Separation Technology in Dialysis
Allan P. Turner M.D.
February 17,2006
Kidney Function
Kidney Function
Terms Used in Dialysis
• Diffusion
• Convection
• Ultrafiltration
• Clearance 100 ml/min100mg/dl
100 ml/min50 mg/dl
Clearance=50 ml/min
100 ml/min100mg/dl
100 ml/min10 mg/dl
Clearance=90 ml/min
Options for RRT• Hemodialysis
– 3X a week for 3-4 hours– diffusive clearance with ultrafiltration of water– faster blood flow rates=less hemodynamic stability
• CRRT(Hemodiafiltration)– a continuous process– used on critically ill patients in US– more convective clearance– lower blood flow rates and smaller filter=greater hemodynamic
stability
• Peritoneal Dialysis– peritoneal membrane used as semipermeable membrane– batch process
Description of Hemodialysis
• Primarily diffusion
• Dialysate– looks like blood of
healthy patient
• 3X week for 3-4 hours
• Blood and dialysate flows are fast– QB=500 ml/min
– QD=800 ml/min
Membrane(Dialyzer)
• Hollow Fiber Design
• Biologic vs synthetic
• Reuse
• Terms– Biocompatibility– High efficiency– High flux
Access
• Difficult
• Trade Offs– rapidity of use
– chance of infection
– patient comfort
– need for addl procedures
Access(PermCath)
• Use immediately
• No needle sticks
• High infection rate
• High recirculation
Access(AV Graft)
• Use in 2-3 weeks• Some infection
risk• 2 needle sticks• low recirculation• numerous
interventions to keep open
Access(AV Fistula)
• 3-18 months to use
• Minimal infection risk
• Can last a lifetime
Anticoagulation• Blood clots
• Heparin – discovered in 1926
• Partial clotting– limits diffusion
• reduces surface area
• Access must stop bleeding
• Calcium– required for clotting
Dialysis Machine
• Blood Circuit– anticoagulate
– deliver blood to membrane
– safely return blood to patient
• Dialysate Circuit– deliver dialysate at proper
temperature, concentration, and pH
– control ultrafiltration
Dialysis Machine(Blood Circuit)
• Roller pump• Heparin syringe
pump• 2 air traps• Air detector• Venous line clamp
Dialysis Machine(Dialysate Circuit)
• Warm, deaerate, mix concentrates, monitor conductivity and pH, pump
• Detect blood leaks
• Generate and monitor ultrafiltration
Dialysis Machine
Dialysis Machine
Dialysate
Urea Clearance
• ?Urea = uremic toxin?
• Diffusion
• Urea: MW=60 (small)
• KoA
• Clearance of urea of 250ml/min
• Native kidneys provide urea clearance of about 90-110ml/min
Urea Clearance
Clearance of Other Solutes• Urea(MW 60), creatinine(MW 113), B12 (MW=1355), ß2
microglobulin (MW=11,800), albumin (MW=80,000)
• Middle molecules
• Diffusion not effective
Hemofiltration• Convection to clear
larger molecules
• Replacement fluids without removed solute
• Costly
Continuous Renal Replacement Therapy(CRRT)
• Critically ill ICU patients– low BP
– can’t tolerate large QB or large filter
– often can’t be systemically anticoagulated
• Continuous– low clearances but runs 24/7
• Anticoagulation– regional anticoagulation instead of systemic
• Combine hemodialysis and hemofiltration– hemodiafiltration
– increases clearances even of middle molecules
– continuous venovenous hemodiafiltration(CVVHDF)
CRRT vs Hemodialysis
QB
150ml/hrDialysate + Ultrafiltration +Replacement fluid
Replacement fluid1000ml/hr
Dialysate40ml/min(2500ml/hr)
QB
500ml/hr
Dialysate + Ultrafiltration
CRRT
HemodialysisDialysate800ml/min(48,000ml/hr)
CRRT Citrate Anticoagulation
DIALYZER
BloodFrompatient
BloodTopatient
C CC CC C
Calcium C
CC
C C
C
CCC
LiverCitrate HCO3
Tri-Sodium Citrate
CRRT
V
QB
QE = QR + QFR + QD
Dialysate: 4 L bag Na+ 140 mEq/L
Cl- 118.5 mEq/L
HCO3 25 mEq/L
K+ 4.0 mEq/L
Mg 1.16 mEql/L
Rate: 1000-2500 mL/hr
QD
100-150 mL/min
(actual QB = QB, machine – QR)
Patient
Ca2+ Gluconate
78 mEq /L (20 g/L) in NS
Rate: 80 mL /hr
PF iCa 2+ (0.25-0.5 mmol /L)
iCa2+
1.1-1.3mmol/L
Gambro PrismaGambro Prisma with withM60 AN69 FilterM60 AN69 Filter
QR
Prefilter Fluid: 4L bag
0.67% Trisodium Citrate Citrate3- 23 mM/L
Na+ 140 mEq/L
Rate: 1000-1500 mL/hr 24 mmol/h citrate
Gambro Prisma Pre-Pump Pre-Dilution Set
V
CRRT
Peritoneal Dialysis(PD)
• Salmon dialysis
• Peritoneal membrane
• Capillaries
• Diffusion, ultrafiltration( ie osmosis), convection, and absorption
PD Membrane
• Pd Membrane– surface area=BSA=1-2 m2
– heteroporus, heterogeneous semipermeable membrane with complex physiology
• Blood Flow– approx. 50-100 ml/min
• 3 pore model– large pores(macromolecules like proteins)– small pores(small solutes)– ultrapores(aquaporins)(water without solute)
PD Ultrafiltration
• Dextrose(3 concentrations) added to provide gradient for UF(osmosis)
• Glucose diffuses into blood and diminishes gradient
• Absorption of dialysate occurs limiting UF
• Newer agents
PD Clearance• High Transporters
– dialyze well– ultrafilter poorly
• ? Icodextran ?
– best with freq. short dwells
• High Avg/Low AVG transporters
• Low Transporters– ultrafilter well– dialyze poorly– best with longer short dwells
• Options– CAPD– CCPD
Future
• Which separation techniques improve mortality
• Less expensive RRT as population grows
• Improve patients quality of life
• Biological systems
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