SEVOFLURANE ANAESTHESIA IN LOW-FLOW SYSTEMS
Michael Nathanson,
Nottingham.

Sevoflurane was first synthesized in 1968, Early animal work and the first reports of the use of sevoflurane in human subjects were encouraging. However, compared with other agents being investigated at that time sevoflurane is a relatively unstable molecule. It undergoes both hepatic metabolism and is broken down by soda lime. Further work on sevoflurane in the US was halted for commercial and scientific reasons. Development continued in Japan where it was released in 1990.
Sevoflurane has good reasons to commend its use in a circle system at low flows - its low blood gas partition coefficient ensures that depth of anaesthesia can be precisely controlled even at low fresh gas flows and it is expensive to use at high fresh gas flows.
Approximately 5% of administered sevoflurane undergoes hepatic metabolism to inorganic fluoride ions and hexafluoroisopropanol. Although hexafluoroisopropanol is potentially hepatotoxic it is conjugated so rapidly that liver damage seems theoretically impossible. Despite several studies showing that peak serum fluoride concentrations after sevoflurane anaesthesia can be in excess of 50 umon/1, there is no data to show that these levels of inorganic fluoride are detrimental either to patients with normal renal function or to those
with renal failure. Serum fluoride levels are not related to fresh gas flow rate.
Sevoflurane reacts with the strong bases in carbon dioxide adsorbents to form fluoromethyl-2,2-difluoro-l-Ctrifluoromethyl) vinyl ether, otherwise known as Compound A. Although five breakdown products may be formed experimentally only Compounds A and B have been found in anaesthesia circuits.
Compound A has been measured in the inspiratory limb of anaesthetic circle systems under a variety of conditions. The mean peak concentration ranged from 2.1 to 32.0 ppm. The maximum individual peak Compound A concentration detected was 60.8 ppm. The factors thought to affect the concentration of sevoflurane degradation products include: temperature of the carbon dioxide absorbent, fresh gas flow, patient's carbon dioxide elimination, concentration of sevoflurane in the circle system, type of absorbent used, freshness of the absorbent, and water content of the absorbent. In particular higher fresh gas flow rates are associated with decreased concentrations of Compound A, as are reductions in temperature of the absorbent.
Compound A toxicity in rats is both concentration and time-dependent. The concentration of Compound A required to kill 50% of rats (L50) after a 1 h exposure is approximately 1050 ppm, and after a 3 h exposure is 400 ppm. Acute toxicity primarily involves pulmonary and renal damage, Exposure to Compound B at 2,400 ppm for 3 h is not toxic to rats. The threshold level of Compound A to produce renal damage in rats is 150-200 ppm for a 1 h exposure and 50 ppm for a 3 h exposure. The pattern of renal injury seen is a corticomedullary proximal tubular necrosis.
The exact mechanism of Compound A toxicity in rats is unknown. However, Compound A is conjugated in the liver, the conjugate then passes to the kidney where it undergoes a ?-lyase reaction to form potentially nephrotoxic acylating intermediates. The activity of P-lyase in the human kidney is 10% of the activity seen in the rat kidney. If the same pathways exist in humans, this difference in enzyme activity may explain the apparent lack of toxicity of Compound A so far seen in human studies and in clinical use.
A number of groups have looked for evidence of renal or hepatic dysfunction after sevoflurane anaesthesia in a circle system. Although minor changes in some laboratory tests have been seen (for example, rises in bilirubin, AST and ALT), these were clinically insignificant and overall there was no indication of organ toxicity. Furthermore such changes can be seen after anaesthesia with halothane, enflurane and isoflurane. However, routine laboratory tests may be insensitive to mild degrees of organ impairment and more detailed studies are required to confirm these findings.
Molecular sieves may be used as an alternative to carbon dioxide absorbents containing strong bases. Breakdown of sevoflurane does not occur and Compound A concentrations do not rise above baseline (contaminant) levels.
Anaesthetic agents which contain the CF2H- group can react with carbon dioxide absorbent to form carbon monoxide under certain conditions. This group is not present in sevoflurane, Even under extreme conditions of completely dry absorbent and high temperatures, carbon monoxide formation during use of sevoflurane is negligible.
In conclusion, sevoflurane does offer a significant advance over previously available agents and appears to be suitable for use in low flow systems. Although the degradation of sevoflurane in low flow systems is a cause for concern, no toxicity has been detected in man. However, during clinical use Compound A concentrations do approach levels found to be nephrotoxic in rats and further work including sensitive tests of renal and hepatic function are required. The use of molecular sieves may offer an acceptable alternative to soda lime in the future.


