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Hemodialysis and the Artificial Kidney

Hemodialysis and the Artificial Kidney. Kidney failure - affects 200 000 patients worldwide 15 000 in Canada Hamilton?. Arterial blood. Venous blood. Waste. What sort of things are excreted? Urea - 30 g/day Creatinine - 2 g/day Salt - 15 g/day Uric Acid - 0.7 g/day Water - 1500 mL/day

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Hemodialysis and the Artificial Kidney

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  1. Hemodialysis and the Artificial Kidney

  2. Kidney failure - affects 200 000 patients worldwide • 15 000 in Canada • Hamilton? Arterial blood Venous blood Waste

  3. What sort of things are excreted? • Urea - 30 g/day • Creatinine - 2 g/day • Salt - 15 g/day • Uric Acid - 0.7 g/day • Water - 1500 mL/day • Unknown • Kidney failure • accumulation of waste • acidosis, edema, hypertension, coma

  4. Nephrons • Functional units of the kidney • 1.2 million per kidney • Filtration and removal of wastes • Reabsorption of water, proteins, other essentials into the blood

  5. Actively Secreted Substances • Hydroxybenzoates • Hippurates • Neutrotransmitters (dopamine) • Bile pigments • Uric acid • Antibiotics • Morphine • Saccharin

  6. Reabsorbed Substances • Glucose • Amino acids • Phosphate • Sulfate • Lactate • Succinate • Citrate

  7. Filtration and Reabsorption of Water by the Kidneys

  8. What does this mean in terms of dialysis? • Purpose - removal of wastes from the body • Kidney should be the ideal model for hemodialysis • Water retention / removal • Salt retention / removal • Protein retention

  9. Artificial Kidney • Removes waste products from the blood by the use of an extracorporeal membrane process • Waste products pass from the blood through the membrane into the dialysate

  10. Membrane Material • Permeable to waste products • Impermeable to essential blood components • Sufficiently strong • Compatible with blood

  11. Mechanisms of Transport through the Membrane • Diffusion (true dialysis) • movement due to concentration gradient • If concentration is higher in the blood and the species can pass through the membrane, transport occurs until the concentrations are equal • Slow • If dialysate concentration is higher, the flow goes toward the blood

  12. Convection • Massive movement of fluid across membrane • Fluid carries dissolved or suspended species that can pass through the membrane • Usually as a result of fluid pressure (both positive and suction pressure) • Principal means of water and electrolyte removal (ultrafiltration) • Can also remove water by adding glucose to dialysate (osmotic gradient)

  13. Membrane Materials • Wettability - usually hydrophilic for transport of dissolved materials • Permeability • Mechanical strength • Blood compatibility

  14. Recall from mass transfer: Js = solute flux PM = diffusive permeability Dc = concentration difference c = average membrane conc ss = reflection coefficient Jv = volume flux

  15. Design Considerations • Should be: • Efficient in removing toxic wastes • Efficient in removing water (ultrafiltration or osmosis) • Small priming volume (<500 mL) • Low flow resistance on blood side • Convenient, disposable, reliable, cheap

  16. Performance - Engineering Approach • Use of film theory model • resistance to mass transfer in fluids is in thin stagnant films at solid surfaces • Leads to concept of mass transfer coefficients Blood Dialysate db dm dd

  17. Assume linear profiles in the films and in the membrane • Define a partition coefficient a At steady state, the fluxes in the membrane and in the films are equal

  18. At steady state, the fluxes in the membrane and in the films are equal N - weight of solute removed /time area D’s are diffusion coefficients

  19. Recall from mass transfer that concentrations in the membrane and in the films are difficult to measure • When the system is at steady state we can manipulate this equation along with the partition coefficient to give an equation that is based on the easily measurable concentrations CB and CD

  20. Overall concentration difference Also And using the definition of a

  21. Ko is the overall mass transfer coefficient It includes two fluid films and the membrane

  22. Note also that Ko can be defined in terms of resistances to mass transfer Analogous to electricity (and like heat transfer), resistances in series are additive RB represents limitation for small molecules RM represents limitation for large molecules RD can be neglected when high flowrate on dialysate side is used

  23. This is a model based on molecular mass transfer • Gives concentrations and flux • We are interested in the amount of waste that can be removed in a period of time (efficiency of the system) • To do this we need to do an overall balance on the dialyzer

  24. QD,CD CD+dCD • Consider a differential element of the dialyzer dW CB+dCB QB,CB dx (dA)

  25. Equating the dW’s Integrate assuming constant Ko

  26. Ko describes performance of dialyzer • Combines • diffusivity of molecule • permeability of membrane • effects of flow (convection etc) • Similar model to that obtained in heat transfer

  27. Performance -Clinical Approach • Clearance / dialysance - more clinical than fundamental QB, CBi CBo CDo QD, CDi Clearance defined as: W- weight of solute removed/time

  28. C* is volume of blood completely “cleared” of solute per unit time • Maximum value of QB

  29. Dialysance • Defined by: Allows for possible presence of solute in inlet dialysate

  30. Extraction ratio • Measurement of efficiency • Can show

  31. If z is small (QB<QD) Assuming Cdi = 0

  32. Analysis for countercurrent flow • Similar analysis for cocurrent flow with slightly different results • Countercurrent flow more commonly used

  33. Assume • QB = 200 mL/minute • QD = high • A = 1.0 m2 • urea Ko = 0.017 cm/minute

  34. Time required for treatment • Model patient as CSTR (exit conc. = conc. in tank - well mixed) • Mass balance on patient – can show CBo CBi

  35. Integrate to yield

  36. Consider: • Curea0 = 150 mg/dL • Require Curea = 50 mg/dL • Using previous data we find that required t is approximately 8 h

  37. Hemofiltration • Cleansing by ultrafiltration • Materials removed from the blood by convection • Analogous to glomerulus of natural kidney

  38. Features • Same equipment as hemodialysis • Leaky membrane required • Water lost is replaced either before or after filter (physiologic solution) • No dialysate needed • Clearance less dependent on molecular weight - better for middle molecules • Generally faster than hemodialysis

  39. Hemoperfusion / Hemoadsorption • Blood passed over bed of activated charcoal • Waste materials adsorbed on charcoal • No dialysate • Relatively simple • Little urea removal, no water removal • Used in combination with hemodialysis / hemoperfusion

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