Manchester 1994
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THE HISTORY OF CLOSED CIRCUIT ANAESTHESIA

Dr. D C White

The Rev Stephen Hales was not only a clergyman but also a physiologist, botanist and sanitary engineer. In 1727 he published a book entitled Statistical Essays containing the figure above which shows a recognisable closed circuit. Two valves direct the breath through the 'bladders' which were partitioned by four cloth diaphragms which were soaked in 'highly calcined sal tartar'. This was to absorb the sulphurous streams' in the expired air. The capacity of this apparatus was 4 - 5 quarts and Hales found he could breath through it for up to 8.5 mins. In describing this apparatus, which preceded the discovery of CO2 by 30 years, Hales wrote "Great care must be taken that the inspiration be as free as possible by making large passages and valves to play more easily".

Although Hales understood that respiration involved the expiration of waste products he did not appreciate the uptake of anything from the 'common air'. In fact he wrote that the inspired air was "not rendered effete by consumption of vivifying spirit" but considered that the I acid fuliginous vapours' destroyed the elasticity of the air. This was 47 years before the discovery of O2 by Priestley in 1774.

The classic researches of Lavoisier between 1774 and 1785 revealed respiration as the uptake of O2 and the output of CO2 . From the beginning of this work it was clearly understood that CO2 was absorbed by caustic alkalis. The precipitation of calcium carbonate from hydroxide solution (lime water) was Lavoisier's method of identifying CO2 which he termed 'chalky acid gas'.

A number of workers at about this time described the maintenance of life in closed systems using caustic alkalies to absorb CO2. These included Scheele (bees), Lavoisier and Seguin (guinea pigs) and Regnault and Reiset (dogs). The purification of expired oxygen by passage through lime water by Ingen Housz also dates from this period.

With his 1727 description of the closed circuit Hales had noted that "such an instrument might be of use when it was necessary to enter a place filled with noxious deadly vapour". It was more than a hundred years before his suggestion was further pursued. In 1852 a serious firedamp explosion occurred in a Belgian coal mine in which 60 coal miners were killed. The casualties could not be reached because of the irrespirable atmosphere. Following this accident the government offered a reward for the invention of a device which would permit rescuers to enter tunnels filled with toxic gas. A competition was set up for this prize to be judged by the Belgian Academy of Medicine of which the distinguished physiologist Theodor Schwann was a member. By the end of 1853 nine papers bad been submitted but the Academy decided that none deserved the prize.

However Schwann began to work on the problem and by 1856 had a working apparatus through which he was able to breath for I hr 45 mins. This apparatus, which had many novel features, was supplied with oxygen from two cylinders carried on the back. CO2 was removed by pieces of lime dipped in caustic soda. These were contained in a canister of complex design in which the gas followed a circuitous route to prolong contact with the absorbent. For reasons which are not clear this apparatus was not described in print until 1877. The absorbtion canister design was the basis for the canisters used in Drager equipment until the 1920s.

The first person to use CO2 absorbtion with anaesthesia was John Snow who had observed that patients recovered from inhalational anaesthesia because they excreted anaesthetic in expired air. He breathed oxygen to and fro from a large bag via his spiral inhaler into which he had put 'solution of potassa'. Snow reported in 1850 that rebreathing expired gas in this way considerably prolonged the duration of action obtained from small amounts of ether or chloroform.

Credit for the first clinical use of CO2 absorbtion must go to the dentist Alfred Coleman, a pioneer of nitrous oxide anaesthesia at the National Dental Hospital in London. In 1869 he published in the British Journal of Dental Science the description of a closed circuit for use with nitrous oxide. A tin box (the economizer) containing 'pieces of lime' was mounted on top of the nitrous oxide cylinder. After a few breaths of pure nitrous oxide the valve on the facepiece was closed and the patient breathed to and fro across the economizer and into a reservoir bag of nitrous oxide which was topped up via a valve from the cylinder. Colman's reason for using this system was to economise in expensive gas and he continued for many years to use and recommend the method with little success in persuading his colleagues to do the same.

