Solute clearance

Higher solute clearance allows intermittent treatment. 

Soluble oil metal-working fluids, 1 22 Soluble oils, 15 240 Soluble silicates, 22 451-452 dissolution of, 22 455-456 history and applications of, 22 452 Soluble starch synthases, 12 492 Soluble titanium glycolate complexes, 25 87 Solute clearances... 

Because elimination clearances are additive/ total solute clearance during hemodialysis (CLp) can be expressed as the sum of dialysis clearance (CLd), and the patienfs renal clearance (CLr) and nonrenal clearance (CLjvp) ... 

By analogy with Equation 6.6/ the contribution of CLec to total solute clearance during continuous renal replacement therapy is given by... 

In the postinjury period an inverse relationship exists between solute clearance by the body and ADH levels in the plasma. It is suggested that ADH depresses solute-linked losses of urine by altering either the glomerular filtration rate or the renal plasma flow. It is sometimes necessary to attempt to overcome the ADH response in the postoperative period, and this can be done only by increasing the solute load to the body. Urea and mannitol will produce a modest diuresis, but the infusion of modest amounts of sodium (75-100 mEq) is the simplest method of producing more urine in the presence of excess ADH (C4, C8). 

The assessment of adequacy of dialysis treatment for individual patients in the clinical setting includes consideration of the patient s well-being, cardiovascular risk, nutritional status, and degree of achievable ultrafiltration. It also includes estimates of a number of laboratory parameters— such as hemoglobin, phosphate, and albumin— and clearance of the small solutes urea and creatinine. During die following discussion, the term adequacy will refer to small solute clearances obtained from both dialysis and residual renal function (RRF), For practical reasons, HD adequacy is calculated using urea as the small solute. 

The role of solute removal in defining adequacy of PD is complicated by the concern that solute removal by PD may not be clinically equivalent to an equal quantitated solute removal by RRF. For PD, small solute clearance targets have often been established on the assumption that peritoneal and renal clearances are equivalent and therefore additive. However, most studies that exammed the relationship between small solute clearances and mortality rates noted that patient survival was directly correlated with renal clear-... 

An adjunct to the assessment of adequacy in PD patients is the peritoneal equilibration test (PET). The PET assesses peritoneal membrane transport characteristics in terms of solute clearances and ultrafiltration. The results are used to select a dialysis regimen appropriate to the transport characteristics of the patient (e.g., high transporters may do better on short dwell APD regimens). The PET is typically undertaken at the same time as adequacy assessments in PD patients. 

Diaz-Buxo JA, Lowrie EG, Lew NL, Zhang SM, Zhu X, Lazarus JM. Associates of mortality among peritoneal dialysis patients with special reference to peritoneal transport rates and solute clearance. Am J Kidney Dis 1999 33 523-34. 

The Canada-USA cooperative study (CANUSA) evaluated 680 PD patients in 14 centers who began dialysis between 1990 and 1992 64 Decj-eases of 0.1 in weekly Tf/Purea or 5 L/1.73 m per week in Clcr were associated with 5% to 7% increases in the risk for death. No plateau was observed. Based on this finding it would appear that the greater the urea and creatinine clearances, the greater the rate of patient survival. One criticism of the CANUSA study was that it was assumed that the renal and peritoneal contributions to Kt/V were equivalent. Interestingly, the CANUSA data were reanalyzed to assess the contribution of residual renal function to the clinical results reported in CANUSA. When solute clearance was subdivided into that contributed by residual renal function and that from peritoneal clearance, the former was found to be a more significant predictor of patient survival. 

