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Basic principles of hemodialysis final
This document discusses the basic principles of hemodialysis. It covers: 1) Hemodialysis aims to remove waste, correct electrolytes, and remove excess fluids via diffusion, convection, and ultrafiltration across a semi-permeable membrane. 2) Countercurrent blood-dialysate flow maintains the concentration gradient to increase solute removal efficiency. 3) Clearance depends on factors like molecular weight, blood/dialysate flow rates, and dialyzer properties. Higher blood flows and matching dialysate flows can improve clearance.
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Basic principles of hemodialysis final
- 1. Hesham Elsayed Professor ofNephrology Ain shams university – Cairo Egypt BASIC PRINCIPLES OF HEMODIALYSIS
- 2. HD : aFiltration Therapy Enhance the Clearance HD Removal by Filtration Removal by Adsorption Flux HDF 17 % Solute permeability Solute Dragging
- 3. Principles of Hemodialysis Hemodialysisis a treatment which aims to 1- Remove accumulated metabolic waste products and 2-To correct blood electrolyte composition by means of an exchange between patient's blood and a dialysate fluid Across a semi-permeable membrane in a countercurrent mechanism . 3-To remove excess fluids by means of ultrafiltration
- 4. Basic Principals ofDialysis Principles related to solute removal (mass transfer) Diffusion Convection Principles related to water removal Ultrafiltration
- 5. Membrane Blood Dialysate Countercurrent Blood-Dialysate Flow Dialysate FlowQD 2 X QB The counter-current approach is used for intermittent haemodialysis to provide maximum diffusive gradient as fresh dialysate fluid is continuously exposed to solute-laden blood Hemodialysis utilizes counter current flow, where the dialysate is flowing in the opposite direction to blood flow in the extracorporeal circuit. Counter-current flow maintains the concentration gradient across the membrane at a maximum and increases the efficiency of the dialysis Blood Flow QB
- 6. In the end, the concentration gradient remains the same; the solute-depleted blood is exposed to clean dialysate, and the concentration gradient remains unchanged. in the middle , the concentration gradient remains the same; the blood is depleted of solute at the same rate as the dialysate is enriched by it. In the first part , the exiting dialysate fluid is already concentrated, but less so than the blood. Thus there is still a concentration gradient. Countercurrent Blood-Dialysate Flow Magnitude of the concentration gradient the Urea 100 mg/dl 50 25
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- 8. Blood & dialysateflow Blood in Blood out Dialysate out Dialysate in Hollow fiber Casing or jacket Dialysate Header Diffusion process all through the Dialyzer length Maximumspeed
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- 10. Blood Flow QBml/min Dialysate Flow QD ml/min Where QBi is Blood flow inlet in ml/min QBo is Blood flow outlet in ml/min QDi is Dialysate flow inlet in ml/min QDo is Dialysate outflow in ml/min Where CBi is concentration of solute in Blood inlet in mg/dl CBo is concentration of solute in Blood outlet in mg/dl CDi is concentration of solute in Dialysate inlet in mg/dl CDo is concentration of solute in Dialysate outlet in mg/dl
- 11. Clearance from Plasma thenfrom RBCS Urea diffuse easily Creatinine less diffuse from RBC
- 12. CA: filter inletconcentration CD: dialysate concentration QD: dialysate flow CV: filter outlet concentration
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- 14. Effect of Hematocritfrom 20 – 40 % Urea diffuse from Plasma and Blood , creatinine less and Phosphate Much less - 8% - 13%
- 15. Calculation of Clearance Dialyzer clearance Urea pre = 200 Urea post = 20 Clearance = 180 ml /min Whole session clearance Urea pre predialysis = 200 Urea post Dialysis = 40 URR = 80%
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- 17. Diffusion and Backdiffusion Movementof molecules from the blood side is called Clearance or diffusion Movement of molecules from the Dialysate side is called BackDiffusion Removal of Toxins Removal of excess K+ Backdiffusion of Bicarbonate Glucose and Calcuim
- 18. Semipermeable Membrane HD willallow certain molecules to pass in Both directions while it retains other molecules to pass Albumin and Cells will not pass
- 19. Diffusion Diffusion isdefined as the spontaneous passive transport of solutes from blood to dialysate (and vice versa, i.e., backdiffusion) across the dialysis membrane through a concentration gradient . Equilibrium Diffusion is Bi-directional From Blood to Dialysate = Diffusion or Clearance From Dialysate to Blood = Back-Diffusion
- 20. Basic Principals ofDialysis Diffusion The rate of diffusive transport depends upon: 1- The diffusion coefficients of the solute in blood, in membrane and in dialysate. 2-The concentration difference across the Membrane. 3-The surface area of the membrane. The transport rate of solute is inversely proportional to the Molecular Wright (MW) So smaller solute can diffuse easier than bigger molecules .