The same seems to have been the case with Dennis Jackson (1878 - 1980) who designed a series of closed circuit machines between 1915 and 1926 in America which were in many ways ahead of their time. Jackson, although medically qualified, was primarily a pharmacologist. His machines, which he constructed himself, were developed and used as much or more in the laboratory than in the operating theatre. His first machine (1915), which underwent considerable refinement in the following ten years, made use of two systems. The primary circuit was made up of relatively narrow bore tubing round which gas was driven by an electrically driven fan. This circuit contained a wash bottle of caustic alkali and was fed by oxygen and nitrous oxide from cylinders and with liquid ether from a burette. The primary circuit gas passed into, and was evacuated from, a large bag into and from which the patient breathed through a large, low resistance port.

At the same time as this Jackson also described a to and fro system for animal use only. Caustic soda was contained in a pie can and a rubber bathing cap was stretched across the top of the tin so that it acted as a flexible reservoir. Ports on either side of the tin led to the patient and to the gas supply. Direct liquid injection of ether from a burette was employed in this system also. Jackson's series of machines culminated in a combined anaesthetic machine and ventilator published in 1926. It is not clear how much clinical use this machine had but in 1930 Jackson did publish the case of a patient with a cerebral abscess kept alive by a ventilator for 22 hours.

Wash bottles in which gas was bubbled through caustic alkali solutions were not well suited to anaesthetic machines. Soda lime, in which the active chemicals are incorporated into sodium silicate granules, was developed in America for use in gas masks in response to the use of gas in the First World War. 'Re American engineer R. C. Wilson standardised the method of manufacture, hardness and optimum granule size publishing his work in 1920.

The American anaesthesiologist Ralph Waters began to correspond with Jackson about closed circuit anaesthesia in 1920 and in 1924 he published his description of the now familiar Waters canister containing soda lime. The original paper showed this to and from system fed with oxygen and nitrous oxide and the can bearing a small stopcock through which ether chloroform or ethyl chloride could be injected as in Jackson's original apparatus.

The invention of the now conventional circle system is usually credited to Brian Sword in 1930 but in fact one of Jackson's later machines incorporated valves in the facepiece so that the patient could circulate the gas if there were an electrical failure. These systems, to and fro and circle, did not attain widespread use until the introduction of cyclopropane suggested by Lucas and Henderson in 1929 and reported on by Waters and his colleagues in 1934. The closed system was particularly suited to cyclopropane which could be added to the system through a rotameter. A closed or low flow system was necessary because the maximum flow of cyclopropane was 750 or 1000 mls/min.

From this time onwards closed, or more usually low flow anaesthesia was widely used and only minor changes mostly in canister design, were made. The availability of CO2 monitoring (Luft 1943) assisted in this and between 1955 and 1967 a series of papers resulted in larger., double, prepacked and otherwise modified canisters appearing.

The introduction of halothane (1956) calibrated vaporizers (1957) and the MAC concept (1965) led to a reduction in the use of C02 absorption systems. The reason for this is that during closed circuit anaesthesia the concentration of anaesthetic inhaled by the patient is the resultant of a number of factors. In contrast with high gas flows and a calibrated vaporizer the anaesthetist knows the inspired concentration and, with knowledge of MAC, can set this to what it should be.

The pendulum is now swinging back again. The growing availability of agent monitors gives information on inspired concentrations without calibrated vaporizers. 'Re introduction of new and more expensive agents together with increased financial pressures has sharpened interest in economy. A wish to reduce pollution of the environment has played a part in this. Finally, closed systems are suited to various forms of electronic control which will play a part in the future development of inhalational anaesthesia.

If desflurane had the good fortune to possess a lower MAC value I would have been able to say that it would be difficult to design a more user-friendly low flow volatile agent. It is unfortunate that desflurane is so irritative to the airways during the inhalational induction phase because such high concentrations are required. However the interposition of the circle system between the vaporizer and the patient goes a long way towards preventing sudden high desflurane concentrations being presented to the patient at this time with a consequent reduction in upper respiratory tract irritation.

