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Understanding UF Rate
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How is the UF Rate controlled without adjusting Blood flow rate or Dialysate flow rate? Just to be clear, I know how to set the machine, but what is going on inside the dialyzer to control how much water leaves the fibers if nothing else has changed?
 
Posts: 11 | Registered: 05 July 2015Report This Post
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The UF rate is controlled by volume of dialysate.

With the UF rate set to zero, a measured amount of dialysate is sent to the dialyzer. When the machine "withdraws" the dialysate, the machine is designed so that an equal amount comes out.

The machine has a UF pump that pulls out a specific volume of dialysate out of the circuit during the withdrawal phase.

This UF volume is replaced with water from the patient's blood. The machine generates the necessary transmembrane pressure (TMP) to pull this UF volume from the patient's blood.
 
Posts: 64 | Registered: 15 April 2006Report This Post
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quote:
The machine generates the necessary transmembrane pressure (TMP) to pull this UF volume from the patient's blood.


My physics might be weak but I thought the pressure differential would be a function of flow rates. How does the machine generate the necessary TMP if the Qb and Qd doesn't change? For example: if, mid treatment, I want to lower the net UF, the UF rate will be reduced but the blood and dialysate flow rates would not change. So what is going on in the dialyzer to prevent water from leaving the blood?
 
Posts: 11 | Registered: 05 July 2015Report This Post
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The "pressure differential" (TMP?) is not a function of the flow rates in volumetric-control dialysis machines.

The only way for water to leave the patient's blood is through the dialysate. The machine controls the volume of dialysate both in and out of the dialyzer.

If you want an explanation beyond this, perhaps someone can suggest a link that has an animation that demonstrates the process. It's hard to understand until you see it in operation.
 
Posts: 64 | Registered: 15 April 2006Report This Post
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Alan,

The principle behind volumetric control is that, fluid can not be compressed. Since the dialysate can't be compressed, flow rates, and even positive pressure on the blood side do not play into the the UF process.

In order to allow controlled UF to occur, the machine will remove "X" amount of dialysate from the circuit post dialyzer. This removal results in a negative pressure, or void, creating the TMP and allowing fluid to cross the membrane from the blood to the dialysate.

Chuck


DISCLAIMER : My opinions and views are mine and may not be the same as my employer.
 
Posts: 1787 | Location: Baltimore, MD USA | Registered: 24 October 2001Report This Post
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This is really interesting, thanks for the great replies. I'm going to flaunt my ignorance again and ask about a statement in Gary's reply:
quote:
The only way for water to leave the patient's blood is through the dialysate.

What is going on during sequential mode (UF Only)? Water is leaving the blood but the dialysate flow is off.
 
Posts: 11 | Registered: 05 July 2015Report This Post
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Is this anywhere close to right? Snarky comments about my artistic abilities are also welcome.
 
Posts: 11 | Registered: 05 July 2015Report This Post
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Is this anywhere close to correct?

This message has been edited. Last edited by: Alan,

uf_diagram.bmp (137 Kb, 14 downloads)
 
Posts: 11 | Registered: 05 July 2015Report This Post
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It's close. The UF circuit is prior to the balancing chamber. The UF volume removed from the circuit never enters the balancing chamber so the flow exiting the chamber would still be 600.

A balancing chamber is actually 2 chambers with each of those chambers separated by a flexible/moving diaphragm. As one side of one of the chambers is filling with fresh dialysate, it is pushing used dialysate out the other side and down the drain. At the same time, the other chamber is doing the exact opposite, it is filling with used dialysate from the dialyzer and pushing fresh dialysate out the other side to the dialyzer. The chambers have sensors that have to be satisfied at the exact same time or the machine is not happy and will alarm.

Chuck


DISCLAIMER : My opinions and views are mine and may not be the same as my employer.
 
Posts: 1787 | Location: Baltimore, MD USA | Registered: 24 October 2001Report This Post
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quote:
Originally posted by Alan:
This is really interesting, thanks for the great replies. I'm going to flaunt my ignorance again and ask about a statement in Gary's reply:
quote:
The only way for water to leave the patient's blood is through the dialysate.

What is going on during sequential mode (UF Only)? Water is leaving the blood but the dialysate flow is off.


If you turn off the dialysate flow (UF-only mode), then a different set of rules apply.

All dialyzers have an ultrafiltration coefficient (K(UF)), which I believe is expressed in ml/mmHg/hr (it's been a long time since I thought about this, so this may need some correction or refinement). Let's say the dialyzer's UF coefficient is 5. That means if the patient dialyzes in UF-only mode and has a venous pressure of 50 mmHg, the patient would lose 250 ml in an hour.

In UF-only mode, if you increase the blood flow rate, the venous pressure will also likely increase. That would increase the UF rate.
 
Posts: 64 | Registered: 15 April 2006Report This Post
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quote:
Originally posted by Gary Peterson:
quote:
Originally posted by Alan:
This is really interesting, thanks for the great replies. I'm going to flaunt my ignorance again and ask about a statement in Gary's reply:
quote:
The only way for water to leave the patient's blood is through the dialysate.

What is going on during sequential mode (UF Only)? Water is leaving the blood but the dialysate flow is off.


If you turn off the dialysate flow (UF-only mode), then a different set of rules apply.

All dialyzers have an ultrafiltration coefficient (K(UF)), which I believe is expressed in ml/mmHg/hr (it's been a long time since I thought about this, so this may need some correction or refinement). Let's say the dialyzer's UF coefficient is 5. That means if the patient dialyzes in UF-only mode and has a venous pressure of 50 mmHg, the patient would lose 250 ml in an hour.

In UF-only mode, if you increase the blood flow rate, the venous pressure will also likely increase. That would increase the UF rate.


I'm not certain but, I think this may be the case with something like CVVH but, a conventional HD machine still uses the UF pump and controls how much fluid is removed.

Chuck


DISCLAIMER : My opinions and views are mine and may not be the same as my employer.
 
Posts: 1787 | Location: Baltimore, MD USA | Registered: 24 October 2001Report This Post
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That seems likely. I default back to the 1970s on stuff like this.
 
Posts: 64 | Registered: 15 April 2006Report This Post
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