A simulation study on transcellular fluid shifts induced by hemodialysis

Comparison of Body Fluid Volumes Determined by Kinetic Modeling and by Bioimpedance Spectroscopy

<b><i>Introduction:</i></b> The bioimpedance spectroscopy (BIS) method is used in individual patients requiring body fluid volume measurement. In a hemodialysis facility, however, regular screening of body fluid volumes is also necessary. Such screening, by kinetic modeling, may become possible by calculating distribution volumes of urea and uric acid from regular blood test results. <b><i>Objective:</i></b> The aim is to compare uric acid distribution volumes with BIS-extracellular volume, urea distribution volume with BIS-total body water, and difference between urea and uric acid distribution volumes with BIS-intracellular volume. <b><i>Methods:</i></b> We reanalyzed stored blood test data of 53 hemodialysis patients obtained together with BIS data of the same patients in our previous study. <b><i>Results:</i></b> Significant correlations were found between urea distribution volume and total body water predicted by the BIS method, between uric acid distribution volume and extracellular volume predicted by the BIS method, and between the difference of uric acid distribution volume from urea distribution volume and intracellular volume predicted by the BIS method. In Bland-Altman analysis, comparison of each pair showed no systematic error. The mean difference between each pair was minimal. <b><i>Conclusion:</i></b> Fluid volumes in different body compartments can be estimated by kinetic modeling as well as by the BIS method.

Correcting hyponatremia by fluid sodium modulation in continuous renal replacement therapy with regional citrate anticoagulation

A 40-year-old man presented with severe hyponatremia with a serum sodium of 102 mmol/L and concomitant acute kidney injury complicated by severe acidosis. He was started on continuous renal replacement therapy (CRRT) with regional citrate anticoagulation. We present the equations and strategy used to calculate and adjust the sodium concentration of the dialysate and replacement fluids to increase serum sodium levels by ≤8 mmol/L/day. The equations were based on fundamental chemistry principles and applicable to common CRRT solutions with 140 mmol/L of sodium. This simple strategy for CRRT fluid sodium titration required only one adjustment per day, and the serum sodium levels increased safely within the daily targets set. Although the citrated-replacement fluid was diluted for sodium adjustment, the citrate anticoagulation protocol was still able to achieve the targeted circuit ionized-calcium levels and provided adequate anticoagulation without issues related to frequent clotting and other electrolyte abnormalities.

Monitoring transcellular fluid shifts during episodes of intradialytic hypotension using bioimpedance spectroscopy

Background

Transcellular fluid shifts during dialysis treatment could be related to the frequency and severity of intradialytic hypotension (IDH). We investigated that (i) in addition to ultrafiltration, extracellular fluid (ECF) is further depleted by transcellular fluid shifts and (ii) changes in intracellular fluid (ICF), which have been overlooked so far, or if they were considered, are not understood, might be due to these fluid shifts.

Methods

Thirty-six patients were categorized as haemodynamically stable, asymptomatic IDH or unstable (symptomatic IDH) according to their changes in systolic blood pressure and associated clinical symptoms. Their intradialytic changes in body fluids were studied using bioimpedance spectroscopy measurements and compared among groups.

Results

For IDH-prone patients, data showed a rapid drop in ECF that was more than expected from the ultrafiltration rate (UFR) profile and was associated with a significant increase in ICF (P = 0.001). Study of accumulative loss profiles of ECF revealed a loss in ECF up to 300 ml, more than that predicted from UFR for unstable patients.

Conclusions

The considerable discrepancy between the expected and measured loss in ECF might provide evidence of transcellular fluid shifts possibly induced by changes in plasma osmolarity due to haemodialysis. Moreover, the results suggest a pattern of fluid removal in IDH-prone patients that significantly differs from that in haemodynamically stable patients.

Quantitative Estimation of Dietary Intake in Patients on Hemodialysis

A simple method to calculate the amount of dietary (protein, sodium and potassium) intake in hemodialyzed patients was developed. In 8 nutritionally stable patients, the amount of dietary intake was monitored conventionally by a dietary record method. In contrast, assuming that the amount of dietary intake was equal to the amount of accumulation in the body, the former was calculated as the change in the product of serum concentrations and total body fluid volume, which was estimated based on the sex and body build of each patient. The urea accumulation was converted to the protein intake. The interdialytic dietary protein and sodium intake calculated by this method, 120 ± 10 g and 240 ± 40 mEq, respectively, was not significantly different from that obtained by the dietary record, while the interdialytic potassium accumulation, 60 ± 7 mEq, was significantly smaller than the dietary intake, 110 + 9 mEq, obtained by the record method, though the correlation was significant. Thus, the amount of protein and sodium intake can be calculated simply without diet research or body fluid volume measurements. Although potassium intake can not be calculated exactly because of intestinal loss, this simple method gives us a rough estimate. In addition, multiple regression analysis showed that the amount of energy intake obtained by the record method may be explained by the protein and sodium intake estimated by simple calculation.

A Digital Computer Model for Optimal Programming of Hemodialytic Treatment

A mathematical model of hydroelectrolyte exchanges and arterial pressure regulation in the human body during dialysis has been set up. It is conceived as a tool for a new dialysis unit which will be able to “interpret” the signals supplied by suitable instruments connected to the patient and modify the machine set-points in real time in order to obtain clinical results defined by the physician. The main aim is the prevention of hypotensive episodes during treatment. An experimental protocol has been developed for parameter estimation of each patient during a single dialysis. Clinical tests illustrated the model's ability to fit the patient's state during dialysis. This is the first step in the more general task of validation of the model, necessary for the achievement of a closed-loop dialysis unit.

