Filtration fraction as a measure of ultrafiltration

This chapter is related to the aims of Section H3(i) from the 2017 CICM Primary Syllabus, which expects the exam candidate to "describe the principles of dialysis and renal replacement fluid". Filtration fraction is one of the principles of dialysis which calls for at least some brief discussion, at least in terms of the ways in which it affects the filter function.

In brief summary:

• The filtration fraction is the volume of plasma removed from the dialysed blood by ultrafiltration.
• The official definition is "the ratio of ultrafiltration rate to plasma water flow rate".
• A filtration fraction of 25% represents 25% of the plasma water removed by ultrafiltration
• Practically, one should have a filtration fraction no greater than 30%
• An FF of 30% corresponds to a post-filter blood haematocrit of 0.40, which is a practical maximum. For CRRT, the maximum FF is probably lower, ~ 15%.
• A post-filter haematocrit which is too high will tend to degrade the life of the filter and promote clot formation and membrane rupture

Generally, this area has been largely ignored by the college. It does not appear in any of the past papers, and the exam candidate can safely omit this from their list of revision topics. If for whatever reason some understanding of these issues is required,  Neri et al (2016) is good for the basic definitions of the concepts. William Clark's 2008 article is a more indepth exploration. For a practical overview of "what can my filter do" and "what settings do I want", Penn et al (2009) have written a great overview of convective volumes and filter characteristics.

Definition of filtration fraction

The filtration fraction is defined as the ratio between the ultrafiltration flow rate QUF and the plasma flow rate QP (Neri et al, 2016), such that:

The ultrafiltrate flow rate is usually known (i.e you prescribed it) but the plasma flow rate is somewhat more difficult to quantify One might be tempted to substitute blood flow rate for plasma flow rate, but then a fair amount of blood (by weight or volume) is not plasma and so blood flow is probably not a very effective surrogate in this equation. However, we do have blood flow as a value. In order to force it into the equation, it needs to be adjusted for haematocrit. If pre-dilution fluid is used, it also needs to be included in the equation.

Thus:

This brings up the important issue of haematocrit, which is the topic of the next subheading.

Safe levels of haemofiltration

Let us say you have for whatever reason decided that you want to remove vast amounts of fluid from your patient; and let's say that the patient is somehow haemodynamically capable of withstanding massive fluid removal. At 200ml/min blood flow rate, 12,000ml of blood flow though the fiter every hour, and therefore with a haematocrit of 0.40 theoretically there is 7200ml of plasma water  available for removal. Every hour. Would it be possible to remove all that water?

Of course, it would not. 

The effect of increasing the filtration fraction is haemoconcentration at the end of the filter. If all the water is removed, the end of the filter would be full of cells with a haematocrit of 1.0. This would be unreasonable. The filter would clot long before all the water was removed (for a point of reference, the haematocrit of a bag of packed red blood cells is 0.55-0.70).

However, when one wishes to remove water, one would like to do so rather quickly to minimise the exposure of the patient to the risks of the extracorporeal circuit. Ergo, there must be some "ideal" filtration fraction which maximises the fluid removal while minimising the effects of the increased haematocrit on the filter lifespan (or, conversely, the effect of the filter on the patient's lifespan - as haemolysis may occur, not to mention loss of blood into the circuit).

Thus, the realistic limits of filtration fraction are defined by pragmatic haematocrit-related concerns. A haematocrit which is too high would lead to an unacceptably high rate of filter clotting. The end-filter haematocrit limit for safe filtration has been established; it appears to be a haematocrit of approximately 0.40, which corresponds to a filtration fraction of approximately 0.25-0.30 (i.e. a quarter to a third of all the blood water removed by filter's end). This recommendation comes from a variety of sources (eg. Joannidis et al, 2007) who all seem to quote the same numbers, but without any reference as to where they got it from, suggesting that this is just another unquestioned piece of nephrology folk wisdom. In fact it probably comes from the early eighties, from practoical studies like the paper by Kramer et al (1980) and Lauer et al (1983). These authors found practical limits at a filtration fraction of around 35%-40%, and a postfilter haematocrit of 0.45. A theoretical limit is generally believed to be a filtration fraction of around 45%, a level at which most filters can be expected to develop "operational instability" (Jenkins et al, 1992). The authors perfused filters with bovine blood and found that the highest postfilter haematocrit at which the circuit would remain patent for sustained periods was around 0.50. At a postfilter haematocrit of 0.55, "the prefilter pressure abruptly rose to 450 mm Hg... When the prefilter pressure reached 950 mm Hg, the pump could not maintain prefilter blood flow. Visual inspection of the hemofilter showed areas of membrane rupture."

So, a filtration fraction of 35% seems to be the practical limit. In real-life intermittent haemodialysis, most nephrologists seem to stick to lower values. Penn et al (2009) offer an overview of randomly sampled Dutch Norwegian and Canadian practice - it seems an FF of 25-30% is the upper level of normal for those parts of the world.