Inhalational agents

Measuring and monitoring

P Beatty

Introduction

The monitoring of inhalational agent concentration in the anaesthetic breathing system is always instructive. In the context of low flow anaesthesia it is a key issue, especially when the agents to be used are expensive or where precise control of end-tidal agent concentration is required. However, in principle, measuring inhalational concentration is no different from measuring any gas or vapour. The same methods of assessing the number of molecules of agent per unit volume can be used as in non-rebreathing systems, the difference is that low gas flows impose on the clinician the need to measure the gas concentrations both in the breathing system and in the patient lungs more rigorously.

Let us consider the restraints on an inhalational agent monitor for use in a low flow system, from the point of view of writing an ideal specification for the monitoring system. Removal of gases from the breathing system for analysis, unless they can be returned to the breathing system without risk, effectively wastes fresh gas. Thus whether a monitor allows the sample gas to be returned is an important issue. For similar reasons it is desirable that the monitor be compatible with other systems in

particular other gas concentration monitors. If an instrument can be combined with others or incorporates monitors of other gases this reduces the need for an extra gas sample to be removed from the breathing system. Flexibility is important also. A monitor that can measure a limited range of agents and cannot be adjusted or calibrated for newer agents is not going to be cost-effective. Similarly a monitor that is prone to breakage, expensive to calibrate or difficult to repair will also be expensive. The last factor to consider is speed of response. How fast does a monitor really need to be?

The answer to that depends on how a flow is to be used and how much control, particularly on end-tidal concentration, is desired. The lower the flow the more need for fast measurement. Simulations of capnograph traces suggest that for breathing rate of 12 breaths per minute, i.e. 0.2 Hz, a faithful respiratory curve requires a monitor with an upper frequency limit of 3 Hz or a rise time of 300 ms. Higher breathing rates mean that rise time may well have to decrease and at 50 breaths per minute should be nearer to 60 ms.

amount of absorbed infra-red. Oxygen does not absorb infra-red but the device uses para-magnetic properties of the gas by exciting oxygen using an oscillating magnetic field and detecting the sound on a separate machine.

Mass spectrometry detects the number of molecules in a gas sample after ionisation of the sample by electron bombardment. The ions created are injected into a mass filter and the number with a specific mass charge ratio detected give a measure of the total number of parent molecules for that ion in the gas sample. Mass spectrometers are very expensive because they require high vacuum systems. Their inlet systems are also complex and they chemically change the sample. They are, however, very fast and can measure all gases, even mixtures of the inhalational agent as well as water vapour.

Raman spectroscopy is a newcomer in the field. It relies on Raman Stokes scattering of photons of light by gas molecules. When a photon collides with a molecule it can be either by scattered like a billiard ball or absorbed by the molecule, then re-emitted at a different wavelength. The wavelength and energy depend on the rotational and vibrational configuration of the molecule and thus the spectrum obtained gives a finger print for the molecule. A Raman spectrometer measures the shift by exciting the gas sample to be measured using a laser and measuring the wavelengths of the scattered light in a double grating spectro-photometer. The spectrum of intensity of scattered light in a particular wavelength is characteristic of a particular molecule and the size of the peak produced proportional to the

number of molecules in the sample. Raman scattering is not demonstrated by single atoms so can measure all the gases normally present in the breathing system with the exception of argon which may be present as the main noble gas fraction of air.

The most familiar technique used for monitoring is that of infra-red spectroscopy. It uses the same principle as is used in the familiar capnograph and shares with capnographic technology the same cells and measurement systems. This has made the infra-red measurement of inhalational agents extremely cost-effective. The gas samples are not changed chemically and can be returned to the breathing system easily.

The relative specifications of the different methods are compared in Table 1.

The Future

What sort of inhalational anaesthetic detection technology are we likely to see in the future?

At present in infra-red technology the side-stream techniques dominate over mainstream monitors, where the detection system is placed inside the breathing system. There is some indication in the capnograph market that this may change and therefore we would expect to see mainstream monitors, for which returning gas sample to the breathing system is unimportant, emerge.