Total Body Fluid Volume Determination Based on Urea Kinetics in Hemofiltration as an Index of Basal Body Weight in Uremic Patients

Assuming that urea is distributed uniformly within the total body water, urea-space or total body fluid volume was determined in six uremic patients based on urea kinetics in hemofiltration.

The total body fluid volume before hemofiltration was 36.0 ± 3.6 L (61.8 ± 2.6% BW) and after hemofiltration 32.5 ± 3.4 L (59.3 ± 2.8% BW), suggesting that the total body fluid volume was nearly normalized by hemofiltration. It is concluded that urea-space, easily measurable based on urea kinetics during hemofiltration, is useful in evaluating the fluid balance in patients undergoing artificial kidney therapy.

Conductivity: On-Line Monitoring of Dialysis Adequacy

Cardiovascular disease and the inadequacy of delivered dialysis are the main factors determining morbidity and mortality in dialysis patients. We have already demonstrated that a conductivity kinetic model makes it possible to match interdialytic sodium loading and intradialytic sodium removal (the main factor determining cardiovascular morbidity) without the need for blood samples and, thus, in routine clinical practice.

The aim of the present study was to test the possibility of using the conductivity method also to determine Kt/v without blood or dialysate sampling.

In 18 steady-state patients, the urea distribution volume (V) was kinetically determined once using ionic dialysance (D) values instead of those of effective urea clearance. One month later, the Kt/V was determined by using the current D and T values and the predetermined V (Dt/V), then compared with the equilibrated Kt/V computed by means of the SPVV kinetic model (eqKt/V). The mean value of Dt/V was 1.18 ± 0.15; while of eqKt/V it was 1.18 ± 0.16, with a mean difference of 0.00 ± 0.07.

The conductivity method therefore seems to be very promising not only for monitoring the sodium balance, but also for quantifying delivered dialysis. Since its simplicity and low-cost make it suitable for use at each dialysis session, the conductivity method could therefore lead to significant progress in dialytic practice by contributing to the elimination of the two main causes of morbidity and mortality in dialysis patients.

An Algorithm for the Rational Choice of Sodium Profile during Hemodialysis

The incidence of intradialytic disequilibrium syndrome and symptomatic hypotension has increased significantly among dialysis patients over the last ten years. Profiled hemodialysis (PHD) is a new technique, based on the intradialytic modulation of dialysate sodium concentration, which aspires to reduce to previous imbalances.

This paper presents a new algorithm for the determination of a rational dialysate sodium profile during PHD. A mathematical model of solute kinetics, monocompartmental for sodium and bicompartmental for urea is used. The algorithm allows the sodium profile to be elaborated a priori before each dialysis session, respecting the individual sodium mass removal and weight gain. A procedure allowing the adjustment of the method to the individual characteristics, on the basis of routine measurements performed before each session is also presented.

The method was validated during seven dialysis sessions. Comparison between data measured in vivo and those predicted by the model showed standard deviations corresponding to the range of laboratory measurement errors: 1.50 mEq/L for sodium and 0.87 mmol/L for urea. In vivo implementation of PHD by our algorithm allows one to remove an amount of sodium close to that established a priori on the basis of patient's need.

Monitoring Sodium Removal and Delivered Dialysis by Conductivity

As cardiovascular stability and the delivery of the prescribed dialysis “dose” seem to be the main factors in determining the morbidity and mortality of hemodialyzer patients today, it is of paramount importance to match hydro-sodium removal with interdialytic load and to verify the delivered dialysis at each session. A specially designed Biofeedback Module (BM - COT Hospal) allows the automatic determination of plasma water conductivity and effective ionic dialysance with no need for blood samples. Using BM, we evaluated the validity of “conductivity kinetic modelling” (CKM) and the possibility that this may substitute “sodium kinetic modelling”. Moreover, we evaluated the “in vivo” relationship between ionic dialysance and effective urea clearance. Our results demonstrate that: 1) CKM makes it possible to obtain programmed end-dialysis plasma water conductivity with an error of less than ± 0.14 mS/cm, roughly equivalent to a sodium concentration of ± 1.4 mEq/L. 2). Ionic dialysance and effective urea clearance are not equivalent but, as the interrelationship between these is known, the BM allows the routine monitoring of delivered dialysis.

A Simple Mathematical Model of Intradialytic Sodium Kinetics: “in vivo” Validation During Hemodialysis with Constant or Variable Sodium

A simple mathematical model of the intradialytic relationship between natraemia and dialysate sodium concentration is presented. The model includes a bicompartmental description of sodium, urea and fluid kinetics and an algebraic characterization of diffusive/convective mass-transfer across the dialysis membrane. Its ability to provide realistic responses has been validated comparing model predictions by a priori parameter tuning against quantities measured during in vivo sessions with both constant and variable dialysate sodium concentration. A quantitative analysis of model predictions indicates that the mean deviation between data calculated by the model and those measured in vivo is 1.32 mEq/l for sodium and 0.76 mmol/l for urea, values which do not greatly exceed the measurement errors of current instruments. The model's predictive capacity thus proves reliable. The ability of the model to calculate the amount of sodium removed and the time course of intra-extracellular volumes during the dialysis session makes it possible to forecast the patient's clinical tolerance to a given sodium dialysate concentration.

