Kidney Health Initiative ESKD Data Standards Project

The end-stage kidney disease (ESKD) Data Standards Project was launched by the Kidney Health Initiative (KHI) with the goal of standardizing dialysis-related measurements for research use. KHI is a public-private partnership between the American Society of Nephrology, US Food and Drug Administration, and organizations with an interest in kidney disease. KHI promotes safe and effective patient-centered therapies for people with kidney disease. In 2018, KHI established a workgroup with expertise in nephrology, nursing, quality management, ESKD data, organizational management, and clinical research. The workgroup identified 5 topic areas and 8 specific measures for the development of standards on the basis of the existing ESKD Measurement Specification Manual published by the Centers for Medicare & Medicaid Services. The topic areas were ultrafiltration rate, vascular access, dialysis small solute clearance (3 data standards), hospitalization (2 data standards), and mortality. The research standards were approved by the workgroup, reviewed by external reviewers, and opened to public comment. The data standards attempt to achieve balance between brevity and completeness in the face of knowledge gaps. The ESKD Data Standards are publicly available on the KHI website (https://khi.asn-online.org/projects/project.aspx?ID=78).

Mortality and Hospitalizations among Sickle Cell Disease Patients with End-Stage Kidney Disease Initiating Dialysis

BACKGROUND

Sickle cell disease (SCD) is the most common inherited hematological disorder and a well-described risk factor for end-stage kidney disease (ESKD). Mortality and hospitalizations among patients with SCD who develop ESKD remain understudied. Furthermore, prior studies focused only on SCD patients where ESKD was caused by SCD. We aimed to describe mortality and hospitalization risk in all SCD patients initiating dialysis and explore risk factors for mortality and hospitalization.

METHODS

We performed a national observational cohort study of African American ESKD patients initiating dialysis (2000-2014) in facilities affiliated with a large dialysis provider. SCD was identified by diagnosis codes and matched to a reference population (non-SCD) by age, sex, dialysis initiation year, and geographic region of care. Sensitivity analyses were conducted by restricting to patients where SCD was recorded as the cause of ESKD.

RESULTS

We identified 504 SCD patients (mean age: 47 ± 14 years; 48% females) and 1,425 reference patients (mean age: 46 ± 14 years; 49% females). The median follow-up was 2.4 (IQR 1.0-4.5) years. Compared to the reference, SCD was associated with higher mortality risk (hazard ratio 1.66; 95% confidence interval [CI]: 1.36-2.03) and higher hospitalization rates (incidence rate ratio 2.12; 95% CI: 1.88-2.38) in multivariable analyses. Exploratory multivariable mortality risk models showed the largest mortality risk attenuation with the addition of time-varying hemoglobin and high-dose erythropoietin, but the association of SCD with mortality remained significant. Sensitivity analyses (restricted to ESKD caused by SCD) also showed significant associations between SCD and mortality and hospitalizations, but with larger effect estimates. High-dose erythropoietin was associated with the highest risk for mortality and hospitalization in SCD.

CONCLUSIONS

Among ESKD patients, SCD is associated with a higher risk for mortality and hospitalization, particularly in patients where SCD is identified as the cause of ESKD.

Conversion from Intravenous Vitamin D Analogs to Oral Calcitriol in Patients Receiving Maintenance Hemodialysis

BACKGROUND AND OBJECTIVES

In the United States, intravenous vitamin D analogs are the first-line therapy for management of secondary hyperparathyroidism in hemodialysis patients. Outside the United States, oral calcitriol (1,25-dihydroxyvitamin D₃) is routinely used. We examined standard laboratory parameters of patients on in-center hemodialysis receiving intravenous vitamin D who switched to oral calcitriol.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

We conducted a retrospective cohort study of adult patients treated within Fresenius Kidney Care clinics. During a 6-month period (December 2013 to May 2014), we identified patients on an intravenous vitamin D analog (doxercalciferol or paricalcitol) who switched to oral calcitriol and matched them to patients receiving an intravenous vitamin D analog. Mean serum calcium, phosphate, and intact parathyroid hormone (iPTH) concentrations were examined for up to 12 months of follow-up. We used Poisson and Cox proportional hazards regression models to examine hospitalization and survival rates. The primary analysis was conducted as intention-to-treat; secondary analyses included an as-treated evaluation.

