Principles of CRRT PDF

Principles of CRRT CATHY LANGSTON The start of hemoperfusion was a mistake Peter Kramer in 1977, Germany Femoral vein catheter mistakenly placed in femoral artery Pete took advantage of an idea he’d been thinking about Hemofiltration: CRRT was born as a purely convective therapy...

Principles of CRRT CATHY LANGSTON The start of hemoperfusion was a mistake Peter Kramer in 1977, Germany Femoral vein catheter mistakenly placed in femoral artery Pete took advantage of an idea he’d been thinking about Hemofiltration: CRRT was born as a purely convective therapy From Peter Kramer, circa 1977 Ronco CCN 2009 Two years later, a blood pump was added It took 10 years to treat the first neonate (37 years ago) Amicon Minifilter, Vicenza 1985 Ronco CCN 2009 The 1st CRRT machines evolved in the 1980’s  Short blood lines with no gadgets to reduce resistance  Position collection bag to apply negative pressure  Concept of filtration fraction  Changes in filter geometry  Addition of CAVHD Fancier machines keep being developed Prismaflex NxStage PrisMax New machine for pediatric patients – and veterinary patients! Type of Therapy Prolonged Intermittent Continuous Therapies Therapies Intended for critically ill Evolved for more efficient hemodynamically use of time unstable patients over an extended period of time Filter (Hemofilter) Semipermeable Membrane Dialyzer (Hemodialysis) Semipermeable Membrane  Transmembrane Pressure Ppre + Pout TMP = − Peff 2  Fouling – proteins stick to membrane, decrease permeability  Diffusion Methods of Convection Solute Removal Ultrafiltration Adsorption Diffusion Blood Dialysate dc Jd = −D  Jd (Solute diffusive flux) dx  mass removal rate normalized to membrane surface area  Concentration gradient dc = C1-C2  D = diffusion coefficient  dx - distance  Typical operating situation with CRRT  Dialysate is saturated for small solutes  Less clearance for larger molecules Membrane Convection Blood Dialysate  Sieving coefficient Ultrafiltrate concentration CUF SC = SC = Blood concentration (CPi + CPo)/2  CUF – solute concentration in ultrafiltrate  CPi & CPo – blood inlet and outlet solute concentration  Convective flux QUF Jc = A x CPi x SC  QUF – ultrafiltration rate, A – membrane surface area Membrane Convection Blood Dialysate  Concentration polarization – affects high MW, not small solutes  Solutes accumulate by membrane, create gradient for diffusion  Concentration polarization (enhances diffusion) vs fouling (impairs diffusion) Membrane Adsorption Blood Dialysate  Solutes stick to membrane  Adsorption vs fouling Membrane Ultrafiltration Blood Dialysate  QUF is function of TMP  QUF (total removed) vs QNET (part that is not replaced)  Effluent = UF + dialysate Membrane Blood Flow Arteriovenous Venovenous Requires arterial catheter Requires dual lumen and venous catheter catheter or two venous Arterial pressure determines catheters blood flow speed Requires blood pump; blood flow speed set by operator  C refers to continuous AV refers to arteriovenous V V refers to venovenous CRRT Abbreviations H means hemofiltration HD means hemodialysis HDF means hemodiafiltration  Access CAVH Hemo- filtration Replacement Fluid Effluent Return Convection  Access CVVH Pump Hemo- filtration, Replacement Fluid Post Filter Effluent Return Convection  Replacement Access Fluid CVVH Pump Hemo- filtration, Pre-Filter Effluent Return Convection  Access CVVHD Pump Dialysate Hemo- dialysis Effluent Return Diffusion  Access CVVHDF Pump Dialysate Hemo- diafiltration Replacement Fluid Effluent Return Convection Diffusion  Access SCUF Pump Slow Continuous Ultrafiltration Effluent Return Ultrafiltration Modalities of  Intermittent hemodialysis (3-5 hours) IHD  Continuous therapies CRRT RRT  Continuous hemofiltration CVVH  Continuous hemodialysis CVVHD  Continuous hemodiafiltration CVVHDF  Hybrid therapies (6-12 hours)  Sustained low efficiency dialysis SLED  Extended daily dialysis EDD  Sustained low efficiency daily diafiltration SLEDDf  Prolonged Intermittent CRRT PIRRT  Slow Continuous Ultrafiltration SCUF  Peritoneal dialysis PD, CAPD  High Volume  Larger volume of fluid removal/replacement Hemofiltration  Langston term: “therapeutic water exchange”  Replacement fluid composition determines blood concentrations  No phosphate ? ? Convective Clearance “Solvent Drag” Every ml of effluent = 1 ml plasma clearance Convective flux determined by QUF Ultrafiltration rate (Replacement Fluid Rate) Jc = A x CPi x SC Blood solute concentration Membrane sieving properties Less dependent on molecular size  Fluid Flow Characteristics Blood in hollow fiber Dialysate and ultrafiltrate Blood in hollow fiber Blocks pores – decreases efficiency Velocity fastest in center of diffusion and convection Resistance to flow at edges near walls Cells and proteins sludge against membrane on blood side Solutes