Electrolytes and Fluid Management in Hemodialysis and Peritoneal Dialysis

The kidney is a complex and vital organ, regulating the electrolyte and fluid status of the human body. As hemodialysis (HD) and peritoneal dialysis (PD) are forms of renal replacement therapy and not an actual kidney, they do not possess the same physiologic regulation of both fluid and electrolytes. Precise regulation of fluid and electrolytes in the HD and PD population remains a constant challenge. In this review, fluid status of both HD and PD will be examined, as well as sodium, potassium, phosphorous, and calcium. Each electrolyte will be analyzed by its physiological significance, the complications that arise when a proper balance cannot be maintained, and methods to correct these imbalances. An overview of the fluid compartments and volume of distribution within the body will be discussed. Ultrafiltration, a modality used in both forms of renal replacement therapy, will be defined, along with its impact on fluid status. Fluid assessment will be addressed, along with proper maintenance of fluid homeostasis. By having an understanding of the pathophysiology behind the fluid and electrolyte abnormalities that occur in end-stage renal disease, one can direct proper management with medications, diet, and alterations in dialysis to provide patients with the most optimal form of renal replacement therapy available.

Bioimpedance monitoring of cellular hydration during hemodialysis therapy

Introduction The aim of this paper is to describe and demonstrate how a new bioimpedance analytical procedure can be used to monitor cellular hydration of End Stage Renal Disease (ESRD) patients during hemodialysis (HD). Methods A tetra-polar bioimpedance spectroscope (BIS), (UFI Inc., Morro Bay, CA), was used to measure the tissue resistance and reactance of the calf of 17 ESRD patients at 40 discrete frequencies once a minute during dialysis treatment. These measurements were then used to derive intracellular, interstitial, and intravascular compartment volume changes during dialysis. Findings The mean (± SD) extracellular resistance increased during dialysis from 92.4 ± 3.5 to 117.7 ± 5.8 Ohms. While the mean intracellular resistance decreased from 413.5 ± 11.7 to 348.5 ± 8.2 Ohms. It was calculated from these data that the mean intravascular volume fell 9.5%; interstitial volume fell 33.4%; and intracellular volume gained 20.3%. Discussion These results suggest that an extensive fluid shift into the cells may take place during HD. The present research may contribute to a better understanding of how factors that influence fluid redistribution may affect an ESRD patient during dialysis. In light of this finding, it is concluded that the rate of vascular refill is jointly determined with the rate of "cellular refill" and the transfer of fluid from the intertitial compartment into the intravascular space.

Optimal dialysate sodium-what is the evidence?

Oligo-anuric patients with end-stage kidney disease are dependent on hemodialysis to achieve and maintain the desired goal of euvolemia. The dialysis prescription, in addition to sodium and fluid restriction, is therefore a critically important factor in the care of hemodialysis patients. Various dialysate sodium concentrations have been favored throughout the history of dialysis, but the "optimal" concentration remains unclear. In this manuscript, we examine the historical context of changes to the dialysate sodium prescription, review the evidence of its associated effects, discuss 'individualization' of dialysate sodium, and highlight the need for definitive trials that are powered for important clinical outcomes.

Postdialysis serum sodium changes and systolic blood pressure in patients undergoing online hemodiafiltration and high-flux hemodialysis

Background

Because hemodiafiltration (HDF) involves large amounts of ultra-filtration and substitution fluid infusion, its effects on serum electrolytes may be different from those of hemodialysis (HD). Serum sodium and blood pressures were compared between patients undergoing online HDF and high-flux HD (HFHD).

Methods

Thirty-two of 101 patients on HFHD switched voluntarily to online HDF. Their pre- and postdialysis serum measurements were compared with those of the remaining 69 HFHD patients.

Results

Online HDF patients had lower pre- and postdialysis systolic blood pressures (SBPs) than HFHD patients (predialysis, 136±21 vs. 145±19 mmHg, P<0.05; postdialysis, 129±22 vs. 142±25 mmHg, P<0.05). Pre- and postdialysis serum sodium concentrations were not significantly different between online HDF and HFHD (predialysis, 138±2 vs. 137±3 mEq/L; postdialysis, 134±2 vs. 134±2 mEq/L). However, the change in serum sodium concentration after dialysis was greater in online HDF than HFHD patients (−3.7±2.2 vs. −2.5±2.8 mEq/L, P<0.05). The change in serum sodium concentration was correlated with postdialysis SBP (r=0.304, P<0.005) and pulse pressure (r=0.299, P<0.005). Predialysis SBP (r = 0.317, P<0.005) and pulse pressure (r=0.324, P=0.001) were also correlated with the postdialysis serum sodium change.

Conclusion

Compared with HFHD, online HDF has a greater serum sodium lowering effect. This might contribute to the ability of online HDF to stabilize both pre- and postdialysis SBP.

Effect of lowering dialysate sodium concentration on interdialytic weight gain and blood pressure in patients undergoing thrice-weekly in-center nocturnal hemodialysis: a quality improvement study

BACKGROUND

Patients on in-center nocturnal hemodialysis therapy typically experience higher interdialytic weight gain (IDWG) than patients on conventional hemodialysis therapy. We determined the safety and effects of decreasing dialysate sodium concentration on IDWG and blood pressure in patients on thrice-weekly in-center nocturnal hemodialysis therapy.

STUDY DESIGN

Quality improvement, pre-post intervention.

SETTINGS & PARTICIPANTS

15 participants in a single facility.

QUALITY IMPROVEMENT PLAN

Participants underwent three 12-week treatment phases, each with different dialysate sodium concentrations, as follows: phase A, 140 mEq/L; phase B, 136 or 134 mEq/L; and phase A(+), 140 mEq/L. Participants were blinded to the exact timing of the intervention.

OUTCOMES

IDWG, IDWG/dry weight (IDWG%), and blood pressure.

MEASUREMENTS

Outcome data were obtained during the last 2 weeks of each phase and compared with mixed models. The fraction of sessions with adverse events (eg, cramping and hypotension) also was reported.

