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D Schneditz

Publications and source records attributed to D Schneditz.

At least 37 records · Page 2Linked to original sources
PubMed1995

Solute disequilibrium and multicompartment modeling.

Mathematical models that simulate the exchange of solute between multiple body compartments have been used to study the distribution, elimination, and transport of urea, water, electrolytes, and other substances in the dialysis patient. Within a compartment, such substances are assumed to be uniformly distributed while exchange between compartments or with the environment may occur in a number of different ways. Diffusion in response to concentration gradients between, for example, intracellular and extracellular spaces, and convection due to blood flow have been identified as the most important transport mechanisms. Any system with more than one compartment may develop nonuniform solute distribution or solute disequilibrium between compartments. The minimum number of compartments required to model a kinetic process such as urea removal during hemodialysis depends on the accuracy and temporal resolution required, with higher resolution calling for more compartments. A two-compartment model is adequate for most clinical purposes. The physiological meaning or anatomic counterparts of the mathematical compartments remain uncertain as both flow and diffusion transport mechanisms contribute to the disequilibrium. Processes such as access and cardiopulmonary recirculation may be represented as additional compartments with small distribution volumes and high mass transport rates. Failure to recognize the effect of multiple compartments will result in an inaccurate measurement of dialysis dose and an inadequate hemodialysis prescription with a predictably poor clinical outcome. Allowance for compartment effects is particularly important in patients receiving treatment with a high ratio of dialyzer clearance to total body water, now commonly encountered during short-time, high-efficiency dialysis.

Biological Transport↗
PubMed1995

Is intercompartmental urea clearance during hemodialysis a perfusion term? A comparison of two pool urea kinetic models.

Analyses of intradialytic and postdialytic urea profiles call for models that consider delayed urea transfer from different parts of the body to the blood. There are two different approaches to the problem. In the classical cell membrane model it is assumed that the two compartments refer to the serial (s) arrangement of extracellular and intracellular volumes, whereas in the regional blood flow model the two compartments are identified as parallel (p) organ systems with high or low perfusion. In the cell membrane model, delayed urea removal from peripheral body compartments is governed by intercompartmental clearance (Kc) which is a function of cell membrane permeability, whereas in the regional blood flow model delayed urea removal is related to low perfusion (QL) of the large muscle/skin/bone compartment. Both models were compared in a set of 16 high-efficiency hemodialysis treatments. Modeled volumes (Vm,s = 31.2 +/- 9.5 L; Vm,p = 30.0 +/- 8.3 L) and modeled dose of hemodialysis (Kt/Vm,s = Kt/Vm,p = 1.12 +/- 0.33) were the same for both models. However, volumes modeled by either technique were significantly lower than anthropometric volumes (V alpha = 35.0 +/- 6.4 L). These data suggest that at this point the two models are experimentally indistinguishable. Moreover, the main system parameters of both models, Kc (0.54 +/- 0.16 L/min) and QL (0.63 +/- 0.15 L/min) showed a strong linear dependence (QL = 0.921 Kc + 0.139, r2 = 0.884), whereas no relation could be found between Kc and Vm. Therefore, delayed transport that has up to now been characterized by membrane permeability may also be explained by peripheral perfusion.

Adult↗
PubMed1993

Characterization of "refilling types" by continuous blood volume monitoring during hemodialysis.

Fluid removal during HD is frequently associated with acute hypotension due to insufficient mobilization of extravascular fluid and subsequent hypovolemia. Large variability in vascular refilling makes dialysis therapy difficult and requires a better understanding of fluid distribution in the individual hemodialysis (HD) patient. Blood volume monitoring was performed by continuous measurement of blood density with a DMA 46 Density Meter (Fa. Chempro, PAAR, Austria) in six patients on regular HD treatment. A body filtration coefficient (CF = extra/intravascular fluid shift) was calculated using a computer model by Schneditz et al (1990) and blood density was measured during a 60-minute ultrafiltration period (1/3 x delta kg/hr = 19 +/- 4 ml/min). Concerning blood density differences (delta f%) and body filtration coefficient (CF) there was a wide inter-individual range (delta f = 2.8-8.0%, CF = 3-9 ml/mm Hg/min), but there was a good intraindividual reproducibility of delta f and CF. A negative correlation (r = -0.95) between delta f and CF could be established. The severity of hypotensive episodes and frequency of interventions correlated well with delta f and CF; severe symptoms occurred with a delta f > 6% and a CF < 4 ml/mm Hg/min. These results suggest that improvement in dialysis therapy can be achieved by blood volume monitoring and classification of "refilling types." By blood volume-controlled computerized sodium and UF profiles, a reduction of hypotensive episodes and emergency intervention might be possible.

Blood Volume↗
PubMed1992

Nature and rate of vascular refilling during hemodialysis and ultrafiltration.

