DIALYSIS MACHINE - DIALYSATE DELIVERY SYSTEM.

 THE HYDRAULIC CIRCUIT/DIALYSATE DELIVERY SYSTEM.

 INTRODUCTION

The hydraulic circuit is a part of the hemodialysis machine component just like the extracorporeal and electronic circuit respectively. Why is the hydraulic mechanics important in a hemodialysis machine of any type and model?? This is because the hydraulic circuit takes care of the fluid movement in this case the dialysate fluid (a mixture of water 34 bicarbonate concentrate 1.83 and acid concentrate 1). We need pumps to deliver dialysate to the dialyzer and also it is important to monitor the dialysate of certain parameters like ph, temperature, and conductivity. The hydraulic circuit controls the water, bicarbonate and acid concentrate for dialysate. It checks the conductivity, temperature, ph., flow rate and pressure.

Simple diagram of dialysate pathway 

 DIALYSATE DELIVERY SYSTEMS

The hemodialysis machine monitors and delivers dialysate using two major systems in the hydraulic circuit;

     1.      The Monitoring System.

     2.      The Proportioning System.

THE MONITORING SYSTEM.

What is to monitor? To monitor is to watch over something. The HD machine has a program for viewing and checking the dialysate as it flows in the hydraulic circuit. The importance of monitoring this dialysate is for the safety of the dialysis patient to avoid any complications like pyrogenic reactions during treatment and long-term harm of the same patients that could even lead to death overtime if the dialysate is not monitored properly. Approximately half of the patient complication in dialysis is related to the quality of water used for preparing dialysate...The remaining 50% were related to user errors or machine malfunctions.

Parameters to be monitored include;

                                        i.            Conductivity

                                      ii.            Temperature

                                     iii.            PH

                                     iv.            Flow rate

                                       v.            Blood leak

                                     vi.            Bacteria’s and endotoxins

                                    vii.           Unwanted material/particles like fats and tissues from the patient.

                        I.   CONDUCTIVITY.

Conductivity is the measurement of current flow which indicates the total electrical charge of a solution in our case the dialysate. The dialysate will carry in itself a composition of electrolytes like Na K CA Mg Cal Bicarb.  All these ions carry in themselves some electrical charge giving the dialysate ability to conduct current. By use of a conductivity cell/sensor, we are then able to measure the electrical charge of our dialysate solution.

A pair of electrodes or a sensor cell #7 are placed in the dialysate. The machine applies a charge and measures current. A thermistor is placed just adjacent the cell…NTC PTC characteristics (a resistor whose resistance varies rapidly and predictably with temperature and as a result can be used to measure temperature).NB. A temperature sensor#3#109 works hand in hand with conductivity as high in temperature results in a high in conductivity and vice versa.  A high in temperature will cause a high in conductivity and vice versa since the application of heat helps in a bombardment of the molecules/ions thus enhancing expansion and less of heat aids in the contraction of the molecules. Temperature, in this case, helps in bombarding of the ions in the dialysate i.e. Na, K+, Ca, Cl, ACETIC ACID, etc.  And in providing free space.

 Diagram showing a conductivity cell.

The conductivity limit of the final dialysate should be between 13.5-14.5 micro siemens. When the dialysate is outside the limits i.e. above/below the set limits, it then triggers a conductivity alarm and stops the flow of dialysate to the dialyzer by closing dialyzer valve24a and opening bypass valve26 and then to the drain. The circuit sets off noise and light alarms to alert and notify the staff. The bypass valve 26 keeps the wrong dialysate from reaching the patient. A high in conductivity above the set limits could actually cause hypertension to the patient because sodium in both A and B parts increases conductivity of the solution and vice versa.

Diagram showing Fresenius 4008s conductivity cell and temperature sensor.

  II. TEMPERATURE.

The measure of how cold or hot a substance is, in this case, a solution called dialysate with temperature sensors *109 *3 for dialysate and sensor*2 for RO water from the plant. A heat sensor*2 monitors our RO water temp before flowing to the multifunction block...In the same manner, another heat sensor *3*109 monitors dialysate temperature before reaching the patient in the dialyzer. Both the temp sensor and conductivity are placed together while monitoring the dialysate. A high in temperature will cause a high in conductivity and vice versa since the application of heat helps in the bombardment of the molecules/ions thus enhancing expansion and less of heat aids in the contraction of the molecules. The temperature, in this case, helps in bombarding of the ions in the dialysate i.e. NA K CA CL ACETIC ACID, etc.  and in providing free space.