RESULTS

IDWG, IDWG%, and predialysis systolic blood pressure decreased significantly by 0.6 ± 0.6 kg, 0.6% ± 0.8%, and 8.3 ± 14.9 mm Hg, respectively, in phase B compared with phase A (P < 0.05 for all comparisons). No differences in predialysis diastolic and mean arterial or postdialysis blood pressures were found (P > 0.05 for all comparisons). The proportion of treatments with intradialytic hypotension was low and similar in each phase (P = 0.9). In phase B compared with phase A, predialysis plasma sodium concentration was unchanged (P > 0.05), whereas postdialysis plasma sodium concentration decreased by 3.7 ± 1.9 mEq/L (P < 0.05).

LIMITATIONS

Modest sample size.

CONCLUSION

Decreasing dialysate sodium concentrations in patients undergoing thrice-weekly in-center nocturnal hemodialysis resulted in a clinical and statistically significant decrease in IDWG, IDWG%, postdialysis plasma sodium concentration, and predialysis systolic blood pressure without increasing adverse events. Prolonged exposure to higher than required dialysate sodium concentrations may drive IDWG and counteract some of the purported benefits of "go-slow" (longer session length) hemodialysis.

Dialysate sodium and sodium gradient in maintenance hemodialysis: a neglected sodium restriction approach?

BACKGROUND

A higher sodium gradient (dialysate sodium minus pre-dialysis plasma sodium) during hemodialysis (HD) has been associated with sodium loading; however, its role is not well studied. We hypothesized that a sodium dialysate prescription resulting in a higher sodium gradient is associated with increases in interdialytic weight gain (IDWG), blood pressure (BP) and thirst.

METHODS

We conducted a cross-sectional study on 1084 clinically stable patients on HD. A descriptive analysis of the sodium prescription was performed and clinical associations with sodium gradient were analyzed.

RESULTS

The dialysate sodium prescription varied widely across dialysis facilities, ranging from 136 to 149 mEq/L, with a median of 140 mEq/L. The mean pre-HD plasma sodium was 136.7 ± 2.9 mEq/L, resulting in the majority of subjects (n = 904, 83%) being dialyzed against a positive sodium gradient, while the mean sodium gradient was 4.6 ± 4.4 mEq/L. After HD, the plasma sodium increased in nearly all patients (91%), reaching a mean post-HD plasma sodium of 141.3 ± 2.5 mEq/L. We found a direct correlation between IDWG and sodium gradient (r = 0.21, P < 0.0001). After adjustment for confounders and clustering by facilities, the sodium gradient was independently associated with IDWG (70 g/mEq/L, P < 0.0001). There were no significant associations among sodium gradient and BP, whether measured as pre-HD systolic (r = -0.02), diastolic (r = -0.06) or mean arterial pressure (r = -0.04). Post-HD thirst was directly correlated with sodium gradient (r = 0.11, P = 0.02).

CONCLUSION

Sodium gradient is associated with statistically significant and clinically meaningful differences in IDWG in stable patients on HD.

The association of the sodium "setpoint" to interdialytic weight gain and blood pressure in hemodialysis patients

INTRODUCTION

Hemodialysis patients lack the normal mechanisms to regulate body water volume and osmolality. The dialysis treatment is expected to adequately regulate both body water volume and body Na+ content, which is the primary action determining body water osmolality. Data in subjects with normal renal function indicate that an individual has a specific osmolality value above which thirst is generated and fluid will be ingested. This specific osmolality value or "setpoint" varies among individuals, but is quite reproducible within an individual. It was postulated that hemodialysis patients also may have a Na+ 'setpoint', which if increased by the use of higher dialysate Na+ concentration, might be associated with increased interdialytic weight gain and blood pressure.

METHODS

Monthly laboratory and treatment data were abstracted on 58 hemodialysis patients and included pre- and post-dialysis serum Na+ concentrations, interdialytic weight gain and blood pressure over 9 to 16 months. The Na+ concentrations were averaged to determine the individual Na+ 'setpoint' and the Na+ gradient (Dialysis Na+ concentration - mean Na+ concentration) was determined for each patient.

RESULTS

Linear regression analyses showed that there was a statistically significant association between the magnitude of the Na+ gradient and interdialytic weight gain and blood pressure.

CONCLUSIONS

These data suggest that interdialytic weight gain in individual patients may be associated with the use of dialysate Na+ concentration in excess of the patient's desired Na+ 'setpoint'. More individualization of dialysate Na+ concentration may be indicated.

Optimization of dialysate sodium in sodium profiling haemodialysis

Sodium profiling haemodialysis is a modified method of sodium gradient dialysis during which dialysate sodium follows a time-dependent profile. Sodium profiling haemodialysis has claimed to reduce intradialytic discomforts such as hypotension, muscle cramps, and disequilibrium syndrome. Having the low sodium period is an essential part of the sodium profiling haemodialysis to compensate for the sodium gain during the high sodium period. In spite of this, however, the incidence of interdialytic complications that results from the excessive sodium gain has been reported in previous literature. Making the prediction of optimal dialysate sodium concentration for isonatric dialysis is practically very difficult since too many variables influence the sodium gradient, including the initial plasma sodium and tonicity and/or dialysis dynamics that differ from patient to patient and from treatment to treatment. As for sodium profiling haemodialysis, complexities are added further since details of profile, such as type and form of profile, or initial, terminal, or time-distribution of dialysate sodium are varied considerably. We have recently reported that the intradialytic sodium balance and interdialytic weight gain are directly related to the time-averaged concentration of dialysate sodium (TACNa). The dialysate sodium can be optimized using this concept of TACNa for sodium profiling dialysis. TACNa should be approximately 0.5-0.8 mmol/L lower than patient's predialysis serum sodium concentrations to achieve a sodium balance neutral dialysis. In that study the optimal TACNa, seems to be between 137.8 and 143.5 mmol/L. Such an optimal value should be defined for the individual centres based on their profile protocols for clinical use. In the future, dialysate sodium should be optimized based on the exact prediction of the postdialysis plasma sodium levels.

