Keywords

Introduction

Patients with chronic kidney disease stage 5 (CKD5) have increased systemic inflammation and oxidative stress irrespective of whether they are managed conservatively or treated by dialysis. Progressive loss of residual renal function leads to the accumulation of uraemic toxins (Table 13.1). Some of these toxins, such as p-cresol and indoxyl sulfate are formed as by-product of tyrosine and phenylalanine, and tryptophan metabolism respectively, by bacteria in the gastrointestinal tract, whereas other uraemic toxins, including carbamylated albumin and other proteins accumulate due to increased production and others such as advanced glycosylation end (AGEs) products, β2 microglobulin, plasma light chains and circulating cell free DNA simply accumulate due to reduced renal clearance. However, in addition to inflammation driven directly as a consequence of the retention products of uraemia and treatments, there are additional pro-inflammatory factors (Table 13.2).

Table 13.1 List of azotaemic toxins which predominantly affect the vascular endothelium
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Table 13.2 Patients with chronic kidney disease are at increased risk of both acute and chronic infections
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Several of these uremic toxins, including p-cresol, indoxyl sulfate, homocysteine, AGEs and β2 microglobulin have been reported to be independent risk factors for cardiovascular disease in the CKD5d patient [1–3]. After cardiovascular causes, infectious diseases are the next most common cause of death for dialysis patients with increased mortality rates being greatest for sepsis, followed in descending order by peritonitis, influenza, tuberculosis and pneumonia [4]. Patients with CKD are more susceptible to some infections, as the azotaemic state alters innate immunity, with reports of reduced monocyte Toll like receptor (TLR) 4 expression [5], reduced B lymphocyte cell populations [6], and impaired polymorphonuclear chemotaxis and phagocytosis [7] (Table 13.3). It has also been proposed that changes in the gastro-intestinal microbiota, due to the azotaemic milieu and changes in diet accompanied by increased intestinal permeability to endotoxin, results in a persistent activation of the innate immune system, with induction of regulatory mediators of the immune system which then suppress both innate and adaptive immunity [8]. Additionally, immune responses may also be impaired by poor nutritional status, malnutrition and vitamin D deficiency [9].

Table 13.3 Changes in innate and adaptive immunity in patients with chronic kidney disease stage 5
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Inflammation leads to protein energy wasting (PEW), combining central appetite suppression, increasing risk of depression, insulin resistance with increased muscle breakdown and reduced physical activity. Inflammation leads to an increased endothelial permeability and expansion of extracellular water, which in turn leads to macrophage recruitment and activation, increasing local inflammation and the production of reactive oxygen species, AGEs and advanced oxidised protein products (AOPPs) [10, 11]. This then leads to a vicious cycle which can be difficult to break in clinical practice.

Removal of Uremic Toxins by Hemodiafiltration (HDF)

HDF provides additional convective clearance compared to standard hemodialysis (HD). Small water soluble compounds such as uric acid are effectively removed by diffusion, so HDF, especially in predilution mode is less effective for urate clearance than HD. However, larger molecules such as asymmetric dimethylarginine (ADMA), with a molecular weight of just over 200 D, is more effectively cleared by postdilution HDF than HD [12]. Similarly both phosphate and β2 microglobulin clearance are increased by postdilution HDF compared to highflux HD [13]. Small peptide hormones such as leptin and FGF23 have increased clearance with on-line HDF [14, 15]. Both HDF and high flux HD have been reported to reduce circulating AGEs during a single treatment session, however postdilution HDF removes some 50 % more, and only HDF has been shown to produce a reduction in serum AGEs levels over time [16]. Similarly postdilution HDF clears more mitochondrial and cell free DNA fragments, during a treatment session, than highflux HD [17].

Studies measuring protein bound solute clearance have not demonstrated an advantage for postdilutional HDF over other dialytic modalities in removing p-cresyl or indoxyl sulfate [13, 18]. However, a recent report has suggested that predilution HDF infusing a combination of hypertonic sodium infusate, coupled with a hyponatremic dialysate increases protein bound solutes by altering protein binding, so increasing the free proportion and allowing greater clearances [19].

