Does anyone know which brands (and models) of dialysis machines compensate for the pre-pump arterial pressure when displaying a blood pump speed?

In our unit we are using Fresenius 4008H machines and you program the blood flow and on the screen it gives you the effective blood flow, (I presume this is compensating for arterial flow as this looks like what is happening)

I spoke with someone at Fresenius and they confirm that while this feature is available in the 4008 machine in Europe, the US version of the Fresenius machine, 2008H does not have this capability. However, I understand this is one of the new features to be included in the next generation Fresenius machine, the 2008K, which is a year or more away from being available.

Does anybody know the calculation that the dialysis machine companies use to determine the true blood volume pumped? Are blood pump speed and pre-pump arterial pressure the only variables? If they are, we should be able to calculate it ourselves.

I can't tell you what the machine manufacturers use to calculate the difference between RPM blood flow and actual delivered blood flow in the presence of negative pressure, but I can give you a reference article that shows the calculation for determining this value. Check out the article titled "Blood Flow : An Important Determinant of Urea Clearance and Delivered Kt/V" by Richard A. Ward PhD. in the January 1999 issue of Advances in Renal Replacement Therapy. Dr. Ward has published many times regarding these types of issues and is recognized as an industry expert. As a matter of fact he will be presenting at the ASAIO Meeting in San Diego in June on the topic of extracorporeal circuits.

His equation for calculating actual delivered QB is as follows for blood flow rates between 200 and 400 ml./min.:

QB = QBN - 0.22 X Pre-pump pressure


For blood flows > 400 ml./min. :

QB = QBN - 0.31 X Pre-pump pressure

By the way, QBN means "blood flow indicated on the blood pump display".

Thanks, Patt, for the calculations. I suspect these calculations are intended to for quick back-of-the-envelope figuring. If you use them precisely, the best blood pump setting for everyone would be 400 ml/min, with big drops in actual blood flow occurring at a 401 ml/min blood pump setting.

Just rounding numbers, it seems that if you have a prepump arterial pressure of -100, you deliver approx 5% less than the blood pump setting.

If you have a prepump arterial pressure of -200, you deliver approx 13% less than the blood pump setting.

If you have a prepump arterial pressure of -250, you deliver approx 17% less than the blood pump setting.

It appears the newer dialysis machines compensate for this, while others display an adjusted total blood volume (but don't adjust the displayed blood pump speed). I think most units (like ours) are blissfully ignorant of these facts.

It looks like a logarithmic rate of change as pressure increases. Anyone still remember calculus? =0)