It is a common thread in the development of recent new techniques that they have depended on new optical spectroscopy methods of one form or another. Laser technology continues to advance and one would expect to see new laser based optical spectroscopy methods emerge in the near future.

Except where Raman spectroscopy is used, it is unlikely that nitrogen measurement will become routine. Thus the practitioner of low flow anaesthesia can expect still to have to do some informed guesswork about the content of nitrogen in circle systems under very low flows.

Various artificial intelligence technologies, some of which have been applied to alarm systems, are likely to make an impact in monitoring in general within the next ten years. They will make alarm systems more reliable and therefore make low flow anaesthesia easier with a higher degree of confidence for the inexperienced user. Developments in this area are probably the most important in the encouragement of the application of low flow anaesthesia.

Table 1 Comparative specifications of various inhalational agent monitors.

Minimum Fresh Gas Flows for Non-Absorber Breathing Systems

G Meakin

Manchester

 "Low- flow anaesthesia" is a term applied to techniques in which fresh gas flows less than the alveolar ventilation are used. Depending on the specific technique, fresh gas flows may vary between 250 ml/min and 3 litres/min for an adult [1,2]. Given this definition of low flow anaesthesia, none of the non-absorber breathing systems can be regarded as low flow systems. However, greater efficiency in the use of these systems is possible through improved understanding of their fresh gas economics and improved design.

The non-absorber breathing systems, or the semi-closed systems, were classified by Mapleson in 1954 into five types A-E [3]. The most important of these are A, the Magill system, and D which exists in two forms, the Bain and the Jackson Rees. The two latter are functionally identical and may be referred to jointly as the T-piece systems (4)

All these systems rely on adequate gas flow to prevent rebreathing, and it has been suggested that system A requires a fresh gas flow which is only equal to the patient's minute volume to prevent rebreathing completely, while systems B-E require fresh gas flows 2-3 times as great. The greater efficiency of the Mapleson A compared with the other systems can largely be explained by the presence of the reservoir bag on the inspiratory limb of the system. This permits the flow of gas to the patient to be interrupted during expiration, thereby keeping the fresh gas and expired gases separate.    Further economy in the use of fresh gas is possible by allowing rebreathing of dead space gas, which is stored in the inspiratory limb of the system when the fresh gas flow is equal to the patient’s alveolar ventilation.

The original suggestion that the T-piece systems require a fresh gas flow greater than twice minute volume to prevent rebreathing during spontaneous ventilation was based on a theoretical model employing a square wave of respiration [3].     When more realistic models incorporating changes in the respiratory waveform, respiratory depression due to anaesthesia and allowing for a small amount of rebreathing to occur into the dead space at the end-inspiration, it is clear that lower flow rates can be used. In one such study, Meakin and Coates measured the respiratory effects of reducing the fresh gas flow two 10 one times the predicted minute volume in 12 children breathing from a Bain system [5]. They found no significant effect on minute volume or end-tidal CO. tension, because the degree of rebreathing was small enough (12.6%) to be accommodated in the dead space at the end of inspiration.  The formula V_F = weight^0.5 is convenient for spontaneously breathing patients and yields flow rates which are 25^% greater than the predicted minute volume [6].

A number of problems can be identified with the Magill system during controlled ventilation. Firstly, the expiratory valve is sited inconveniently close to the head of the patient:   secondly, the valve opens during  lung  inflation  reducing  the efficiency of the system by allowing fresh gas and dead space gas to be vented. The first problem can be overcome by adding an expiratory limb to resite the valve on the anaesthetic machine [7]. The second problem can be solved by enclosing the reservoir bag and the expiratory valve a rigid plastic box. The system that results is an enclosed afferent reservoir system (EAR), or more specifically, an enclosed Mapleson A system [8-10].