As such, HDF generally offers advantages over standard HD in terms of clearance of the small and middle sized water soluble azotaemic toxins. Although convective clearance would in theory be greater with predilution mode for middle sized molecules, high convective volumes also dilute the concentration gradient and reduce diffusional losses. However for most middle sized solutes, clearance is equal or greater with postdilutional mode, as the concentration entering the dialyzer is higher, and membrane adsorption is also increased. When used in conventional pre or postdilution mode, HDF does not offer any increased clearance of protein bound azotaemic toxins.

Does Hemodiafiltration Reduce the Inflammatory Effect of Hemodialysis

As blood passes out through the patient’s vascular access into the extracorporeal circuit (ECC), across the dialyzer, through the venous air detector chamber and then returns through the access, leukocytes, monocytes and platelets are activated. As the dialyzer has the greatest surface area of the extracorporeal circuit, this is the main site of activation. Complement proteins are also activated by dialyzers, with different dialyzer membrane compositions activating complement by different pathways; with polysulphone dialyzers causing classic complement pathway activation or lectin pathway activation and cellulosic dialyzers causing alternative pathway activation. Cellular activation leads to transcription of several proinflammatory cytokines, including TNF\( \alpha \), IL1β, IL-6, and IL-8, as well as chemokine receptors CXCR4 CCR7 CX3CR1, and other inflammatory mediators such as TWEAK, TRAIL and pentraxin 3. Monocyte and leukocyte activation also leads to surface blebbing and release of microparticles which trigger thrombin generation and clotting, and activation of the kinin-bradykin system. As bradykinin generation is pH dependent, then on-line priming with bicarbonate solutions increases pH and reduces bradykinin generation compared to priming with 0.9 % saline with haemodialysis [20]. HDF using ultrapure fluids has been reported to induce less monocyte and leukocyte activation and cytokine release compared to HD [21, 22].

HDF has been reported to reduce the frequency of hypotensive episodes during dialysis sessions compared to HD. Intermittent hypotensive episodes can potentially result in hypoperfusion and visceral ischemia. Although most interest has centred on reduction in cardiac blood supply and cardiac “stunning” during dialysis, other organs including the gastro-intestinal tract also suffer from ischaemia. Ischaemia, per se induces inflammatory changes. However intestinal ischaemia also leads to alteration in gut permeability, and so allows the potential for the passage of bacterial derived endotoxin into the portal circulation. As such, some of the reduction in inflammatory changes reported with on-line HDF, may be consequent on a reduction in the inflammatory response to dialysis, due to the combination of improved dialysis water quality, reduced production or increased clearance of cytokines and inflammatory mediators generated by the passage of blood through the extracorporeal circuit, and reduced gut ischemia and endotoxin translocation.

Dialysis Water Risk in Hemodiafiltration

As large volumes of dialysis water are infused directly into the patient during on-line HDF treatments, then water quality is of paramount importance, and should comply with both microbiological standards for endotoxin and bacterial contamination to ensure ultra-pure water grade (<0.1 colony forming bacterial/ml and <0.03 EU/ml) as well as chemical purity [23]. In part some of the reports of reduced inflammatory changes associated with HDF may simply reflect switching to ultrapure dialysis water.

Bacterial may form biofilm in the pipes supplying water to a dialysis unit, or contaminate bicarbonate or electrolyte mixtures. Although the current endotoxin filters will remove endotoxin and large bacterial DNA fragments [24], smaller fragments may pass through. Small fragments of bacterial DNA, up to 20 base pairs can potentially cross the current highflux dialyzers from the dialysate into the plasma water [25]. Bacterial DNA differs from human DNA in terms of methylation, and as such bacterial DNA fragments are detected and directly activate Toll like receptor 9 and provoke an inflammatory reaction.

Effects of Hemodiafiltration on Inflammation and Oxidative Stress

As renal function declines, the clearance of inflammatory mediators declines, and as such HDF, by adding convective clearance, may be expected to reduce the inflammatory milieu and oxidative stress of chronic kidney disease. Hence reports of HDF reducing circulating levels of IL-6 and TNF\( \alpha \), associated with a reduction in circulating proinflammatory monocytes (CD14+CD16+ positive cells) and C creative protein [26, 27]. Similarly HDF has been reported to reduce markers of oxidative stress, such as p22phox (the subunit of NAD(P)H oxidase), PAI-1, and oxidised plasma low density lipoproteins [28]. Others have demonstrated a reduction in reactive oxygen metabolites, and an increase in total anti-oxidant activity in both whole blood and lymphocytes [29, 30] and also increased heme-oxygenase-1, a protein involved in protection against the effects of oxidative damage and inflammation compared to patients treated by standard HD [31]. However longer term studies showed that changes in anti-oxidant activity were more modest, than those reported in short term studies, with if anything a reduction in the antioxidant capacity of lymphocytes, with reduced concentrations of superoxide dismutase [32].

