Mod 1: Principles of CRRT
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Questions and Answers

What occurs when the filtration fraction (FF) is higher than 25%?

  • Lower protein levels on the filter
  • Decreased risk of filter clotting
  • Less fluid removal
  • Decreased permeability of the filter (correct)

In Continuous Renal Replacement Therapy (CRRT), the blood flow is generally less than the therapy fluid rate.

False (B)

What does K represent in the estimation of Kt/V?

Clearance rate of solutes

In intermittent hemodialysis (IHD), if the blood flow is slow, the blood is completely cleared of _______.

<p>solute</p> Signup and view all the answers

Which of the following accurately describes the relationship between blood flow rate (Qb) and solute clearance in IHD?

<p>Higher Qb can lead to less thorough clearance (A)</p> Signup and view all the answers

Match the clearance mechanisms to their definitions:

<p>Diffusion = Movement of solute from high to low concentration Ultrafiltration = Fluid movement driven by pressure gradients Convection = Solute movement due to drag from flowing fluid Hemodiafiltration = Combination of diffusion and convection for solute removal</p> Signup and view all the answers

Hemodiafiltration balances the benefits of both diffusive and convective modes.

<p>True (A)</p> Signup and view all the answers

What is the typical effluent rate for adult CRRT therapy at 2 L/hr?

<p>33 ml/min</p> Signup and view all the answers

What is the main factor that determines the convective flux in ultrafiltration?

<p>Ultrafiltration rate (B)</p> Signup and view all the answers

In pre-filter dilution, the hemoconcentration and clotting are decreased significantly.

<p>True (A)</p> Signup and view all the answers

What does FF stand for in the context of filtration?

<p>Filtration Fraction</p> Signup and view all the answers

The formula for calculating the Filtration Fraction (FF) is FF* = (QUF/Qp) x 100, with the calculated value indicating whether it is too ______ or not.

<p>high</p> Signup and view all the answers

Match the types of dilution with their characteristics:

<p>Pre-Filter Dilution = 30-40% decreased efficiency compared to postdilution Post-Filter Dilution = Shorter filter life due to clotting</p> Signup and view all the answers

Which of the following is a disadvantage of post-filter dilution?

<p>Decreased solute transfer (D)</p> Signup and view all the answers

What is the primary purpose of a semipermeable membrane in therapy?

<p>To allow selective passage of solutes (A)</p> Signup and view all the answers

Fouling refers to solutes sticking to the membrane and enhancing its permeability.

<p>False (B)</p> Signup and view all the answers

Solutes are concentrated against the membrane on the effluent side during filtration.

<p>True (A)</p> Signup and view all the answers

What is the primary benefit of having a higher ultrafiltration rate?

<p>Enhanced solute movement out of red blood cells</p> Signup and view all the answers

What is the role of transmembrane pressure (TMP) in solute removal?

<p>TMP drives the movement of solutes across the membrane.</p> Signup and view all the answers

The equation for calculating Filtration Fraction is FF = (QUF / (Qb x [1- HCT])) x 100, which relates ______ flow to ultrafiltration.

<p>plasma</p> Signup and view all the answers

The primary mechanism used in ultrafiltration is _____ pressure.

<p>transmembrane</p> Signup and view all the answers

What happens if the filtrate velocity is fastest in the center of the hollow fiber?

<p>Cells sludge against the membrane (C)</p> Signup and view all the answers

Match the following solute removal mechanisms with their descriptions:

<p>Diffusion = Movement due to concentration gradient Convection = Movement driven by fluid flow Adsorption = Solutes stick to the membrane surface Ultrafiltration = Fluid removal driven by pressure</p> Signup and view all the answers

Which of the following represents continuous therapies?

<p>High volume hemofiltration (B)</p> Signup and view all the answers

Larger molecules achieve higher clearance during typical operating situations with CRRT.

<p>False (B)</p> Signup and view all the answers

What is the effect of concentration polarization on solute removal?

<p>It creates a gradient that can enhance diffusion but may impair solute clearance for higher molecular weight solutes.</p> Signup and view all the answers

In hemodialysis, solute removal is primarily achieved through _____ and _____ mechanisms.

<p>diffusion, convection</p> Signup and view all the answers

Which therapy requires an arterial and venous catheter?

<p>HD (B)</p> Signup and view all the answers

Dispersion can occur when polymers accumulate by the membrane, creating a uniform solution.

