SAFER DESIGN OF ANAESTHETIC EQUIPMENT

Transcription

1 Br. J. Anaesth. (1987), 59, SAFER DESIGN OF ANAESTHETIC EQUIPMENT P. W. THOMPSON "Doubtless God could have made a better berry {strawberry), but doubtless God never did". (Butler, W. in Walton, I. The Compleat Angler, Part I, Chapter 5.) Many anaesthetists might fairly feel that the anaesthetic machine of today has reached a degree of satisfactory performance which makes it totally acceptable and take a view that further sophistication and refinement would not justify the further research and development input required, to say nothing of the financial implications. There are many books on the history of anaesthesia which incorporate information on the development of anaesthetic machines, a subject which has recently been reviewed by Thompson and Wilkinson in this Journal (1985). Accumulated literature on anaesthetic apparatus hazards is even more abundant and it is, indeed, unusual for any issue of an anaesthetic journal not to include some report, however brief, of yet another equipment incident. However, vast as the accumulation of this literature now is, there are, fortunately, relatively few such incidents which have resulted in death or permanent damage to patients. In their report on anaesthetic mortality Lunn and Mushin (1982), analysing 6060 deaths within 6 days of operation, identified a group of 58 deaths "for which the assessors believed anaesthesia was totally responsible". Examples of events quoted by assessors in this group listed one failure of a defibrillator and one failure of a ventilator as equipment faults to which death could be attributed. However, they considered that inadequate provision of essential monitoring instruments was a factor which could not be ignored and they were critical of failure to use such facilities when they were available. It is not the purpose of this paper to review either the history of equipment development or the hazard literature yet again. Nevertheless, as PETER W. THOMPSON, F.F.A.R.C.S.; Department of Anaesthesia, University Hospital of Wales, Heath Park, Cardiff CF4 4XW. long as the anaesthetic apparatus itself is the centrepiece, it cannot be discussed without also looking at the role of the user vis-i-vis safety, the place of monitoring, the maintenance of equipment, and standardization. The Role of the User Looking more closely at the role of the anaesthetist in connection with equipment accidents, the conclusions of some writers cannot be ignored. Craig and Wilson (1981) collected reports of anaesthetic misadventures from a District General Hospital and analysed 81 of these. They concluded that human error was more frequently responsible than equipment failure and that failure to perform a normal check of anaesthetic apparatus was the most frequently associated factor. Cooper and colleagues (1978), analysing "preventable anaesthesia mishaps" examined 359 incidents. Equipment failure was involved in 40 % and, of these, 12% (six incidents) were associated with functional failure of anaesthetic machines, not including the breathing system a figure which may suggest a high degree of reliability and effective maintenance. They pointed out "on the other hand it is conceivable that anaesthesia machines may be failing in ways that are not easily or rapidly detected by most anaesthetists". Throughout the literature it is argued that no equipment of any type can basically be better or safer than the person using it. For example, while it is generally recognized that the automatic pilot on modern aeroplanes is less liable to judgmental errors than human pilots, there is still a margin of error in that they require the accurate insertion of appropriate data in the first place. The incorrect input of, for example, wind speed, or the misinterpretation of some information in a flight plan, can result in a major disaster. Sophisticated as such equipment now is, the published figures onflyingaccidents internationally still place human error as a major contributory factor. Indeed, work in fields of safety investigation, such as rail or flying accidents, or mountain climbing, could well be

2 914 BRITISH JOURNAL OF ANAESTHESIA applied to the specialty of anaesthesia. The contributors to Pilot Error (Hurst, 1977) and Pilot Error: Human Factors (Hurst and Hurst, 1982) emphasize repeatedly points which are of fundamental importance to safety in anaesthesia. To give just three examples: "Too many pilots let themselves be carried away by an emergency to such an extent that solving the emergency takes priority over flying the aircraft"; "Man can only, in fact, attend to one thing at a time"; and " Individuals may deal with pieces of information quickly and badly or may concentrate entirely on one source of information to the exclusion of all others. They may confuse information obtained from two or more sources or may even seek to escape from the situation by ignoring all the inputs, possibly by indulging in a totally irrelevant activity". The importance of the user cannot be more succinctly expressed than it was by Schreiber (1985) writing in the U.S.A.: "It is, therefore, absolutely necessary that an anaesthesia system be operated under the constant surveillance of a trained operator and it must be understood that the anaesthesia system cannot be designed to automatically respond to a change in a patient's condition or to