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Understanding Sorbent Dialysis
By: Dr. John Agar
Barwon Health, Geelong, Victoria, Australia
Sorbent dialysis was introduced in the early 1970’s. In a sorbent dialysis treatment, used dialysate is circulated through a cartridge packed with specialized chemicals. The sorbent cartridge regenerates the dialysate so that it can be used for the entire dialysis treatment.
It is common to find confusion between the terms sorbent dialysis and hemoperfusion, which is used in an ICU for the removal of drugs and other poisons in overdose treatments. The important difference between the two processes is that in hemoperfusion, the blood passes over an adsorptive column of activated charcoal or other polymer adsorptive substances. In sorbent dialysis, the dialytic process is unaltered. The sorbent cartridge simply regenerates the dialysate.
What is a Sorbent?
A sorbent is a material that binds another substance or compound to it by a physical and/or chemical reaction.
A sorbent – which can be either solid or liquid in form – permits (or even encourages) the binding of an extremely thin layer of molecules to its surface by:
...and is especially effective if its surface area is large. |
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For example, a common adsorbent, zirconium phosphate, possesses sodium and hydrogen bonds on its surface which are preferentially exchanged for other more active monovalent and divalent cations such as K+, Ca++ and Mg++.
The Difference between Standard Single Pass and Sorbent Dialysis Systems
For a standard dialysis system, there is need for:
- A power source
- A continuous flow water source
- Water treatment equipment (filtration + reverse osmosis)
- A proportioning system
- Per-use sterilization and regular decalcification of the internalized water and dialysate circuits
- An effluent drain
For a sorbent system, there is need for:
- A power source
- A sorbent cartridge
Sorbent dialysis does not need a continuous water source, water filtration or reverse osmosis water treatment equipment, and does not need an effluent drain.
The Sorbent Cartridge – the ‘Key’ to Sorbent Therapy
The key to sorbent therapy is the capacity for the effluent or used dialysate (which usually is drained to waste in single-pass systems) to be passed through a cartridge containing a variety of adsorbents and to emerge cleaned and purified, ready for re-presentation to the dialyzer. This allows the required volume of dialysate to be hugely diminished.
After leaving the dialyzer, the effluent dialysate in a sorbent system is no different from the dialysate that exits the used dialysate port of any standard single-pass system. In a single-pass system, the effluent dialysate is piped to a waste drain and away. By contrast, in a sorbent system the effluent dialysate is presented to the sorbent cartridge. Effluent dialysate enters the bottom of the cartridge and is purified as it passes through 5 contiguous layers.
Layer 1:
The first layer consists of activated charcoal. This removes, by absorption, heavy metals, oxidants, chloramines, creatinine, uric acid, a variety of middle molecules including B2 microglobulin and other organics. Nothing is released or produced in exchange; these substances are simply absorbed by the activated charcoal. Wikipedia describes activated charcoal as:
“a material with an exceptionally high surface area (where) just one gram has a surface area of approximately 500 m2 and includes a large amount of microporosity. Sufficient activation may come solely from the high surface area, though further chemical treatment (may be) used to enhance its absorbing properties.”
Layer 2:
The second layer contains urease, which, again, is described in Wikipedia as:
“an enzyme that catalyzes the hydrolysis of urea into carbon dioxide and ammonia (by) the following reaction: (NH2)2CO + H2O ⇔ CO2 + 2NH3.”
Ammonium carbonate is released by this layer, the total production of which is transferred by the forward fluid flow to layer 3.
Layer 3:
In the third layer, the now significantly cleansed and modified effluent dialysate is passed over adsobent zirconium phosphate. This has available sodium and hydrogen abundantly distributed over its massive surface area which exchanges for preferentially adsorbed calcium, magnesium, potassium, other cations and metals and, importantly, ammonium. Thus the ammonium created in the second layer is removed in the third in exchange for H+ and Na+.
Layer 4:
In the fourth layer, the now largely cleansed ‘dialysate’ is subjected to a combination of zirconium oxide and zirconium carbonate. Both zirconium oxide and zirconium carbonate adsorb phosphate, fluoride, heavy metals, and anions exchanging them for sodium, bicarbonate, and a small amount of acetate.
At the end of this journey through the cartridge, the effluent dialysate has been cleansed of all the metabolic wastes removed from the blood across the dialyzer membrane - the emergent fluid now being water + Na+ + H+ + HCO3- + a small amount of acetate.
Importantly, the cartridge acts as a bacterial filter for any bacterial contamination and an endotoxin adsorbent for any endotoxin. The removal of bacteria and endotoxin is so efficient that the bacterial counts in the emergent fluid are < 1cfu/ml and the detectable endotoxin is < 0.3EU/ml – both approaching those levels required of ultrapure water and far better than those for either the AAMI or European water standards.
Re-constituting the Dialysate
One last step is needed. Much like a ‘concentrate’ of chemicals is needed in single-pass systems to mix with R/O water to make the final dialysate, a composite chemical mix of potassium, calcium, and magnesium is needed to re-form the final dialysate before ‘re-presentation’ to the dialyzer. This is because the cartridge effectively removes these electrolytes during dialysate cleansing.
... then ... round goes the dialysate again ... and again ... and again.
So...How Does All This Work in Practice?
A container is filled with 6 liters of water and powdered electrolytes and the process of circulation through the cartridge is begun. This water is purified and decontaminated, any impurities being removed as it circulates past the cartridge layers. Once the sorbent cartridge is primed, dialysis can begin.
Conclusions
Greater mobility, the ability to use a small amount of water, the suitability for mobile or multi-site dialysis, the advantages for arid regions, the major reduction in R.O. maintenance requirements and costs...all these and more suggest that the resurgence of interest in sorbent systems is well-founded and the future for such systems seems bright.