An index of plasma refilling in hemodialysis patients

The Changes in Relative Blood Volume during Dialysis Are Characterized by Ultrafiltration Rate and Predialysis Blood Test Values

INTRODUCTION

Continuous monitoring of relative blood volume (percentage BV) in hemodialysis (HD) is critical for determining dry weight and preventing intradialytic hypotension. However, the cause of the BV variation remains unknown. This research aimed to examine factors that influence the percentage BV.

METHODS

We devised a formula based on coefficients ("a," "τ," and "b") to predict changes in percentage BV. "a" denotes a significant decrease in percentage BV in the early stages of HD. "τ" represents the transition from early to late phase of HD. "b" denotes the slope of the decrease in percentage BV in the late phase of HD. We measured the percentage BV in 18 patients with end-stage renal disease. The coefficients were estimated by fitting experimental data from patients using a least squares optimization algorithm. A correlation analysis of these parameters and patient predialysis data was performed.

RESULTS

Ultrafiltration rate (UFR) was found to be negatively correlated with "b" (r = -0.851, p < 0.01). However, UFR was not significantly related to "a." Predialysis serum total protein level was negatively correlated with "a" (r = -0.531, p = 0.042). Predialysis serum albumin and predialysis sodium were not significantly correlated with "a" and "τ." Plasma osmolarity did not have a significant relationship with "a" and "τ."

DISCUSSION/CONCLUSION

UFR influenced the decrease in percentage BV in the late phase but did not influence the decrease of percentage BV in the early phase. "a" was associated with predialysis serum total protein level but not with plasma osmolality or predialysis sodium. This implies that colloid oncotic pressure is important for plasma refilling immediately after dialysis begins. During the change of percentage BV, the decrease in the early phase of dialysis was not related to UFR, but related to other parameters, especially predialysis total protein level. A decrease in the late phase of dialysis is related to UFR.

Advances in the Physiology of Transvascular Exchange and A New Look At Rational Fluid Prescription

The Starling principle is a model that explains the transvascular distribution of fluids essentially governed by hydrostatic and oncotic forces, which dynamically allow vascular refilling according to the characteristics of the blood vessel. However, careful analysis of fluid physiology has shown that the principle, while correct, is not complete. The revised Starling principle (Michel-Weinbaum model) provides relevant information on fluid kinetics. Special emphasis has been placed on the endothelial glycocalyx, whose subendothelial area allows a restricted oncotic pressure that limits the reabsorption of fluid from the interstitial space, so that transvascular refilling occurs mainly from the lymphatic vessels. The close correlation between pathological states of the endothelium (eg: sepsis, acute inflammation, or chronic kidney disease) and the prescription of fluids forces the physician to understand the dynamics of fluids in the organism; this will allow rational fluid prescriptions. A theory that integrates the physiology of exchange and transvascular refilling is the "microconstant model", whose variables include dynamic mechanisms that can explain edematous states, management of acute resuscitation, and type of fluids for common clinical conditions. The clinical-physiological integration of the concepts will be the hinges that allow a rational and dynamic prescription of fluids.

Vascular refilling coefficient is not a good marker of whole-body capillary hydraulic conductivity in hemodialysis patients: insights from a simulation study

Refilling of the vascular space through absorption of interstitial fluid by micro vessels is a crucial mechanism for maintaining hemodynamic stability during hemodialysis (HD) and allowing excess fluid to be removed from body tissues. The rate of vascular refilling depends on the imbalance between the Starling forces acting across the capillary walls as well as on their hydraulic conductivity and total surface area. Various approaches have been proposed to assess the vascular refilling process during HD, including the so-called refilling coefficient (Kr) that describes the rate of vascular refilling per changes in plasma oncotic pressure, assuming that other Starling forces and the flow of lymph remain constant during HD. Several studies have shown that Kr decreases exponentially during HD, which was attributed to a dialysis-induced decrease in the whole-body capillary hydraulic conductivity (L_pS). Here, we employ a lumped-parameter mathematical model of the cardiovascular system and water and solute transport between the main body fluid compartments to assess the impact of all Starling forces and the flow of lymph on vascular refilling during HD in order to explain the reasons behind the observed intradialytic decrease in Kr. We simulated several HD sessions in a virtual patient with different blood priming procedures, ultrafiltration rates, session durations, and constant or variable levels of L_pS. We show that the intradialytic decrease in Kr is not associated with a possible reduction of L_pS but results from the inherent assumption that plasma oncotic pressure is the only variable Starling force during HD, whereas in fact other Starling forces, in particular the oncotic pressure of the interstitial fluid, have an important impact on the transcapillary fluid exchange during HD. We conclude that Kr is not a good marker of L_pS and should not be used to guide fluid removal during HD or to assess the fluid status of dialysis patients.