an operator error". Pre-use check of equipment Before using any equipment it behoves the user to satisfy himself of its fitness for the purpose in mind and of his understanding of its correct operation. It is appropriate, therefore, to give some consideration to the role of the anaesthetist in checking his anaesthetic equipment. The place of the check list in the anaesthetist's code of practice has been discussed by Cundy and Baldock (1982) and Heath (1983). Lunn and Mushin (1982) reported that, in 17.8% of the cases they considered, the anaesthetic machine was not tested before use. A variety of procedural drills has been produced, some published in books or journals or in manufacturers' manuals, others limited to their author's own hospital. There is really nothing new in suggesting that the anaesthetist should check his equipment. From the earliest days such simple measures as "a quick sniff to make sure that the ether was not chloroform" was a regular part of practice. However, increasing awareness of apparatus problems and pipeline accidents, some of them major, has highlighted the importance of the "cock-pit drill". The subject has been well reviewed recently by Ward (1985). Once again it must be stressed that any safety check is dependent on the careful implementation of it by the user. The writer has repeatedly warned "Do not adjust your mind, there is a fault in reality". It is all too easy to see what it is expected to see and to overlook an obvious error. Particular attention may be drawn to the simply expressed "Protocol for checking an anaesthetic machine before use" issued in 1980 by the Faculty of Anaesthetists of the Royal Australasian College of Surgeons (Protocol, 1980). Rapid reminders were listed as (1) gases, (2) rotameters, (3) vaporizers, (4) pre-circuit leaks, (5) breathing system selection, (6) circle system. Whatever procedure is adopted, it is essential that it is followed meticulously. A superficial slapdash check is no better, and could even be worse, than no check at all. Present developments The main thrust of design development at present seems to be aimed at increased safety, which is to be achieved by greater accuracy of controls (particularly important when low flow breathing systems are to be used) and increased information about what is being given to the patient, and the condition of the patient. Incorporation of Monitoring and Safety Devices Although monitoring is the subject of a separate paper in this issue, it is impossible to consider the safer design of anaesthetic equipment without referring to it. Over the years a wide range of views has been expressed as to the value of monitoring equipment. At the one extreme there is the view that a finger on the pulse will tell all, while at the other end Schreiber (1985) and Schreiber and Schreiber (1986) have reviewed the subject and the value of such equipment in considerable detail. There are some who consider that the ingenuity and resourcefulness devoted to the elimination of equipment hazards may be misdirected in concentrating on the development of warning and safety devices. This philosophy was stressed by Bruner (1984), who felt compelled to go so far as to say "I would specifically indict safety developments and regulation on three grounds. First, proposed safety devices add to the complexity of apparatus; Second, what is the rationale for insisting upon the utilisation of a putative safety device that has a higher probability of failure (in the 'no alarm' mode) than the occurrence of the event that the safety device is supposed to detect? Third...the

3 SAFER DESIGN OF ANAESTHETIC EQUIPMENT FIG. 1. The Drager Narkomed 3 anaesthetic machine. (Courtesy of North American Drager.) present tenor of the crusade in regard to safety is diversionary not only of money and time but also of attention... the screen of the ECG monitor is the wrong place to look for trouble!" This view would probably not receive widespread agreement in today's medico-legal climate, and the importance of adequate monitoring is being increasingly recognized. For example, Schreiber (1985) correlated the possible causes of hypoxia during anaesthesia against safety devices and other indicators. He attempted to establish a rating of the importance of the various safety devices and indicators and stressed that it is essential that all devices and indicators are scanned periodically. In his tabular analysis correlating hypoxia causes against safety devices and other indicators (which did not include the carbon dioxide monitor), he concluded that the oxygen analyser had the greatest potential for alerting the anaesthetist to hypoxic situations. He was in no doubt that "the use of an oxygen 915 analyser with any anesthesia breathing system is essential since the reliability of oxygen sensors has become acceptable" and that "the use of an oxygen analyser with an anesthesia system is the single most important measure to prevent hypoxia". In further correlation of causes of apnoea against safety devices and other indicators (Schreiber, 1985), he showed that the ventilatory flowmeter and the carbon dioxide monitor rated most highly. He discussed in detail the importance of the anaesthetist being aware of the safety limitations of such devices and pointed out that the siting of the oxygen sensor was important. For example, monitoring the oxygen concentration in the