 

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Anaesthesia with Xenon
Dr. M.J. Halsey

Oxford

This year is the 50th Anniversary of the first report of the anaesthetic effects of Xenon . The consensus that has emerged over the years is that were it not expensive to administer, Xenon might well be an "ideal anesthetic". Because it has a low blood/gas partition (0.17) and a MAC of 71% , it has the highly desirable features of being sufficiently potent to be used alone and of permitting rapid induction and emergence from anaesthesia. The gas is nonflammable, and appears to be associated with only minor toxicity and minimal cardiovascular and respiratory depressant effects. There are a number of limitations to this encouraging perspective, which will be discussed at this meeting to determine both the potential niche for the agent, and the work that is still required to realise that potential.
The year 1998 will be the 100th Anniversary of the discovery of Xenon itself. It would be particularly appropriate if new developments in low flow anaesthesia over the next two years resulted in Xenon being less expensive to administer and thus ceasing to be a "stranger" to anaesthetic practice.
Selected References

Early Animal and Human Studies include:
Lawrence-JH, Loomis WF Tobias CA Turpin FH Preliminary observations on the narcotic effect of xenon. J Physiol 194C); 105; 197-204
Cullen SC and Gross EG The anaesthetic properties of xenon in animals and human beings Science 1951; 113; 580-2
Pittinger CB, Moyers J, Cullen SC Featherstone RM Gross EG Clinicopathologic studies associated with xenon anesthesia. Anesthesiology 1953; 14: 10-17
Braken A, Burns THS, Newland US. A trial of xenon as a non explosive anaesthetic Anaesthesia 1956; 11: 40-49 Cardiovascular and Respiratory Studies include:
Luttropp-HH; Komner-B; Pcrhag-L; Eskilsson-J; Fredriksen-S; Wcrncr-0 Left vcntricular performance and cerebral haemodynamics during xenon anaesthesia. A transoesophageal ediocardiograpliy and transcranial Doppler sonography study. Anaesthesia. 1993 ; 48: 1045-9
Boomsina-F; Ruprcht-J; Man-in-'t-Veld-AJ; de-Jong-1-H; U/.oljic-M; I-aclimann-B. HaciinxJyuuiiiic and ncurohumoral effects of xenon anaesthesia. A comparison with nitrous oxide. Anaeslhesia. 1990; 45: 273-8
van-Woerkens-LJ; Lachmann-B; van-Daal-GJ; Schairer-W; Tcnbrinck-R; Vcrdouw-PD; Erdmann-W. Influences of different routinely used muscle relaxants on oxygen delivery to and oxygen consumption by tlie heart during xcnon-ancslhcsia. Adv-Exp-Med-Biol, \W); 248: 673-8
Zhang-P; Oliara-A; Masljinio-'I'; Inianaka-H; Uchiyama-A; Yoshiya-1 Pulmonary resistance in dogs: a comparison of xenon with nitrous oxide. Can-J-Anaesth. 1995 ; 42: 547-53
Khan-MA; Alkalay-I; Suetsugu-S; Stein-M Acute changes in lung mechanics following pulmonary emboli of various gases in dogs. J-Appl-Physiol. 1972 ; 33: 774-7
CNS and Analgesia studies include:
Sclabassi-RJ; Lofink-RM; Guthkelch-AN; Gur-D; Yonas-H Effect of low concentration stable xenon on the EEG power spectrum. Electroenccphalogr-Clin-Neurophysiol. 1987 ; 67: 340-7