RESULTS

A total of 2280 patients who switched to oral calcitriol were matched to 2280 patients receiving intravenous vitamin D. Compared with patients on intravenous vitamin D, mean calcium and phosphate levels in the oral calcitriol group were lower after the change to oral calcitriol. In contrast, iPTH levels were higher in the oral calcitriol group. At 12 months, the percentage of patients with composite laboratories in target range (calcium <10 mg/dl, phosphate 3.0-5.5 mg/dl, and iPTH 150-600 pg/ml) were comparable between groups (45% versus 45%; P=0.96). Hospital admissions, length of hospital stay, and survival were comparable between groups. An as-treated analysis and excluding those receiving cinacalcet did not reveal significant between-group differences.

CONCLUSIONS

Among patients receiving in-center hemodialysis who were switched to oral calcitriol versus those on an intravenous vitamin D analog, the aggregate of all mineral and bone laboratory parameters in range was largely similar between groups.

The Effect of Hemoglobin Concentration on Peritoneal Mass Transfer and Drain Volumes in Continuous Ambulatory Peritoneal Dialysis

Objective

To determine whether a correlation exists between hemoglobin levels and peritoneal mass transfer or drain volumes in continuous ambulatory peritoneal dialysis (CAPD) patients.

Design

Prospective study of two groups of CAPD patients, identified on the basis of their stable hemoglobin levels. Group A -hemoglobin less than 8.5 g/dL; Group B hemoglobin greater than 10.5 g/dL. Peritoneal mass transfer and drain volumes were measured for each patient, after which a subgroup of Group A was treated with rHuEPO (forming Group C) and measurements repeated once hemoglobin had risen by at least 2.0 g/dL.

Setting

Single renal unit of a university teaching hospital. Patients:Twenty-seven patients established on CAPD, selected according to their stable hemoglobin level. Group A -14 patients; Group B -13 patients; Group C (subgroup of A) -8 patients.

Main outcome measures

Difference between peritoneal mass transfer or drain volume in Group A versus Group B, and in Group C before and after rHuEPO therapy. Serum biochemical parameters in Group C before and after rHuEPO therapy.

Results

No statistically significant differences in any of the parameters measured were found between groups A and B, or before and after rHuEPO therapy in Group C.

Conclusions

Peritoneal transfer of small solutes and water is not influenced by hemoglobin level, and does not change following otherwise effective treatment with rHuEPO.

One-Year Historical Cohort Study of the Phosphate Binder Sucroferric Oxyhydroxide in Patients on Maintenance Hemodialysis

Objective:

The high pill burden of many phosphate binders (PBs) may contribute to increased prevalence of hyperphosphatemia and poor nutritional status observed among patients undergoing maintenance hemodialysis therapy. We examined the real-world effectiveness of sucroferric oxyhydroxide (SO), a PB with low pill burden, in managing serum phosphorus in patients with prevalent hemodialysis over a 1-year period.

Design:

Historical cohort analyses of de-identified electronic medical records.

Subjects:

In-center hemodialysis patients switched from another PB to SO therapy as part of routine care with 12 months of uninterrupted SO prescriptions recorded, and documented serum phosphorus levels were eligible for inclusion. Clinical data were extracted from a pharmacy service, FreseniusRx, database and Fresenius Kidney Care clinical data warehouse.

Main outcome measures:

Comparisons were made between the 91-day period before SO initiation (i.e., baseline) and the 4 consecutive 91-day intervals of SO treatment (Q1-Q4). Clinical measures included achievement of target phosphorus levels (#5.5 mg/dL) and mean number of PB pills/day.

