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The ultrafiltration coefficient: this old 'grand inconnu' in dialysis.

Ficheux A, Ronco C, Brunet P, Argilés À - Nephrol. Dial. Transplant. (2013)

View Article: PubMed Central - PubMed

Affiliation: RD - Néphrologie and Groupe Rein et HTA, EA3127, Institut Universitaire de Recherche Clinique IURC - UM1 , Montpellier 34090, France.

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It became quite clear from the very beginning that membranes differ in their clearance capacities of the different solutes, basically depending on thickness and pore size... However, increasing the pore size and reducing thickness is almost forcedly associated to a water permeability increase... Young nephrologists, who have only lived the ultrafiltration controller era, have just ignored KUF... They simply did not need it... The ultrafiltration coefficient of the filtrating device, in our case, the dialyser is(1)KUF=KUFs×S,which following Darcy's law can be defined as follows:(2)KUF=QUFΔPwhere ΔP is the pressure difference between the two faces of the membrane; ΔP is the resultant of the hydrostatic pressure and the pressure induced by the constituents of the fluid (osmotic and oncotic pressures)... Although π will change with increasing filtration, it is considered constant over the measured range and the general formula is often amended as follows :(3)KUF=QUFTMP−πIn this setting, the filtration is always from the blood side to the external or dialysate side for the whole length of the dialyser's fibres (see Figure 1A) and it was well adapted to the low-permeability dialysers... To determine the KUF of a high-permeability dialyser, the AAMI recommends the use of an ultrafiltration setting with an ultrafiltration pump to regulate the QUF and to measure QUF over the manufacturer's specified range; this pump closes the ultrafiltrate circuit... By doing so, particularly in the high-permeability dialysers, the filtration of fluid inside the dialyser is both directions: from blood to dialysate and also from the dialysate side to blood to obtain a resultant QUF programmed and no extra ultrafiltration flow... The filtration flows have a characteristic KUF within the dialyser which follows the following formula:QUF=∫∫0SΔP⋅KUF⋅dSIt is of note that both ΔP and QUF vary alongside the dialyser fibres under the influence of plasma protein concentration and oncotic pressure, haematocrit and blood viscosity... GKD-UF is the resultant KUF obtained with the resultant QUF and the resultant pressures in the system... It does not rely on every point measurements alongside the membrane of the dialyser but on the global values... The parabolic model of GKD-UF variation differs from the linear model of KUF over QUF... We have already commented that the values inside the dialyser are difficult to measure and do not follow simple laws... In a moment that convection is gaining the protagonist place in dialysis, KUF is doing its come back to the scene... Simple methods to quantify the hydraulic permeability of a given system, such as GKD-UF should be welcomed as (i) they are informative of the conditions of the system, (ii) they are not incompatible with the assumptions and formulas but simplify them by measuring a global component and (iii) they represent an objective parameter easily available to drive convection with a better understanding of the constraints the fluid (blood) is submitted to in the system.

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Ultrafiltration profiles derived from albumin concentration along the length of the dialysers. (A) Maximal ultrafiltration is observed at the proximal end of the dialyser with a subsequent decrease to zero at the distal end. (B) Maximal ultrafiltration is observed at the proximal end of the dialyser with a subsequent decrease to zero at different points of the polysulphone (×1) and cuprophane (×2). From these points, backfiltration begins reaching its maximum at the distal end of the dialyser. Despite different profiles are observed, cumulative ultrafiltration and cumulative backfiltration are equal. (Modified from ref. [19], reprinted by permission from Macmillan Publishers Ltd).
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GFT493F1: Ultrafiltration profiles derived from albumin concentration along the length of the dialysers. (A) Maximal ultrafiltration is observed at the proximal end of the dialyser with a subsequent decrease to zero at the distal end. (B) Maximal ultrafiltration is observed at the proximal end of the dialyser with a subsequent decrease to zero at different points of the polysulphone (×1) and cuprophane (×2). From these points, backfiltration begins reaching its maximum at the distal end of the dialyser. Despite different profiles are observed, cumulative ultrafiltration and cumulative backfiltration are equal. (Modified from ref. [19], reprinted by permission from Macmillan Publishers Ltd).

Mentions: The requirements defined by the Association for the Advancement of Medical Instrumentation (ANSI/AAMI RD16:1996), on which the FDA based its exigencies to homologate a dialyser up to 2010 include the description of the KUFin vivo and in vitro with a limited variability in its values (10% as reported by Keshaviah et al., 17% in most of the dialysers and 20% mandatory). They proposed the measurements of KUF to be performed without circulating dialysate following Keshaviah's method [14] which was set in an open dialysate side circuit and assuming a positive filtration from the blood side to the dialysate side all throughout the dialyser. They fixed TMP at 0, 100 and 300 mmHg and the maximum tolerated by the membrane and collected the ultrafiltrate; they considered KUF as the slope of the regression line of TMP over QUF. The TMP at QUF = 0, TMP0 is the value accepted as equal to the amount of pressure that opposes the production of fluid and is taken as equal to the oncotic pressure π. Although π will change with increasing filtration, it is considered constant over the measured range and the general formula [2] is often amended as follows [15]:(3)KUF=QUFTMP−πIn this setting, the filtration is always from the blood side to the external or dialysate side for the whole length of the dialyser's fibres (see Figure 1A) and it was well adapted to the low-permeability dialysers.FIGURE 1:


The ultrafiltration coefficient: this old 'grand inconnu' in dialysis.