Clark-DL; Rosner-BS Neurophysiologic effects of general anesthetics. I. The electroencephalogram and sensory evoked responses in man. Anesthesiology. 1973 ; 38: 564-82
Yagi-M; Mashimo-T; Kawaguchi-T; Yoshiya-I Analgesic and hypnotic effects of subanaesthetic concentrations of xenon in human volunteers: comparison with nitrous oxide. Br-J-Anaesth. 1995 ; 74: 670-3
Toxicity studies include:
Lane-GA; Nahrwold-ML; Tait-AR; Taylor-Busch-M; Cohen-PJ; Beaudoin-AR Anesthetics as teratogens: nitrous oxide is fetotoxic, xenon is not. Science. 1980 ; 210: 899-901
Aldrete-JA; Virtue-RW Prolonged inhalation of inert gases by rats. Anesth-Analg. 1967 ; 46: 562-5

Low Flow Applications include:
Luttropp-HH; Rydgren-G; Thomasson-R; Wemer-0. A minimal-flow system for xenon anesthesia. Anesthesiology. 1991 ; 75: 896-902
Luttropp-HH; Thomasson-R; Dahm-S; Persson-J; Werner-0. Clinical experience with minimal flow xenon anesthesia. Acta-Anaesthesiol-Scand. 1994 ; 38: 121-5
Pharmacological investigations include:
Hom-JL; Janicki-PK; Franks-JJ Nitrous oxide and xenon enhance phospholipid-N-methylation in rat brain synaptic plasma membranes.Life-Sci. 1995 ; 56: PL455-60
Franks-JJ; Horn-JL; Janicki-PK; Singh-G. Halothane, isoflurane, xenon, and nitrous oxide inhibit calcium ATPase pump activity in rat brain synaptic plasma membranes. Anesthesiology. 1995 ; 82: 108-17
Markoe-AM; Anigstein-R; Schulz-RJ Effects of inert gases and nitrous oxide on the radiation sensitivity ofHeLa cells.
Public-Health-Rep. 1970 ; 85: 200

Berger-EY; Pecikyan-FR; Kanzaki-G Anesthetic gases ;and water struacture. The effect of xenon on tritiated water flux across the gut. J-Gen-Physiol. 1968 ; 52: 876-86
Keyes-M; Luniry-R Binding of anesthetics to proteins: linkage between the sixth-ligand site of heme iron ion and the nonpolar binding sites of myoglobin. Fed-Proc. 1968 ; 27; 895-7
Gottlicb-SF; Cymennan-A; Metz-AV Jr Effect on xenon, krypton and nitrous oxide on sodium active transport through frog skin with additional observations on sciatic nerve conduction. Aerosp-Med, 19o8 ; '59: -149 ^3
Eger-EI; Brandstater-B; Saidman-LJ; Regan-MJ; Scveringhaus-JW; Munson-ES Equipotent alveolar concentrations of methoxyflurane, halothane, diethyl ether, fluroxene, cyclopropane, xenon and nitrous oxide in the dog Anesthesiology. 1965 ;26: 771-7
Schoenbom-BP; Fealherstone-RM Molecular forces in anesthesia. Adv-Pharmacol. 1967,5: 1-17

