WAG Treatment and CO₂ Absorbers: New Technologies for Pollution and Waste Prevention

There are three potential methods for destruction of N₂O: oxidation, reduction or catalytic splitting. Oxidation produces NOx (x>0.5) species, which also impact the environment negatively. Reduction, on the other hand, removes oxygen to produce N₂ and another oxidized substance (depending on the catalyst). Catalytic splitting splits N₂O into N₂ and O₂. Catalytic splitting technology is currently used in Sweden to prevent waste N₂O release into the outdoor atmosphere from labor and delivery suites. It was adapted from Japanese technology and scaled up to facility-level scavenging system installation.²  The Swedish technology treats 95 percent of collected N₂O. The word “collected” should be emphasized, as a significant amount of N₂O escapes due to the nature of delivery (patient-controlled face mask). N₂O used in the O.R. can have a much higher rate of collection when administered in a semi-closed circle system; however, leaving fresh gas flows on while intubating after mask induction, and mask anesthetics, are significant opportunities for pollution outside of the scavenging system. Destruction of N₂O could, in theory, be accomplished via a single catalytic splitting unit servicing multiple L&D rooms or O.R.s, and potentially PACUs if installed in HVAC systems.

Sequestration refers to a method to prevent release of anesthetic agents into the environment. Activated charcoal has been shown to effectively adsorb volatile anesthetics only. There has been some interest in its use as a reflector, meaning both adsorption and desorption of anesthetic gas. Along with the anesthetic gas, adsorption of methanol and acetone also occurs, and the potential for desorption and delivery of these waste products back to the patient is problematic.³  If activated charcoal is used solely for anesthetic gas sequestration, there is still the problem of storage and disposal. Saturated charcoal will quickly desorb and release agents back into the environment, so its greatest utility is for indoor pollution control in settings that lack scavenging systems; this technology is not, however, effective for outdoor pollution prevention. Desorption accompanied by destruction and/or reclamation is necessary for activated charcoal to be truly effective at preventing the release of volatile anesthetic gases into the atmosphere. Cold trap condensation is another form of sequestration. It recaptures agents in liquid form by cooling them to below their dew point. This could potentially allow for reclamation of the volatile anesthetic gases for destruction or reuse.

Blue-Zone Technologies Ltd. (Toronto, Ontario, Canada) produces the Deltasorb^® canister system for use on a single machine, and Centralsorb^® is a more centrally located system for collecting halogenated agents. Both utilize honeycomb-shaped silica zeolite crystals (Deltazite^®) to maximize surface area. Deltasorb canisters were found to be effective for 100 percent adsorption of isoflurane for up to eight hours.⁴  Deltazite^® allows for desorption of halogenated agents as liquids for potential processing and reuse.⁴  Reuse of halogenated ethers is not currently approved by the FDA, and storage of collected wasted volatiles is an issue. This technology is not suitable for N₂O waste handling.

Standard scavenging systems consist of vacuum technology that limits indoor occupational exposure to anesthetic gases, though eventually releases these directly to the outdoor environment. The most common systems are continuously on, even if the anesthesia machine is not in use. This greatly increases energy consumption. The Dynamic Gas Scavenging System (Anesthetic Gas Reclamation, Inc.) provides a more efficient scavenging system that can be installed on any anesthesia machine.⁵  The use of a pressure valve/switch ensures that the system operates only when the anesthesia machine is in use. This reduces room air entrainment, energy use and the cost of running the scavenging system.

Carbon dioxide absorbers in semi-closed circuits allow for use of fresh gas flows well below minute ventilation, unlike historic open-circuit systems. One need only replace the oxygen consumed by the patient. Low-flow anesthesia allows for reduction in the amount of anesthetic gas used and therefore wasted into the atmosphere. Low-flow sevoflurane presents its own challenge due to the theoretical risk from the formation of compound A. Newer CO₂ absorbers have calcium hydroxide, as opposed to historic sodium hydroxide or potassium hydroxide (Amsorb^®, Armstrong Medical Ltd). These produce compound A only when desiccated, thus permitting low-flow sevoflurane without the concern for compound A toxicity.^6,7 

Lithium hydroxide is a promising CO₂ absorbent that also does not produce compound A (Litholyme™, Allied Healthcare Products, Inc.). It was initially thought to undergo an exothermic reaction that produced too much heat to be clinically useful. A new formulation of lithium hydroxide, chemically bound to water (SpiraLith™, Micropore, Inc., Elkton, MD), was shown not to have a significant increase in temperature when compared to calcium hydroxide-containing absorbents.⁸  Lithium hydroxide does not absorb volatile anesthetics, decreasing the amount of time necessary to reach equilibrium in the breathing system and thus the amount of volatile agents used.⁹  It also does not undergo a significant weight change between its fresh and desiccated forms, signifying that it will perform similarly in both forms.¹⁰ 

Memsorb¹¹  is a new technology in development that does not rely on chemical reactions, but rather uses polymeric membranes similar to ones used in oxygenators for cardiac surgery. The membranes selectively allow CO₂ to leave the rebreathing circuit while maintaining anesthetic vapor, and no compound A is produced.

WAG pollution is a concern. In addition to behaviors that limit its impact (namely, careful selection of least-polluting drugs and close attention to lowest fresh gas flows), new technology can help limit WAG release into the atmosphere. Both WAG capturing and destruction technologies are already in limited use. WAG capturing technologies are effective for collecting volatile anesthetics, but do not handle N₂O, and storage is a concern. WAG destruction technologies are in routine use for N₂O in other nations, (e.g., Sweden); however, they would need to be modified to handle volatile agents. Newer CO₂ absorber technologies can permit lower fresh gas flows without concern for compound A. All such technologies are promising to aid in the prevention of WAG pollution.