Time-averaged concentration of dialysate sodium relates with sodium load and interdialytic weight gain during sodium-profiling hemodialysis

BACKGROUND

Factors determining sodium level during sodium-profiling hemodialysis rarely have been studied. We hypothesized that the time-averaged concentration of dialysate sodium (TAC(Na)) is related to intradialytic sodium load and interdialytic complications.

METHODS

Eleven patients underwent 6-week periods of (1) conventional hemodialysis with a dialysate sodium concentration of 138 mmol/L (TAC(138)) and (2) sodium-profiling hemodialysis with a dialysate sodium concentration of 150 to 138 mmol/L (TAC(Na), 140 mmol/L [TAC(140)]) and (3) 155 to 130 mmol/L (TAC(Na), 147 mmol/L [TAC(147)]). Serum sodium level, weight gain, 24-hour blood pressure, and intradialytic and interdialytic discomfort were compared.

RESULTS

Serum sodium levels increased during the TAC(140) and TAC(147) periods (P < 0.05 compared with predialysis serum sodium). Intradialytic change in sodium level correlated positively with TAC(Na) (r = 0.945; P < 0.001). Regression analysis indicates that positive sodium load occurred with TAC(Na) more than 137.8 mmol/L. Interdialytic weight gain increased in proportion to TAC(Na) (P < 0.05 compared with each other period), with a positive correlation (r = 0.823; P < 0.001). TAC(Na) causing interdialytic weight gain less than 3 kg was estimated to be less than 143.5 mmol/L. Intradialytic hypotension decreased, but interdialytic discomforts increased during the TAC(147) period (P < 0.05 compared with TAC(138) and TAC(140)). Mean 24-hour blood pressures and pressure loads increased during the TAC(147) period (P < 0.05 compared with TAC(138) and TAC(140)). Mean diastolic blood pressure correlated positively with TAC(Na) (r = 0.354; P < 0.05).

CONCLUSION

TAC(Na) is a factor determining sodium load and interdialytic complications during sodium-profiling hemodialysis. Defining the optimal TAC(Na) for individual centers based on their protocols will be helpful to avoid sodium load and excessive weight gain.

Sodium and body fluid homeostasis in profiling hemodialysis treatment

Acute adverse side-effects of hemodialysis such as hypotension, muscle cramps, osmotic imbalance and thirst are induced by the interference with fluid and electrolyte balance occurring during treatment. Changes in osmolarity due to alterations of plasma sodium concentration during hemodialysis strongly influence fluid distribution between extracellular and intracellular fluid volume. Increased sodium dialysate concentration induces fluid shift from the intracellular to the extracellular compartment. This shift leads to a more efficient ultrafiltration by increasing plasma refilling volume but also to an increased thirst. Treatment of hypotension, cramps and nausea with hypertonic saline solution leads also to a considerable retention of sodium. Profiling hemodialysis consists in deliberately changing ultrafiltration and dialysate. sodium in order to combine an efficient ultrafiltration with a balanced sodium handling and to prevent side-effects during treatment. Continuous measurement and control of blood volume seems to be the best method to prevent hypotensive episodes. Profiling of sodium should not be the cause of a positive sodium balance. The clinical benefits of sodium profiling to the patients have still to be proven.

Role of sodium in hemodialysis

Sodium chloride is the most abundant salt in extracellular fluid. In normal individuals, the tonicity exerted by dissolved sodium chloride determines plasma osmolality and indirectly determines intracellular tonicity and cell volume. Uremic patients retain nitrogenous wastes and have an elevated plasma osmolality. While urea exhibits osmotic activity in serum, no sustained gradient can be established across cell boundaries because it readily diffuses through cell membranes. Thus, sodium remains the major indicator of body tonicity and determines the distribution of water across the intracellular-extracellular boundary, subsequent cell volume, thirst, and, among patients with renal insufficiency, systemic blood pressure. As a result of highly conserved plasma tonicity control systems, uremic subjects demonstrate remarkable stability of their serum sodium. Dialysate is a synthetic interstitial fluid capable of reconstituting extracellular fluid composition through urea extraction and extremely efficient solute and solvent (salt and water) transfer to the patient. Subtle transdialyzer gradients deliver and remove large quantities of trace elements, solvent, and solute to patients, creating a variety of dialysis "disequilibrium" syndromes manifest as cellular and systemic distress. Every dialysis patient uses dialysate, and the most abundant chemicals in dialysate are salt and water. Despite its universal use, no consensus on dialysate composition or tonicity exists. This can only be explained if we believe that dialysate composition is best determined by matching unique dialysis delivery system characteristics to specific patient requirements. Such a paradigm treats dialysate as a drug and the dialysis system as a delivery device. Understanding the therapeutic and toxic profiles of this drug (dialysate) and its delivery device (the dialyzer) is important to safe, effective, goal-directed modifications of therapy. This article explores some of the historical rationale behind choosing specific dialysate tonicities.