The non-English literature on Xenon includes:
(abstracts accessible via Medline)
Burov-NR; Kasatkin-IuN; Ibraginiova-GV; Shulunov.MV; Kosaclicnko-VM. (Comparative assessment of the hormonal status during N20 and xenon anesthesia using similar methods], Anesteziol-Reanimatol. 1995 (4); 57-60
Burov-NE; Kornienko-LIu; Dxhabarov-DA; Mironova-II; Moroxova-VT; Agceva-LA; Ostapchenko-DA; ShuluiK/v .MV [Effect of xenon anesthesia on morphology and the blood coagulation system] Anesteziol-Reanimatol. 1993 (6); 14-8
Burov-NE; Ivanov-GG; Ostapchenko-DA; D/.habarov-DA; Kornienko-LIu; Sliulun<Jv-MV | Hcmodynamies and function of the myocardium during xenon anesthesia) Anesteziol-Reanimatol). 1993 t(5): 57-9
Oku-.S; Karasawa-J; Kuriyama-Y; Salou-K; Yahagi-N; Okumura-P; Kikuchi-H; et al. [Minimal-flow xenon and semiclosed circuit anesthesia for computed tomographic measurement of local cerebral blood flow (LCBF)] No-To-Shinkei. 1984; 36: 813-9
 

 

 

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Clinical Studies of Carbon Dioxide Absorbtion During Anaesthesia
J Murray
Belfast

Antoine Lavoisier's greatest contribution to the study of respiration was contained in a memoir read in 17S'), From experiments carried out partly on guinea-pigs confined within vessels of caustic alkali to absorb carbon dioxide, and partly upon his assistant Sequin, Lavoisier showed that respiration is the same in any concentration of oxygen provided carbon dioxide is removed. Over sixty years later John Snow wrote the following about ether:
" It follows as a necessary consequence of this mode of excretion of vapour, that, if
its exhalation by the breath could anyway be stopped, its narcotic effects ought to be
much prolonged "

Snow proceeded to demonstrate this point upon himself by rigging up a primitive
closed-circuit apparatus, which included an absorber for carbon dioxide.

"A solution of caustic potash was employed for the purpose of absorbing the carbonic acid gas generated by respiration as the air passed to and fro over a large extent of its surface....."
Thirty years later Paul Bert, Professor of physiology at the Sorbonne, described his experimental methods for assessing the toxic and lethal effects of chloroform. These were to place an animal in a closed vessel already filled with chloroform and potash and sufficiently large to obviate asphyxia. He wrote:
The use of potash to absorb carbon dioxide must be absolutely rejected...at least in experiments with chloroform, which it rapidly breaks down ".

Since 1914 when Dr D E Jackson first applied the carbon dioxide absorption principle of rebreathing to inhalational anaesthesia, various absorbent materials have been produced in an effort to gain maximum efficiency of carbon dioxide absorption from anaesthetic atmospheres. Soda lime (sodium and calcium hydroxide) and baralyme (barium and calcium hydroxide) are the chemical absorbents which have gained wide acceptance. Soda lime has been used for almost 80 years. During this time many changes have been made in an attempt to prevent excessive heating and to minimise crumbling and the formation of dust. The percentage of sodium hydroxide in soda lime has been reduced to 5%. The remainder of the material comprises calcium hydroxide with a moisture content which varies between 2-18%.

Despite an interval of 80 years, soda lime still remains an effective method of removing carbon dioxide from closed and semi-closed anaesthetic systems. However, it is far from ideal; strong bases such as sodium and potassium hydroxide promote the dehalogenation and alkaline hydrolysis of many inhaled anaesthetics [1]. A historical example was the formation of dichloracetylene when trichloroethylene (Trilene) was used in the presence of soda lime. The re-inhalational of dichloracetylene from the absorber resulted in cranial nerve palsies in some individuals before the problem was recognised. Although trichloroethylene is no longer available as an inhalational anaesthetic, interactions with soda lime also occur with fluorinated agents such as halothane. The presence of strong bases such as sodium hydroxide and potassium hydroxide promote the dehalogenation and alkaline hydrolysis of some anaesthetic agents. Fortunately, the majority of the degradation compounds are short-lived and are not thought to exert a significant toxic effect in clinical practice.

More recently, sevoflurane has been shown to undergo degradation in the presence of soda lime to an olefin (CF2=C(CF3)OCH2F; compound A) which has nephrotoxic potential in rats [2].