Results:

Among 530 analyzed patients, the proportion achieving target serum phosphorus levels increased by >100% 1 year after switching to SO therapy, that is, from 17.7% at baseline to 24.5%, 30.5%, 36.4%, and 36.0% at Q1 through Q4, respectively (P < .0001 for all). Reductions in serum phosphorus were observed at all follow-up timepoints (P <.0001), irrespective of baseline PB. From a mean baseline PB pill burden of 8.5 pills/day, patients experienced an average 50% pill burden reduction during SO treatment (P <. 0001). Phosphorus-attuned albumin and phosphorus-attuned protein intake (normalized protein catabolic rate) improved significantly after transition to SO (P < .0001). The effectiveness of SO was evident in prespecified subgroups of interest (i.e., black/African-American patients, Hispanic/Latino patients, and women).

Conclusion:

Among patients on hemodialysis, switching to SO resulted in a 2-fold greater likelihood of achieving target phosphorus levels while halving daily PB pill burden. Increases in phosphorus-attuned albumin and protein intake suggest improved nutritional status.

Real-World Scenario Improvements in Serum Phosphorus Levels and Pill Burden in Peritoneal Dialysis Patients Treated with Sucroferric Oxyhydroxide

BACKGROUND

A database analysis was conducted to assess the effectiveness of sucroferric oxyhydroxide (SO) on lowering serum phosphorus and phosphate binder (PB) pill burden among adult peritoneal dialysis (PD) patients prescribed SO as part of routine care.

METHODS

Adult PD patients (n = 258) prescribed SO through a renal pharmacy service were analyzed. Baseline was 3 months before SO prescription. SO-treated follow-up was for 6 months or until either a new PB was prescribed, SO was not refilled, PD modality changed, or patient was discharged. In-range serum phosphorus was defined as ≤5.5 mg/dL.

RESULTS

At baseline, mean serum phosphorus was 6.59 mg/dL with 10 prescribed PB pills/day. The proportion of patients achieving in-range serum phosphorus increased by 72% from baseline to month 6. Prescribed PB pills/day decreased by 57% (10 at baseline to 4.3 at SO follow-up, p < 0.0001). The mean length of SO follow-up was 5.1 months; SO follow-up ended for 38, 27, and 50 patients at months 4, 5, and 6, respectively, due to no further PB fills, and for 10, 11, and 4 patients at months 4, 5, and 6, respectively, due to another PB prescribed. In patients with baseline serum phosphorus >5.5 mg/dL who achieved in-range serum phosphorus during SO follow-up for ≥1 quarter, a notable improvement in serum phosphorus (6.54 to 5.10 mg/dL, p < 0.0001) was observed, and there was a 53% reduction in PB pill burden (9.9 to 4.7, p < 0.0001).

CONCLUSION

Among PD patients prescribed SO as part of routine care, improvements in serum phosphorus control and >50% reduction in PB pills/day were observed.

Anemia management in patients receiving chronic hemodialysis

Anemia treatment in hemodialysis-dependent (HDD) CKD patients involves adequate supply of iron and an erythropoiesis-stimulating agent (ESA). Despite widespread usage of these agents, there is no generally accepted "standard dosing algorithm" for treating anemia in HDD-CKD patients. The new anemia Quality Incentive Program (QIP) introduced by the Centers for Medicare & Medicaid Services represents a motivation to standardize and harmonize iron and ESA regimens with interactive electronic algorithms and novel modes of deliveries for IV iron and ESA doses. In addition, quality assessment and performance improvement programs at dialysis facilities include achieving measurable improvement in health outcomes, healthcare cost, and reductions in medical errors. Thus, the Corporate Medical Advisory Board for Fresenius Medical Services (FMS) is evaluating an anemia algorithm that will be incorporated into the automated workflow of a new clinical system at FMS clinics. In the future, such systems might communicate with medication pumps incorporated into state-of-the-art HD machines, thereby eliminating manual data entry of medication orders and other potential errors related to data entry or administration of medications such as ESA and IV iron. In addition, the CritLine III TQA Monitor, which allows real-time blood volume, oxygen, and anemia monitoring during HD in acute and chronic settings, may become an integrated diagnostic tool to improve volume and anemia management through better fluid management and ESA dose adjustment algorithms. These novel interactive electronic algorithms, delivery and monitoring methods, and data transfer may be integrated in the Pharmatech process to meet patient-specific anemia therapy.