Does the plasma refilling coefficient change during hemodialysis sessions?

The filtration coefficient in the Starling equation is an important determinant of plasma refilling during hemodialysis. A method for calculating from clinical data an estimate of the filtration coefficient, called the refilling coefficient, was proposed in the past. The assumption behind this method was that the only drive for refilling is the increase in plasma oncotic pressure, and the remaining Starling forces have negligible effect. The refilling coefficient was observed to decrease during hemodialysis, and this was interpreted as a change in the filtration coefficient. The purpose of our study was providing an alternative explanation for the behavior of the refilling coefficient and, using clinical data and mathematical modeling, to predict the values of the immeasurable Starling forces and provide the theoretical basis for the interpretation of the refilling coefficient as the filtration coefficient. Blood volume and bioimpedance data from 23 patients undergoing hemodialysis were used to calculate the refilling coefficient according to the original formulation and to fit a two-compartment model of protein and fluid transport. The changes in the other Starling forces were non-negligible, ranging from 19% to 60% of plasma oncotic pressure. The results showed that the decrease observed in the refilling coefficient is likely caused by neglecting important changes in the Starling forces while deriving the equation for the refilling coefficient. When these Starling forces were taken into account, constant filtration coefficient and dynamic refilling coefficient provided an equivalent description of the data in most cases. However, this was not true for a subgroup of sessions, which suggests that additional factors may also be responsible for the observed decrease in the refilling coefficient.

Hemodialysis Ultrafiltration Rate Targets Should Be Scaled to Body Surface Area Rather than to Body Weight

The association between higher ultrafiltration rates and poor outcomes in hemodialysis patients has received increased attention, to the point that various regulatory entities are considering adding ultrafiltration rate as a quality measure to be monitored and controlled. Most of the discussion to date has focused on ultrafiltration rate scaled to body weight, or more correctly, body mass (ml/hour per kg). One outcome study suggests that ultrafiltration rate might best be not scaled at all to body size, as modestly higher ultrafiltration rate in very small-size patients may be associated with some survival benefit, probably via increased dietary intake. Outcomes studies also suggest that the risk of exceeding a weight-scaled ultrafiltration target may be magnified in very large patients, and that body weight-scaled ultrafiltration targets in such patients should be set a lower level. Here, we present an analysis, based on physiological hemodynamic arguments, that it would be better to scale ultrafiltration rate to body surface area rather than to body mass. Whatever ultrafiltration rate is scaled to, attempts to restrict ultrafiltration rate by limiting interdialytic weight gain in small, possibly malnourished patients, should be done cautiously, to prevent an inadvertent lowering of intake of calories and dietary protein.

Modelling Transcapillary Transport of Fluid and Proteins in Hemodialysis Patients

Background

The kinetics of protein transport to and from the vascular compartment play a major role in the determination of fluid balance and plasma refilling during hemodialysis (HD) sessions. In this study we propose a whole-body mathematical model describing water and protein shifts across the capillary membrane during HD and compare its output to clinical data while evaluating the impact of choosing specific values for selected parameters.

Methods

The model follows a two-compartment structure (vascular and interstitial space) and is based on balance equations of protein mass and water volume in each compartment. The capillary membrane was described according to the three-pore theory. Two transport parameters, the fractional contribution of large pores (α_LP) and the total hydraulic conductivity (LpS) of the capillary membrane, were estimated from patient data. Changes in the intensity and direction of individual fluid and solute flows through each part of the transport system were analyzed in relation to the choice of different values of small pores radius and fractional conductivity, lymphatic sensitivity to hydraulic pressure, and steady-state interstitial-to-plasma protein concentration ratio.