Electrolyte and pH dependence of heart rate during hemodialysis: a computer model analysis

The influence of hemodialysis-induced modifications in extracellular fluid characteristics on heart rate was investigated by using a detailed computer model of sinus-node electrical activity. Changes similar to those occurring in the course of hemodialysis in extracellular concentrations of sodium (from 138 to 140 mM), potassium (from 6 to 3.3 mM), and calcium (from 1.2 to 1.5 mM) ions as well as in pH (from 7.31 to 7.4) and intracellular volume were simulated. The model predicted that such changes may largely influence the rhythm of the sinoatrial node pacemaker, causing the heart rate to range from 69 to 86 bpm. Heart rate increases after removing potassium (up to 7 bpm) and also after calcium perfusion (up to 11 bpm) whereas restoring pH slows heart beat (up to 6 bpm). Extracellular sodium has no significant influence, but the heart rate strictly depends on intracellular sodium concentration (5 bpm/mM). A complex dependence of heart rate on electrolytes and pH was also recognized. Providing extracellular potassium concentration is maintained above 5 mM, heart rate exhibits low sensitivity to changes in calcium and potassium. When potassium concentration is reduced below 4.5 mM, heart rate sensitivity to calcium and potassium increases significantly to 10 and 30 bpm/mM, respectively. A sustained increase in heart rate always corresponds to an increase in intracellular sodium concentration.

Simulation study of the intercompartmental fluid shifts during hemodialysis

Hypotension is the most frequent complication during hemodialysis. An important cause of hypotension is a decrease in the intravascular volume. In addition, a decrease in plasma osmolality may be a contributing factor. Modeling of sodium and ultrafiltration (UF) may help in the understanding of underlying relationships. We therefore simulated, in a mathematical model, the intercompartmental fluid shifts during standard hemodialysis (SHD), diffusive hemodialysis (DHD), and isolated ultrafiltration (IU). We analyzed the relative theoretical effect of hydration status, dialysate sodium concentration, the initial plasma concentrations of sodium and urea, and tissue permeation to solutes on the magnitude and direction of intracellular and intravascular volume changes. This theoretical analysis shows that the transcellular fluid shifts taking place during hemodialysis treatment are, to a great part, due to inhomogeneous distribution of regional blood flow and tissue fluid volumes. During hemodialysis treatment, the cellular fluid shifts in tissue groups with relatively high perfusion and small volume occur from the intra- to the extracellular spaces. However, the fluid shift in tissue groups with a low perfusion and large volume takes place in the opposite direction. The UF volume and rates, and the size of the sodium (Na+) gradient between the dialysate and blood side of the dialyzer membrane are the most important factors influencing the fluid shifts. Higher UF volumes and flow rates cause an increasing decline in the plasma volume in both SHD and IU. High dialysate sodium concentration (150 mEq L(-1)) helps plasma refilling slightly when compared with a normal dialysate sodium concentration (140 mEq L(-1)). However, a high dialysate sodium concentration is associated with a high plasma sodium rebound, which in turn may lead to interdialytic water intake resulting from thirst and may cause increased weight gain and hypertension.

Modeling arterial hypotension during hemodialysis

A mathematical model of the hemodynamic response to hemodialysis is presented. This model includes the dynamics of sodium, urea, and potassium in the intracellular and extracellular pools; fluid balance equations for the intracellular, interstitial, and plasma volumes; systemic and pulmonary hemodynamics; and the action of several short-term arterial pressure control mechanisms. The control mechanisms are triggered by information coming from both arterial and cardiopulmonary pressoreceptors, and they work on systemic arterial resistance, heart rate, and systemic venous unstressed volume. Moreover, the model hypothesizes that decreasing left atrial pressure below a given threshold causes a paradoxical withdrawal of the sympathetic drive and a consequent vasodepressor syncope. The model is used to simulate the pattern of the main hemodynamic quantities (systemic arterial pressure, heart rate, total systemic resistance, and cardiac output) during hemodialysis in several groups of patients (both hypotension resistant and hypotension prone) whose data were drawn from the clinical literature. The simulation results point out that the model is able to reproduce a variety of different conditions, including no hypotension, moderate hypotension, and severe hypotension with ultimate vasodepressor syncope, by adjusting a few parameters with clear physiological meanings. Hypotension is principally imputed to a loss of the sympathetic mechanisms working on systemic resistance and to an impairment of vascular refilling at the capillary wall. The results suggest that hypotension during hemodialysis is a complex phenomenon that depends on the superimposition of several concomitant factors working together that can lead to a variety of distinct individual patterns.

Sodium ramping in hemodialysis: a study of beneficial and adverse effects

Sodium ramping has been introduced as a technique to decrease side effects occurring during hemodialysis. We studied sodium ramping in 414 dialysis sessions in 23 patients by randomizing 2-week blocks of dialysis to either steady dialysate sodium of 140 mEq/L, linear sodium ramping during dialysis from 155 mEq/L to 140 mEq/ L, or stepwise ramping (sodium of 155 mEq/L for 3 hours and 140 mEq/L for 1 hour). We studied the number and severity of hypotensive and hypertensive episodes. A hypotensive episode was defined as an abrupt decline of systolic blood pressure of more than 50 mm Hg, a decrease in blood pressure accompanied by symptoms requiring intervention, or systolic blood pressure of less than 90 mm Hg even without symptoms. A hypertensive episode was defined as a sudden increase in systolic blood pressure of over 30 mm Hg. We also recorded other side effects (headache, cramps, nausea, vomiting, dizziness, thirst, fatigue, weight gain, and blood pressure) during, immediately after, and between dialysis sessions. There was no major difference between the two ramping protocols, but compared with standard dialysis, both decreased total number of side effects from 4.0 to 3.0 (P = 0.057); the number of hypotensive episodes decreased from 1.3 to 0.7 (P = 0.036). The lowest blood pressure was 114/66 mm Hg during control and 123/69 mm Hg during ramping (P < 0.0001). The frequency of cramps during dialysis decreased from 0.9 to 0.5 (P = 0.006). There was no difference in headache, nausea, or vomiting. The number of hypertensive episodes increased from 0.045 to 0.086 during ramping (P = 0.125). Of 23 patients, only five (22%) had a marked decrease in symptoms; two of the three most symptomatic patients showed no significant improvement. Between dialysis sessions, patients complained of more fatigue and thirst (P < 0.0001 and P = 0.0028, respectively), and interdialytic weight gain following ramping was 5.1% of body weight compared with 4.4% without ramping (P < 0.0001). Blood pressure also increased following ramping, from 143/79 mm Hg to 152/81 mm Hg (P = 0.001). Ramping can decrease the overall number of side effects, but increases interdialytic symptoms, weight gain, and hypertension. In most instances, it simply changes the time the side effects occur. Only 22% of patients have significant benefit. These patients can be identified only through trial and error, as no model of these patients can be created.