International Quotidian Dialysis Registry: annual report 2009

The International Quotidian Dialysis Registry (IQDR) is a global initiative designed to study practices and outcomes associated with the use of hemodialysis regimens of increased frequency and/or duration. Several small studies suggest that compared with conventional hemodialysis (HD), short-daily, nocturnal, and long conventional HD regimens may improve surrogate endpoints and quality of life. However, methodologically robust comparisons on hard outcomes are sorely lacking. The IQDR represents the first-ever attempt to aggregate long-term follow-up data from centers utilizing alternative HD regimens worldwide, and will have adequate statistical power to examine the effects of these regimens on multiple clinical endpoints, including mortality. To date, the IQDR has enrolled patients from Canada, the United States, Australia, and New Zealand, with plans in place to begin linking with additional commercial databases and national registries. This fifth annual report of the IQDR describes (1) a proposed governance structure that will facilitate international collaboration, stakeholder input and funding; (2) data sources and participating registries; (3) recruitment to date and patient baseline characteristics; and (4) an agenda for future research.

The International Quotidian Dialysis Registry: Annual report 2008

Alternative hemodialysis (HD) schedules, including short-daily and nocturnal HD, continue to proliferate, with the hope of offering improved patient outcomes. Three nights per week and every other night, nocturnal HD are now being provided to more patients worldwide, both at home and in-center. However, alternative HD schedules are still experimental in most centers, and studies establishing the efficacy of these therapies with respect to major clinical outcomes are needed. Endorsed by the National Institutes of Health, the International Quotidian Dialysis Registry is an international collaboration that was established in 2002 to prospectively study large numbers of patients treated with alternate HD schedules. The Registry will ultimately allow alternate HD modalities to be compared to conventional thrice-weekly HD with respect to clinical endpoints, including mortality, using a prospective cohort study. To date, the Registry has enrolled 182, 1193, and 740 subjects from Canada, the United States, and Australia, respectively. This report is the fourth annual update and describes recruitment progress, baseline characteristics of enrolled patients, and worldwide prescription patterns.

Impact of the Change in CMS Billing Rules for Erythropoietin on Hemoglobin Outcomes in Dialysis Patients

On April 1, 2006, new Centers for Medicare and Medicaid Services (CMS) rules for billing erythropoietin (EPO) for dialysis patients went into effect. Two key provisions of the rules were to cap the dose for a single patient at 500,000 IU/month and to mandate a 25% reduction in dose for patients whose latest hemoglobin (HGB) or hematocrit (HCT) in the prior month exceeded 13 g/dl or 39%, respectively. The purpose of this article is to document the effect of the rules change on HGB outcomes in a single large dialysis provider whose computer system was modified to enforce the rules. HGB and EPO doses for 5 months following the implementation were analyzed retrospectively. The most noteworthy observation is that while the rule appears to have reduced the percentage of patients with an HGB of >13 g/dl slightly, it has also increased the percentage of patients with HGB in the medically undesirable range of <11 g/dl.

The Epidemiology of Systolic Blood Pressure and Death Risk in Hemodialysis Patients

BACKGROUND

This study compares the associations of predialysis systolic blood pressure (SBP) with mortality risk in both incident and prevalent hemodialysis (HD) cohorts by using both conventional and time-varying Cox analyses, thus addressing limitations of prior studies.

METHODS

A total of 56,338 incident patients starting HD therapy during 1997 to 2001 and 69,590 prevalent HD patients on January 1, 2002, were grouped into the following categories: (1) SBP less than 120 mm Hg, (2) 120 < or = SBP < 140 mm Hg, (3) 140 < or = SBP < 160 mm Hg, (4) 160 < or = SBP < 180 mm Hg, (5) 180 < or = SBP < 200 mm Hg, and (6) SBP of 200 mm Hg or greater. Conventional and time-varying models evaluated 1-year and 3-year (incident patients only) survival.

