Know your dialysis: Conductivity

Yesterday, we saw how Ultrafiltration and dry weight are really important parameters to keep an eye on before your dialysis treatment. Today, let us look at another important parameter - conductivity.

Ever had to hear a scolding from your technician or nurse for coming back with too much fluid weight gain? All of us probably have! Now, guess what? Chances are that they are responsible for this! Seriously. Read on.

The conductivity setting in a dialysis machine controls how much Sodium is present in the dialysate. What is the dialysate?

A schematic representation of a dialyzer

Ok, let's get to some basics. I am sure you know that the dialyzer is the artificial kidney that does the actual work of cleaning our blood of the excess fluid and toxins. How does this actually happen? There are two compartments in the dialyzer - the blood compartment and the dialysate compartment. The blood flows through the blood compartment (what else did you expect?) which contains hundreds of 'hollow fibres' which are very thin pipe like structures which have many, many pores on their walls. All around these hollow fibres is the dialysate compartment through which a special solution flows. The excess fluid and the toxins flow through these pores from the blood compartment to the dialysate compartment.

A diagrammatic representation of how substances in solutions move from areas of high concentration (right in the picture above) to areas of low concentration.

Why doesn't the blood itself leak out of the pores? That's because the pores are very, very tiny and the size of the blood cells are larger than the size of the pores. The pores are designed so that only the toxins and water can pass through them.

Now, it is possible that things from the dialysate also pass through these pores and enter the blood. It all depends on the size of the substance and the concentration difference on the two sides of the pores. Any substance can only move from one side to the other if the concentration (the amount of the substance per unit volume) is more on one side than the other. If the concentrations are the same, no movement will happen.

Now coming back to Sodium. The dialysate must be ideally designed such that the concentration of Sodium in it is around the same as that of a healthy human body. This is around 135 to 145 mEq/liter. The dialysate, therefore must also be maintained around this level.

The dialysate is prepared by mixing three liquids inside the machine - the Acid solution (Part A), the Bicarbonate solution (Part B) and RO water in a certain proportion.

The conductivity setting of the dialysis machine directly corresponds to the level of sodium in the dialysate. A higher conductivity means a higher sodium level in the dialysate and vice versa. By altering the conductivity desired, we can tell the machine what sodium level we would like the blood to be exposed to. The machine does this by altering the proportion in which it mixes these three liquids.

As in anything related to dialysis, as indeed, in medicine, every individual is different and there is no one single number for conductivity that is suited to all. Generally, a high conductivity causes excess sodium to be present in the dialysate and as a direct consequence, in our blood while a lower conductivity cause lower amounts of sodium to be present in the dialysate and in our blood. Excess sodium causes excess thirst and causes us to drink more water while low sodium causes low blood pressure and cramps as well.

So, if you come back with excess fluid weight gain, it is quite likely that your conductivity setting was higher than you need it to be. Blame the techs or this! Not yourself!

But I am sure most of us are lucky enough that our technician or nurse knows all this and sets the conductivity that is suitable for our body. I am sure most of them understand the problems related to excess sodium and low sodium and most of us don't have to ever worry about this at all. Right? :-)

For the minority among us whose technician or nurse do not know about this, start with a conductivity of anything between 13.8 to 14 and then if you are getting cramps or low blood pressure, go up a point or two (0.1 or 0.2) at a time. If, on the other hand, you are feeling too thirsty and feel you can drink the water in all the rivers in the country and still not tire, try going down a notch at a time (0.1 o 0.2) and see how it feels! Always remember to discuss any changes you make with your nephrologist since he or she is the most aware of your overall condition. What is explained here is only a general guideline!

Remember, it YOUR health on the line. It is YOUR body that is being dialyzed. The least you can expect is that YOU be consulted on matters related to this. This will obviously make sense only when you are aware of these things.


Anonymous said…

Very good description and explanation Doc.
I am reading a lot of stuff online on Dialysis and the technique and technology behind it,and your blog is helping me understand few bits. reading all this because I am working on a machine as a software engineer :-)
so to help me understand the overall system doing the reading bit.

Thanks for the blog..

I hope I will find more interesting teachings in other posts...this is my first post of yours :-)