The temperature of the water and dialysate in the hemodialysis machine should be between 34-40°C near the normal physiological levels of 37°C.Too hot dialysate of above 44°C can cause a patient’s RBCs to rapture (hemolysis).Too cold dialysate of below 34°C can cause rigors, shivers, chills to the patient and also cause clotting of the patients’ blood thus interrupting the flow of blood in the extracorporeal circuit. If the dialysate is too hot or too cold above/below the set limits, the circuit sets off noise and light alarms. The circuit also opens bypass valve 26 closes dialyzer valve 24a to shunt dialysate to the drain and avoiding it flow to the dialyzer.

Diagram showing Fresenius 4008s conductivity cell and temperature sensor.

                     III FLOW RATE.

By using the various pressure measuring points in the hydraulic circuit, one is able to check the flow rate and pressures in the circuit. It is important to check the flow of fluid to their desired pressures as low/excess flows and pressures would cause errors in the circuit. Blockages, leakages or even presence of air in the hydraulic circuit would reduce/increase pressures above the set limits thereby causing errors in the system. At this point, if the machine is in treatment mode, it automatically goes into a bypass for the safety of our patient. The machine automatically gives Cyclic PHT errors and Balancing errors on the display of the screen.

There are Four Pressure Measuring points in the hydraulic circuit of a functioning Haemodialysis Machine namely:

       A.   Reduced water inlet pressure approx. 20 psi 1.0bars,   0.9 to 1.4 Bar. Water alarm during T1 test signifies Water inlet pressure <1.0 Bar

       B.      Balancing chamber loading pressure.

       C.      Flow pump pressure approx. 2bars.

       D.    The pressure of  the degassing pump -0.8bars   0.81 to 0.85 Bar

Diagram showing a Fresenius 4008s pressure measuring point A.

                       

                      IV BLOOD LEAK DETECTOR(BLD).*8

The blood detector is a safety in the HD used to check for any presence of blood in the used dialysate. The dialyzer membranes are tiny, very fragile and can tear easily if not handled properly causing blood mix with the non-sterile dialysate. The patient could also have a major blood loss which is a drawback since most patients are already Anaemic by the time they start dialysis due to difficulty in production EPO as a result of kidney abnormalities. Additional loss of blood is a big drawback for these patients. The BLD can recognize very small amounts of blood losses of 0.5ml per minute.

The BLD shines/transmits light through the used dialysate and onto a receiver the photocell/photoresistor which activates an alarm in case of any blood losses. The red blood cells scatter light and trigger an alarm which then deactivates the blood pump to stop blood from flowing. The bypass valve 26 opens and dialyzer valve 24a closes causing dialysate flow to the drain. The blood leak detector works on the principle of Photoelectric cells.  

Diagram showing BLD in Fresenius 4008s machine

                     V. PH METER.

A ph. meter is used to measure how acidic or alkaline (basic) a solution is, in this case, the dialysate. It’s used to measure the concentration of hydrogen ions in the dialysate, PH of the dialysate should be between 7.0-7.4 near the normal physiological ph. levels of the blood (7.35-7.45).

Some machines will have a ph. monitor while others don’t. Of importance is to make sure that your bicarbonate is at the right proportion to clear the metabolic acidosis of the patient. From our previous awards in Diploma and Certificate Studies in Biomedical engineering, it was extensively noted and taught concerning the principle behind ph. meter electrodes and their characteristics.

                    VI. DIASAFE FILTER *114

The diasafe filter helps keep bacteria and endotoxins (dead colonies of bacteria) out of reach to the patient. Any presence of bacteria and endotoxins in the dialysate can cause serious or even deadly complications to the patient. Please note that the diasafe is NOT an option/alternative when it comes to treatment BUT it is mandatory to have one that is working and functional. Any absence of the same is jeopardy and is an insult to the Dialysis field.

Diasafe filter life is checked by Disinfection Cycle Count.

Diagram showing diasafe filter in Fresenius 4008 machine.

          VII. DIALYSATE FILTER *73

This is a filter located just after the dialyzer responsible for removal of any unwanted particles/material like fats and tissues that could clog the circuit as dialysate flows back to the HD machine. It is a safety in the hydraulic circuit, it is also externally attached to the used dialysate Hansen Tubing.