RESULTS

Nine percent and 26.0% of incident patients and 5.7% and 20.1% of prevalent patients were in categories 1 and 2, respectively. Their associated 1-year hazard ratios (HRs) were 2.63 to 3.68 and 1.57 to 1.68 compared with category 4, the reference group. HRs for categories 3, 5, and 6 were not different from category 4. Time-varying models magnified category 1 and 2 HRs to 5.54 to 7.42 and 1.92 to 2.21, such that 25% to 35% of patients in the target SBP range (< 140 mm Hg) had the greatest risk. A "reversed J-shaped" risk profile emerged in the time-varying models, with very high SBP (category 6) associated with HRs of 1.52 to 1.55, but only 1% of patients were in category 6. Three-year outcomes were similar.

CONCLUSION

Epidemiological characteristics of predialysis SBP consistently differ from those in the general population despite different analytic perspectives. The data suggest a need for greater investigative, diagnostic, and therapeutic focus on HD patients with normal and prehypertensive blood pressure ranges.

Evaluating a new method to judge dialysis treatment using online measurements of ionic clearance

New technology now supports direct online measurements of total dialysis dose per treatment, Kt. An outcome-based, nonlinear method for estimating target Kt in terms of ionic clearance measurements and body surface area (BSA) has been described recently. This is a validation study of the new method that evaluates the relationship between the (actual Kt-target Kt) difference and death risk. Patients with Kt measurements during March 2004 were identified (N=59,644). Target Kt was determined for each patient using the new method. Patients were then grouped by (actual Kt-target Kt) decile. They were also grouped by (actual URR-target URR) decile. Cox analysis-based risk profiles were constructed using those groupings. The (actual Kt-target Kt) difference profiles suggested improving death risk as Kt increased from below target to equal target. Risk ratios then flattened and remained so until (actual Kt-target Kt) reached the highest decile at which it appeared to improve, suggesting a possible biphasic profile. The (URR-target URR) risk profile was U-shaped. Death risk was related to the difference between the actual Kt and a target Kt value selected using the new nonlinear method. The method is therefore valid for prescribing and monitoring hemodialysis treatment.

Dedicated outpatient vascular access center decreases hospitalization and missed outpatient dialysis treatments

Dedicated outpatient vascular access centers (VAC) specializing in percutaneous interventions (angiography, thrombectomy, angioplasty and catheter placement) provide outpatient therapy that can obviate the need for hospitalization. This paper reports the impact of one VAC staffed by interventional nephrologists on vascular access-related hospitalization and missed outpatient dialysis treatments. We performed a retrospective analysis of vascular access-related hospitalized days and missed vascular access-related outpatient dialysis treatments from 1995 to 2002 in 21 Phoenix Arizona Facilities (5928 cumulative patients) and 1275 cumulative Fresenius Medical Care North America (FMCNA) facilities (289,454 cumulative patients) to evaluate the impact of the introduction of a VAC in Phoenix. Vascular access-related hospitalized days/patient year and missed dialysis treatments/patient year declined from 1997 to 2002 across all access types. The decline was greater in Phoenix and coincided with the creation of a VAC in 1998. By 2002, there were 0.57 fewer hospitalized days/patient year and 0.29 fewer missed treatments/patient year than in the national sample (P<0.01). In 2002, the relative risk for vascular access hospitalized days was 0.38 (95% confidence interval (CI) 0.27-0.5) (P<0.01) and for vascular access-related missed outpatient dialysis treatments was 0.34 (95% CI 0.24-0.49) (P<0.01) in Phoenix vs FMCNA after adjustment for age, gender, diabetic status duration of dialysis and access type. VAC development was associated with a significant decrease in vascular access-related hospitalization and missed outpatient dialysis treatments. Further studies are necessary to demonstrate this effect in other communities.

Epoetin alfa use in patients with ESRD: an analysis of recent US prescribing patterns and hemoglobin outcomes

BACKGROUND

It is unknown to what degree physicians adjust erythropoietin doses to achieve hemoglobin levels (11.0 to 12.0 g/dL [110 to 120 g/L]) recommended by the National Kidney Foundation-Kidney Disease Outcomes Quality Initiative (NKF-K/DOQI) for patients with end-stage renal disease receiving hemodialysis. Our objective is to examine epoetin alfa prescribing patterns for achieving the target hemoglobin level range in this population.

