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

Preliminary Study of Wound Oxygenation Comparing Skin Substitutes and Dressings

Background

Improving oxygen delivery to challenging wound types has been shown to optimize and accelerate several key contributors to healing. This study aims to compare selective skin substitutes and primary dressings and evaluate their ability to transfer oxygen to the wound.

Methods

Visual and quantitative methods were employed to measure gas and fluid movement across several skin substitutes, including a bilayer nylon and silicone dressing coated with porcine gelatin and aloe vera (CNS), a porous bovine collagen-glycosaminoglycan (GAG) matrix dressing coated with silicone (UBC), and a urethane biodegradable temporizing matrix (PFD).

Results

Fluids did not move across solid silicone membranes or urethane foam while oxygen movement across solid silicone membranes was inversely proportional to the thickness of the membrane. Oxygen moved across the coated nylon and silicone dressing 5.63 times faster than across the bovine-GAG scaffold and 2.0 times faster than the biodegradable temporizing matrix of polyurethane.

Conclusions

The coated nylon and silicone matrix functioned like a membrane oxygenator, potentially augmenting atmospheric oxygen delivery to healing wounds.

A novel pump-free ultrafiltration rate modulation system for continuous renal replacement therapy applications

Purpose

Extracorporeal ultrafiltration is an attractive alternative to diuretics for removing excess plasma water in critically ill patients suffering from fluid overload. In continuous renal replacement therapy (CRRT), ultrafiltration occurs in isolated form (SCUF) or supplemented by replacement fluid infusion (CVVH) and the net fluid removal rate is controlled by peristaltic pumps. In this work, a pump-free solution for regulating the ultrafiltration rate in CRRT applications is presented.

Methods

The system consists of a motorized clamp on the ultrafiltration line, whose intermittent opening is modulated with a closed-loop control system based on monitoring of ultrafiltrate collected and any replacement fluid infused. The system was tested on two platforms for SCUF and CVVH, with “low-flux” and “high-flux” hemofilter, with various ultrafiltration setpoints and patient net weight loss targets.

Results

In all configurations the set ultrafiltration rate was achieved with a maximum error of 5% and the values recorded were kept within ± 100 ml/h with respect to the setpoint, as recommended by international standard IEC 60601-2-16. The net fluid removal trend was highly correlated with that expected (95%<R2<99%) and the weight loss target was reached in the expected time. For low ultrafiltration rates (60-150 ml/h) the system accuracy was better with the “low-flux” hemofilter.

Conclusion

The developed clamp system represents a valid alternative to state-of-the-art solutions with peristaltic pumps in terms of performance, with potential usability advantages. The compliance with safety requirements given by international standard IEC 60601-2-16 is a prerequisite for clinical use.

Flummoxed by flux: the indeterminate principles of haemodialysis

In haemodialysis (HD), unwanted substances (uraemic retention solutes or ‘uraemic toxins’) that accumulate in uraemia are removed from blood by transport across the semipermeable membrane. Like all membrane separation processes, the transport requires driving forces to facilitate the transfer of molecules across the membrane. The magnitude of the transport is quantified by the phenomenon of ‘flux’, a finite parameter defined as the volume of fluid (or permeate) transferred per unit area of membrane surface per unit time. In HD, as transmembrane pressure is applied to facilitate fluid flow or flux across the membrane to enhance solute removal, flux is defined by the ultrafiltration coefficient (KUF; mL/h/mmHg) reflecting the hydraulic permeability of the membrane. However, in HD, the designation of flux has come to be used in a much broader sense and the term is commonly used interchangeably and erroneously with other measures of membrane separation processes, resulting in considerable confusion. Increased flux is perceived to reflect more ‘porous’ membranes having ‘larger’ pores, even though other membrane and therapy attributes determine the magnitude of flux achieved during HD. Adjectival designations of flux (low-, mid-, high-, super-, ultra-) have found indiscriminate usage in the scientific literature to qualify a parameter that influences clinical decision making and prescription of therapy modalities (low-flux or high-flux HD). Over the years the concept and definition of flux has undergone arbitrary and periodic adjustment and redefinition by authors in publications, regulatory bodies (US Food and Drug Administration) and professional association guidelines (European Renal Association, Kidney Disease Outcomes Quality Initiative), with little consensus. Industry has stretched the boundaries of flux to derive marketing advantages, justify increased reimbursement or contrive new classes of therapy modalities when in fact flux is just one of several specifications that determine membrane or dialyser performance. Membranes considered as high-flux previously are today at the lower end of the flux spectrum. Further, additional parameters unrelated to the rate of diffusive or convective transport (flux) are used in conjunction with or in place of KUF to allude to flux: clearance (mL/min, e.g. of β₂-microglobulin) or sieving coefficients (dimensionless). Considering that clinical trials in nephrology, designed to make therapy recommendations and guide policy with economic repercussions, are based on the parameter flux they merit clarification—by regulatory authorities and scientists alike—to avoid further misappropriation.