fresh gas flow downstream of the common gas outlet of the anaesthetic machine could represent only a limited safety potential with some modified breathing systems such as the coaxial. Another method of tackling this problem favoured by Schreiber is the "oxygen ratio monitor". This device activates an alarm if the percentage of oxygen concentration in the mixture of oxygen and nitrous oxide decreases to less than a certain predetermined value. A further development of this device is the oxygen ratio monitor controller (ORMC), the function of which is similar, but which has the additional capability to control the nitrous oxide flow in order to maintain a safe percentage of oxygen in an oxygen-nitrous oxide mixture. Schreiber's philosophy has been the guiding factor in the development of the latest range of anaesthetic equipment produced by the North America Drager Company, the Narkomed 3 System. This apparatus (fig. 1) aims to eliminate the array of stand-alone monitors by building them into the anaesthetic machine. Six integrated monitors, based on a data network, are available, from a total of eight, for display of patient-status data. The eight options are: oxygen concentration, pulse oximetry, ventilatory volumes, anaesthetic concentrations, breathing pressures, carbon dioxide, arterial pressures and temperatures. A comparable philosophy has been developed in the U.K. by Sykes and Sugg, of the Nuffield Department of Anaesthetics, Oxford, and Penlon Limited, respectively, on the basic premise that, ideally, all functions of an anaesthetic machine would be monitored continuously. Since this can only be done really effectively by electronic control, they have incorporated this approach in their Penstar apparatus (fig. 2). Even a cursory glance at their fresh gas and vaporizer circuit (fig.

4 916 BRITISH JOURNAL OF ANAESTHESIA BREATHNO SVSTEM ALARMS VENTItATOR FIG. 2. The Penlon "Penstar" anaesthetic machine. (Courtesy of Penlon Limited.) PULSED MAGNETIC VALV c ^UPSTREAM CONSTANT-PRESSURE VALVE & FLOWMETER. -DOWNSTREAM CONSTANT-PRESSURE VALVE. MIXING CHAMBER: Gas Analysis ^.Sample. \To Breathing System. Vapour Analysis h Sample. Temperature Signal. TEMPERATURE SENSOR. up N2O Air PIPELINE INE CONNIECTIONS 3ofl COPPER KETTLE VAPORIZERS WITH LIQUID AGENT RESERVOIRS.- FIG. 3. The fresh gas and vaporizer system of the Penlon "Penstar (Courtesy of Penlon Limited.) ' anaesthetic machine. 3) and one of their breathing systems diagrams (fig. 4) will make clear the degree of sophistication which is called for by this approach. Gas flows, for example, are pulsed electronically in 10-ml quanta and anaesthetic vapours are treated similarly. A centralized alarm panel allows the display of up to eight warnings (fig. 5). A third apparatus which illustrates the new approach is the "Ohmeda Excel Series" (fig. 6). The stress is again on safety, and the main features listed by the makers are: (1) Flowmeter safety link 25. This is a physical chain link between the oxygen and nitrous oxide flow control valves which operates so as to

5 SAFER DESIGN OF ANAESTHETIC EQUIPMENT 917 O, FLUSH BUTTOI O, FRESH GAS I 60cm»H,0 PRESSURE RELIEF VALVI. X IMj BI-DIRECTIONAL FLOWMETER. PATIENT. DRIVE GAS II Spill Control Slflnal SPILL VALVE. Vihr.Actlv.ttonSffln.l- FiG. 4. The Mapleson D breathing system of the Penlon "Penatar" anaesthetic machine. (Courtesy of Penlon Limited.) I I Inspired O,% MnuuVbiUtms STOP WATCH FIG. 5. The display panel of the Penlon "Penstar" anaesthetic machine. (Courtesy of Penlon Limited.) maintain, at all times, a minimum of 25% (nominal, subject to a tolerance of±4%) oxygen in oxygen-nitrous oxide mixtures. (2) Preferential oxygen. Ensures that oxygen is the last gas to enter the mixed gas flow to the patient (ISO5358, 1980). (3) Oxygen failure warning system. Provides an audible warning when oxygen supply pressure fails and shuts off the remaining gasflow. The patient can breathe room air through the machine if able to do so. (4) Carbon dioxide flow limitation. Normal carbon dioxide flow valves allow up to litre min" 1 toflowthrough the system when fully opened. Accidents have occurred because of this feature. In this apparatus, a physical restriction prevents aflowgreater than 2.0 litre min" 1. (5) Pin-indexed flowmeter tube blocks. Means

6 918 BRITISH JOURNAL OF ANAESTHESIA (c) Gas and electric failure. (d) Oxygen concentration. The integrated ventilator contains an inbuilt oxygen concentration monitor and alarms (audible and visual). (e) Patient cable management. Patient cables are ducted through the machine to the patient to remove visual obstruction (other cable management systems similarly manage patient cables from other monitors placed on the monitor shelves). Another approach to the prevention of hypoxic mixtures is the development of the blender. Several anaesthetic machines such as the Quantiflex now incorporate mixers which are so constructed that it is impossible to give less than, say, 30% oxygen in the inspired mixture. However, there will certainly be debates as to the level at which the minimum should be set. For example, some might say that they would wish to go as low as 18% oxygen while some would still desire to be able to give pure nitrous oxide. This raises the possibility of incorporating bypasses or mutes for safety devices and arguments as to whether such bypasses or mutes should be banned in