The K/DOQI clinical practice guidelines suggest that the adequacy of PD be assessed by using measured Kt/V and CEj three times in the first 6 months of dialysis (i.e., at months 1, 4, and 6). The reasoning behind this frequency is to accurately establish a baseline creatinine and urea excretion rate. Thereafter the KtA and Clcr should be measured every 4 months, at months 10, 14, and so on. The rationale for this is that it is imperative to detect subtle decreases in residual renal function and noncompliance and to make the necessary alterations to the prescribed PD dose to compensate for them. It is recommended that the first PET be conducted within the first month of treatment. Because solute clearance is dependent on peritoneal membrane transport properties, the guidelines also recommend that a PET be conducted within the first month of treatment. Future PET assessment is only recommended for patients with suspected changes in peritoneal membrane transport function, particularly when usual efforts to increase the PD dose are not successful. 

A major objective of fundamental studies on hollow-fiber hemofliters is to correlate ultrafiltration rates and solute clearances with the operating variables of the hemofilter such as pressure, blood flow rate, and solute concentration in the blood. The mathematical model for the process should be kept relatively simple to facilitate day-to-day computations and allow conceptual insights. The model developed for Cuprophan hollow fibers in this study has two parts (1) intrinsic transport properties of the fibers and (2) a fluid dynamic and thermodynamic description of the test fluid (blood) within the fibers. 

When the permeable species is partially rejected by the membrane (as in fractionation), a higher wash volume turnover will be required to achieve the same degree of clearance. Figure 3.96 may be used to predict solute clearance for a specified membrane retention (R). 

Clearance determinations discussed so far all require measurement of concentrations in carefully timed urine and plasma samples. In addition, useful approximations to relative solute clearance values can be obtained by simplified procedures. The best known of these simply takes the plasma level of urea, or preferably creatinine, as a measure of the GFR. Indeed, if creatinine excretion (UV in g/day) is constant, the GFR (=UV/P) theoretically is inversely proportional to Pcr/ the creatinine concentration is plasma any increase in P r above a normal level of around 1 mg/dl should therefore reflect a corresponding fall in GFR. In practice the method is not very sensitive in the normal range of plasma creatinine levels (<1.4 mg/dl) a better correlation between measured creatinine clearance (Cc) and that predicted on the basis of Pcr is obtained at higher plasma levels, that is, lower Gcr values [6,21]. 

In hemodiafiltration, diffusive solute removal is superimposed to purely convective transport across HF membranes by circulating a dialysate in the shell of a hemofilter counter currently with respect to the blood. Superimposition of diffusive transport, driven by a concentration gradient, to convection augments the poor LMW solute clearance typical of HF, while still retaining the good MMW solute clearance typical of HF (Legallais et al., 2000). Different techniques to deliver the replacement fluid have been proposed to... 

The terminology for characterizing dialysis membranes is somewhat unique to the dialysis field. Instead of being characterized in terms of hydrauhc penneabUity, diffusive membrane permeabihties, and solute rejection coefficients, dialyzers arc generally characterized in terms of an ultrafiltration coefficient (Kuf), solute clearances, and the product of the mass transfer coefficient times the surface area (KoA). 

The clearance rate of a solute, or solute clearance, is defined as the mass removal rate divided by the concentration of the solute in the blood and is expressed in units of miUUiters/minute. Thus, the clearance represents the equivalent volume of blood fully cleared of the solute each minute and cannot exceed the blood flow rate to the dialyzer. The term clearance was originally used as a measure of the performance of the natural kidneys, which operate continuously and rely primarily on convection for solute removal. 

Figure 19.3 shows a typical hemodialyzer. Device properties such as the fiber length, membrane surface area, number of fibers, hollow fiber packing density, and header design aU affect solute clearances. 

The plans to lower adequacy targets for PD notwithstanding (J. Burkhart, personal communication), we believe that higher solute clearances will be needed if PD is to grow as a modality. Newer APD techniques are the answer to these requirements. Higher adequacy targets, as well as abetter quality of life, will be more easily delivered by APD. Improved dialysis solutions and continuous flow techniques are further steps in the evolution of APD. 

Freida PH, Issad B, Allouache M. 1998. Relationships between fill volume, small solute clearances, and net ultrafiltration during a standardized APD program. Petit Dial Int 18(1) 124. 

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