Mass Transfer Characteristics of Haemofiltration Modules-Experiments and Modeling

Reliable mathematical models are important tools for design/optimization of haemo-filtration modules. For a specific module, such a model requires knowledge of fluid- mechanical and mass transfer parameters, which have to be determined through experimental data representative of the usual countercurrent operation. Attempting to determine all these parameters, through measured/external flow-rates and pressures, combined with the inherent inaccuracies of pressure measurements, creates an ill-posed problem (as recently shown). The novel systematic methodology followed herein, demonstrated for Newtonian fluids, involves specially designed experiments, allowing first the independent reliable determination of fluid-mechanical parameters. In this paper, the method is further developed, to determine the complete mass transfer module-characteristics; i.e., the mass transfer problem is modelled/solved, employing the already fully-described flow field. Furthermore, the model is validated using new/detailed experimental data on concentration profiles of a typical solute (urea) in counter-current flow. A single intrinsic-parameter value (i.e., the unknown effective solute-diffusivity in the membrane) satisfactorily fits all data. Significant insights are also obtained regarding the relative contributions of convective and diffusive mass-transfer. This study completes the method for reliable module simulation in Newtonian-liquid flow and provides the basis for extension to plasma/blood haemofiltration, where account should be also taken of oncotic-pressure and membrane-fouling effects.

Informed decision-making in delivery of dialysis: combining clinical outcomes with sustainability

As the prevalence of chronic kidney disease is expected to rise worldwide over the next decades, provision of renal replacement therapy (RRT), will further challenge budgets of all healthcare systems. Most patients today requiring RRT are treated with haemodialysis (HD) therapy and are elderly. This article demonstrates the interdependence of clinical and sustainability criteria that need to be considered to prepare for the future challenges of delivering dialysis to all patients in need. Newer, more sustainable models of high-value care need to be devised, whereby delivery of dialysis is based on value-based healthcare (VBHC) principles, i.e. improving patient outcomes while restricting costs. Essentially, this entails maximizing patient outcomes per amount of money spent or available. To bring such a meaningful change, revised strategies having the involvement of multiple stakeholders (i.e. patients, providers, payers and policymakers) need to be adopted. Although each stakeholder has a vested interest in the value agenda often with conflicting expectations and motivations (or motives) between each other, progress is only achieved if the multiple blocs of the delivery system are advanced as mutually reinforcing entities. Clinical considerations of delivery of dialysis need to be based on the entire patient disease pathway and evidence-based medicine, while the non-clinical sustainability criteria entail, in addition to economics, the societal and ecological implications of HD therapy. We discuss how selection of appropriate modes and features of delivery of HD (e.g. treatment modalities and schedules, selection of consumables, product life cycle assessment) could positively impact decision-making towards value-based renal care. Although the delivery of HD therapy is multifactorial and complex, applying cost-effectiveness analyses for the different HD modalities (conventional in-centre and home HD) can support in guiding payability (balance between clinical value and costs) for health systems. For a resource intensive therapy like HD, concerted and fully integrated care strategies need to be urgently implemented to cope with the global demand and burden of HD therapy.

Reliable fluid-mechanical characterization of haemofilters: Addressing the deficiencies of current standards and practices

Facile methods for accurate fluid-mechanical characterization of haemofilters (HF) are indispensable for haemofiltration process improvements, equipment design/optimization, and reliable module specifications. Currently employed methods, implemented through specific experimental in vitro protocols, are assessed herein in detail, considering the conditions prevailing during haemofiltration. Minimum number of key parameters required to fully describe the common countercurrent flow field, in the HF active section, include membrane permeance K and friction coefficients in lumen and shell side (f_f and f_s ). It is shown that the countercurrent flow mode itself is incapable of yielding these parameters, based on externally measured flow rates and pressures. Similarly, the relevant ISO protocol is deficient as it can only provide rough underpredictions of permeance K. The causes of such inherent deficiencies of current standards and practices are analyzed. In contrast, a recently developed methodology, accounting for the (heretofore ignored) pressure drop in module headers and combining a mechanistic theoretical model with experimental data from 2 special haemofilter operating modes, yields an accurate determination of the key parameters (K, f_f , f_s ). Additionally, it permits a full description of flow field for Newtonian liquids, for both constant and axially varying viscosity in fiber-lumen due to the transmembrane flux. Development of new reliable standards is suggested, facilitated by the insights gained in this work.