<p>False (B)</p> Signup and view all the answers

What does the sieving coefficient indicate in the context of convection?

<p>The sieving coefficient indicates how effectively solutes are removed from blood during ultrafiltration.</p> Signup and view all the answers

Continuous therapies like _____ use a blood pump to set flow speeds.

<p>CVVH</p> Signup and view all the answers

What is the concentration gradient in the context of diffusion?

<p>The difference between the solute concentrations on either side of the membrane (B)</p> Signup and view all the answers

Flashcards

Filtration Fraction Target

The desired percentage of ultrafiltrate (fluid removed) compared to the total blood volume processed.

High Filtration Fraction

A higher percentage of fluid removal from blood than is ideal.

QNetUF

Net patient fluid removal during dialysis.

QUF (Total Fluid Removed)

Sum of all fluids removed; including pre-blood pump fluid (PBP), rinse fluid, and net fluid removal.

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Diffusion Clearance (IHD)

Process of removing waste products from blood by passing blood through a dialyzer with a higher fluid rate.

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Diffusion Clearance (CRRT)

Process of removing solute from blood for CRRT, flow of blood higher than therapy fluid rate.

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Kt/V (Dialysis Dose)

Measure of dialysis adequacy; calculated using fluid removed. It is an essential indicator of the effectiveness of hemodialysis.

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CVVHDF

Hemodiafiltration that balances diffusion and convection for solute removal. Enables predilution and postdilution simultaneously.

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Convective Clearance (Jc)

The rate at which solutes are removed from the blood by convective flow through a membrane.

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Ultrafiltration Rate (QUF)

The rate at which fluid is filtered from the blood.

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Pre-Filter Dilution

Adding fluid before the blood passes through the filter during dialysis to prevent blood thickening.

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Post-Filter Dilution

Adding fluid after the blood passes through the filter to enhance solute removal.

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Filtration Fraction (FF)

The proportion of plasma filtered across a membrane during dialysis.

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Solvent Drag

The movement of solutes with the flowing solvent across the membrane during dialysis.

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Unstirred Layer

A thin layer of fluid surrounding the membrane where solute movement is slower due to reduced mixing

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Hemoconcentration

Increased concentration of blood components due to fluid removal.

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Membrane Sieving Properties

How easily different sized solutes pass through the dialysis membrane.

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Blood flow (Qb)

The volume of blood flowing through the dialysis circuit per unit time measured in mL/min

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Continuous Therapies

Treatment methods for critically ill, hemodynamically unstable patients over an extended period.

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Prolonged Intermittent Therapies

Evolved for more efficient use of time, used for critically ill patients that need less frequent therapies.

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Semipermeable Membrane

A membrane allowing some substances to pass through but not others.

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Transmembrane Pressure (TMP)

Pressure difference across a semipermeable membrane.

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Fouling

Protein buildup on the membrane, reducing permeability.

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Diffusion

Solute movement from high to low concentration.

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Convection (in context of therapy)

Movement of solutes due to a pressure gradient.

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Ultrafiltration

Removal of fluid from blood under pressure using a semipermeable membrane.

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Adsorption

Solutes sticking to the membrane surface.

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Arteriovenous (AV) access

Therapy requiring an arterial and venous catheter access for blood flow.

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Venovenous (VV) access

Therapy requiring two venous catheters, or a dual lumen catheter for blood flow.

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CRRT

Continuous renal replacement therapy, which encompasses different types of therapies for renal failure with continuous blood flow.

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Study Notes

Principles of CRRT

  • CRRT stands for Continuous Renal Replacement Therapy
  • CRRT is used for critically ill, hemodynamically unstable patients over an extended period.
  • Hemoperfusion started as a mistake in 1977, Germany
  • A femoral vein catheter was mistakenly placed in the femoral artery
  • CRRT started as a purely convective therapy, later a blood pump was added
  • It took 10 years to treat the first neonate, in 1985

Methods of Solute Removal

  • Diffusion: Solutes move from high concentration to low concentration
  • Convection: Solutes move due to fluid flow.
  • Ultrafiltration: Removal of fluid by applying pressure.
  • Adsorption: Solutes stick to the membrane

Semipermeable Membrane

  • TMP = (Pin + Pout)/2 - Peff
  • Fouling: proteins stick to membrane causing decreased permeability