- 21. Basic Principals ofDialysis Diffusion solute movement against concentration gradient ` Blood side Dialysate side Semipermeable Membrane Larger solute difuse slower Small solutes diffuse easier
- 22. Diffusion – RandomMolecule Movement Diffusive resistance ` Diffusion process is slower in : 1- Bigger molecules . 2-Thicker Dialysis membrane 3-low Temperature 4-Low concentration gradient
- 23. Dialyzer KoA Themass transfer area coefficient (KoA), expressed in mL/min, for a given solute is the clearance of the dialyzer at infinitely high blood and dialysate flow rates on a theoretical basis. Therefore, KoA is a measure of the maximum solute removal capacity of the dialyzer and has been considered as an intrinsic property of the dialyzer membrane.
- 24. Dialyzer KoA KoA = Qb.Qd Qb- Qd Ln 1-Kd/Qb 1-Kd/Qd Calculated from clearance, blood flow, dialysate flow Kd: Dialyzer clearance Qb: Blood flow Qd: dialysate flow KoA: Mass transfer area coefficient
- 25. Mass transfer areacoefficient KoA Diffusive resistance ` - MW of solute - Membrane Pore size, number & distribution Membrane thickness - Diffusive resistance - Membrane surface area – A - Concentration gradient Ko
- 26. Basic Principals ofDialysis Convection Convection is the simultaneous transport of solvent and solutes from the blood compartment to the dialysate compartment (and vice versa, i.e.,backfiltration) across the dialysis membrane
- 27. HFD versus HDF Removalof MM •Diffusion •convection High Flux •Diffusion •Augmented convection HDF
- 28. HDF Dose Depends on: 1-Blood volume processed. 2- UF and substitution volume Volume of processed Blood Volume of convection volume QBSV
- 29. Basic Principals ofDialysis Convection The convective process requires a fluid movement caused by a transmembrane pressure gradient Solute flux Depends on Ultrafiltration Rate (Qf) Solute concentration in plasma water (Cb) And Sieving coefficient of the solute (SC) Removal rate Through a pressure gradient
- 30. B2m RR% isa linear with substitution volume 45% 50% 58% 65% 70% 78% 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 0 40 60 80 100 120 ml/min
- 31. Basic Principals ofDialysis Adsorption increasing size of middle-sized proteins and other compounds, relatively more clearance is achieved by membrane adsorption compared with loss into the dialysate. A high adsorptive capacity, one of the main features of some dialysis membranes like polymethylmethacrylate (PMMA) can adsorb more LMW proteins than others
- 32. Basic Principals ofDialysis Adsorption Removing PBUT from the blood by means of diffusion and convection (containing the albumin loss) is virtually impracticable; however, PBUT can be removed by using the adsorptive properties of particular biomaterials. PMMA High adsorption capacity
- 33. Clark, W. R.et al. J Am Soc Nephrol 2002;13:S41-S47 Effect on membrane function by protein adsorption With time membrane clogging by proteins will close the membrane pores a phenomenon called “Protein Fouling”
- 34. Basic Principals ofDialysis Ultrafiltration Ultrafiltration is the movement of water across a semi- permeable membrane because of a pressure gradient (hydrostatic, osmotic or oncotic). UFR = rate of water removal / hour QUF of dialyzer = UF capacity / mmHg eg 10 – 80 TMP = UFR / QUF LF = 1000/10 = 100 mmHg is required HF = 1000 / 50 = 20 mmHg is required Volumetric HD Machine
- 35. Basic Principals ofDialysis Ultrafiltration Blood pressure within the hollow fibers is positive, while the pressure outside the hollow fibers is lower. The difference between the blood pressure in the hollow fibers and the surrounding pressure is the TransMembrane Pressure (TMP). The TMP determines the ultrafiltrate production