Ficheux A, Ronco C, Brunet P, Argilés À - Nephrol. Dial. Transplant. (2013)

Ultrafiltration profiles derived from albumin concentration along the length of the dialysers. (A) Maximal ultrafiltration is observed at the proximal end of the dialyser with a subsequent decrease to zero at the distal end. (B) Maximal ultrafiltration is observed at the proximal end of the dialyser with a subsequent decrease to zero at different points of the polysulphone (×1) and cuprophane (×2). From these points, backfiltration begins reaching its maximum at the distal end of the dialyser. Despite different profiles are observed, cumulative ultrafiltration and cumulative backfiltration are equal. (Modified from ref. [19], reprinted by permission from Macmillan Publishers Ltd).
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Related In: Results  -  Collection

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GFT493F1: Ultrafiltration profiles derived from albumin concentration along the length of the dialysers. (A) Maximal ultrafiltration is observed at the proximal end of the dialyser with a subsequent decrease to zero at the distal end. (B) Maximal ultrafiltration is observed at the proximal end of the dialyser with a subsequent decrease to zero at different points of the polysulphone (×1) and cuprophane (×2). From these points, backfiltration begins reaching its maximum at the distal end of the dialyser. Despite different profiles are observed, cumulative ultrafiltration and cumulative backfiltration are equal. (Modified from ref. [19], reprinted by permission from Macmillan Publishers Ltd).
Mentions: The requirements defined by the Association for the Advancement of Medical Instrumentation (ANSI/AAMI RD16:1996), on which the FDA based its exigencies to homologate a dialyser up to 2010 include the description of the KUFin vivo and in vitro with a limited variability in its values (10% as reported by Keshaviah et al., 17% in most of the dialysers and 20% mandatory). They proposed the measurements of KUF to be performed without circulating dialysate following Keshaviah's method [14] which was set in an open dialysate side circuit and assuming a positive filtration from the blood side to the dialysate side all throughout the dialyser. They fixed TMP at 0, 100 and 300 mmHg and the maximum tolerated by the membrane and collected the ultrafiltrate; they considered KUF as the slope of the regression line of TMP over QUF. The TMP at QUF = 0, TMP0 is the value accepted as equal to the amount of pressure that opposes the production of fluid and is taken as equal to the oncotic pressure π. Although π will change with increasing filtration, it is considered constant over the measured range and the general formula [2] is often amended as follows [15]:(3)KUF=QUFTMP−πIn this setting, the filtration is always from the blood side to the external or dialysate side for the whole length of the dialyser's fibres (see Figure 1A) and it was well adapted to the low-permeability dialysers.FIGURE 1:

View Article: PubMed Central - PubMed

Affiliation: RD - Néphrologie and Groupe Rein et HTA, EA3127, Institut Universitaire de Recherche Clinique IURC - UM1 , Montpellier 34090, France.

AUTOMATICALLY GENERATED EXCERPT
Please rate it.

It became quite clear from the very beginning that membranes differ in their clearance capacities of the different solutes, basically depending on thickness and pore size... However, increasing the pore size and reducing thickness is almost forcedly associated to a water permeability increase... Young nephrologists, who have only lived the ultrafiltration controller era, have just ignored KUF... They simply did not need it... The ultrafiltration coefficient of the filtrating device, in our case, the dialyser is(1)KUF=KUFs×S,which following Darcy's law can be defined as follows:(2)KUF=QUFΔPwhere ΔP is the pressure difference between the two faces of the membrane; ΔP is the resultant of the hydrostatic pressure and the pressure induced by the constituents of the fluid (osmotic and oncotic pressures)... Although π will change with increasing filtration, it is considered constant over the measured range and the general formula is often amended as follows :(3)KUF=QUFTMP−πIn this setting, the filtration is always from the blood side to the external or dialysate side for the whole length of the dialyser's fibres (see Figure 1A) and it was well adapted to the low-permeability dialysers... To determine the KUF of a high-permeability dialyser, the AAMI recommends the use of an ultrafiltration setting with an ultrafiltration pump to regulate the QUF and to measure QUF over the manufacturer's specified range; this pump closes the ultrafiltrate circuit... By doing so, particularly in the high-permeability dialysers, the filtration of fluid inside the dialyser is both directions: from blood to dialysate and also from the dialysate side to blood to obtain a resultant QUF programmed and no extra ultrafiltration flow... The filtration flows have a characteristic KUF within the dialyser which follows the following formula:QUF=∫∫0SΔP⋅KUF⋅dSIt is of note that both ΔP and QUF vary alongside the dialyser fibres under the influence of plasma protein concentration and oncotic pressure, haematocrit and blood viscosity... GKD-UF is the resultant KUF obtained with the resultant QUF and the resultant pressures in the system... It does not rely on every point measurements alongside the membrane of the dialyser but on the global values... The parabolic model of GKD-UF variation differs from the linear model of KUF over QUF... We have already commented that the values inside the dialyser are difficult to measure and do not follow simple laws... In a moment that convection is gaining the protagonist place in dialysis, KUF is doing its come back to the scene... Simple methods to quantify the hydraulic permeability of a given system, such as GKD-UF should be welcomed as (i) they are informative of the conditions of the system, (ii) they are not incompatible with the assumptions and formulas but simplify them by measuring a global component and (iii) they represent an objective parameter easily available to drive convection with a better understanding of the constraints the fluid (blood) is submitted to in the system.

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