Hemodialysis (HD)
Hemodialysis (HD)
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AKA intermittent hemodialysis (iHD)
External links:
Hemodialysis simulator 🔒 available on OpenPediatrics (free account required)
ASPN Webinar: Initiation, withholding, and withdrawal of dialysis in children
Recommended reading:
Table of contents:
Related topics:
Special thanks to:
- Dr. Cynthia Wong
- Dr. Jordan Symons
Basic setup
Basic setup
Blood from the patient perfuses an extracorporeal circuit and enters the tiny capillaries of the hollow fiber dialyzers
Dialysate passess on opposite side of membrane
High efficiency system:
Blood and dialysate move at rapid speeds, so particles move quickly
Particle removal is mostly by diffusion
Fluid removal by ultrafiltration (hydrostatic pressure across the dialyzer membrane)
Small particles are also forced across the membrane by hydrostatic pressure
Image: Jordan Symons, MD
Image: Jordan Symons, MD
Effect of pore size on membrane selectivity
Effect of pore size on membrane selectivity
Small and middle sized (based on molecular weight) molecules tend to traverse the membrane fairly easily
"Small" molecules are <500 Da
"Middle" molecules are 500-60,000 Da
"Large" molecules >60,000 Da
Small molecules may be bound to larger molecules such as albumin
When this is the case, the small molecules will behave like the larger molecules they are bound to
Underlying concepts
Underlying concepts
Ultrafiltration (UF)
Ultrafiltration (UF)
The removal of plasma water from whole blood across a semipermeable membrane (the dialysis filter) down a pressure gradient
To increase the UF rate in HD, we decrease the pressure of the dialysate, which increases the hydrostatic pressure gradient or transmembrane pressure (TMP).
When moving down this pressure gradient, the movement of water "drags" the dissolved solutes along with it (this "solvent drag" is called convection).
Ultrafiltrated water is isotonic
- Contrast this with loop diuretics, which produce a hypotonic urine.
Solids and large molecules are left behind. The exact composition of the ultrafiltrated fluid depends on the TMP and on the characteristics of the filter membrane.
In dialysis, we use this term to quantify the net removal of water.
Convection
Convection
Convection is the mass transfer of solutes down a pressure gradient in ultrafiltrated fluid
Pressure is applied to one side of the semipermeable membrane, forcing water and the dissolved molecules across the membrane
To establish this pressure gradient, one can apply positive pressure on one side of the membrane or negative pressure applied to the other side; in dialysis, we apply negative pressure on the dialysate side of the membrane to force water and solutes over from the blood side of the membrane.
Unlike diffusion, particle movement is not dependent on size: if molecules can fit through the pores of the membrane, they will flow equally well
Diffusion (hemodialysis)
Diffusion (hemodialysis)
Diffusion is the movement of particles down a concentration gradient
Particles move from an area of higher concentration (on one side of the membrane) to an area of lower concentration (on the other side of the membrane)
Particles move through random (Brownian) motion
Small molecules diffuse more quickly than larger molecules
Diffusion occurs in both directions across the membrane
If the concentration of the molecules are the same on both sides (i.e., equilibrium), the net movement is zero
Diffusion is the main form of solute removal in HD.
Convection (hemofiltration) vs Diffusion (hemodialysis) vs Combination (hemodiafiltration)
Convection (hemofiltration) vs Diffusion (hemodialysis) vs Combination (hemodiafiltration)
Given the same dose of dialysis, diffusion and convection are similarly effective at removing small solutes, but convection is better at clearing large molecules
Removing molecules and fluids
Removing molecules and fluids
Permeability surface area product (K0A)
Permeability surface area product (K0A)
K0A ("K-O-A") is a property of the dialyzer
Product of permeability (K0) & surface area (A): K0A = K0 & A
Permeability (K0) = transfer coefficient of the membrane for a given solute
Dependent upon pores per surface area, thickness of the membrane, and design of the dialyzer (i.e., degree of contact between blood and dialysate columns across the membrane)
Describes maximum ability of dialyzer to "clear" a given substance (how well the filter allows molecules to move from the blood into the dialysate)
The in vitro K0A is specified by the manufacturer of the dialyzer
In vitro K0A is often 20-30% higher than in vivo measurements
K0A urea is the maximum possible urea clearance (mL/min) at infinite QB and QD
High efficiency dialyzers have a urea K0A >600 mL/min, whereas conventional dialyzers have a urea K0A of <500
Clearance (KD)
Clearance (KD)
Clearance (KD) describes ability of a dialyzer to remove a substance from the blood under a given set of circumstances (the dialysis prescription)
Some define high efficiency dialyzers as having a Kurea of >210 mL/min
Clearance is a function:
Size and permeability of the filter selected (K0A)
Blood flow rate (QB)
Dialysate flow rate (QD)
Effects of K0A blood flow on clearance
At higher blood flow rates, larger dialyzers are able to achieve better clearance (greater efficiency) than smaller dialyzers at the same blood flow rates
At lower blood flow rates (<100 mL/min), there is no advantage to using a large dialyzer
In children, we may not be able to achieve a high blood flow rate, so a smaller dialyzer may be more appropriate
Smaller dialyzers require smaller extracorporeal blood volume - less blood outside of the patient that is liable to clot
Effects of blood flow and molecular weight on clearance
Smaller molecules (e.g., urea) diffuse easily, and clearance (KD) increases with QB and QD
Larger molecules (e.g., vitamin B12) diffuse slowly, and clearance does not increase as much as blood or dialysate flow rates increase
In other words, large molecules have a lower KD (relative to small molecules) for a given QB and QD
Ultrafiltration (UF)
Ultrafiltration (UF)
Removal of water due to the effects of hydrostatic pressure
Solutes are removed by convection at the same time
The composition of the solutes in the water is determined by the characteristics of the dialyzer (flux)
The flux of the dialysis membrane is how "leaky" it is: high flux membranes allow mid-sized molecules through, whereas low-flux filters only permit small molecules.
High flux dialyzers are arbitrarily defined as having a β2-microglobulin clearance of >20 mL/min
Low flux dialyzers have a β2-microglobulin clearance of <10 mL/min
- Vitamin B12 (MW 1.335 kD) is used as a surrogate for middle molecules and β2-microglobulin (MW 11.8 kD) is used a surrogate for large weight molecules
Ultrafiltration coefficient (KUF) describes the ultrafiltration capability of a dialyzer
KUF is expressed in mL/hour/mmHg
As this suggests, one can increase ultrafiltration rate by using a larger/more porous dialyzer (higher KUF) or by increasing the transmembrane pressure (TMP) across the membrane
However, too much TMP can crack the dialyzer
References
References
[AMA formatted citations]
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