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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.

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 flo

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 intrato 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.

In conventional intermittent hemodialysis, the overall mass transfer coefficient (Ko) 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 (Ko) 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.

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.

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.

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)