Unknown said…
I'm unclear on the proportionality of the dialysate.
Part A: Acid
Part B: Bicarb
Part C? RO Water? What exactly is this?
Kamal D Shah said…
RO water is water purified by a Reverse Osmosis system.
Anonymous said…
beautiful explanation.
Girish said…
Nice explanation dear Kamal ji,
It explained very descriptively. Please tell me one more thing. What does it mean when the machine gives alarm of high conductivity? Is it mixing more acid or more carbonate? And what might be the most probable cause?
Kamal D Shah said…
Dear Girish ji, high conductivity could mean less RO water added while mixing the bicarbonate solution. This is one of the many causes but probably the most common one.
Girish said…
Well said dear sir. That's right. There was problem in the duct that sucks the acid solution and only bicarbonate was being added to the flow. That might be increasing conductivity perhaps. Now All done right. Working!
Thanks for help!
Karthik said…
Sir, what will happen if the conductivity option was stopped in the dialysis machine?
Kamal D Shah said…
What exactly do you refer to you when you say conductivity option was stopped? Conductivity is a measure of the dialysate. How can it be stopped? I am not sure I understand.
Cricket said…
I only just read this and imagine the answer was found another way...but if the conductivity was turned down on the machine, and alarms ignored...that is basically turning the bicarb off. for the dialysate to be complete you MUST have the bicarb on which contains more than just sodium. It has all the electrolytes in it that needs balancing in the body since the kidneys do not do it. The Dr orders the conductivity settings, as well as the temperature of the solution (Too warm can rupture your blood cells and too cool will actually cause the process of osmosis--the cleaning of toxins out of your blood--to go slower as well as cause crenation of your blood cells.) Everything that is programmed into the dialysis machine is directly by the DOCTORS orders and CANNOT be altered EXCEPT by the doctor. The nurse on staff will call the dr to see what they say if a patient is requesting a change. Testing of the blood is done weekly and the dr updates the orders as needed depending on your sodium, hemoglobin, ect test results.

So basically, if the bicarb/conductivity is will pull all of it from the body and kill you. Theres a utube clip of greys anatonmy where the dr is fired for using a 0k bath...basically no conductivity, so it pulls all her potassium and almost killed the patient. Check it out. ITs very true.

Dialysis Tech and daughter of dialysis recipient.
Anonymous said…
Which part is responsible for conductivity during dialysis
Unknown said…
Which part of Dialysate is responsible for conductivity
Kamal D Shah said…
Not sure about that.
Unknown said…
Hi,Mr. KAMAL SHAH thanks aMillion for sharing.Needed this info for my Nephro docs.easy for me to explain.
Kamal D Shah said…
Responding to the earlier comment on which part of the dialysate is responsible for conductivity. The answer is both parts - Part A and Part B. Both contain Sodium and so would contribute.

The person just above this comment - I appreciate your comment and glad this post helped you.
Unknown said…
Hi,l would say a volumetric control dialysis machine with balancing chamber would be safe for dialysis treatment.
Because if the conductivity is out of range machine will go to bypass.
Diffusion stops.☺
Anonymous said…
there is a "normal conductivity level" that is targeted for all treatments. The Nephrologists orders a specific treatment. His/her order include the type of bath (1k,2k,3k), Sodium setting (135 - 145), and bicarb setting. The machine mixes these according along with the dialysate quality water (RO). The net result is to produce an admixture with a conductivity of 13.5 to 15.5. The machine is able to test conductivity and it is the staff responsibility to independently test in order to compare: actual (staff test) versus theocratical (machine test). The independent test should be within a 0.4 range of the machine. The conductivity is a method of making sure the dialysate is properly mixing in the machine. If there is any problems with "thirst, cramping or low bp" it could be related to the Sodium settings for the treatment. The machine will always blend the components in order to keep the conductivity level of the dialysate safe. Investigate your ordered sodium level for your treatment if you are having those issues. The alarm is a way of telling you that there could be something wrong with the "mixing" of the dialysate during treatment.
karthik said…
Can conductivity levels have effect over bp during dialysis and non dialysis days?
Karthik said…
Which is the correct vale to refer on 4008s ...the display or the prescribed sodium?
My tech is confusing me that irrespective of the led display the prescribed sodium vale is the amount the machines uses...

My understanding isupposes that the led display shows 14.4, lowering the prescribed sodium (140)by 0.4 points gives a target sodium of 14, though the prescribed sodium value is 13.6 now
Kamal D Shah said…
Hi Karthik, the conductivity level does impact BP during dialysis and non dialysis days.

The correct value to refer to is the LED indicator. That is the value going through the dialyser and interacting with the blood. The prescribed sodium should ideally correspond to the LED but it does not always happen due to several factors. You are right when you say you can lower the LED value by reducing the prescribed sodium. Again ideally it should go down or up directly proportion to the increase or decrease in prescribed sodium but small variations could be there.
Karthik said…
Hi sir. Thank you for your guidance... Now with conductivity at 13.9 the bp stays almost normal (130/80 on non dialysis days.

Is there a way to fix the limits on 4008s machine so that it's in 13.8 to 14.2, without having the tech/nurse to just center the limits when alarm the basic menu i found doo such option.
Kamal D. Shah said…
No such option. It depends on the way the Part A and Part B are mixed. If they are mixed properly then it should not fluctuate. There should not be a need to centre and so one normally.
canoli said…
Do you know if dialysis technicians make note of the dialysate solution used when replenishing the empty containers? Also, what can happen if the wrong dialysate is given after a low conductivity alarm sounds?
Thank you for taking the time to answer the questions!
Kamal D Shah said…

While they make a mental note, I don't think they not this separately while replenishing empty containers. Also, most centres I know will fill containers with enough liquid to last a full session.

If a wrong dialysate is given after a low conductivity alarm sounds, it all depends on the reason for the wrong conductivity and what specifically was wrong with the dialysate that was given.