Dialysate sodium delivery can alter chronic blood pressure management

Low dialysate sodium concentrations can reduce postdialysis thirst and serum sodium activity, but patients typically experience dialysis hypotension, fatigue, disequilibrium, and cramps. "High-sodium" hemodialysis minimizes dialysis disequilibrium but increases the serum sodium activity of most patients. Programmed "variable-sodium" dialysis can minimize dialysis discomfort but may also alter the sodium kinetics from those of "high-sodium" dialysis. We designed a cross-over study with random order assignment to determine whether a "variable-sodium" dialysis program could reduce the blood pressure of dialysis patients without increasing dialysis morbidity. Dialysis with a dialysate sodium of 140 mEq/L was compared with dialysis with a programmed exponential decrease of dialysate sodium from 155 mEq/L to 135 mEq/L. Dialysate sodium was then held constant at 135 mEq/L for the final half hour of dialysis. Eighteen patients completed the 7-month study, each receiving 3.5 months of experimental and 3.5 months of standard therapy. Programmed "variable-sodium" dialysis resulted in a reduction in antihypertensive drug use without alterations in predialysis blood pressure, interdialytic weight gain, ultrafiltration tolerance, or the frequency of symptomatic dialysis cramps or hypotension. Patients did, however, have lower postdialysis standing blood pressures and higher postdialysis target weights during programmed "variable-sodium" dialysis.

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.

Sodium modeling in hemodiafiltration

A computer model was developed to simulate sodium and water kinetics during hemodiafiltration (HDF), acetate-free biofiltration (AFB) and hemodialysis (HD). Multiple regression analysis of the results of 3,240 simulated applications of the model (1,620 HDF, 1,080 AFB, 540 HD) showed that, during HDF and AFB, there is a close correlation (R2 = 0.92 and 0.91) between plasma water sodium concentration [( Na+P]) and a set of three variables: 1) the sodium gradient between plasma water and dialysate, 2) the sodium concentration of the substitution fluid and 3) ultrafiltration (UF) rate. With HD, a close correlation (R2 = 0.94) was found between changes in [Na+P] and combined changes in sodium gradient and the UF rate. On this basis, a regression equation was formulated for each procedure which allowed a reliable prediction of final [Na+P] to be made on the basis of knowledge of the imposed Na gradient, the programmed infusion (during HDF and AFB), and the UF rate. Clinical validation of the model was obtained in 12 patients: predicted final [Na+P] agreed well with the values measured by means of direct potentiometry (141.9 vs. 142.1 mEq/liter; P = NS), with a mean difference (-0.16 mEq/liter) and limits of agreement (+0.8 to -1.03 mEq/liter) fully acceptable for clinical purposes.(ABSTRACT TRUNCATED AT 250 WORDS)

Removal of intracellular waste products by hemofiltration

Because the total amount of waste products removed by hemofiltration can be measured exactly, it was divided into two components, that removed from the extracellular compartment and that removed from the intracellular compartment, using inulin as a marker for extracellular fluid in five uremic patients treated with hemofiltration in the postdilution mode. The amount removed from the extracellular compartment as a proportion to the total amount removed from the whole body by hemofiltration was 86.5% +/- 10.6% for guanidinosuccinic acid, 69.1% +/- 15.6% for sodium, 59.4% +/- 3.3% for uric acid, 56.4% +/- 2.1% for inorganic phosphate, 45.6% +/- 5.3% for creatinine, 43.1% +/- 7.2% for potassium, 42.9% +/- 3.1% for guanidinoacetic acid, 42.5% +/- 7.5% for methylguanidine, 37.2% +/- 8.4% for chloride, 36.3% +/- 2.8% for urea, and 6.9% +/- 2.9% for glucose. These results show that extracellular substances such as sodium and guanidinosuccinic acid were removed mainly from the extracellular compartment. On the other hand, glucose was removed only from the intracellular compartment, since blood glucose level is regulated. Although uric acid, inorganic phosphate, creatinine, potassium, guanidinoacetic acid, and methylguanidine are intracellular substances, they accumulated also in the extracellular fluid in renal failure, and were removed from both compartments, intracellular as well as extracellular, by hemofiltration.

Effect of dialysate composition on intercompartmental fluid shift

Effect of dialysate composition on intercompartmental fluid shift and hemodynamics was studied in 12 patients during 1.5 or 2 hours of hemodialysis without net ultrafiltration, using high (H;Na 154 mmol/liter), normal (N;Na 140 mmol/liter) or low (L:Na 126 mmol/liter) concentration dialysate. H dialysate was associated with a small (0.9%) increase in blood volume, a larger increase in plasma volume and a decrease in erythrocyte volume. L dialysate resulted in a 2.3% decrease in blood volume, a larger decrease in plasma volume and an increase in erythrocyte volume. N dialysate gave results which were intermediately between the other two dialysis conditions. There was no difference in the post-dialysis mean arterial pressure between the groups, although heart rate increased more during H dialysis than during the other two conditions. Change in blood and erythrocyte volume correlated significantly with change in plasma Na concentration and osmolality, but not with change in plasma urea concentration. We conclude that dialysate composition affects the movement of water into and out of the plasma and erythrocytes in a manner that can be accounted for by altered plasma concentrations of osmotically active substances.