The change of blood volume, of blood and plasma density (rho b, rho p) following a short ultrafiltration pulse (duration: 20 min; mean rate -35 ml/min) within the first hour of hemodialysis was analyzed in 13 hemodynamically stable patients (30 single measurements). Protein concentration of refilling volume (7 g/liter) was calculated from its density (1009.25 +/- 3.7 kg/m3, at 20 degrees C) and from the linear relationship between plasma density and protein concentration (cp) of uremic plasma samples (rho p = 1007.46 + 0.2422 x cp). The filtration coefficient (Lp,calc) determined from a relation derived from Starling's hypothesis was 5.6 +/- 1.4 ml/(min.mm Hg.50 kg lean body mass); N = 13, mean +/- SD, minimum 3.2, maximum 8.0. A model describing the dynamics of blood and plasma volume was developed. It was fit to on-line measurements of relative blood volume changes by variation of the filtration coefficient and of initial blood volume (Lp,fit, Vb,fit). The linear regression between Vb,fit and blood volume determined from anthropometry (Vb,calc) was highly significant (r = 0.79, N = 30, P < 0.001). Compared to Vb,calc, Vb,fit was typically increased by 21 +/- 11%, reflecting a fluid overload at the beginning of the treatment. Lp,fit was not different from Lp,calc. Lp,fit significantly increased with blood volume excess. Due to the small but definite protein content of refilling volume, the model accounts for increased blood volume recovery and occasional overshoot of blood and plasma volumes following ultrafiltration.

Adult↗
PubMed1991

The measurement of blood density to investigate protein deposition at the blood/hollow fiber membrane interface during ultrafiltration.

Concentration polarization and secondary membrane formation change the membrane hydraulic permeability of capillary filters during hemofiltration reducing initial value filtration rates up to 50%. This leads to a significant loss of filter efficiency which must be taken into consideration when designing filters for long-term application such as in implantable artificial kidneys. By measuring blood density uninterrupted over a period of time using the mechanical oscillatory technique it is possible to follow the dynamics of protein deposition at the interface between the blood and capillary walls. The resulting picture of the deposition behaviour can lead to a better understanding of time-dependent filtration where flow and pressure conditions change. Protein deposition (Pt) with polysulfone membranes in relation to the effective capillary surface was in the range of 2.2. mg/(h x cm2) according to a logarithmic function (Pt = a + b x log(t)).

Animals↗
PubMed1990

[Continuous measurement of blood volume changes in hemodialysis using an ultrasound method].

Haemodialysis treatment comprises the removal of surplus body water, mainly by ultrafiltration. Frequent complications, such as hypotension, are believed to be related to an imbalance between blood volume reduction, based upon ultrafiltration, and vascular refilling. The control of fluid balance can be achieved by the measurement of blood volume changes. A new method of determining total blood protein concentration by ultrasonic means facilitates continuous monitoring of blood volume changes during haemodialysis. Blood volume monitoring was undertaken during 38 haemodialysis treatments (19 patients) in order to achieve a better adjustment of the patient's estimated dry weight. The relative change in blood volume was registered in 11 patients who were first ultrafiltrated to their estimated dry weight. In a following session the ultrafiltration was increased by 10%. The relative change in blood volume, normalized to the change in total body water, increased significantly from 1.16 +/- 1.11% (normal ultrafiltration) to 1.67 +/- 0.8% (normal ultrafiltration + 10%) (p less than 0.05). Thus, normalized blood volume reduction may serve as an approximation to adjust the patient's dry weight.

Adult↗
PubMed1990

Methods in clinical hemorheology: the continuous measurement of arterial blood density and blood sound speed in man.

Both blood density and sound speed are closely related to total protein concentration in blood and, as a consequence, to rheologically important parameters of blood. Two methods that permit continuous measurement of these properties, the mechanical oscillator technique and the new ultrasonic technique, were used for measuring blood protein concentration over a continuous period of time in a group of hemodialysis patients and in volunteers. It was seen that the concentration of the components of blood varies considerably. This variability is related to transport phenomena within as well as to the flow of masses across the cardiovascular compartment. From the continuous measurement of concentrations during hemodialysis treatment, relative changes in blood volume can be recorded in order to control the fluid balance of the patient. Rapid fluctuations at the macroscopic scale with periods of 5 to 30 seconds are due to heterogeneities at the microscopic scale and to the particular rheological behaviour of the red blood cells at the level of the capillaries and the small blood vessels. The amplitude of rapid oscillations increased up to 1.2% in terms of hematocrit values when there was rhythmic, spontaneous breathing at various frequencies. The measurement of concentrations at an accessible measuring site may be used to investigate the rheology of blood in the human microvasculature.