Results

The estimated values of LpS and α_LP were respectively 10.0 ± 8.4 mL/min/mmHg (mean ± standard deviation) and 0.062 ± 0.041. The model was able to predict with good accuracy the profiles of plasma volume and serum total protein concentration in most of the patients (average root-mean-square deviation < 2% of the measured value).

Conclusions

The applied model provides a mechanistic interpretation of fluid transport processes induced by ultrafiltration during HD, using a minimum of tuned parameters and assumptions. The simulated values of individual flows through each kind of pore and lymphatic absorption rate yielded by the model may suggest answers to unsolved questions on the relative impact of these not-measurable quantities on total vascular refilling and fluid balance.

A physiologically based model of vascular refilling during ultrafiltration in hemodialysis

An assessment of fluid status can be obtained by monitoring relative blood volume (RBV) during hemodialysis (HD) treatment. The dynamics of RBV is determined by fluid removal from the intravascular compartment by ultrafiltration (UF) and vascular refill from the interstitium. To characterize this dynamics, a two-compartment model describing the short-term dynamics of vascular refilling and UF is developed. Fluid movement between the compartments is governed by lymphatic and microvascular fluid shifts. Further, protein flux is described by convection, diffusion and the lymphatic protein flux. Patient specific parameters are identified based on hematocrit (Hct) measurements by the Crit-Line monitor (CLM). Different measurement frequencies and UF profiles are compared to determine data fidelity and influence on the quality of parameter estimates. This relevant information can be used to assess the (patho)physiological status of hemodialysis patients and could aid in individualizing therapy.

Effect of estimated plasma volume reduction on renal function for acute heart failure differs between patients with preserved and reduced ejection fraction

BACKGROUND

The prognostic relevance of plasma volume reduction (PVR) in acute heart failure patients remains unclear because of the confounding hemodynamic effect of left ventricular ejection fraction impairment on kidney function.

METHODS AND RESULTS

Subjects enrolled in the West Tokyo Heart Failure Registry were examined. The PV at admission and discharge was estimated from the subjects' body weight and its deviation from the ideal body weight. Patients in the top tertile of estimated PVR were classified as PVR+. Of the 381 patients with acute heart failure, 181 (47.5%) had heart failure with preserved ejection fraction (HFpEF). Estimated PVR was associated with worsening renal function in the HFpEF (odds ratio, 3.28; 95% confidence interval, 1.55-6.96; P=0.002) but not in the heart failure with reduced ejection fraction cohort (odds ratio, 1.22; 95% confidence interval, 0.61-2.42; P=0.57). This association in the HFpEF cohort remained significant after adjusting for a history of hypertension and diabetes mellitus and the estimated glomerular filtration rate (odds ratio, 3.34; 95% confidence interval, 1.52-7.33; P=0.003). The use of intravenous diuretics was a significant predictor of PVR in the HFpEF and heart failure with reduced ejection fraction groups.

CONCLUSIONS

The effect of estimated PVR differs by HF type, and the estimated PVR during hospitalization is a predictor of worsening renal function in patients with HFpEF but not in heart failure with reduced ejection fraction.

CLINICAL TRIAL REGISTRATION

URL: http://www.umin.ac.jp/ctr/index-j.html. Unique identifier: UMIN000001549.

Effect of intradialytic change in plasma volume on blood pressure in patients undergoing maintenance hemodialysis

BACKGROUND

Hypervolemia is a common complication in patients on hemodialysis (HD). To determine the effect of volume change on blood pressure in HD population, this cohort was conducted.

MATERIALS AND METHODS

The study population was composed of 60 non-diabetic patients on maintenance HD, with mean age of 59.95±15.28 years. They were divided into hypertensive group A (n=26) and normotensive group B (n=34). Data were collected by a questionnaire. Pre and post-dialysis blood levels of urea, sodium, total protein, and hemoglobin were measured and intradialytic change of plasma volume were calculated. Data analyses were performed by the SPSS v.16.