The amount of sodium removed by hemodialysis

The amount of sodium removed by hemodialysis was estimated, without using radioisotopes, as the change in total osmotically active cations, which is the product of the serum sodium concentration and urea-space. The extracellular and total body fluid volumes were measured using 35SO4 and 3H2O, respectively, in five stable hemodialysis patients under four different conditions. Urea-space determined, based on urea kinetics, was consistent with total body fluid volume measured by 3H2O. The amount of sodium removal, estimated as the change in the product of the serum (Na+) and urea-space, was equal to the change in the sodium content, which is the product of the serum (Na+) and extracellular fluid volume measured by 35SO4. Sodium removal may be divided into two components, diffusion and ultrafiltration.

Heterogeneity of the cardiovascular response to hemofiltration

Twenty-one stable maintenance hemodialysis patients were studied in a crossover format with hemofiltration to determine whether the lower incidence of symptomatic hypotension noted with hemofiltration could be correlated with changes in baroreflex function as tested using the cold pressor test and amyl nitrite inhalation study. Baroreflex function remained abnormal and unchanged in all patients in the face of a reduced incidence of symptomatic hypotension. Subdivision of the patients into frequent (greater than 1 episode/treatment) and infrequent (less than 1 episode/treatment) reactors during the hemodialysis control period resulted in the infrequent reactors, showing a significant increase in episodes of symptomatic hypotension/hemofiltration treatment where a significant reduction was noted with the frequent reactors. No clear correlation could be made between the incidence of symptomatic hypotension and the pre- to post-treatment change in body temperature. The presence of pretreatment hypertension, another previously identified correlate of symptomatic hypotension with hemodialysis, also could not be corroborated. Further, changes from baseline predialysis values in mean arterial pressure noted with hemofiltration could not be correlated with a changed incidence of symptomatic hypotension. We conclude that previously identified correlates of symptomatic hypotension noted in the hemodialysis setting may be dissociated during treatment with hemofiltration and that there is a heterogeneous patient response to this treatment. These data suggest that there are additional, as yet undetermined, pathophysiologic events that underly the symptomatic hypotension of artificial kidney treatment.

Clinical validation of a predictive modeling equation for sodium

Changes in plasma sodium (Na) concentration during hemodialysis were predicted by changes in Na concentration of the dialysate at equilibrium with the plasma, according to the formula C't = CD - (CD - C'0) [(V0 - QFt)/V0]A/QF, where C'0 and C't are the Na concentration of the dialysate at equilibrium with the plasma at times 0 and t, respectively; QF is the ultrafiltration flow rate; V0 is the initial total body water; and CD is the Na dialysate concentration. This modeling involves only one parameter, A, which is the effective sodium dialysance and depends on the dialyzer, the QF, the plasma water flow rate, and the actual Donnan coefficient. Parameter A was evaluated after 1 h of dialysis. Seven routine 4-h dialysis sessions were performed in which the Na concentration of dialysate at equilibrium with the plasma was measured at varying times. The mean (+/- SEM) difference between predicted and measured values was delta C = 0.5 +/- 0.2 mmol/L. These data support the validity of the model that allows the monitoring of Na dialysate concentration to obtain a prescribed Na plasma concentration at the end of a dialysis session.

Sodium modeling during hemodialysis: a new approach

Sodium volume modeling during hemodialysis encounters several difficulties. First, the actual sodium distribution volume is the extracellular water, whereas the ultrafiltration flow reflects the variation of total body water. Thus, a two-pool model must be considered. This will complicate the model by increasing the number of parameters and boundary conditions. An alternative is to consider the total body water as the apparent distribution volume of loaded or removed sodium, which leads to a single-pool model. Second, convective sodium transfer induced by ultrafiltration is not negligible compared with diffusive sodium transfer. Therefore, sodium transfer modeling must simultaneously take into account the diffusive and the convective part, with the coupling part related to both processes. Third, the Donnan effect due to nondiffusible anionic plasma proteins modifies the sodium transfer through the membrane. Adequate sodium volume modeling should be a compromise between oversimplification, resulting in discrepancies between calculated values and experimental data, and overcomplexity, involving a great number of parameters and boundary conditions, which leads to a model unsuitable for clinical application. A single-pool model is proposed with only one parameter that is estimated during the first period of the hemodialysis session.

Prediction of postdialysis serum sodium concentration and transcellular fluid shift without measuring body fluid volumes

The postdialysis levels of serum sodium concentration, urea concentration, and osmolality, as well as the magnitude of both transcellular fluid shifts and sodium removal, were predicted based on computer modeling without measuring body fluid volumes. A 4-h hemodialysis was performed in five patients at a constant ultrafiltration rate of 0.5 L/h using dialysate with normal (141 mEq/L) or high (150 mEq/L) Na+ concentration. The serum sodium concentration, urea concentration, and osmolality, as well as intracellular and extracellular fluid volumes, were determined before and after hemodialysis. The model predictions without measurement of body fluid volumes were in excellent agreement with measured values, suggesting clinical validity. The model may be useful in clinical practice to control the postdialysis levels of sodium and water content by computerized hemodialysis.