HDF fluid removal - 5008

[Ian Wilde]

It may seem obvious, but when you think about it - is it?

How does the 5008 remove the 20 or so litres in a HDF treatment?  Not via the UF pump!?

[Chris Bates]

Never thought about it, but 20 litres over 4 hours is only 83 ml/min which doesn't seem that much for the UF pump to do - a pulse every 0.7 seconds

[Ian Wilde]

I'm sure watching the flow diagram over the years though I've never seen the uf pump working very hard though. I've asked fresenius for some clarification but as of yet nothing has come through...

[Chris Eaton]

There is no other way than the UF pump!
It's now a stepper motor with a known volume per stroke, so previously you'd see multiple strokes each equating to 1 ml of fluid removal, but now the pump can rotate the diaphragm in and out 10 times in a second if it needs to. This still gives the accuracy required and explains why you can't see much noticeable difference in duty cycle
or i'm talking crap!
 Peas owt

[Chris Eaton]

Sorry, that reply sounded rude!
That was not my intention.
I'm a lovely chap..
Peas owt

Chris (polite is my middle name) Eaton x

[Ian Wilde]

Been discussing it this morning in the workshop and we have come up with this:

Rather than thinking of the HDF volume being removed and then added back in at the bubble trap we are now looking at is as the HDF replacement fluid being added first and then removed after that.  

The balance flow must deliver a volume (X+Y).  X to the dialyser and then Y goes off through valve 31 to the pre or post dilution port on the bloodlines.  

The waste from the dialyzer is then pulled though by P03 at a rate which is dialsysate flow + HDF so that "what goes in must come out".

Because the sub fluid is going back in on the blood side of the dialyzer, the machine has no option but to make up the difference in the dialysate side by pulling it across.

Does that make any sense or am I totally mistaken?

[Chris Eaton]

Sorry Wildeymon,
Can't agree, never could! even if polite is my middle name . The balance flow is exactly that(what goes in comes out)
ergo the fluid provided at one side balances the other, no matter how hard P03 runs it can't remove any more than is put in from the clean side.
The only way to increase the volume removed on the effluent side is to created a deficit in volume into the balance flow on this side, therefore the UF pump bypasses what returns to the balance flow, hence the increase in TMP, hence the fluid drag across the membrane of the dialyser!
the UF pump rotates instead of pumping as such so;
to remove 20L = say 2 L fluid loss we have 22L in 4 hours
this equates to 5500ml/h or 91.66mls/min
the volume of a UF pump is around 1ml so we have 90 ish rotations/min
or 9 rotations every 6 seconds, I would  imagine this only shows as 1 illumination of the pump on the diagnostic flow diagram.

But I happily stand to be corrected

Peas owt

Chris

[Chris Eaton]

Just looked again, you could be right, and in fact probably are!
Ignore me, I'm losing it!

Peas owt

Chris

[Ian Wilde]

 
Believe me it took some getting away from the traditional Uf pump removes a mill so it must drag that from across the dialyzer.
You almost need the extra corporeal circuit adding to the flow diagram to help visualize whats going on.
I've always taken it for granted that the UF pump did all the work.

Even now, this added knowledge won't hellp me fix it any easier!

[Chris Eaton]

Darn tootin,
To be honest I'd forgot that the sub port was post balance flow, this creates the deficit pre dialyser, and accuracy is irelivent from the sub port as its balenced.
Clever it is..
Peas owt
Chris

[Chris Bates]

D'oh.
No wonder they don't let me near machines any more.

[Chris Bates]

Senility is rampant here.

I did a flow diagram including the blood flows, and various measurements a couple of years ago on this subject.
Lost the diagram so I've recreated it - attached

I couldn't get the same answer twice from Fresenius as to whether indicated dialysis fluid flow included the substitutuion flow so I measured and made the table attached - hopefully.

HDF_Dialysis_Fluid_and_Blood_Flows.docx
Subst_flow_and_dialysis_fluid_flow_measured_values.xlsx

Chris

[Ian Wilde]

Nicely done Chris

[Chris Bates]

Ta.
This is better though - shows pre and post dilution and the effects of autoflow now.

HDF_Dialysis_Fluid_and_Blood_Flows.docx

[John Moore]

I like this a lot