Other recent concerns with the use of soda lime as a carbon dioxide absorbent include the accumulation of nitrogen, methane and carbon monoxide gas within the breathing system [3-5]. In particular the reaction between enflurane isoflurane and desflurane with soda lime has resulted in carbon monoxide poisoning in some individuals [6,7]. Experimental evidence suggests that carbon monoxide is formed when these inhaled
anaesthetics are used with dry soda lime thereby producing formates probable precursors of carbon monoxide. The exact reaction has not yet been identified, however, in the presence of soda lime, enflurane, isoflurane and desflurane could be degraded in trace quantities to fluoroform, the fluorinated analogue of chloroform, and a fluorinated analogue of trichlorethylene: CFCl=CF2 (chlorotrifluorethylene). In theory, these compounds might decompose to carbon monoxide under alkaline conditions. To overcome this, the available literature suggests using fresh, or normally wet, soda lime.
As a result of these problems, a greater incentive has developed to explore carbon dioxide removing agents in clinical practice. The economic demands of the marketplace will dictate the use of cost-effective methods of anaesthetic administration. This clearly highlights the use of low-flow and closed system anaesthesia and the effective and safe absorption of an acid gas.
Possible alternatives:
It is important to state that many methods of removing carbon dioxide from expired air are possible. Some remain impractical at present but rapid advances are being made in the fields of aviation, space and underwater medicine and those methods currently regarded as far-fetched may ultimately become accepted. The ideal carbon dioxide "scrubber" must meet the following requirements:
a) effective removal ofC02 (10-15 litres of C02/100g)
b) removal of C02 only
c) non-degradable in the presence of inhaled anaesthetics
d) present in a suitable form for anaesthesia (granular, dustless, non-toxic)
e) ideally provide heat, moisture and be bacteriostatic.

1) Zeolite molecular sieves
Carbon dioxide adsorbtion by synthetic zeolites is an alternative to soda lime which hitherto has not been evaluated for anaesthesia. Naturally occurring zeolites have been used in industry and medicine for many years, e.g. in petroleum refining, for water purification and as oxygen concentrators. Molecular sieves are crystalline metal aluminosilicates having a three-dimensional interconnecting network of silica and alumina tetrahedra. Natural water ofhydration is removed from this network by heating to produce uniform cavities which selectively adsorb molecules of a specific size. The cavities are 2-8A wide and are presented in either powder or bead form. Two of the commonest synthetic zeolites (4A, 5A and 13X) have pore diameters of between 4.0A and 7.44A . Carbon dioxide is a polar molecule with a diameter of less than 4.2A.
It is retained in 4A, 5A and 13X sieves by Van der Waal's forces rather than chemical bonding, allowing the process to be reversed by changes in temperature and pressure. A number of studies will be described regarding the removal of carbon dioxide and the interaction of4A, 5A and 13X molecular sieves with nitrous oxide and current inhaled anaesthetics. Data will also be presented regarding the degradation of sevoflurane when soda lime or molecular sieves were used for the removal of C02.
2) A Non-regenerative (chemical) method:
As previously stated, little incentive has existed to explore alternate chemical strategies to replace soda lime. It is effective, inexpensive and allows for excellent heat and moisture preservation during anaesthesia. However, the presence of strong alkali (NaOH and/or KOH) means the material is caustic and chemically reactive with regard to certain inhaled anaesthetics. The degradation products produced (compound A, carbon monoxide) raise doubts about the safe administration of these drugs in low-flow or closed system anaesthesia.
A number of recent in vitro and vivo studies have examined the potential toxicity of compound A when sevoflurane was administrated during low-flow anaesthesia with soda lime [8-11]. These studies report peak concentrations of compound A ranging from 8-60 ppm. This wide variation made be due several factors. Firstly there is chemical reaction between sevoflurane and the strong bases contained in modem soda lime. This reaction is not quantitatively precise as different types of soda lime produce differing amounts of compound A [8]. The material used in UK practice differs in its composition to that used in North America and Japan. The greater part of soda lime comprises Ca(OH)2 (94%). In addition to calcium hydroxide, the United States Pharmacopoeia specifies either NaOH, or KOH, or both, whereas the British Pharmacopoeia specifies either NaOH or KOH but not both.
Both these alkali are included as activators and it is these salts which are responsible for the degradation of sevoflurane to compound A and the formation of formates from isoflurane, enflurane and desflurane. These chemical reactions are directly related to alkalinity, rather than to a specific formulation [5-7].
For the remaining half of this paper data will be presented concerning a new chemical formulation for absorbing carbon dioxide. I will demonstrate using carbon dioxide break-through curves and gas chromatographic analyses that this material has exciting potential without the aforementioned drawbacks of soda lime.