Inflammation is linked to endothelial dysfunction, with release of endothelial microparticles. Reports have suggested that CKD patients treated by HDF have lower circulating endothelial microparticles [33]. Although inducible monocyte nitric oxide synthase activity was shown not to be altered by HDF [31], the response to endothelial nitric oxide appears to be improved with increased brachial artery flow-mediated vasodilatation and carotid artery distensibility with HDF [34].

Clinical Effects of Hemodiafiltration

If HDF leads to a reduction in microinflammation in CKD patients, then the question arises as to whether this translates into clinically relevant measures. Comparative studies using ultrapure dialysate water comparing haemodialysis with hemodiafiltration have shown a variable effect on serum albumin, with some studies reporting an increase with HDF [35], and more recent reports not showing any differences in serum albumin over time [36, 37]. This may be due to the potentially greater losses of albumin with higher convection volume exchanges used in the more recent studies [36, 37]. Earlier studies also reported an improvement in nutritional status with HDF, as assessed by body mass index and fat mass [35]. However more importantly two studies observed that treatment with HDF led to an increase in lean body mass, measured by bioimpedance and DEXA techniques [35, 38].

Although earlier studies reported that treatment with HDF increased the response to erythropoietins, and reduced erythropoietin resistance [39, 40], this was not supported by more recent studies [41]. However both the targets for haemoglobin, and biologically available iron, have changed over time and as such lower doses of erythropoiesis stimulating agents are now used in clinical practice, which may well explain why the initial reports showed a positive effect for HDF when much higher doses were used compared to the current day. See also Chap. 12.

Inflammation has been linked to a greater prevalence of low mood and depression. Studies which have investigated whether HDF improves quality of life have produced varied results, with one study reporting that quality of life scores improved with HDF [38], whereas another failed to show any significant benefit [42] (Table 13.4).

Table 13.4 Reported benefits of hemodiafiltration treatment on inflammation compared to haemodialysis
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Summary

Observational and randomised trials of HDF generally have reported that the introduction of HDF generally decreases the inflammatory milieu and increased oxidative stress of CKD. Results do vary from study to study, not only due to differences in patient recruitment, but also secondary to differences in dialysis water quality and HDF mode – predilution, mid-dilution and postdilution and mixed pre and postdilution, but more importantly the convective volume exchanged. In terms of translating these improvements in reducing microinflammation, then the earlier trials when higher haemoglobin targets and greater use of erythropoietins were required did suggest that HDF was associated with lower erthyropoietin requirements. In addition two prospective trials reported improved nutritional status with HDF with objective changes in body composition as demonstrated by bioimpedance and DEXA scanning. There have been few studies which investigated whether switching from HD to HDF improved patient quality of life, and the results have been somewhat contradictory [38, 42]. However the trials differed in terms of the mode of HDF and convective volumes delivered to be able to compare studies.

More recently randomised controlled trials have reported an overall survival benefit for HDF. Several studies have shown that the survival benefit was dependent upon the amount of convective clearance delivered [47, 48]. In addition as the survival benefit was predominantly for cardiovascular disease, then HDF by reducing microinflammation could potentially reduce vascular disease by modifying atheroma.

Teaching Points

  • CKD patients not yet on dialysis already show signs of (micro)inflammation and oxidative stress

  • Its causes are multifactorial, and result from reduced renal clearance, abnormal metabolic pathways and increased intestinal permeability for bacterial endotoxins

  • During HD, the (micro)inflammatory state is aggravated by cellular and humoral activation within the ECC

  • HDF may reduce (micro)inflammation by a reduction in IDH, and consequently, less intestinal hypoperfusion and a lower passage of bacterial endotoxins into the portal circulation

  • In addition, several cytokines and other inflammatory mediators in the MMW range are better removed by HDF than by (highflux) HD

  • Despite these findings, large recent RCTs failed to show clear differences in serum CRP and albumin levels between HD and HDF patients