I have a physician who doe's not want his patients running on a machine with a cond. greater than 14.5 ( even when the SVS is on ). He is saying that with a conductivity of 14.5, this also means the sodium is 145. Can anyone please shed some light on this issur. He is also asking for documentation to back up any answers.

nu 2 this,

Tell him that the only way for the conductivity not to exceed 14.5 is to not use the SVS modeling at all. SVS does what it says... raises the sodium at different rates. Raising the sodium, raises your conductivity.

Conducivity is calculated thusly:

Condo= (sum of all cations)-6 / 10

have your doctor fill out the equation and he'll see that a 14.5 conductivity does not mean a 145 sodium meq

Fred, Thanks for this, and not to sound really stupid, but could you please explain a little better for me?

I will assume you have a Fresenius machine. Look in the dialysate screen and you will see the TCD box. The number in that box is in mS/cm which when you look at the base sodium box which is where the base sodium is set that number is in mEq/l. Two different units of measurement but mean the same thing.

Not stupid at all. Very good topic actually.

All right, here goes. While the conductivity of the dialysate is almost entirely generated by the sodium content, there are other factors too. The potassium, magnesium, and calcium play a part. Here's how to use the equation and figure out what the theoretical conductivity (TCD) of the machine should be.

Add up all of the cations (ions with a positive charge) in the acid and bicarb solutions. They would be the sodium, potassium, magnesium, and calcium measured in units called meq/L. Then subtract 6 and divide by 10 (to move the decimal point). For example:

My acid has: Na=100 meq/L, K=2 meq/L, Ca=2.5 meq/L, Mg=1 meq/L
My bic has: Na=37 meq/L

If I add these up I get 142.5. I subtract 6 and get 136.5, then divide by 10 to get 13.65. This is the TCD that the machine calculates (it would probably round this up to 13.7). Now, if I add more sodium by initiating the sod. var., I raise the condo by that amount. This can be calculated by using the same equation. So the moral of the story is that while the condo is mostly derived from the sodium content, it does not mean that a 14.5 condo = 145 meq/L of sodium. Hope this helps nu.

It is an interesting topic. I'm just curious if anyone knows why the Ca and Mg are treated the same as the other cations in the formula. They have an associated +2 charge as opposed to the +1 associated with the other ionic species.
Also, where does this subtraction of 6 originate? i.e. What does it represent?

Fred,
Thank you SOOOOOOO much for the info. It now makes sense to me. however, is there any way you can give me the source of this information, as he will want to know where it came from.
Thanks again!

<nu 2 this>, I was taught this same formula YEARS ago by John Sweeney in Baxter school. Tell your MD he needs NO higher authority than John Sweeney!

If you do SVS you also increase the other ions too.

quote:

Originally posted by kevin:
If you do SVS you also increase the other ions too.

I don't believe this is true. When SVS is initiated, you are adding more bicarb to the mix. The only ion in bicarb is sodium.

You can do SVS without bicarb if you use acetate concentrate.Even with bicarb u use more acid con if u use SVS.