METHODS

Monthly hemoglobin levels and epoetin alfa doses from 2 large databases were retrospectively analyzed. One data set comprised 31,267 patients from the Fresenius Medical Care-North America (FMC-NA) database, and the other comprised 128,761 patients based on claims for Medicare services.

RESULTS

Longitudinal evaluation of the FMC-NA data set showed that hemoglobin levels in patients administered epoetin alfa cycled in and out of the NKF-K/DOQI hemoglobin target range, and doses were decreased in 98.8% of patients with persistent hemoglobin levels greater than 12.0 g/dL (> 120 g/L). Hemoglobin levels in patients from the Medicare data set that initially were outside the target range migrated into the range with epoetin alfa dose titration. FMC-NA patients with a 3-month average hemoglobin level less than 11.0 g/dL (< 110 g/L) were administered significantly greater epoetin alfa doses than those with average hemoglobin levels greater than 12.0 g/dL (> 120 g/L; 21,838 versus 13,503 U/wk; P < 0.0001). Less than 0.4% of patients administered epoetin alfa were persistently anemic (hemoglobin < 11.0 g/dL [< 110 g/L]) and were administered persistently high doses (> 30,000 U/wk), but failed to respond with a 0.5-g/dL or greater (> or = 5-g/L) increase in hemoglobin levels.

CONCLUSION

In these analyses, few hemodialysis patients experienced persistent anemia while being administered high epoetin alfa doses. Physicians appeared to appropriately adjust doses to achieve hemoglobin levels recommended by the NKF-K/DOQI guidelines.

Potential of dual-skinned, high-flux membranes to reduce backtransport in hemodialysis

BACKGROUND

Potential backfiltration of cytokine-inducing material is a clinical concern during hemodialysis conducted with high-flux membranes. Novel hollow-fiber membranes were developed that had asymmetric convective solute transport properties, aimed at reducing the passage of potentially harmful molecules from dialysate to blood, while maintaining the desired fluid and solute movement from blood to dialysate.

METHODS

Sieving coefficient as a function of molecular weight was measured in vitro using polydisperse dextrans. Measurements were conducted using two different flat-sheet membranes in series or using hollow fiber membranes having two integrally formed skin layers. Based on measured experimental parameters, model calculations simulated the performance of a clinical-scale dialyzer containing these new membranes versus that of a commercially available high-flux dialyzer.

RESULTS

Asymmetric convective solute transport was demonstrated using both commercial flat-sheet and newly developed hollow-fiber membranes. For two flat-sheet membranes in series, the extent of asymmetric transport was dependent on the order in which the solution was filtered through the membranes. For the hollow-fiber membranes, the nominal molecular weight cut-off was 20 kD in the blood-to-dialysate direction and 13 kD in the dialysate-to-blood direction. For this membrane, model calculations predict that clearance of a beta2-microglobulin-sized molecule (11,800 D) would be significantly greater from blood to dialysate than in the reverse direction, even under conditions of zero net ultrafiltration.

CONCLUSION

A novel hollow-fiber dialysis membrane was developed that allows greater convective solute transport from blood to dialysate than from dialysate to blood.

The effect of blood contact and reuse on the transport properties of high-flux dialysis membranes

The effect of blood contact and reprocessing using bleach on the convective transport of both neutral and positively charged dextrans was determined for cellulose triacetate (CT), polyacrylonitrile (PAN), and polysulfone (PS) dialyzers (Fresenius USA, F60B, Concord, CA). For neutral dextrans, blood contact reduced the convective permeability, determined by differences in the sieving coefficient profile for both the PAN and PS, but not for CT dialyzers. Reprocessing of the dialyzers with bleach (up to 15 reuses) did not affect the convective transport of dextrans through CT or PAN, but did enhance the permeability of the blood contacted PS dialyzers. However, sieving coefficients for the blood contacted and reprocessed PS (F60B) dialyzers were significantly lower than those for the other dialyzers studied, approaching zero for dextrans larger than 18 k molecular weight. Sieving coefficients for positively charged, diethylaminoethyl (DEAE) dextrans were a function not only of solute size, but also of the membrane's capacity for adsorption of charged molecules. The majority of smaller, filtered DEAE dextrans adsorbed to the PAN membrane. Adsorption of DEAE dextrans to PAN was not observed for larger dextrans, or for DEAE dextrans of any size with CT, despite the lower permeability of both membranes for DEAE dextran compared to that for neutral dextrans.

