Example: bachelor of science

Principles of Dialysis: Diffusion, Convection, and ...

1 Principles of Dialysis: Diffusion, Convection, and dialysis MachinesChronic renal failure is the final common pathway of a numberof kidney diseases. The choices for a patient who reaches thepoint where renal function is insufficient to sustain life are 1) chronic dialysis treatments (either hemodialysis or peritoneal dialysis ),2) renal transplantation, or 3) death. With renal failure of any cause,there are many physiologic derangements. Homeostasis of water andminerals (sodium, potassium, chloride, calcium, phosphorus, magne-sium, sulfate), and excretion of the daily metabolic load of fixedhydrogen ions is no longer possible. Toxic end-products of nitrogenmetabolism (urea, creatinine, uric acid, among others) accumulate inblood and tissue.

1 Principles of Dialysis: Diffusion, Convection, and Dialysis Machines C hronic renal failure is the final common pathway of a number of kidney diseases.

Tags:

  Failure, Dialysis, Kidney

Information

Domain:

Source:

Link to this page:

Please notify us if you found a problem with this document:

Other abuse

Transcription of Principles of Dialysis: Diffusion, Convection, and ...

1 1 Principles of Dialysis: Diffusion, Convection, and dialysis MachinesChronic renal failure is the final common pathway of a numberof kidney diseases. The choices for a patient who reaches thepoint where renal function is insufficient to sustain life are 1) chronic dialysis treatments (either hemodialysis or peritoneal dialysis ),2) renal transplantation, or 3) death. With renal failure of any cause,there are many physiologic derangements. Homeostasis of water andminerals (sodium, potassium, chloride, calcium, phosphorus, magne-sium, sulfate), and excretion of the daily metabolic load of fixedhydrogen ions is no longer possible. Toxic end-products of nitrogenmetabolism (urea, creatinine, uric acid, among others) accumulate inblood and tissue.

2 Finally, the kidneys are no longer able to function asendocrine organs in the production of erythropoietin and 1,25-dihy-droxycholecalciferol (calcitriol). dialysis procedures remove nitrogenous end-products of catabo-lism and begin the correction of the salt, water, and acid-base derange-ments associated with renal failure . dialysis is an imperfect treatmentfor the myriad abnormalities that occur in renal failure , as it does notcorrect the endocrine functions of the for starting dialysis for chronic renal failure are empiricand vary among physicians. Some begin dialysis when residual glomerularfiltration rate (GFR) falls below 10 mL/min m2body surfacearea (15 mL/ m2in diabetics.) Others institute treatmentwhen the patient loses the stamina to sustain normal daily work andactivity.

3 Most agree that, in the face of symptoms (nausea, vomiting,anorexia, fatigability, diminished sensorium) and signs (pericardialfriction rub, refractory pulmonary edema, metabolic acidosis, foot orwrist drop, asterixis) of uremia, dialysis treatments are urgently W. as Treatment of End-Stage Renal DiseaseFUNCTIONS OF THE kidney AND PATHOPHYSIOLOGY OF RENAL FAILUREF unctionSalt, water, and acid-base balanceWater balanceSodium balancePotassium balance Bicarbonate balanceMagnesium balancePhosphate balanceExcretion of nitrogenous end productsUreaCreatinineUric acidAminesGuanidine derivativesEndocrine-metabolicConversion of vitamin D to active metaboliteProduction of erythropoietinReninDysfunctionSalt, water, and acid-base balanceFluid retention and hyponatremiaEdema, congestive heart failure , hypertensionHyperkalemiaMetabolic acidosis, osteodystrophyHypermagnesemiaHyperphosph atemia, osteodystrophyExcretion of nitrogenous end products?

4 Anorexia, nausea, pruritus, pericarditis, polyneuropa-thy, encephalopathy, thrombocytopathyEndocrine-metabolicOsteo malacia, osteodystrophyAnemiaHypertensionFIGURE 1-1 Functions of the kidney and pathophysiology of renal 1-2 Statue of Thomas Graham in GeorgeSquare, Glasgow, Scotland. The physico-chemical basis for dialysis was firstdescribed by the Scottish chemist ThomasGraham. In his 1854 paper On OsmoticForce he described the movements of various solutes of differing concentrationsthrough a membrane he had fashionedfrom an ox bladder. (FromGraham [1].)UreaUreaCreatinineCreatinineHCO3 HCO3 Ca2+Ca2+K+K+Na+Na+BloodMembraneDialysate FIGURE 1-3 Membrane fluxes in dialysis . dialysis is the process of separating elements in a solution bydiffusion across a semipermeable membrane (diffusive solute transport) down a concentra-tion gradient.

5 This is the principal process for removing the end-products of nitrogenmetabolism (urea, creatinine, uric acid), and for repletion of the bicarbonate deficit of themetabolic acidosis associated with renal failure in humans. The preponderance of diffusionas the result of gradient is shown by the displacement of the of dialysis : Difusion, Convection, and dialysis MachinesAcidifiedconcentrateBicarbonatec oncentrateConductivitymonitorMembrane unitHeparinpumpBloodpumpPatientPumpWater DrainHeatexchangerSpentdialysateHeaterPu mpMix 1 DeaeratorMix 2 SpentdialysatepumpBloodleakdetectorAirem bolusdetectorVolumebalancesystemUltrafil tratepumpFIGURE 1-4 Simplified schematic of typical hemodialysis system. In hemodialysis,blood from the patient is circulated through a synthetic extracorporealmembrane and returned to the patient.