Continuous renal replacement therapy principles

Continuous renal replacement therapy (CRRT) is an extracorporeal blood purification therapy that aims to support kidney function over an extended period of time. One of the main objectives of CRRT is the removal of excess fluid and solutes retained as a consequence of acute kidney injury. Because prescription of CRRT requires goals to be set with regard to the rate and extent of solute and fluid removal, a comprehensive understanding of the mechanism by which solute and fluid removal occurs during CRRT is essential. Basic mechanisms of fluid transport and solute removal (ultrafiltration, diffusion, convection, and adsorption) and the factors influencing these processes in CRRT are described. From the combination of the different transport mechanisms, a number of CRRT modalities are identified and described. Finally, these principles are applied to provide a brief overview of the concept of effluent-based CRRT dose.

Ultrafiltration in critically ill patients treated with kidney replacement therapy

Management of fluid overload is one of the most challenging problems in the care of critically ill patients with oliguric acute kidney injury. Various clinical practice guidelines support fluid removal using ultrafiltration during kidney replacement therapy. However, ultrafiltration is associated with considerable risks. Emerging evidence from observational studies suggests that both slow and fast rates of net fluid removal (that is, net ultrafiltration (UFNET)) during continuous kidney replacement therapy are associated with increased mortality compared with moderate UFNET rates. In addition, fast UFNET rates are associated with an increased risk of cardiac arrhythmias. Experimental studies in patients with kidney failure who were treated with intermittent haemodialysis suggest that fast UFNET rates are also associated with ischaemic injury to the heart, brain, kidney and gut. The UFNET rate should be prescribed based on patient body weight in millilitres per kilogramme per hour with close monitoring of patient haemodynamics and fluid balance. Dialysate cooling and sodium modelling may prevent haemodynamic instability and facilitate large volumes of fluid removal in patients with kidney failure who are treated with intermittent haemodialysis, but the effects of this strategy on organ injury are less well studied in critically ill patients treated with continuous kidney replacement therapy. Randomized trials are required to examine whether moderate UFNET rates are associated with a reduced risk of haemodynamic instability, organ injury and improved outcomes in critically ill patients.

Acute cardiorenal syndrome in acute heart failure: focus on renal replacement therapy

Almost half of hospitalised patients with acute heart failure develop acute cardiorenal syndrome. Treatment consists of optimisation of fluid status and haemodynamics, targeted therapy for the underlying cardiac disease, optimisation of heart failure treatment and preventive measures such as avoidance of nephrotoxic agents. Renal replacement therapy may be temporarily needed to support kidney function, mostly in case of diuretic resistant fluid overload or severe metabolic derangement. The best timing to initiate renal replacement therapy and the best modality in acute heart failure are still under debate. Several modalities are available such as intermittent and continuous renal replacement therapy as well as hybrid techniques, based on two main principles: haemofiltration and haemodialysis. Although continuous techniques have been associated with less haemodynamic instability and a greater chance of renal recovery, cohort data are conflicting and randomised controlled trials have not shown a difference in recovery or mortality. In the presence of diuretic resistance, isolated ultrafiltration with individualisation of ultrafiltration rates is a valid option for decongestion in acute heart failure patients. Practical tools to optimise the use of renal replacement therapy in acute heart failure-related acute cardiorenal syndrome were discussed.

Measuring intradialyser transmembrane and hydrostatic pressures: pitfalls and relevance in haemodialysis and haemodiafiltration

Background

Post-dilutional haemodiafiltration (HDF) with high convection volumes (HCVs) could improve survival. HCV-HDF requires a significant pressure to be applied to the dialyser membrane. The aim of this study was to assess the pressure applied to the dialysers in HCV-HDF, evaluate the influence of transmembrane pressure (TMP) calculation methods on TMP values and check how they relate to the safety limits proposed by guidelines.

Methods

Nine stable dialysis patients were treated with post-dilutional HCV-HDF with three different convection volumes [including haemodialysis (HD)]. The pressures at blood inlet (B_i), blood outlet (B_o) and dialysate outlet (D_o) were continuously recorded. TMP was calculated using two pressures (TMP2: B_o, D_o) or three pressures (TMP3: B_o, D_o, B_i). Dialysis parameters were analysed at the start of the session and at the end of treatment or at the first occurrence of a manual intervention to decrease convection due to TMP alarms.

Results

During HD sessions, TMP2 and TMP3 remained stable. During HCV-HDF, TMP2 remained stable while TMP3 clearly increased. For the same condition, TMP3 could be 3-fold greater than TMP2. This shows that the TMP limit of 300 mmHg as recommended by guidelines could have different effects according to the TMP calculation method. In HCV-HDF, the pressure at the B_i increased over time and exceeded the safety limits of 600 mmHg provided by the manufacturer, even when respecting TMP safety limits.