Types of Therapy

  • Continuous Therapies are intended for critically ill, hemodynamically unstable patients over an extended period of time.
  • Prolonged Intermittent Therapies are evolved for more efficient use of time

Blood Flow

  • Arteriovenous: Requires arterial and venous catheters, arterial pressure determines blood flow speed.
  • Venovenous: Requires dual lumen catheters or two venous catheters, requires a blood pump, operator sets blood flow speed

Abbreviations

  • C refers to continuous
  • AV refers to arteriovenous,
  • V refers to venovenous,
  • H means hemofiltration
  • HD means hemodialysis
  • HDF means hemodiafiltration

CAVH

  • CAVH (Continuous Arteriovenous Hemofiltration)
  • A diagram showing the flow of replacement fluid, access, return, and effluent

CVVH

  • CVVH (Continuous Venous Hemofiltration) Post Filter: A diagram showing the flow of replacement fluid, access, pump, return, and effluent

CVVHDF

  • CVVHDF (Continuous Venous Hemofiltration with Diafiltration) Post Filter: A diagram showing the flow of replacement fluid, access, pump, return, and effluent

SCUF

  • SCUF (Slow Continuous Ultrafiltration): A diagram showing the flow of replacement fluid, access, pump, return, and effluent

Modalities of RRT

  • Different ways to perform renal replacement therapy.

High Volume Hemofiltration

  • Larger volume of fluid removal/replacement
  • Replacement fluid composition determines blood concentrations.
  • No phosphate

Convective Clearance

  • "Solvent Drag": Every ml of effluent = 1 ml plasma clearance
  • Convective flux determined by ultrafiltration rate, blood solute concentration and membrane siiving properties.
  • Less dependent on molecular size.

Fluid Flow Characteristics

  • Velocity fastest in center
  • Resistance to flow at edges near walls
  • Cells and proteins sludge against membrane on blood side
  • Solutes concentrated against membrane on effluent side
  • Blocks pores, causing decreased diffusion and convection efficiency
  • Unstirred layer decreases diffusion efficiency

Pre-filter Dilution

  • Advantages include decreased hemoconcentration, reduced clotting, mass transfer alterations, reduced RBC and protein concentration, and increased flow through the blood compartment.
  • Increased membrane shear rate enhances solute movement.

Post-Filter Dilution

  • Advantages include maximal efficiency.
  • Disadvantages include hemoconcentration inside the filter, clotting, decreased solute transfer, and decreased filter life.

Filtration Fraction

  • FF is the proportion of plasma filtered across the membrane.
  • Target FF<25% for hemoconcentration. Higher FF leads to a higher risk of filter clotting and more protein on the filter membrane, thus decreasing permeability.

Definitions

  • QNetUF = net patient fluid removal
  • QUF = all fluid removed (QR + QPBP + QNetUF)
  • PBP = pre-blood pump

Diffusion

  • IHD (Intermittent Hemodialysis) uses dialysate in excess of blood flow (Qd = 500 ml/min) with blood flow rate (Qb = 50-300 ml/min)
  • CRRT (Continuous Renal Replacement Therapy): Blood flow (Qb) exceeds therapy fluid, (Qd + Qr + QNetUF) 2 L/hr = 33 ml/min typical adult, 10L/hr = 166 ml/min max. Qb = 50-180 ml/min.

Diffusive Clearance Rates

  • clearance values given for IHD and CRRT

Hemodiafiltration - CVVHDF

  • Balances good and bad of diffusive and convective modes.
  • New machines can perform predilution and postdilution simultaneously

Estimating Kt/V

  • Formulas for various CRRT methods.

Comparing CRRT to IHD

  • CRRT is not blood flow dependent, whereas IHD is.
  • Higher blood flow rates in CRRT reduce clotting.

Bacteriological Safety of Therapy Fluid

  • Dialysate and Replacement Fluids (prepackaged) are essentially sterile.
  • More expensive than dialysate made by the dialysis machine, which requires water purification.
  • Endotoxins can cause inflammation

Wrap-Up Questions

  • List methods of clearance (solute removal)
  • Define CVVH, CVVHD, CVVHDF, SCUF in regards to clearance methods
  • Define filtration fraction
  • What are the advantages and disadvantages of pre vs post-filter dilution?
  • What determines “dose” in CRRT?

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