relevant equipment standards. A middle course would be the provision in such equipment of maximum time delays. Cost of equipment FIG. 6. The Ohmeda Excel Series anaesthetic machine. f Courtesy of Ohmeda.) that it is impossible to transpose flow tubes during servicing or repair. (6) Selectatec Tec 4 vaporizer system. Features a physical interlock which allows only one vaporizer (and hence one agent) to be used at any one time. A unique gas flow channelling system also prevents the uptake of even minute amounts of a second agent if one is mounted on the machine. (7) Non-interchangeable internal pipework connections. (8) Stringent leak test procedures. (9) Integrated electronic ventilator (OAV 7700S) which contains multiple alarms for the following: (a) Disconnect (both pressure and expired minute volume based alarms). This alarm system will pick up small leaks in the patient breathing system. (b) Over pressure. Alarms and cycles the ventilator into the expiratory phase at an operator-set pressure threshold. The brief descriptions of three new anaesthetic machines given above might well raise an immediate query in the reader's mind, "Surely the cost is prohibitive, isn't it?" No effective answer can be given to this, since it is quite impossible ever to quantify the price of increased safety. However, it should be pointed out that the machine with incorporated monitoring devices is unlikely to be more expensive than a simpler version with added stand-alone equipment which may, coincidentally, be more liable to accidental damage. Modifications of equipment Cooper and colleagues (1978) drew attention to the dangers which may arise from local modifications of anaesthetic equipment. They cited an example where all anaesthetic machines in one hospital were modified by replacement of the oxygen flow control knob with a large custommade knob designed to provide obvious distinction by touch from the nitrous oxide knob. Eight incidents were reported in which the oxygen flow was inadvertently decreased, the cause being

7 SAFER DESIGN OF ANAESTHETIC EQUIPMENT 919 movement of this special knob by impact from an object placed on the machine's surface. Recently, the author left an anaesthetized dental patient in the care of a trainee. The patient was breathing spontaneously through a nasotracheal tube with a scavenging valve connected to the catheter mount in a Magill, Mapleson A system. Two lengths of corrugated breathing tube were in use. Some 10 min later the author was called urgently because the patient was waking up and the ECG showed serious abnormalities. The cause was detected at once the trainee had moved the scavenging valve from its original position back to the reservoir bag in the mistaken belief that he was using a Bain's system! Fortunately no harm was done, but the incident serves only too well to emphasize the necessity for anyone, at any level of experience, using any form of anaesthetic apparatus, to have a clear understanding of the equipment and to appreciate the importance of refraining from making unauthorized modifications. Maintenance The importance of maintenance cannot be overemphasized and manufacturers of equipment are strongly recommended, and indeed, by some published International or British Standards, required, to state in their accompanying literature the maintenance procedures and service intervals which they recommend. Such maintenance is normally carried out under contract with the manufacturer or an associate or by in-house maintenance. Many manufacturers are prepared to give factory training to in-house technicians insofar as their own equipment is concerned and to provide supplies of appropriate spare parts. Whoever or however such maintenance is carried out, it is essential that full checks are performed at the end of the procedure and that clear notices are displayed on the machine that it has been serviced, so that the first user may satisfy him/herself as to its performance and also be alerted should any untoward or unexplained incident occur during use. Ward (1985) discussed some of the problems of equipment maintenance and drew attention to the danger of sending equipment to the hospital engineers who "are often ignorant of the principles involved in anaesthetic equipment". Their well-meaning assistance or interference has, only too often, led to accidents resulting from inappropriate repairs or alterations. It is not unusual for the Safety Committee of the Association of Anaesthetists to be informed of post-maintenance faults or to hear of hazards arising from screws etc., which had been inserted by "Mr Nobody". Bearing in mind that this is so, the extreme danger of referring equipment to unsuitable engineers or attempting do-it-yourself maintenance cannot be over emphasized. Finally, it must not be forgotten that previously safe anaesthetic apparatus may be rendered highly dangerous by errors during servicing and maintenance (DHSS, 1982; Bonsu and Stead, 1983; Judkins and Sage, 1983). At the time of writing there is no universally agreed code of practice for ensuring that first users of serviced equipment are made fully aware that they are the first. However, it is anticipated that, in the U.K., agreement will be reached in the near future between the interested parties about this problem. The whole subject of management of equipment is discussed fully in Health Equipment Information