Blood Flow Velocity↗
PubMed1989

Sound speed, density and total protein concentration of blood.

The sound speed and the density of a series of blood samples was measured in a temperature range from 20 degrees C to 40 degrees C. An expression was derived from a least square fit which related the sound speed in human blood in this temperature range to the blood density and to the total protein concentration, respectively. The data were obtained from the measurement of sound speed and density by means of a newly developed Density and Sound Analyzer (DSA-48, A. Paar K.G., 8054 Graz, Austria). The relationship of sound speed, density and temperature permits the calculation of the total protein concentration with an accuracy of 1 g/kg. The resolution of the measurement is in the order of 0.1 g/kg.

Blood Proteins↗
PubMed1987

Quick measurement of hematocrit and erythrocyte sedimentation-rate by means of a density tracking method.

An instrument to track the sedimentation properties of red blood cells quickly and automatically and to evaluate the hematocrit as well as the plasma density of a blood sample in the same operation is described. Erythrocyte-sedimentation basically is due to the density difference between the red blood cells and the plasma and the new method makes use of the mechanical-oscillator-technique, where the resonant frequency of the bending-type oscillations of a U-shaped glass tube is related to the density of the fluid with which the tube is filled. The oscillating U-tube is part of a sedimentation-tube of special dimensions and particular spatial orientation. The tube is tilted at 60 degrees, the tip of the U-tube being raised above the horizontal plane. Filling the sedimentation-tube with anticoagulated blood first permits recording of the density of the homogeneous suspension. Then, after a few minutes, the onset of erythrocyte-sedimentation produces a decrease in recorded density. The final value is related to the density of the supernatant plasma-fluid. Hematocrit, blood and plasma density of the undiluted blood sample as well as the sedimentation-rate of the anticoagulated sample are automatically calculated from the recorded sedimentation-curve. The particular arrangement of the tilted sedimentation-tube facilitates a determination of the maximum sedimentation-rate within 15 min at most.

Blood Sedimentation↗
PubMed1987

Viscoelastic properties of whole blood. Influence of fast sedimenting red blood cell aggregates.

Red blood cell (RBC) aggregation is known to be of deciding influence on erythrocyte sedimentation-rate (ESR) and on whole blood viscoelastic properties. The rheological behaviour of blood collected from a control-group with normal ESR is compared to the viscoelastic behaviour of blood collected from two groups with high to very high ESR, whose individuals are suffering from chronical polyarthritis and Morbus Bechterew, respectively. The rheological properties are evaluated by means of an oscillating-flow capillary-rheometer where the viscous (eta') and elastic (eta") component of the complex viscosity (eta) is measured at a constant frequency of 2 Hz. Correcting for the varying hematocrit of the different blood samples according to an exponential equation, the viscoelastic data are found to be elevated in the groups with high ESR. For the viscous properties this is only due to the increase of the plasma viscosity. A correction for the plasma viscosity, however, shows that the viscous properties at low shear- rates (2s-1) are significantly reduced, whereas elastic properties in a range of medium shear-rates (10s-1 to 50s-1) are significantly increased (P less than 0.001, t-test of Student). This result is discussed to be due to the high packing density of the RBC in fast sedimenting aggregates. High packing density reduces the effective volume of the RBC but increases the stiffness of the aggregates.

Adolescent↗
PubMed1985

Rheological discrimination between native, rigid and aggregated red blood cells in oscillatory flow.

The viscoelastic behaviour of hardened or aggregated red blood cells is compared with the flow pattern of native red blood cells, all suspended in buffer solution at a hematocrit of 45%. The rheological properties are investigated under oscillatory shear at the constant frequency of 2Hz. Variation of the amplitude covers a range of shear-rates from 0.5/s to 200/s. It can be seen that rigidification of the red cells by treatment with glutardialdehyde leads to changes of the flow properties in the range of shear-rates above 10/s, whereas aggregate formation due to addition of dextran distinctly alters the flow properties in the range of shear-rates below 10/s.

Blood Viscosity↗
PubMedDate not supplied

Influence of tonicity on the viscoelastic properties of blood during isovolemic dilution.

