Increase in blood volume during dialysis without ultrafiltration.

Combined dialysis and ultrafiltration leads to more frequent episodes of hypotension than isolated ultrafiltration. It has been suggested that decreased plasma volume preservation could be responsible for this phenomenon. The present study evaluates the effects of diffusive dialysis on the changes in relative blood volume (RBV). Six stable hemodialysis patients, without the need of ultrafiltration, were studied during 10 sessions of diffusive dialysis (bicarbonate) lasting 4 h. RBV was monitored continuously by measurement of hematocrit. During the 1st and 2nd h RBV increased by 2.4+/-1.4 and 2.5+/-0.8% respectively, returning to baseline levels at the end of dialysis. No changes in blood pressure or heart rate were noted. We conclude that during diffusive dialysis without ultrafiltration RBV is increased. A decrease in vascular resistance, or changes in regional blood distribution could explain these findings.

Variability of relative blood volume during haemodialysis.

BACKGROUND: A decrease in blood volume is thought to play a role in dialysis-related hypotension. Changes in relative blood volume (RBV) can be assessed by means of continuous haematocrit measurement. We studied the variability of RBV changes, and the relation between RBV and ultrafiltration volume (UV), blood pressure, heart rate, and inferior caval vein (ICV) diameter. METHODS: In 10 patients on chronic haemodialysis, RBV measurement was performed during a total of one hundred 4-h haemodialysis sessions. Blood pressure and heart rate were measured at 5-min intervals. ICV diameter was assessed at the start and at the end of dialysis using ultrasonography. RESULTS: The changes in RBV showed considerable inter-individual variability. The average change in RBV ranged from -0.5 to -8.2% at 60 min and from -3.7 to -14.5% at 240 min (coefficient of variation (CV) 0.66 and 0.35 respectively). Intra-individual variability was also high (CV at 60 min 0.93; CV at 240 min 0.33). Inter-individual as well as intra-individual variability showed only minor improvement when RBV was corrected for UV. We found a significant correlation between RBV and UV at 60 (r= -0.69; P<0.001) and at 240 min (r= -0.63; P<0.001). There was a significant correlation between RBV and heart rate (r= -0.39; P<0.001), but not between RBV or UV and blood pressure. The level of RBV reduction at which hypotension occurred was also highly variable. ICV diameter decreased from 10.3+/-1.7 mm/m(2) to 7.3+/-1. 5 mm/m(2). There was only a slight, although significant, correlation between ICV diameter and RBV (r= -0.23; P<0.05). The change in ICV-diameter showed a wide variation. CONCLUSIONS: RBV changes during haemodialysis showed a considerable intra- and inter-individual variability that could not be explained by differences in UV. No correlation was observed between UV or changes in RBV and either blood pressure or the incidence of hypotension. Heart rate, however, was significantly correlated with RBV. Moreover, IVC diameter was only poorly correlated with RBV, suggesting a redistribution of blood towards the central venous compartment. These data indicate that RBV monitoring is of limited use in the prevention of dialysis-related hypotension, and that the critical level of reduction in RBV at which hypotension occurs depends on cardiovascular defence mechanisms such as sympathetic drive.

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.

Continuous arterio-venous haemodiafiltration: hydraulic and diffusive permeability index of an AN-69 capillary haemofilter.

The dependence of uraemic solute clearance on the hydraulic and diffusive permeability index of an AN-69 capillary haemofilter is investigated during the treatment of patients with continuous arterio-venous haemodiafiltration (CAVHD). A mathematical model is presented to calculate solute clearance and the hydraulic and diffusive permeability index parameters from clinical data and to predict the blood flow rate entering the extra-corporeal circuit from the manufacturer's specifications and blood viscosity. By measuring the flow rates, the patient's mean arterio-venous pressure difference and uraemic solute clearance under different clinical and operational conditions, mathematical model equations are evaluated. During the average survival time of an AN-69 capillary haemofilter of about five days, it is found that both the hydraulic permeability index and the diffusive permeability index decline over treatment time, independent of the haemofilter resistance to blood flow. The measured haemofilter resistance to blood flow is three times higher than the haemofilter resistance predicted from the manufacturer's specifications and blood viscosity. Predicting the blood flow rate entering the extra-corporeal circuit from the arterial haematocrit, plasma protein concentration and temperature and the manufacturer's specifications is not reliable.

An analytical solution to solute transport in continuous arterio-venous hemodiafiltration (CAVHD).