- 36. Basic Principals ofDialysis Ultrafiltration Transmembrane pressure (TMP) = Positive pressure in Blood side + Negative pressure in Dialysate side in mmHg Maximum UF Higher pressure Lower UF Lower pressure
- 37. Back Filtration A situationwhere there is a transport of fluid from the dialysis fluid side into the blood side is called backfiltration. This is a phenomenon encountered with High Flux Dialysis . And is due to excessive UF through the first half of dialyzer that follow a pressure drop in the second distal part of the dialyzer
- 38. HD Backfiltration Pressure Drop Backfiltration volume is risky for Pyrogen transfer `… volume ??
- 39. Basic Principals ofDialysis Ultrafiltration The magnitude of net filtration in Haemodialysis is determined by 1- The hydraulic permeability of the dialyzer 2-The surface area of the membrane, and by the geometry of the dialyzer . 3-the hydrostatic and oncotic forces acting on the blood and dialysate sides of the membrane.
- 40. Ultrafiltration and BackFiltration in different modalities Low Flux dialyzer has low KUF so need more TMP Backfiltration occur due to obligatory fluid loss and pressure drop HDF remove excess fluids so needs high TMP all through the dialyzer length
- 41. Ultrafiltration and BackFiltration High Flux Dialysis
- 42. HD clearance ofsolutes In a clinical setting, the removal of a solute is measured in terms of clearance, the term being defined as the volume of blood or plasma from which the solute is completely removed in unit time. ml/min.
- 43. HD clearance ofsolutes During transit across the dialyzer, most solutes are removed from plasma water (about 93% of blood volume, depending on plasma protein concentration). the clearance of the solute decreases as the hematocrit increases (since the plasma volume decreases). There is a difference between the manufacture data ( in vitro test ) from the actual data in clinics ( in vivo test ).
- 44. HD clearance ofsolutes • Solute Generates slowly then transport within the body Generation • Generation is slow enough for equilibration to occur between extracellular (interstitial and plasma) and cellular water Equilibration • During HD blood levels fall sharply but blood re- equilibrates as urea is recruited from the body HD clearance
- 45. HD clearance ofsolutes Diffusion K = CLEARANCE FOR A GIVEN SOLUTE IN ML/MIN QB = Blood flow rate ml/min CBi = cpncentration of solute at Blood inlet mg/dl CBo = concentration of solute at blood outlet mg/dl
- 46. HD clearance ofsolutes convection
- 47. HD clearance ofsolutes Dialyzer Flux or permeability • Flux • Measure of ultrafiltration capacity • Permeability • Measure of the clearance of the middle molecular weight molecule (eg, B2-microglobulin) • General correlation between flux and permeability • Efficiency • Low and high efficiency are based on the urea KoA value • Low efficiency: KoA <500 mL/min • High efficiency: KoA >600 mL/min
- 48. Solute Removal duringHD Hemodialysis Kinetics Diffusion , convection and Solute Flux index Body distribution Intravascular Intracellular / interstitial Protein bound Free Bound and % Solute MW Small Medium
- 49. Sieving coefficient It refersto the amount of solute removed by convection A sieving coefficient S = Concentration in the ultrafiltrate Concentration in blood A membrane cutoff is defined when the SC of a certain solute is below 0.1
- 50. The solute sievingcoefficient of dialyzer Only High Flux membrane can remove middle molecules
- 51. Factors influencing low-molecular-weight soluteclearance during hemodialysis Dialyzer related factors : 1- Surface area. 2- membrane type and its diffusive permeability 3-membrane porosity. 4-dialyzer rheology or fiber configuration. Dialysis related factors : QB ,QD, Dialysis time . Patient related factors : 1-Vascular access type. 2-Recirculation% 3-Patient Haematocrite.
