Gas separation: processes – Solid sorption – Inorganic gas or liquid particle sorbed
Reexamination Certificate
1999-09-03
2001-05-08
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Solid sorption
Inorganic gas or liquid particle sorbed
C095S900000, C096S108000, C055S516000
Reexamination Certificate
active
06228150
ABSTRACT:
This invention relates to a carbon dioxide absorbent which is intended to be used during low flow or closed circuit anaesthesia.
It has been known since 1789 (A.Lavoisier) that animals confined within closed vessels would continue to respire for a prolonged period, provided the vessel contained a carbon dioxide gas absorber. In the early 1880's, Professor Bert of Paris conducted experiments with small animals in closed vessels using chloroform vapour as anaesthetic and potassium hydroxide as a carbon dioxide absorber and reported toxic effects in the animals which he subsequently showed to be due to the decomposition products of the reaction of the chloroform with potassium hydroxide.
In 1914, D. E. Jackson first applied the carbon dioxide absorption principle of rebreathing to inhalational anaesthesia and therein commenced a search for efficient carbon dioxide absorbents which ended, at that time, with soda lime granules as the material of choice. Soda lime has been used since 1914 in conjunction with rebreathing anaesthesia and during this time many variations on the sodium and/or potassium hydroxide contents of the chemical composition have been made in an attempt to prevent excessive heating of the granules and to minimise crumbling and dust formation when the exothermic gas absorption process takes place.
Soda lime still remains the carbon dioxide absorbent of choice but it is far from ideal. For example, circuits containing organo-halogen anaesthetic vapours tend to chemically react with the sodium and/or potassium hydroxides in the soda lime granules by alkaline hydrolysis, or dehydrohalogenation, with the production of by-products which are very much more toxic than the anaesthetic itself. Just such a situation was reported in 1966 after the introduction of trichloroethylene (Trilene™). When trichloroethylene was used in the presence of soda lime as the carbon dioxide absorber, the alkaline hydrolysis product was dichloroacetylene whose rebreathing resulted in cranial nerve palsies in some individuals.
As would be expected, soda lime also reacts with the vapours of the modern fluoro-chloro anaesthetics. In 1992 the anaesthetic Sevoflurane™ was shown to undergo degradation in the presence of soda lime to a toxic olefin denoted “Compound A” of structure CF
2
═C(CF
3
)OCH
2
F, which olefin is nephrotoxic to rats at concentrations of 60-100 ppm and lethal at concentrations of 350-400 ppm.
On a broader front, the literature teaches that the anaesthetics Enflurane™, Isoflurane™ and. Desflurane™ all react with soda lime under certain conditions and this has resulted in carbon monoxide poisoning in some individuals. The experimental evidence suggests that carbon monoxide is formed when these inhaled anaesthetics are used with dry soda lime, thereby producing formates, the probable precursors of the carbon monoxide. -Whilst the exact reactions have not yet been identified, it is thought that traces of fluoroform (trifLuoromethane; CHF
3
) are produced which produce carbon monoxide under alkaline conditions. It is, therefore, recommended to use fresh or wet soda lime, to avoid generation of carbon monoxide.
U.S. Pat. No. 2,322,206 (published on Jun. 23, 1943) concerns a carbon dioxide absorbent bonded mixture, containing lime and barium hydroxide. The stability of the bonded mixture is ascribed to its non-hygroscocicity. EP-A-0 530 731 (published on Mar. 10, 1993) concerns a carbon dioxide absorbent containing a magnesium compound. When the magnesium compound is magnesium hydroxide, water is present. EP-A-0 530 731 discloses that, in contrast to calcium hydroxide, magnesium compounds hardly decompose certain anaesthetic gases. DE-A-2316214 (published on Dec. 20, 1973) concerns granular carbon dioxide absorbents comprising a travertine porous support, calcium hydroxide, water and a glycol. JP-A-58177137 (published on Oct. 17, 1983) concerns a carbon dioxide absorbent powder mixture for absorbing carbon dioxide generated by foods. The powder mixture may include an alkaline metal or alkaline earth metal hydroxide with an inorganic deliquescent material. The example includes sodium hydroxide and the dehydrate or hexahydrate of calcium chloride.
It is apparent therefore that there is a need in clinical practice for an efficient carbon dioxide removing agent, which is chemically inert (benign) to the anaesthetic itself.
It is an object of the present invention to fulfil this need by providing a carbon dioxide absorbent for use in low flow or closed circuit anaesthesiology comprising calcium hydroxide, the calcium hydroxide being essentially free of sodium and potassium hydroxide; and a hygroscopic or deliquescent inorganic humectant or a mixture thereof.
In a second aspect, there is provided use of a carbon dioxide absorbent in low flow or closed circuit anaesthesiology, the absorbent comprising calcium hydroxide, the calcium hydroxide being essentially free of sodium and potassium hydroxide; and a hygroscopic or deliquescent inorganic humectant or a mixture thereof.
The described carbon dioxide absorbent can be physically improved in terms of granule strength by preferably incorporating in the mixture an effective amount of a compatible setting agent, preferably Plaster of Paris (calcium sulphate hemihydrate), to increase the granule crushing strength.
It will be appreciated that any suitable setting agent may be used in the absorbent of the invention. It will also be appreciated that, as the amount of setting agent is increased, the absorbent's crushing strength is increased but the absorbent's absorptive efficiency is reduced. The amount of setting agent should, therefore, be kept as low as possible, so as to improve the absorbent's crushing strength whilst not significantly reducing the absorbent's absorptive efficiency. Thus, for plaster of paris, it is preferred to incorporate 4-5.5% (w/w), 4.5-5% (w/w) being more preferred.
The carbon dioxide absorbent may additionally comprise a compatible agent for internal generation of hydrogen, for example, a suitable amount of aluminium metal powder, to swell or “aerate” the granules before hardening, thereby improving the reactive surface area of the granules or pellets. Aluminium metal powder is preferred at 0.5-1% (w/w).
The basic concept in the present invention is the use of a hydroxide, preferably a hydroxide of low solubility in water (for example, calcium hydroxide, which is only soluble in water to the extent of 1 part in 900 of water) which nevertheless has a high capacity (reserve of acid neutralising power) for carbon dioxide absorption provided it can remain damp at all times. If such a permanent intrinsic moisture content can be achieved within the matrix of the hydroxide (preferably calcium hydroxide)-containing absorbent, without resorting to the hygroscopic property of the sodium or potassium hydroxide of soda lime, then carbon dioxide absorption can take place in low flow or closed circuit anaesthesiology without the production of either “Compound A” or carbon monoxide. Such permanent intrinsic moisture contents of the described absorbents are achieved by the incorporation of hygroscopic or deliquescent substances in the mix as humectants—these can be either inorganic or organic in nature.
The inorganic humectants must be hygroscopic (absorb atmospheric water) or deliquescent (absorb atmospheric water and dissolve in the water thus absorbed). The preferred inorganic humectants are hydroscopic or deliquescent hydrates. Suitable hydrates can be readily obtained by evaporation of an aqueous solution of a hydrate-forming compound. A preferred inorganic humectant is calcium chloride hexahydrate but a hydrate of magnesium chloride is also suitable. Although magnesium chloride hexahydrate reacts with calcium hydroxide to give magnesium hydroxide, the by-product of the reaction is calcium chloride, so that the matrix of the absorbent becomes a mixture of calcium and magnesium hydroxides kept permanently moist by the calcium chloride of the reaction.
The preferred inorganic humectant is calcium chlo
Armstrong John Raymond
Murray James
Armstrong Medical Limited
Baker & Botts L.L.P.
Spitzer Robert H.
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