RESULTS

Out of 60 patients, 58.3% were male and 41.7% female. Post-dialysis systolic blood pressure (SBP) and diastolic blood pressure (DBP) were significantly lower than pre-dialysis values in both groups (P=0.001, each). No correlation was found between intradialytic change in plasma volume or body weight and alterations of SBP or DBP during HD in the study groups (P>0.05, each). Intradialytic changes of body weight did not correlate to intradialytic changes of plasma volume (P=0.15).

CONCLUSION

HD effectively reduces blood pressure and volume expansion, however, intradialytic changes of plasma volume and body weight do not influence on SBP and DBP.

Changes in hemoglobin and albumin concentration during nocturnal home hemodialysis

The hemoglobin (Hb) and the serum albumin (S.Alb) concentration commonly rise during seated, conventional thrice-weekly 4 to 4.5 hr hemodialysis (CHD) as a result of rapid fluid removal from the intravascular compartment. Conversely, in long, slow, recumbent nocturnal home hemodialysis (NHHD), the intra-dialytic S.Alb concentration has been shown to fall. In normal human physiology, plasma volume expansion rapidly follows recumbency and is sustained until a resumption of an upright position re-induces plasma volume contraction. The plasma protein dilution of recumbency has been suggested as the mechanism behind this finding in NHHD. Our retrospective analysis of 585 consecutive measurements of predialysis and postdialysis S.Alb and Hb taken from 71 NHHD patients confirmed an intra-dialytic fall in S.Alb (0.99% in alternate night NHHD and 1.4% in 6 nights/week NHHD) compared with an 8.4% rise in a control group of 104 CHD patients (p<0.001). Although the NHHD intra-dialytic Hb rose (3.8% in alternate night NHHD and 2.6% in 6 nights/week NHHD), this rise was significantly greater (8%) in CHD patients (p<0.001), and as physiological data confirm that recumbent dilution for albumin is greater than that for Hb, this may provide the explanation. We conclude that NHHD provides a more physiological volume milieu with the normal physiological dilution mechanisms of recumbency still operating despite the slow, steady volume reduction that accompanied longer hour and more frequent dialysis. These mechanisms are subverted, however, in CHD by the more-aggressive plasma contraction needed to attain adequate control of the intravascular volume in the face of shorter hour, less-frequent dialysis.

Hypertension control with daily dialysis

Hypertension is present in 60-90% of patients on maintenance hemodialysis (HD) and it is an important cause of cardiovascular (CV) mortality and morbidity. Frequent and prolonged HD has been uniformly shown to control hypertension in end-stage renal disease (ESRD) patients more effectively than conventional HD. The etiology of hypertension is predominantly volume dependent, but in a subset of patients increased renin, sympathetic overactivity, and endothelial dysfunction may play a role. Intradialytic hypotension precludes attainment of dry weight and hence optimal control of hypertension in conventional HD is challenging. Frequent and prolonged dialysis with gentle and persistent ultrafiltration allows time for refilling of the intravascular compartment and permits normalization of extracellular volume. It is also possible that intensive dialysis enables removal of pressor molecules and improves endothelial function. Improved blood pressure control translates into regression of left ventricular hypertrophy in patients on daily HD. Thus prolonged and frequent dialysis permits better control of hypertension via volume and volume-independent mechanisms and also improves cardiac geometry.

The change in renal replacement therapy on acute renal failure in a general intensive care unit in a university hospital and its clinical efficacy: a Japanese experience

The aim of our study was to examine renal replacement therapies (RRT) that have been used for acute renal failure (ARF) in our intensive care unit (ICU) patients and to compare their outcomes. Sixteen patients who underwent intermittent hemodialysis (IHD), 14 patients who underwent continuous hemofiltration (CHF) in combination with IHD (CHF + IHD), and 38 patients who underwent continuous hemodiafiltration (CHDF) were evaluated. Regarding the effects of blood purification on hemodynamics and renal function, the percentage increase in blood pressure and percent rapid increase in urinary output were the greatest in the CHDF group. The hourly urinary output after the start of initial blood purification increased only in the CHDF group. The survival rate was significantly higher in the CHDF group. These results suggest that CHDF should be the first-line therapy for patients with ARF and that we are moving in the right direction regarding the application of RRT to treat ARF in ICU patients.