6 The opposite side of thatmembrane is washed with an electrolyte solution (dialysate) contain-ing the normal constituents of plasma water. The apparatus contains a blood pump to circulate the blood through the system, proportioningpumps that mix a concentrated salt solution with water purified byreverse osmosis and/or deionization to produce the dialysate, a meansof removing excess fluid from the blood (mismatching dialysateinflow and outflow to the dialysate compartment), and a series ofpressure, conductivity, and air embolus monitors to protect thepatient. Dialysate is warmed to body temperature by a 1-5 The hemodialysis membrane. Most membranes are derived fromcellulose. (The earliest clinically useful hemodialyzers were madefrom cellophane sausage casing.) Other names of these materialsinclude cupraphane, hemophan, cellulose acetate.

7 They are usuallysterilized by ethylene oxide or gamma irradiation by the manufac-turer. They are relatively porous to fluid and solute but do notallow large molecules (albumin, vitamin B12) to pass freely. Thereis some suggestion that cupraphane membranes sterilized by ethyleneoxide have a high incidence of biosensitization, meaning that thepatient may have a form of allergic reaction to the , polyacrylonitrile, and polymethylmethacrylate membranesare more biocompatible and more porous (high flux membranes).They are most often formed into hollow fibers. Blood travels downthe center of these fibers, and dialysate circulates around the outsideof the fibers but inside a plastic casing. Water for dialysis must meetcritical chemical and bacteriologic standards. These are listed inFigures 1-6 and as Treatment of End-Stage Renal DiseaseASSOCIATION FOR THE ADVANCEMENT OF MEDICALINSTRUMENTATION CHEMICAL STANDARD FORWATER FOR HEMODIALYSISS ubstanceAluminumArsenicBariumCadmiumCalc iumChloramineChlorineChromiumCopperFluor ideLeadMagnesiumMercuryNitratePotassiumS eleniumSilverSodiumSulfateZincConcentrat ion (mg/L) 1-6 Association for the Advancement of Medical Instrumentation(AAMI) chemical standards for water for hemodialysis.

8 Beforehemodialysis can be performed, water analysis is for hemodialysis generally requires reverse osmosis treat-ment and a deionizer for polishing the water. Organic materials,chlorine, and chloramine are removed by charcoal filtration.(FromVlchek [2]; with permission.)ASSOCIATION FOR THE ADVANCEMENT OF MEDICALINSTRUMENTATION BACTERIOLOGIC STANDARDSFOR dialysis WATER AND PREPARED DIALYSATED ialysis waterPrepared dialysateColony-forming units/mL<200<2000 FIGURE 1-7 Association for the Advancement of Medical Instrumentation(AAMI) bacteriologic standards for dialysis water and prepareddialysate. Excess bacteria in water can lead to pyrogen water supply systems are designed so that there are nodead-end connections. Because the antiseptic agents (chlorine andchloramine) have been removed in water treatment, the water isprone to develop such problems if stagnation is allowed.

9 (FromBland and Favero [3]; with permission.)dndtdcdx= DAFIGURE 1-8 Factors that govern diffusion, where dn/dt= the rate of movement of molecules perunit time; D = Fick s diffusion coefficient;A = area of the boundary through which molecules move; dc = concentration gradient; and dx = distance through which molecules move. Hemodialysis depends on the process of diffusion for removal of solutes. The amount of material removeddepends on the magnitude of the concentration gradient, the distance the moleculetravels, and the area through which diffusion takes place. For this reason those dialyzers that have a large surface area, thin membranes, and are designed to maximize the effect of concentration gradient (countercurrent design) are most efficient at removing of dialysis : Difusion, Convection, and dialysis Machinesk 6 4 N3 D=3 Blood flow, mL/minClearance, mL/min4000100200300250200150 UreaCreatininePhosphateVitamin B12100500 FIGURE 1-9 Fick s diffusion constant, where D = Fick s diffusion coefficient, k = Boltzman s constant; T = absolute temperature; = viscosity; N = Avogadro s number; M = molecular weight; and = partial molal volume.

10 The diffusion constant is proportional to the temperature ofthe solution and inversely proportional to the viscosity and the size of the molecule removed. FIGURE 1-10 Effect of blood flow on clearance of various solutes, Fresenius F-5 membrane. The amountof solute cleared by a dialyzer depends on the amount delivered to the membrane. Theusual blood flow is 300 400 mL/min, which is adequate to deliver the dialysis prescrip-tion. On institution of dialysis to a very uremic patient the blood flow is decreased to 160to 180 mL/min to avoid disequilibrium syndrome. As time goes on, blood flow can beincreased to standard flows as the patient adjusts to dialysis . Most patients requirehemodialysis at least thrice weekly. From this graph it is also evident that small moleculessuch as urea (molecular weight 60 D) are cleared more easily than large molecules such asvitamin B12(molecular weight 1355 D).


Related search queries