Dialyser Machine an artificial kidney

Dialyser machine an artificial kidney is used in dialysis which is a process for removing waste and excess water from the blood, and is used primarily as artificial replacement or lost kidney function in people with renal failure,

Kidneys maintain the internal balance of salts, excretion of wastes and synthesis of RBC protein erythropoietin and Dialyser mimics the kidney’s function of filtration.

What is the Principle behind Dialysis ?

• It works on the principle of diffusion and ultrafiltration across a Semipermeable membrane(SPM).
• Blood flows by one side of a semi-permeable membrane and a dialysate flows by the opposite side.
• Action similar to kidney functioning for filtration.

Working of Dialysis Machine or Haemodialysis :

• Hemodialysis is performed in a dialysis center of the hospital or clinic, three times a week, with the patient being connected to the dialysis machine by blood lines or catheters.
• The dialysis machine incorporates a computer, a pump, a dialyzer or filter, and dialysate or dialysis solution.
• The machine’s computer monitors blood flow, blood pressure, and how much fluid is removed from the body during the treatment.
• The pump moves the blood from the body, to the dialyzer, and then back to the body.
• The dialyzer acts as a replacement kidney during the treatment.
• It is a hollow plastic tube made up of many tiny filters. A semi permeable (sponge like) membrane separates the blood from the dialysate fluid.
• The dialysate fluid pulls waste products from the blood. This process allows extra body water and wastes to be removed from the blood.
• The dialysate with poisons and extra water flows to the drain, the cleaned blood is returned to the body. The dialysis machine also has many safety detection features.

What are the components of dialyser machine ?

Arterial Pressure Monitor :

• Measures pressure between access and blood pump.
• Negative between access and blood pump but later has positive pressure post-pump.
• Alarms may indicate patient disconnection, separation of blood tubing, or obstruction/kink in the blood circuit.
• Out-of-range pressures trigger the machine to clamp the blood line and activate the appropriate alarms.
• Causes of low arterial pressure alarm:- Fall in blood pressure, Kink between needle and pump, Clot (check for air bubbles), Suction of vessel wall into the needle.
• Causes of  high arterial pressure alarm:- Increase in patient’s blood pressure, Circuit disruption between access and pump, Unclamping of saline infusion line, Blood pump that has torn the pumping segment (check for blood leak).

Venous Pressure Monitor :

• The venous pressure may build up owing to resistance to venous return anywhere between the venous drip chamber and the venous needle
• Venous pressure monitors normally read positive pressures. Out-of-range pressures trigger clamping of the blood line, stopping of the blood pump, and activation of appropriate alarms
• Causes of a low venous pressure alarm:- Disruption of connections anywhere downstream from the blood pump to and including the venous needle and access, Low blood flow (upstream of blood pump).
• Causes of a high venous pressure alarm:- Kink in the venous return line, Clot in the venous drip chamber, Venous access malfunction.

Blood Tubing:

• Blood tubing is made of biocompatible and nontoxic material such as high grade PVC.
• Leaching of phthalate di-(2-ethylhexyl) phthalate (DEHP) from polyvinyl chloride(PVC), a constituent of the blood tubing, may occur into the blood circulation and may lead to liver damage thats why fda recommends minimizing DEHP exposure by using the freshest possible blood products stored at the lowest possible temperature, or by using heparin-coated ECMO circuits.

Pumps:

• Blood pump consists of two or more spring-loaded rollers and a stator supporting the blood tubing. One of the rollers should occlude the tube at all times to prevent uncontrolled flow in the circuit as well as back leak.
• Blood is pumped in the circuit by peristaltic action at a rate of 200 to 600 mL/min.
• It is calibrated to measure blood flow rate (BFR) depending on the internal diameter of the tubing.
• BFR can be calculated by measuring (Rotation per minute) RPM directly and volume of tubing.
• The heparin (anticoagulant) pump is mostly a syringe pump, although a roller pump may be used. Heparin is infused downstream into the positive-pressure segment of the blood circuit.

Air Leak Detectors:

• Air leak detectors are present in order to prevent bubbles of air coming in contact with the blood. It makes use of optical sensors.
• It is placed distally in the venous blood line and monitors and prevents air embolus.
• The usual volume of air needed to result in this complication is 60 to 125 mL.
• Requirements of air detector: Should preferably be ultrasound (US)-based which detects change in ultrasound frequency frequency caused by air foam, (Note: rate of transmission of ultrasound in: blood > saline >air), It Must activate venous line clamp capable of complete occlusion of blood return line to body in order to prevent air embolism.

Blood clamps:

• The blood tubing clamps should be able to withstand pressures up to 800 mmHg.
• They should automatically shut if the circuit is broken or electrical power is lost.

Others:

• Presence of conductivity meter, maintains water to dialysate ratio.
• The blood leak monitor allows detection of blood leaks and prevention of wastage of dialysate, Based on the principle that red blood cells present in the dialysate scatter light, The sensitivity of blood leak monitor is 0.25-0.35 mL of blood per liter of dialysate.

The Dialysate Circuit

• The key components of dialysate circuit include

• Heating
• Deaeration
• Proportioning
• Monitoring
• Ultrafiltration
• Disinfection

Heating:

• It improves mixing with the dialysate concentrate.
• Its heating elements need to be made of stainless steel.
• Internal controls (preset) should limit the temperature range to 33 to 39 degree celsius.

Dialysate deaeration:

• Deaeration (degassing) prevents potential problems caused by air dissolved in the water of the dialysate solution.
• Water is heated to physiologic temperatures and then subjected to negative pressure, thus venting any released air.
• Negative pressure is maintained by a closed loop composed of a blood pump, constricting valve, air trap, and vent

Dialysate proportioning:

• To prepare a dialysate of correct proportion, temperature, and conductivity within specific physiologic limits.
• Bicarbonate is made fresh, because pre-prepared bicarbonate may slowly release CO2 into the air and supports bacterial growth.

Monitoring System:

•  pH monitor- This monitors the ratio of HCO3 to H2CO3 ratio of the dialysate.
• The recommended pH range is 6.8 to 7.6
• Temperature monitor- it should have a short feedback loop to the heater element to allow quick adjustment of the temperature.
• The usual recommended temperature range is 35 to 42 °C.

Requirements for Dialysis machine

1). Air conditioning: 55 to 60% humidity

2). Electricity

• Stable voltage continuous supply is required. Online UPS is recommended.
• It should have a backup for at least 30 minutes. The power capacity of UPS should be able to support all functions of the dialysis machine.
• The use of electrical surge protectors is necessary to protect dialysis machine’s electronics.
• Adequate capacity generator is recommended.

3). Plumbing & drainage:

• All treated water pipelines should be stainless steel grade 316 or medical grade PVC. There should be minimum bends & blind loops should be avoided.
• There should be oxygen & vacuum outlets at the head end of each dialysis  machine.

General Do’s and Don’ts

• Check the pressure on the arterial monitor.
• Replace the SPM (semi permeable membrane) in the dialyser when it is worn off.
• Disinfect the machine with sodium hypochlorite.
• Flow rate of the blood pump must be checked at 400 ml/hr using analyser.
• Blood leaks should be checked.

References and Further reading

1. http://kidney.niddk.nih.gov/KUDiseases/pubs/homehemodialysis/
2. https://www.asn-online.org/education/distancelearning/curricula/dialysis/HemodialysisMachinesYoung.pdf
3. http://lifeoptions.org/catalog/pdfs/teaching/Machine.pdf
4. http://en.wikipedia.org/wiki/Hemodialysis
5. http://en.wikipedia.org/wiki/Step-by-step_description_of_hemodialysis