Surgery – Respiratory method or device – Means for removing substance from respiratory gas
Reexamination Certificate
1999-06-30
2001-08-28
Lewis, Aaron J. (Department: 3761)
Surgery
Respiratory method or device
Means for removing substance from respiratory gas
C128S204180
Reexamination Certificate
active
06279576
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to the field of anesthesia, and in particular to purification of expiration gases during anesthesia. In one aspect it relates to a regenerative device for removal of CO
2
from such gas, and to methods of regeneration of such a device for multiple use thereof. In other aspects the invention relates to a system of anesthesia and to a method of anesthesia.
BACKGROUND OF THE INVENTION
For the purposes of this application the terms absorption and absorbent will include both absorption and adsorption processes.
It is a well established technique to remove CO
2
from expiration gases during anesthesia by means of absorption using mixtures of hydroxides, such as soda lime (a commonly accepted designation of a mixture of sodium and calcium hydroxide). The soda lime is placed in a canister, and expiration gases are passed through the canister, whereby the CO
2
reacts to form carbonates, and thus is removed from the expired air. The reason for wanting to remove CO
2
is that the anesthetic gases used are expensive and it is desirable to reuse the portion not consumed by the patient. The use of hydroxides has been in practice for more than 80 years since D. E. Jackson designed the first machine to be used in a closed system for anesthesia, and numerous improvements have been described in the literature and patents since then.
It should be noted that the processes occurring in this type of devices are in principle irreversible chemical reactions, where the CO
2
and the hydroxides react and carbonates are formed. These carbonates are not easily regenerated such that the hydroxides may be recovered.
DESCRIPTION OF RELATED ART
CO
2
absorbers based on the use of various hydroxides are extensively reviewed by Adriani in “The Chemistry and Physics of Anesthesia”, 1 ed., 1979, pp 151-184, Charles C. Thomas Publishers, Springfield, Ill. USA.
Recently A. M. Holloway in “Anaesth. Intens. Care”, 1994; 22; pp 359-362, discussed possible alternatives to soda lime for CO
2
removal. Inter alia there is discussed the use of monoethanolamine as a regenerative wet CO
2
scrubber, which has been used in submarines. Such systems have the disadvantage that the atmosphere may be contaminated with monoethanolamine and ammonia, and are now being replaced with safer systems. Holloway concludes that CO
2
absorption by a molecular sieve is probably the most promising system that could be introduced into anesthetic practice.
J. P. H. Fee et al in “Anesthesia”, 1995, volume 50, pp 841-845 describes an efficient system based on a molecular sieve for removal of CO
2
. In particular the decomposition of sevoflurane to compound A (CF
2
═C(CF
3
)OCH
2
F) is discussed, and it is noted that marked decomposition is taking place with soda lime, and a detectable decomposition occurs also with the molecular sieve.
In Anesthesia and Analgesia, 1995, p 82, Abstract S425 the use of molecular sieves is described, and it is stated therein that the sieves also absorbs the anesthetic gases, which of course is not a desirable effect.
As indicated above, at present it appears to be a general view in the art of anesthesia that molecular sieves are the most promising candidate for removing CO
2
from expiration air, as an alternative to the well established use of soda lime. None of the workers in the field of anesthesia have acknowledged the prior art dealing with absorption by ion exchange resins as possible candidates in this respect.
There are a number of drawbacks with the use of the absorbent in the form of soda lime or other highly alkaline components.
When the soda lime is used up, it has to be discarded, and the absorbent, possibly containing bacteria or other hazardous micro organisms, is to be regarded as a hazardous waste, and must be treated accordingly. Also the refilling of canisters is a risk operation, in that the granulate being highly alkaline, make a lot of dust that may be irritating to the skin and lungs. Single-use canisters that are not refilled add to the total waste material that has to be discarded.
Another drawback is that the reaction between CO
2
and soda lime is exothermic, and depending on the flow rate through the absorbent bed, rather high temperatures may be reached. This may lead to a reaction between the anesthetic and the absorbent with accompanying decomposition of the anesthetic. Especially the occurrence of Compound A (fluoromethyl-2,2-difluoro-I-(trifluoromethyl)vinyl ether; CF
2
═C(CF
3
)OCH
2
F) is a serious risk factor. The formation of compound A is also observed with the absorbent Baralyme™ (see Anesth. Analg. 1995;81:564-8).
Another highly toxic by-product is carbon monoxide which is believed to be formed from the reaction of anesthetics such as desflurane, enflurane, and isoflurane with desiccated carbon dioxide absorbents of the prior art. With respect to the above consideration, replacement of carbon dioxide absorbent with fresh product is recommended whenever desiccation is suspected. Desiccation is believed to result from high fresh gas flows passing through the carbon dioxide absorber canister over the course of a weekend. In the case of the present invention these problems are eliminated by the use of absorber material of the ion exchange type on which no such reactions take place. The above mentioned problems do not arise with the absorber of the present invention.
A further drawback associated with the use of a molecular sieve for CO
2
absorption is that molecular sieves also absorbs a substantial amount of the anesthetic gases.
Also, of course the irreversible nature of the soda lime based devices is a fundamental drawback, which is overcome by the invention.
SUMMARY OF THE INVENTION
An object of the present invention is therefore to provide a method and device for removing CO
2
from expired air from a patient subject to anesthesia, where the active substance for CO
2
removal is an ion exchange resin. The device shall be regenerative such that the hazardous handling of contaminated spent material and highly alkaline soda lime is eliminated.
The device will have a capability of being regenerated 20 times while maintaining 90% of its activity, preferably 95 times with 90% activity, and 200 times with more than 50% activity left.
The inventors have now surprisingly discovered that it is possible to design a regenerative CO
2
removal system for anesthetic applications, without the prior art disadvantages, and making use of the capacity of ion exchange materials as absorbent. This is in contrast toithe general opinion in this field, that molecular sieves be the most promising option as a substitute for the soda lime based technology.
A very important advantage of the invention is that anion exchange resins do not exhibit the above mentioned behavior of molecular sieves, namely to absorb a substantial amount of the anesthetic gases that are passed through such resins.
The above object is thus achieved with a regenerative CO
2
absorbing device a system for anesthesia and a method of anesthesia as recited in the claims.
Methods and an apparatus of regeneration of a CO
2
absorber is also defined in the claims.
Various embodiments of the different aspects of the invention are defined in the dependent claims.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
REFERENCES:
patent: 3407045 (1968-10-01), Temple
patent: 3498026 (1970-03-01), Messinger et al.
patent: 3785377 (1974-01-01), Jorgensen
patent: 3814091 (1974-06-01), Henkin
patent: 3898987 (1975-08-01), Elam
patent: 3923057 (1975-12-01), Chalon
patent: 4005708 (1977-02-01), Netteland et al.
patent: 4596246 (1986-06-01), Lyall
patent: 47
Lewis Aaron J.
Louis Gibeck AB
Mitchell Teena
Young & Thompson
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