References
1. Mono M, Fujii K, Mukai S, Kodama G Decomposition of halothane by soda lime and the metabolites of halothane in expired gases. Exerpta Medica / International Congress Series 1976; 387: 214-5.
2. Mono M, Fujii K, Satoh N, Imai M, Kawakami U, Mizuno T, Kawai Y, Ogasawara Y, Tamura T, Negislii A, Kumagi Y, Kawai T. Reaction of sevoflurane and its degradation products with soda lime. Toxicity of the by-products. Anesthesiology 1992; 77:1155-67.
3. Morita S, Latta W, Harnbro K, Snider MT. Accumulation of methane, acetone and nitrogen in the inspired gas during closed circuit anesthesia. Anesthesia and Analgesia 1985; 64: 343-7.
4. Rolly G, Versichelen LF, Mortier E. Methane accumulation during closed-circuit anesthesia. Anesthesia and Analgesia 9194; 79: 545-7.
5. Lentz R. Carbon monoxide poisoning during anesthesia poses puzzles. Anesthesia Safety Foundation Newsletter 1994; 9: 13-14.
6. Moon R, Meyer A, Scott D, Fox E, Millington D, Norwood D. Intraoperative carbon monoxide toxicity. Anesthesiology; 73: A1049.
7. Moon R, Ingram C, Brunner E, Meyer A. Spontaneous generation of carbon monoxide within anesthetic circuits, Anesthesiology 1991; 75: A873.

8. Frink EJ, Malan TP, Morgan SE, Brown EA, Malcomson M, Brown BR. Quantification of the degradation products of sevoflurane in two C02 absorbents
during low-flow anesthesia in surgical patients. Anesthesiology 1992; 77: 1064-9.
9. Bito H, Ikeda K. Closed-circuit anesthesia with sevoflurane in humans. Effects on renal and hepatic function and concentrations of breakdown products with soda lime in the circuit. Anesthesiology 1994; 80: 71-6.
10. Gonsowski C T, Laster M J, Eger E I, Ferrell L D, Kerschmann R L. Toxicity of compound A in rats. Effect of a 3-hour administration. Anesthesiology 1994; 80: 556-65.
11. Gonsowski C T, Laster M J, Ferrell L D, Kerschmann RL. Toxicity of compound A in rats. Effect of increasing duration of administration. Anesthesiology 1994; 80: 566-73.

 

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THE SERVO CONTROL OF VOLATILE AGENT DELIVERY
G. ROLLY
Ghent

In non-rebreathing or open circuit conditions, the anaesthetic concentration inhaled by the patient equals that delivered by the vaporizer, whereas the alveolar concentration equilibrates rather fast, depending on the particular inhaled agent, such as described by Eger. Changing the depth of anaesthesia is easily obtained by turning the knob of the vaporiser.
In rebreathing conditions and particularly in low-flow or closed circuit conditions, large differences are present between vaporizer setting (outlet concentration) and inhaled and alveolar concentrations, due to complex gas kinetics and distribution in those systems. Although manual control of the output of the classical vaporizer or manual injection of liquid anaesthetic can be done fairly easily, servo control of the volatile agent delivery is advantageous to obtain a constant alveolar concentration and hence a stable anaesthesia, level.