The influence of isotonic or hypotonic dilution on viscoelastic properties of blood is examined. The viscous, as well as the elastic, properties of blood samples diluted with isotonic saline or pure water, respectively, and of undiluted whole blood samples are compared by means of a dynamic capillary viscosimeter (OCR-D, A. Paar K.G., Austria). The dilution (approximately 17% of the total blood volume) was performed isovolemically and retained the same rbc count. The rbc swelling observed as a consequence of changes in plasma osmolarity was tracked by the high resolution density measurement, according to the mechanical oscillator technique. Since no significant rbc swelling was found in the dilution with isotonic saline, viscoelastic resistance of blood was efficiently reduced in the observed range of shear rates (2 s-1-100 s-1). This decrease is due to reduced plasma protein concentration, which also lowers plasma viscosity by approximately 18%. Although plasma viscosity is significantly decreased in hypotonic dilutions (-12%), flow properties of the rbc suspensions are, in general, significantly impaired. This is due to the osmotic rbc swelling (hematocrit = +8%), which increases viscous resistance within the suspending fluid, as well as elastic resistance of the rbc due to a loss in rbc deformability. It can be concluded that isotonic dilution leads to a decrease in the viscosity of blood, whereas hypotonic dilution--in an order of magnitude which may occur during resorption of water--leads to increased viscous and elastic resistance of the blood.

Blood Viscosity↗
PubMedDate not supplied

Cardiopulmonary recirculation in dialysis. An underrecognized phenomenon.

Access recirculation can be determined by measuring blood temperature or blood water concentration in the dialyzer inlet after injecting a bolus of cold saline into the venous line. In patients with access recirculation, some of the cooled venous blood re-enters the blood inlet line soon after injection, resulting in a sharp transient drop in its temperature. There is also a prompt increase in blood water concentration at the dialyzer blood inlet caused by the dilution effect of the recirculated saline. In this study, data are reported on four patients studied under conditions where no access recirculation could occur because blood was returned to a second access or into a central vein. In these patients, transient cooling of the blood in the dialyzer inlet and a transient hemodilution after venous line injection of cold saline was still observed. These observations can be explained by passage of the injected saline through the heart and pulmonary blood vessels and return of a portion of the cooled blood to the vascular access, bypassing the systemic capillary microcirculation. This "cardiopulmonary recirculation" can cause dilution of urea in dialyzer inlet blood, with resulting errors in urea kinetic modeling and in computing access recirculation.

Arteriovenous Shunt, Surgical↗
PubMedDate not supplied

Formal analytical solution to a regional blood flow and diffusion based urea kinetic model.

At the beginning of hemodialysis (HD), urea in arterial blood drops rapidly, whereas a pronounced postdialytic urea rebound (PDUR) can be observed when HD is discontinued. In a new approach to this observation, the authors suggest that solute flux from remote body compartments to the dialyzer is governed by 1) diffusion of solutes from the tissue to the blood perfusing the tissue, and 2) by regional blood flow distribution, cardiopulmonary recirculation, and access recirculation, respectively. These concepts were incorporated into a variable volume, two compartment model that could be treated as an eigenvalue problem and solved analytically. The resulting equations were used to model intradialytic and postdialytic urea profiles with the help of a commercial spreadsheet program. The significance of hemodynamic model parameters such as cardiac output (CO) and regional blood flow distribution on PDUR was modeled in simulation runs, where PDUR increased from 5% to 15% when CO fell from 7 to 3 l/min with standard treatment parameters (t = 3h, KD = 0.3 l/min, V = 35l, UFV = 2.8l, fQHFS = 0.8, QAc = 0.8l/min). Thus, this urea kinetic model establishes a previously missing link between hemodynamics and solute removal.

Biological Transport, Active↗
PubMedDate not supplied

Overestimation of hemodialysis dose depends on dialysis efficiency by regional blood flow but not by conventional two pool urea kinetic analysis.

In 26 patients, a linear relationship between delta Kt/V (equilibrated minus single pool) and dialysis efficiency K/V was noted (r = -0.72). To determine if such a relationship would be supported by formal urea kinetic analysis, t, Kd, and V were randomly varied in 1,400 simulations using both intracellular/extracellular and regional blood flow 2 pool variable volume models. In the intracellular/extracellular model, delta Kt/V was best correlated with Kd/Kc (r = -0.96), where Kc is the intercompartmental clearance. Kc was not correlated with V, which translated into a lack of correlation between delta Kt/V and V, and a better correlation between delta Kt/V and Kd than between delta Kt/V and K/V. In the regional blood flow model delta Kt/V was best correlated with Kd/QL (r = -0.99), where QL is the perfusion of the low flow compartment. QL was correlated with V because QL is a function of cardiac output, which varies with surface area and therefore with V. In the regional blood flow model, delta Kt/V did correlate with V (r = 0.49), and better with K/V (r = -0.76) than with K (r = -0.47), similar to the results in patients. The slope of delta Kt/V on K/V depended upon fQL (the fractional perfusion of the low flow compartment) and on cardiac index. At an fQL of 0.15 and a cardiac index of 2.85, the theoretical slope was similar to that seen in observational data: delta Kt/V = -0.6 x K/V + 0.03. The results show that the regional blood flow model predicts the observed relation between delta Kt/V and K/V, whereas the intracellular/extracellular model fails in this task unless one arbitrarily ties Kc to V.

Computer Simulation↗