Clinical case-based approach to understanding intradialytic hypotension

The approach to end-stage renal disease (ESRD) patients who develop intradialytic hypotension (IDH) encompasses an understanding of the pathophysiology, appropriate dialysis prescription modification, application of newer pharmacologic therapies, and development of strategies for prevention. Patients should have a "minimal data set" as part of their predialysis assessment. This information is critical to prescription modifications that may help decrease the risk for IDH. Individuals at "high risk" for IDH should be kept to a "safe zone" for dialysis ultrafiltration (</=3% of body weight). Specific maneuvers that may decrease the risk for IDH include adjustment of the dialysate sodium or calcium concentration and dialysate temperature. The first priority for patients developing IDH should be the stabilization of the blood pressure and improvement in the patient's symptomology. Pharmacologic intervention should be considered for patients who require repeat interventions for IDH. "At-risk" patients with a strong cardiac history should undergo an assessment of their cardiovascular status if IDH episodes occur. The use of pharmacologic therapy, ie, midodrine, alone or in combination with prescription modification, can be helpful in decreasing interventions required for IDH. Noncompliance and high interdialytic weight gain in the setting of left ventricular hypertrophy (LVH) and diastolic dysfunction can increase the risk of IDH. Assessment of antihypertensive medications should be performed on a regular basis to determine the correct dosing schedule for patients with hypertension who develop IDH. Coronary flow reserve may be compromised in patients with LVH, adding to the risk for perfusion injury with low blood pressure. Increasing the dialysate calcium concentration may decrease the incidence of arrhythmogenicity in certain patients. Patients with low body temperature may benefit most from cool dialysate. Unit personnel should be aware of the potential link between hypotension and the increased relative risk for death in ESRD patients. Clinical training sessions on IDH risk recognition and appropriate treatment should be implemented within the dialysis unit. Because repeated bouts of IDH can be disruptive to the smooth efficiency of unit operations, attention to prevention as well as acute intervention of IDH is important. Preventive strategies can be developed in each unit to decrease the number of future IDH events. Considering the importance of hypotension in overall patient survival, attention to identifying the percentage of patients in each unit who experience IDH and/or who present with low blood pressure (systolic <110 mm Hg) should be tracked as a quality assurance initiative.

Changes in major blood components after adopting the supine position during haemodialysis

BACKGROUND

In Japan, haemodialysis (HD) is usually performed with patients in the supine position. However, the effects of changing posture on major blood components have not been investigated in HD patients. It is possible that several fluid components change rapidly when patients change from the upright to the supine position. We therefore investigated the effects of posture on blood component analysis.

METHODS

A first blood sample was taken from 10 HD patients 5 min after they adopted a supine position; HD was begun immediately after sampling. Additional blood samples were collected 15 and 30 min later while patients remained in the supine position. On an alternate day, blood samples were taken from these same patients in the supine position, but not during HD. The same procedure was performed in 10 healthy volunteers.

RESULTS

Haematocrit significantly decreased in patients undergoing HD at 15 and 30 min into the HD session. Similar decreases were observed in HD patients not undergoing HD and in normal control subjects. Haematocrit changes at 15 min were not significantly different between the three groups. Serum albumin concentrations decreased in the same way as haematocrit. Consequently, the reductions in haematocrit and albumin concentrations in HD patients during the HD session were not attributable to the HD procedure or to end-stage renal disease, but rather were due to the supine position and consequent haemodilution caused by redistribution of water from the extra- to the intravascular space. Finally, WBC counts decreased significantly at 15 min in both HD patient groups and in normal controls. The relative decrease at 15 min was significantly greater in HD patients undergoing HD (61.4% of baseline) than in those not undergoing HD (88.0%) or in normal controls (94.7%). These differences were probably due to previously reported WBC sequestration in the lungs during the early phase of HD.

CONCLUSIONS

This study suggests that the change from the upright to the supine positions during HD causes changes in blood components that are critical for quality control determinations.