No. 98 (Jan 1982), which was reprinted with amendments in 1984 as Health Equipment Information No. 176 (October 1984). Standardization of anaesthetic equipment In 1931 Hadfield, writing on "Chloroform by the open method" commented: "unfortunately the bore of the lead tops of the bottles differs, and consequently the size of a drop is variable. It is to be regretted that no maker has taken the trouble to issue a drop-bottle with holes in standard sizes like the jets on a motor carburettor ". Over the past few years considerable advances have been made in the development of standards for anaesthetic equipment, and the history of this work was reported on by Hillard (1968), Thompson (1983, 1984) and Greenbaum (1985). A considerable number of International and British Standards have now appeared and, while progress is slow, some real advances have been made. ISO 5358 (1980) deals with "Continuous flow inhalational anaesthetic apparatus (anaesthetic machines) for use with humans" under the following headings: Medical gas cylinder connections, Pipeline inlet connections, Pressure gauges and cylinder contents indicators, Pressure regulators, Machine gas piping, Flow control valves, Flowmeters, Connections for vaporizers, Mixing devices, Flowmeter-controlled vaporizer systems, Concentration-calibrated vaporizers, Non-calibrated vaporizers, Common gas outlet of the machine, Gas power outlets, Oxygen flush valves, and Oxygen supply failure precautions. The importance of this work can hardly be over

8 920 BRITISH JOURNAL OF ANAESTHESIA ensuring that they remain fitted together (Shaw, Davis and Anderson, 1982; Spurring and Small, 1983; Harrison, Tomlinson and Mann, 1984; Heath, 1984; Keddie, 1984). The Little Report (Little, 1983) examined the whole problem of disconnection within breathing systems in great detail and strongly recommended the use of locking connectors at all joints. The Final Report of the U.S. Food and Drug Administration (1986) is now published. The call for locking connectors would seem to be gaining favour generally, but some of the designs produced so far have not always been received as satisfactory. CONCLUSION FIG. 7. Non-fitting (incompatible) expiratory valve units and angle-pieces. emphasized. Figure 7 illustrates a simple situation which was all too common a few years ago, when one could find on an anaesthetic machine three angle-pieces and three expiratory valve units, not one of which would mate with any other. The current U.K. practice of anaesthetizing patients in an anaesthetic room using one apparatus and then transferring to a second one waiting in the operating theatre can still result in sudden connection probems. Only recently the author was, to his great amazement, confronted with a 22-mm McKesson expiratory valve assembly on the second machine, and this in a 10-theatre suite in a new hospital which so far as anyone knew had never had such an item at any time! One fundamental aim of standardization has always been an increase in patient safety. To a considerable extent this may depend on the interchangeability of apparatus made by different manufacturers. Certainly, there lies herein increased safety in that the anaesthetist is less likely at a moment of crisis to find himself unable to connect one part of a breathing system to another. However, while most such components can now be fitted together, there is still much to be done in An attempt has been made to indicate the lines along which designers seem to be thinking. It is stressed yet again that, whatever contributions to safety are made by design improvements, their effect in practice still depends on their understanding and implementation by the user, reliable and regular maintenance, the avoidance of local modifications and the development, as rapidly as possible, of effective National and International Standards for equipment. ACKNOWLEDGEMENTS AND NOTE The author is deeply indebted to P. Schreiber and T. Barford of North American Drager, to L. Smith of Ohmeda, and to B. R. Sugg of Penlon Limited for providing up-to-date information and illustrations of their companies' newest anaesthetic machines. It should be noted that the selection of the three items of equipment so described reflects a purely arbitrary choice by the author seeking to illustrate the paths along which he feels present and future development is taking and will take place. The descriptions are not intended to be full and the mention of one feature, for example, in connection with one machine and not another should not be taken to imply necessarily that it is to be found in that one machine only. The author is deeply indebted to Peter Schreiber for permission to draw quite heavily on his published work, and to Mrs C. Ellis for the tact, patience and skill which she has shown during the preparation of the manuscript. REFERENCES Bonsu, A. K., and Stead, A. L. (1983). Accidental crossconnexion of oxygen and nitrous oxide in an anaesthetic machine. Anaesthesia, 38, 767. Bruner, J. M. R. (1984). Safety in anesthesia: technology vs clinical skills?, in Biomedical Safely and Standards, Special Supplement No 2. Brea, U.S.A.: Quest Publishing Co. Cooper, J. B., Newbower, R. S., Long, C. D., and McPeek, B. (1978). Preventable anesthesia mishaps. Anesthesiology, 49, 399. Craig, J., and Wilson, M. E., (1981). A survey of anaesthetic misadventures. Anaesthesia, 36, 933.