Ultrafiltration and backfiltration during hemodialysis

Ultrafiltration is the pressure-driven process by which hemodialysis removes excess fluid from renal failure patients. Despite substantial improvements in hemodialysis technology, three significant problems related to ultrafiltration remain: ultrafiltration volume control, ultrafiltration rate control, and backfiltration. Ultrafiltration volume control is complicated by the effects of plasma protein adsorption, hematocrit, and coagulation parameters on membrane performance. Furthermore, previously developed equations relating the ultrafiltration rate and the transmembrane pressure are not applicable to high-flux dialyzers, high blood flow rates, and erythropoietin therapy. Regulation of the ultrafiltration rate to avoid hypotension, cramps and other intradialytic complications is complicated by inaccurate estimates of dry weight and patient-to-patient differences in vascular refilling rates. Continuous monitoring of circulating blood volume during hemodialysis may enable a better understanding of the role of blood volume in triggering intradialytic symptoms and allow determination of optimal ultrafiltration rate profiles for hemodialysis. Backfiltration can occur as a direct result of ultrafiltration control and results in transport of bacterial products from dialysate to blood. By examining these problems from an engineering perspective, the authors hope to clarify what can and cannot be prevented by understanding and manipulating the fluid dynamics of ultrafiltration.

Pathways for fluid loss from the peritoneal cavity

During peritoneal dialysis, fluid is transported out of the peritoneal cavity by lymphatic and nonlymphatic pathways, thereby decreasing net ultrafiltration by 40-50% and reducing small solute clearance by 15-20%. The direct lymphatic pathway consists of the diaphragmatic lymphatics, which directly connect the peritoneal cavity to the bloodstream. The interstitial lymphatic and direct blood entry pathways convey fluid that has been driven into the interstitial space of the tissue surrounding the peritoneal cavity by the increased intraperitoneal pressure, and return it to the bloodstream. Since flow through lymphatic pathways is only a portion of the flow through all pathways, total fluid loss is greater than lymph flow. The best technique for estimating lymph flow is direct measurement by cannulation of lymphatic vessels, a technique that is not clinically feasible. The tracer disappearance technique, which measures the rate at which macromolecules leave the peritoneal cavity, is an indirect measure of fluid loss. The tracer appearance technique, which measures the rate at which macromolecules reach the blood from the peritoneal cavity, slightly overestimates lymph flow because some tracer may enter the bloodstream directly from the tissues. Much of the previous controversy over the contribution of the lymphatic pathways to total fluid loss can be resolved by understanding the differences in what these techniques measure.

Effect of high hematocrit and high blood flow rates on transmembrane pressure and ultrafiltration rate in hemodialysis

Removal of prescribed ultrafiltration volumes in hemodialysis requires knowledge of both the ultrafiltration coefficient of the dialyzer and the average transmembrane pressure (TMP) in the dialyzer. While it has been a fairly common practice to assume that the TMP is constant along the length of the dialyzer, it actually decreases linearly from a maximum value at the blood inlet to a minimum value at the blood outlet. In the past, ignoring the difference between arterial and venous TMPs when calculating the dialysate pressure setting did not result in significant errors in ultrafiltration volume. However, with the introduction of erythropoietin therapy and the trend toward high-efficiency dialysis, increases in hematocrit and blood flow rate have led to axial variations in TMP which, if ignored, can lead to inaccurate fluid removal. The goals of this paper are to provide an understanding of how high hematocrits and high blood flow rates affect TMP and ultrafiltration rate, and to provide simple guidelines for ensuring accurate fluid removal. Sample calculations are given on the last page for easy reference.