Conclusions

This study draws our attention to the dangers of using a two-pressure points TMP calculation, particularly when performing HCV-HDF.

Consequences of increasing convection onto patient care and protein removal in hemodialysis

INTRODUCTION

Recent randomised controlled trials suggest that on-line hemodiafiltration (OL-HDF) improves survival, provided that it reaches high convective volumes. However, there is scant information on the feasibility and the consequences of modifying convection volumes in clinics.

METHODS

Twelve stable dialysis patients were treated with high-flux 1.8 m2 polysulphone dialyzers and 4 levels of convection flows (QUF) based on GKD-UF monitoring of the system, for 1 week each. The consequences on dialysis delivery (transmembrane pressure (TMP), number of alarms, % of achieved prescribed convection) and efficacy (mass removal of low and high molecular weight compounds) were analysed.

RESULTS

TMP increased exponentially with QUF (p<0.001 for N >56,000 monitoring values). Beyond 21 L/session, this resulted into frequent TMP alarms requiring nursing staff interventions (mean ± SEM: 10.3 ± 2.2 alarms per session, p<0.001 compared to lower convection volumes). Optimal convection volumes as assessed by GKD-UF-max were 20.6 ± 0.4 L/session, whilst 4 supplementary litres were obtained in the maximum situation (24.5 ± 0.6 L/session) but the proportion of sessions achieving the prescribed convection volume decreased from 94% to only 33% (p<0.001). Convection increased high molecular weight compound removal and shifted the membrane cut-off towards the higher molecular weight range.

CONCLUSIONS

Reaching high convection volumes as recommended by the recent RCTs (> 20L) is feasible by setting an HDF system at its optimal conditions based upon the GKD-UF monitoring. Prescribing higher convection volumes resulted in instability of the system, provoked alarms, was bothersome for the nursing staff and the patients, rarely achieved the prescribed convection volumes and increased removal of high molecular weight compounds, notably albumin.

A reliable method to assess the water permeability of a dialysis system: the global ultrafiltration coefficient

Background

Recent randomized controlled trials suggest that sufficiently high convection post-dilutional haemodiafiltration (HC-HDF) improves survival in dialysis patients, consequently this technique is increasingly being adopted. However, when performing HC-HDF, rigorous control systems of the ultrafiltration setting are required. Assessing the global ultrafiltration coefficient of the dialysis system [GKD-UF; defined as ultrafiltration rate (QUF)/transmembrane pressure] or water permeability may be adapted to the present dialysis settings and be of value in clinics.

Methods

GKD-UF was determined and its reproducibility, variability and influencing factors were specifically assessed in 15 stable patients routinely treated by high-flux haemodialysis or HC-HDF in a single unit.

Results

GKD-UF invariably followed a parabolic function with increasing QUF in dialysis and both pre- and post-dilution HC-HDF (R2 constantly >0.96). The vertex of the parabola, GKD-UF-max and related QUF were very reproducible per patient (coefficient of variation 3.9 ± 0.6 and 3.3 ± 0.3%, respectively) and they greatly varied across patients (31–42 mL/h−1/mmHg and 82–100 mL/min, respectively). GKD-UF-max and its associated QUF decreased during dialysis treatment (P < 0.01). The GKD-UF-max decrease was related to weight loss (R2 = 0.66; P = 0.0015).

Conclusions

GKD-UF is a reliable and accurate method to assess the water permeability of a system in vivo. It varies according to dialysis setting and patient-related factors. It is an objective parameter evaluating the forces driving convection and identifies any diversion of the system during the treatment procedure. It is applicable to low- or high-flux dialysis as well as pre- or post-dilution HDF. Thus, it may be used to describe the characteristics of a dialysis system, is suitable for clinical use and may be of help for personalized prescription.

Nomenclature for renal replacement therapy in acute kidney injury: basic principles

This article reports the conclusions of a consensus expert conference on the basic principles and nomenclature of renal replacement therapy (RRT) currently utilized to manage acute kidney injury (AKI). This multidisciplinary consensus conference discusses common definitions, components, techniques, and operations of the machines and platforms used to deliver extracorporeal therapies, utilizing a "machine-centric" rather than a "patient-centric" approach. We provide a detailed description of the performance characteristics of membranes, filters, transmembrane transport of solutes and fluid, flows, and methods of measurement of delivered treatment, focusing on continuous renal replacement therapies (CRRT) which are utilized in the management of critically ill patients with AKI. This is a consensus report on nomenclature harmonization for principles of extracorporeal renal replacement therapies. Devices and operations are classified and defined in detail to serve as guidelines for future use of terminology in papers and research.