish
Bio medical engg

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