The rationale of anaesthetic vapour administration during closed circuit and low flow conditions is based on the square root of time administration principle developed by Lowe and Ernst. With this formula a predictive value of anaesthetic concentrations can be obtained, but large pharmacokinetic interpatient differences have to be taken into account. Actual measurements of end-tidal anaesthetic concentrations in the patient are much more reliable and are the final cornerstone of the servo control of anaesthetic administration.
One of the first servo controlled administrations was that described by Ross et al. in 1983, whereby he was giving isoflurane. It uses a syringe driver and servo control unit, driven by the output of a Emma Engstrom piezo electric measuring device for anaesthetic vapours, whereby the liquid anaesthetic is injected into the .circuit. A similar setup was also described by
Westenskow :et al. in 1986, but .here an infrared anaesthetic sensor is used for driving the injection of liquid anaesthetic. To enhance the homogenous distribution of the generated vapours, a. circulating pump is included in the circuit. The oxygen delivery in the system is servo controlled to keep the system volume constant.
More recently a servo controlled anaesthetic machine has been described by Humphrey and White in 1991, whereby also a syringe driver is used. Besides anaesthetic agents, Oy concentration is also analysed with a fuel cell sensor and the position of the ventilator bellow is measured as well. Output signals are produced for the liquid injection, and to control the circuit volume and gas composition, using an oxygen and N20 mass flow controller.
The only commercially available anaesthetic machine which uses the principle of servo control of volatile agents is the PhysioFlex apparatus (Physio, The Netherlands; Drager). This apparatus has become available in 1989 for initial trials and has been marketed in 1990.

The PhysioFlex is a valveless, closed circuit system, with a built-in blower rotating the gases unidirectionally in the breathing circuit at a speed of 70 1/min. Four membrane chambers, with a capacity of 625 ml each, are built in parallel into the circuit for controlled ventilation purposes and/or for sensing respiratory movements. Small amounts of oxygen and N-,0 (or air) are administered by built-in computer controlled injection and excess gases are eventually evacuated. Inspiratory oxygen concentration is continuously measured by a paramagnetic oxygen analyzer. The oxygen signal is used to keep the preselected oxygen concentration constant. The concentrations of C0~, N.,0 and volatile anaesthetics are measured by a built-in infrared spectrometer. Liquid anaesthetic is injected by computer-controlled syringe administration, driven by a stepper-motor, at such a rate to maintain the desired end-tidal (alternatively the inspired) concentration constant. Fast response is possible for increasing the desired end-tidal concentration;
for fast lowering of the end-tidal concentration a charcoal filter is temporarily switched into the circulating gas. If a foreign gas concentration in the system exceeds a value of 10 % by volume, a request is made by the computer for a 2-minutes flushing phase with high fresh gas flow, temporarily disrupting the rigid closed circuit conditions.
Several examples, based on mass spectrometric examination, will be given to show that the anaesthetic gas concentration can be reliably obtained, maintained and changed upon request.
References
I/ Lowe H.J. and Ernst E.A., The Quantitative Practice of Anesthesia, Williams and Wilkins, Baltimore, 1981.
2/ Ross J.A.S., Lok R.T., White -D..C. and Howes D.W. Brit.J.Anaest 1983, 55., 1053.
3/ Westenskow D.R., Zbinden A.M., Thomson D.A. and Kohler B.
Control of end-tidal halothane concentration. Brit.J.Anaesth. .1986, 5^, 555-562.
4/ Humphrey S.J.E. and White D.C. A servo-controlled anaesthetic machine. Brit.J.Anaesth. 1991, 66, 400P.
5/ Rendell Baker L. Future direction in anesthesia apparatus. In: Ehrenwerth J., Eisenkraft J.B. Ed. Anesthesia Equipment/ Chicago: Mosby-Yearbook Inc., 1993.