concentrated against membrane on effluent side Unstirred layer – decreases efficiency of diffusion Pre-Filter Dilution  Advantages  Disadvantages  Decreases hemoconcentration  Lower efficiency of solute clearance and clotting  Mass transfer alterations  Example  Decreased RBC and protein  Qb 125 ml/min concentration  QUF 4.5 L/hr (75 ml/min)  Decreases sludging on blood side of membrane  30-40% decreased efficiency compared to postdilution  Increased flow though blood  (i.e., will need 30-40% more compartment replacement fluid)  Increases membrane shear rate  Enhances solute movement out  *Note – unlikely to achieve this QUF of RBC’s rate with postdilution Post-Filter Dilution  Advantages  Disadvantages  Maximal efficiency  Hemoconcentration inside filter  Filter clotting  Decreases solute transfer  Shorter filter life Filtration Fraction FF is the proportion of plasma filtered across membrane Filtration Fraction FF is the proportion of plasma filtered across membrane FF* (QUF/Qp) x 100 = (QUF/(Qb x [1- HCT])) x 100 Example Blood flow (Qb) = 150 ml/min PCV = 40% Plasma flow (Qp) = 90 ml/min Post-filter replacement = 1.8 L/hr (30 ml/min) 30/90 x 100 = 33.3 FF = 33.3% - TOO HIGH! *Post-Filter replacement only Filtration Fraction Target < 25% Higher FF leads to hemoconcentration Higher risk of filter clotting More protein on filter (decreased permeability) Definitions  QNetUF = Net patient fluid removal  = QPFR (Patient Fluid Removal)  QUF = all fluid removed: QR + QPBP + QNetUF  PBP = pre-blood pump (specific to PrismaFlex & PrisMax) Diffusion IHD – dialysate in excess of blood flow At slow Qb, blood is completely cleared of solute Qd = 500 ml/min Qb = 50-300 ml/min At faster Qb, less thorough solute clearance per pass through dialyzer CRRT – blood flow in excess of therapy fluid Qd + Qr + QNetUF = 2 L/hr = 33 ml/min typical adult 10 L/hr = 166 ml/min max Qb = 50-180 ml/min  Diffusive Clearance Rates *theoretical. Not intended to portray real values Sample Clearances with IHD (Qd = 500 ml/min) Qb (ml/min) Clearance* Urea entering Urea exiting Clearance More blood = (mg/dL) (mg/dL) (ml/min) More clearance 50 95% 100 5 48 300 75% 100 25 225 Sample Clearances with CRRT (CVVHD) More fluid = Qb (ml/min) Effluent Rate Clearance*† BUN Effluent urea Clearance More clearance ml/hr (mg/dL) (mg/dL) (ml/min) (ml/min) 50 1000 (17) 100% 100 100 17 300 1000 (17) 100% 100 100 17 300 6000 (100) 100% 100 100 100 †100% if Qb ≥ 3X 300 8000 (133) 80% 100 80 106 Effluent Rate Hemodiafiltration - CVVHDF Balances good and bad of diffusive & convective modes New machines can do predilution and postdilution simultaneously Estimating Kt/V  CVVH, Post-dialyzer replacement  K = QUF  CVVH, Pre-dialyzer replacement  K = QUF ÷ (1+ [QUF/Qb])  CVVHD  K = Qd  CVVHDF, Post-dialyzer replacement  K = QUF + Qd  10 kg Dog Estimating Kt/V 1200 mL/hr Therapy Fluid Qb 120 mL/min Filtration Fraction Net Patient Removal 0 mL PCV = 30% Qp = 84 mL/min  CVVH, Post-dialyzer replacement  CVVH, Post.  K = QUF  K = 20 mL/min FF = 23.8%  CVVH, Pre-dialyzer replacement  CVVH, Pre.  K = QUF ÷ (1+ [QUF/Qb])  K = 17.2 mL/min 20/120 =. 0.16 FF = 19.2% 20/1.16 = 17.2  CVVHD  CVVHD.  K = Qd  K = 20 mL/min. FF = 0%  CVVHDF, Post-dialyzer replacement  CVVHDF, 600 mL/hr Qd, 600 mL/hr Qr.  K = QUF + Qd  K = 10 + 10 = 20 mL/min (post). FF = 11.9% K = 10 + 9.2 = 19.2 mL/min (pre) FF = 10.6% Estimating and Measuring Kt/V  Dialysate Net 0  Replacement Fluid Net 0  Patient Fluid Removal Net Loss  Dialysate +  Replacement +  Patient Fluid Removal  =  Effluent (aka Ultrafiltrate) Clearance Rules of Thumb for CRRT Convection Postdilution 100% 1 ml effluent = 1 ml plasma clearance Predilution Variable 1 ml effluent < 1 ml plasma clearance Diffusion 100 %, if Qb ≥ 3X Effluent Rate 1 ml effluent ≈ 1 ml plasma clearance Comparing CRRT to IHD Parameter CRRT† IHD† Clearance NOT dependent on Dependent on blood blood flow flow Clearance Dependent on therapy NOT dependent on fluid volume dialysate volume Blood flow rate Higher Qb decreases Higher Qb improves clotting clearance, decreases clotting Blood flow rate 10-450 ml/min 10-600* ml/min Therapy fluid rate Limited (10 L/hr max) High (30 L/hr average) †As traditionally considered *Unobtainable. Typical Qb of 300 ml/min = 18 L/hr Bacteriologic Safety of Therapy Fluid  Dialysate and Replacement Fluids – Prepackaged  Essentially sterile  More expensive  Compare to dialysate made by dialysis machine:  Requires water purification machine  200 cfu/mL allowed in water  2000 cfu/mL allowed in dialysate  Endotoxins can cause inflammation Fake Pee, aka Effluent   List methods of clearance (solute removal)  Define CVVH, CVVHD, CVVHDF, SCUF in Wrap-Up  regards to clearance methods Define filtration fraction Questions  What are the advantages and disadvantages of pre vs post-filter dilution?  What determines “dose” in CRRT?