ENCLOSED AFFERENT RESERVOIR BREATHING SYSTEM (EAR)

In this system, fresh gas enters a reservoir bag (bellows) enclosed in a transparent plastic container on the inspiratory limb of the system. The expiratory limb passes to an expiratory valve which discharges to the interior of the container.   Controlled ventilation is effected by increasing the pressure inside the container by means of a self-inflating bag or a ventilator. The increased pressure in the  container squeezes the reservoir bag, while at the same time it holds the expiratory valve tightly shut. In this way, all the fresh gas and any dead space gas stored in the inspiratory tube is delivered to the patient. The valve opens again at end-expiration allowing alveolar gas to be vented.

Meakin and colleagues determined the efficiency and fresh gas requirement of an EAR system during controlled ventilation in children [11].  Provided the minute volume to fresh gas ratio was maintained above 1.5, the system was 92% efficient in the use of fresh gas, and normocapnia or mild hypocapnia was produced accurately with a fresh gas flow equal to the predicted alveolar ventilation (0.6 x weight^0.5).  The efficiency of the EAR during controlled ventilation was about one-third greater than that of the T-piece system studied previously by Nightingale and Lambert, which required a fresh gas flow equal to the predicted minute volume (0.8 weight^0.5) [11,12].  This finding is consistent with the theory that the greater efficiency of the EAR (enclosed Mapleson A) systems is due to conservation of deadspace gas.

SUMMARY:    FRESH GAS REQUIREMENTS OF NON-BREATHING

SYSTEMS EAR (enclosed Mapleson A)

FGF for controlled or spontaneous ventilation = predicted V_A ≈ 0.6 x  √weight (kg)

T-pieces (Bain, Jackson Rees)

FGF for controlled ventilation = predicted V_e ≈ 0.8 x √weight (kg)

FGF for spontaneous ventilation > 2V_E (in theory)

In practice, FGF > predicted V_E, i.e. = √weight (kg)

REFERENCES

1        Bruce W. Anaesthetic breathing systems. In: Scurr C, Feldman S, Soni   N  (eds).      Scientific Foundations of Anaesthesia, 4th edn. Oxford, Heinmann Medical Books 1990, 673-687.

2        Zbinden AM,   Feigenwinter P, Hutmacher M. Fresh gas utilization of eight circle systems.   British Journal of Anaesthesia 1991, 67:492-499.

3        Mapleson WW. The elimination of rebreathing in various semi-closed anaesthetic  systems.     British Journal of Anaesthesia 1954, 26:323-332.

 4.    Rose DK, Byrick RJ, Froese AB. Carbon dioxide elimination during spontaneous ventilation with a modified Mapleson D system:

studies in a lung model. Canadian Anaesthetist's Society Journal 1978, 25: 353-365.

5.     Meakin G, Coates AL.    An evaluation of rebreathing with the Bain system during anaesthesia with spontaneous ventilation.   British Journal of Anaesthesia 1983, 55:487-496.

6.     Meakin G. Fresh gas requirement of the Bain circuit. British Journal of Anaesthesia 1991, 67: 663.

 7.    Lack JA. Theatre pollution control. Anaesthesia 1976, 31: 259-262.

8.     Fletcher IR, Garden E, Healy TEJ, Poole TR.   The MIE Carden ventilator. Anaesthesia 1983, 38:1082-1089.

9.     Miller DM, Miller JC. Enclosed afferent reservoir breathing systems. Description and evaluation. British Journal of Anaesthesia 1988, 60: 469-475.

10.    Miller DM. Breathing systems for use in anaesthesia.   Evaluation using a physical lung model and classification.  British Journal of Anaesthesia, 1988, 60: 555-564.

 11.    Meakin G, Jennings AD, Beatty PCW, Healy TEJ.   Fresh gas requirements of an enclosed afferent reservoir breathing system during controlled ventilation in children. British Journal of Anaesthesia 1992, 68: 43-47.

12.    Nightingale DA, Lambert TF. The behaviour of the Jackson Rees circuit with controlled ventilation. In: Proceedings of the Association of Paediatric Anaesthetists Annual Scientific Meeting, 1978, 21-24.

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