- 52. Clearance by Diffusionin Dialyzer Concentration gradient Blood flow rate Dialysate flow rate Directions of blood and dialysate flow Molecular weight of solute Shape, size of molecule Membrane property Surface area-number of pores Size of pores Distribution of pores Thickness of membrane of Dialyzer for particular salute ie. KoA urea At particular blood and dialysate flow rate
- 53. Increasing Blood flow Upto a limit Clearance of solute is not linear to the increased QB
- 54. Effect of BloodFlow on Clearance
- 55. Clearance depends onsolute MW
- 56. Dialysate flow rateQD ml/min At low blood flow rates (<200 mL/min), no difference exists in urea clearance rates between the two different Qd conditions, because equilibrium in urea concentrations between blood and dialysate is readily achieved. When the blood flow rate is high (>300 mL/min), the higher Qd maintains a higher concentration gradient for diffusion of urea, and therefore, the urea clearance rate is higher.
- 57. Causes of lowclearance values despite use of high efficient dialysis Vascular access related : Low Blood flow High Recirculation rates Time Related factors : Not adherent to prescribed dialysis time. Failure to adjust prescribed time due to repeated alarms , Hypotension episodes and Dialysate Bypass
- 58. Improving clearance bybetter Blood & dialysate flow geometry Blood in Blood out Hollow fiber Casing or jacket Dialysate Header Blood to Dialysate matching to increase the clearance
- 59. Blood Flow speedis higher in the center Dialysate Flow is higher in the periphery
- 60. Dialyzer geometry toenhance the performance Improvement of Blood / Dialysate flow matching spacer yarns consist of multifilament threads integrated into the fiber bundles Improves dialysate distribution throughout the dialyzer Micro Undulation technology for better blood- dialysate matching
- 61. The Japanese classificationof Dialyzer permeability The classification of dialyzers refers to five types, classified to a clearance (in vitro) of β2-microglobulin β2microglobulin (in vitro) I < 10 mL/min II < 30 mL/min III< 50 mL/min IV 50-70 mL/min V ≧70 mL/min Worldwide classification High flux UFR > 20 mL/mmHg/hr β2MG sieving coefficient (SC) >0.6
- 62. Increasing Flux Risks: ET transfer Albumin Loss Dialysis Membrane Is NOT a One Way Street
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- 64. H2O water solublemolecules F mmHg mmHgB B B D B D mmHgB mmHg D B D low-flux high-flux Therapy types Hemodialysis Hemofiltration Hemodiafiltration
- 65. Methods of deliveryof substitution fluids (SF)
- 66. Convective transport isachieved by an effective convection volume of at least 20% of the total blood volume processed FF % = UFR/ QB = > 25 %
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- 71. Post- Dilution HDF SF> 20 L Back filtration Ondemand High TMP `Pre Dilution on Demand Flushing 200 ml saline Maintain UF capacity
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- 73. Dialyzer Hemoconcentration Vascular Access Blood Flow DialysisMachine Blood Purif 2015;40(suppl 1): Hematocrit Protocrit Pressure control Volume control Main Limiting Factors in Achieving High Convective Volume or Qi
- 74.