In conventional intermittent hemodialysis, the overall mass transfer coefficient (Kd) of a dialyser is mostly calculated at zero ultrafiltration and at relatively high dialysate flow rates. In continuous arterio-venous hemodiafiltration (CAVHD), the dialysate flow rates are low as comparable to the rates of ultrafiltration flows, making the dialysis treatment as slow as possible. Therefore the overall mass transfer coefficient (Kd) of a CAVHD hemofilter has to be calculated in the presence of ultrafiltration. A mathematical model of CAVHD is presented in order to calculate the diffusive mass transfer coefficient (Kd) for a solute when blood, filtrate and dialysate flow rates and solute concentrations are known. The ultrafiltration volume flux (Jv) is assumed to vary linearly along the axial direction of the hemofilter. The calculated mass transfer coefficient Kd shows that at high values of dialysate flow and low values of ultrafiltration, the overall mass transfer coefficient (Kd) of a CAVHD hemofilter equals mass transfer coefficient (Kd) of a dialyser in conventional intermittent hemodialysis. Also, the calculated mass transfer coefficient Kd shows no significant differences when the ultrafiltration volume flux is assumed to be constant along the length of the hemofilter if no backfiltration occurs in the hemofilter.

Drug clearance by continuous haemodiafiltration. Analysis of sieving coefficients and mass transfer coefficients of diffusion.

In patients who were treated with continuous arteriovenous haemodiafiltration (CAVHD), using the AN-69 capillary dialyser, we measured the clearance rates of uraemic solutes and drugs at dialysate flow rates of 0, 1 and 3 l/h. By using a mathematical model of CAVHD, we analysed the data in terms of sieving coefficients and diffusive mass transfer coefficients. These parameters proved to be related to drug protein binding and molecular weight. The parameter values may be used to calculate the expected drug clearance rate under different operating conditions.

Blood flow, ultrafiltration and solute transport rate in continuous arteriovenous haemodiafiltration: the AN-69 flat-plate haemofilter.

We measured blood flow, ultrafiltration rate and uraemic solute clearance at different dialysate flow rates during CAVHD using the AN-69 0.43 m2 flat plate haemofilter. As filter performance depends on clinical conditions and operational characteristics, data were analysed in terms of resistance to blood flow, membrane index of ultrafiltration, and diffusive mass transfer coefficients. An attempt was made to construct nomograms that may be used both to predict filter performance and to compare different haemofilters with each other.

A mathematical model of continuous arterio-venous hemodiafiltration (CAVHD).

Continuous arterio-venous hemodiafiltration (CAVHD) differs from conventional hemofiltration and dialysis by the interaction of convection and diffusion, the use of very low dialysate flow rates and by the deterioration of membrane conditions during the treatment. In order to study the impact of these phenomena on diffusive transport, we developed a mathematical model of the kinetics of CAVHD solute transport from plasma water to dialysate. The model yields an expression of the diffusive mass transfer coefficient, Kd, as a function of blood, filtrate and dialysate flow rates and solute concentrations, which can be measured in the clinical setting. This paper gives a description of the model derivation. Kd is demonstrated to vary depending on dialysate flow and duration of treatment.

Solute transport in continuous arteriovenous hemodiafiltration: a new mathematical model applied to clinical data.

A mathematical model of continuous arteriovenous hemodiafiltration is presented, by which the diffusive mass transfer coefficient (Kd) for a solute may be calculated from blood, filtrate and dialysate flow rates and solute concentrations. The model was applied to clinical data obtained with 0.6-m2 AN69 capillary dialyzers that had been used for up to 5 days. The diffusive mass transfer coefficient proved to depend on dialysate flow rate. Furthermore, it was related to the membrane index of ultrafiltration, as measured in the clinic, and to the filter resistance to blood flow. Measurement of these filter characteristics allowed a reasonable prediction of solute clearance.

Determinants of blood flow and ultrafiltration in continuous arteriovenous haemodiafiltration: theoretical predictions and laboratory and clinical observations.

In continuous arteriovenous haemofiltration (CAVH) or haemodiafiltration (CAVHD), it is important to obtain an adequate blood flow through the haemofilter to minimise the risk of excessive haemoconcentration and clotting. In this study we determined the resistance to blood flow of the extracorporeal device as well as the hydraulic permeability of the filter membrane is intensive care patients treated with CAVHD. Data were obtained for CAVH catheters and Scribner shunts and for a polyacrylonitrile (AN-69) plate filter, an AN-69 capillary filter and a polysulphone (PS) capillary filter. In accordance with recent literature we also predicted the resistance to flow by using Poiseuille's law and a formula for the estimation of blood viscosity. Although with all three filters an adequate blood flow was usually obtained, the resistance to blood flow was 2-3 times greater than the predicted value. With continued use of the filter, resistance to blood flow remained largely unchanged. When, in the laboratory, the AN-69 capillary filter was perfused with saline and with a viscous sucrose solution, the resistance to flow was only 1.4 time the predicted value, a difference that might result from small deviations of the capillary diameter. When perfused with blood, the resistance was 2.6 times greater than the predicted value. This was largely explained by gross underestimation of blood viscosity in these patients. By combining laboratory data on filter resistance during saline perfusion and a more accurate estimation of blood viscosity, a reasonably accurate prediction of blood flow rate would be feasible. In the clinic the hydraulic permeability of the filters decreased with time.(ABSTRACT TRUNCATED AT 250 WORDS)