9 SAFER DESIGN OF ANAESTHETIC EQUIPMENT 921 Cundy, J., and Baldock, G. J. (1982). Safety check procedures to eliminate faults in anaesthetic machines. Anaesthesia, 37, 161. DHSS (1982). Maintenance-induced faults. Safety, Information Bulletin, 4, 13. Department of Health and Social Security. Food and Drug Administration (1986). Accidental Breathing System Disconnections Final Report. National Technical Information Service, U.S. Department of Commerce, 5285 Port Royal Road, Springfield, VA Greenbaum, R. (1985). National and international standards for anaesthetic equipment. Br. J. Anaesth., 57, 709. Harrison, M. J., Tomlinson, P. A., and Mann, M. S. (1984). Disconnexions. Anaesthesia, 39, 721. Health Equipment Information (1982). Management of equipment. No. 98, January. (1984). Management of equipment. No. 176, August. Heath, M. L. (1983). Safety check procedures to eliminate faults in anaesthetic machines. Anaesthesia, 38, 297. (1984). Accidents associated with equipment. Anaesthesia, 39, 57. Hillard, E. K. (1968). British standards relevant to anaesthesia. Br. J. Anaesth., 40, 702. Hurst, R. (1977). Pilot Error. A Professional Study of Contributory Factors. St Albans: Granada. Hurst, L. (1982). Pilot Error: The Human Factors. New York: Aranson. ISO 5358 (1980). Continuous Flow Inhalational Anaesthetic Apparatus {Anaesthetic Machines) for Use with Humans. Geneva: ISO. Judkins, K. C, and Sage, M. (1983). Routine servicing of the Cape-Waine ventilator. Anaesthesia, 38, Keddie, F. S. (1984). Possible connexion failure. Anaesthesia, 39, 721. Little, A. D., Inc. (1983). Accidental Breathing System Disconnections. Interim report to Food and Drug Administration. Contract No Cambridge, Massachusetts. Lunn, J. N., and Mushin, W. W. (1982). Mortality Associated with Anaesthesia. London: Numeld Provincial Hospitals Trust. Protocol (1980). Protocol for checking an anaesthetic machine before use. Faculty of Anaesthetists of the Royal Australasian College of Surgeons. Schreiber, P. (1985). Safety Guidelines for Anaesthesia Systems, Introduction and pp. 28, 54. Telford, Pennsylvania: North American Drager. Schreiber, J. M. (1986). Electronic Surveillance During Anesthesia. Telford, Pennsylvania: North American Drager. Shaw, A., Davis, P. D., and Anderson, I. M. (1982). Tapered connectors in patient breathing systems. Anaesthesia, 37, 201. Spurring, P. W., and Small, L. F. G. (1983). Breathing system disconnexions and misconnexions. Anaesthesia, 38, 683. Thompson, P. W. (1983). Joseph Clover Centenary Lecture. "Out of this nettle..." Ann. R. Coll. Surg. Engl., 65, 14. Thompson, P. W. (1984). Everyone needs standards. Crit. Care Int., 3, 13. and Wilkinson, E. J. (1985). Development of anaesthetic machines. Br. J. Anaesth., 57, 40. Ward, C. S. (1985). Anaesthetic Equipment, 2nd Edn. London: Bailliere Tindall.