APPENDIX A STERILIZATION BY STEAM UNDER PRESSURE Effective sterilization by steam is so important in surgery and there is such widespread ignorance of the technical and other factors involved that the following memorandum is included for general information. Important and helpful recommendations are made in two reports of the Medical Research Council's Working Party (1959, 1960), that include a considerable bibliography. Howie published an informative and readable article in 1961 and much else has been written, notably the symposium published by the Pharmaceutical Press in 1961. The principles and practice of sterilization by steam under pressure General Steam produced under pressure comes off at an increasingly high temperature as the pressure is raised. If it is admitted into a chamber that is artificially heated to an even higher temperature than that of the steam, the steam becomes superheated. It then acts like hot air and loses its sterilizing efficiency. If it is exposed to a lower temperature, some of the steam condenses to water and it becomes wet. For satisfactory autoclaving, steam must be delivered from a steady high pressure supply, reduced to the desired pressure, freed from moisture in a separator and admitted to a chamber, of which the jacket is at a similar pressure, or, more precisely, a similar temperature, to avoid superheating or cooling. Steam in this state is dry, saturated steam and is at the water/ steam phase boundary, i.e. it will condense at once on reaching a cooler surface, thereby giving up its latent heat of vaporization. Its supply is the engineer's business. For institutions where a good supply of steam from a central boilerhouse is not available, it is possible to obtain an apparatus in which a boiler is incorporated. This is heated by electricity, gas or oil, below the autoclave chamber that it feeds by simple downward displacement. Sterilization by steam under pressure kills bacteria and spores by a combination of moisture and heat. The process is effective in predictable periods of time that vary inversely with the temperature, assuming that the apparatus is working well and that it is being correctly operated. If these conditions are not satisfied, goods labelled " sterile" may not in fact be sterile, the load may be wet at the end of the sterilization cycle, or rubber goods, especially gloves, may be damaged or even unusable. 275 THE DESIGN OF OPERATING THEATRE SUITES Filling the chamber The chamber is filled with steam by one of two physical processes, namely: 1. Downward displacement, or 2. High pre-vacuum. 1. In downward displacement, the steam is admitted at the top of the chamber and, because it is lighter than air, lies on top of the contained air in the chamber. Gradually the steam fills the chamber from above downwards, the air being driven out through the chamber drain, at the bottom of the chamber. This is a relatively slow process. The time taken for steam to fill the chamber and enter the content that is generally known as the " load " is called the penetration time. 2. When a high pre-vacuum is used, the chamber and its load are almost completely emptied of air before steam is admitted. The steam therefore penetrates all parts of the chamber and its load very quickly. The penetration time is therefore so short that it scarcely adds to the total time of the sterilizing cycle. In order to achieve an effective vacuum, an efficient pump must be employed and the gasket of the chamber door and the valves must close accurately. A big chamber, double-ends and a rectangular door all add to the risk of leakage and even a very small leak prevents possibly the drawing, and almost certainly the maintenance, of a high vacuum. A satisfactory vacuum must be related to the atmospheric pressure, which should be neutralized to within at most half an inch, or 20 mm. Hg. Residual pockets of air prevent access of steam to parts of the load that do not become penetrated and are not exposed to the necessary conditions of moisture and temperature. In order to assist the complete removal of air in the pre-vacuum stage, some manufacturers are now providing for the admission of a small jet of steam during the pumping down process. This is too little to prevent the drawing of the vacuum, but has the effect of washing out potentially residual air. The holding of the vacuum for even a few seconds is prevented by the smallest leak that permits the re-entry of air to the chamber. It is known that the corners of rectangular doors offer relatively weak spots with increased risk of leakage and uneven wear of gaskets. Many users, however, prefer rectangular chambers because of their actual or supposed greater convenience in loading. A small door tends to leak less than a bigger door and a small chamber can be exhausted more easily than a larger one. A well filled chamber sterilizes more efficiently than a chamber with a disproportionately small 276 THE DESIGN OF OPERATING THEATRE SUITES load, for reasons that are discussed later under the heading " The small load phenomenon ". For all these technical reasons a small rather than a large autoclave is commendable. It is perhaps wise not to go above 10 or 12 cubic feet capacity, and as a matter of convenience and expediency two (small) autoclaves may be better than one (large). A partial vacuum, such as is drawn by a steam venturi pump in many old autoclaves to something in the region of half an atmosphere, does not remove all the air and so does not ensure rapid and complete penetration. It is to be regarded as little more than a curtain-raiser to a downward displacement performance. Moisture and latent heat Some part of the steam admitted to the chamber condenses on the load, supplying moisture and surrendering its latent heat. The amount of steam actually admitted to the chamber is therefore much greater than could be accounted for by the latter's simple cubic capacity. The chamber drain At the lowest level in the chamber there is a drain through which steam, air and moisture escape through a near-to-steam trap. This trap, or valve, remains open until such time as pure steam begins to emerge from the chamber, the air and moisture having all escaped. The valve closes more or less completely and thereafter opens only when more air or condensate reaches it. This minimal " bleeding" continues throughout the holding phase. When pure steam comes through, the temperature on the discharge line, near the chamber drain, should correspond with that in the chamber, and in the load, and should be that calculated from the chosen pressure. When this temperature is reached, the time allowed for penetration is ended and the holding time begins. This is the standard method of reading and recording the temperature in the chamber. An alternative method, in which the temperature actually within the load is read, is described later, and other techniques may yet be devised. The sterilizing time is inversely proportional to the pressure and temperature levels and is read from a previously calculated scale. A variable amount of extra time, usually up to 50 per cent, is commonly added as a safety margin and the total is known as the holding time. At the end of the holding time the steam is allowed to escape from the chamber. Drying is either a very quick or a very slow process, depending upon whether the system in operation includes a high vacuum. 277 THE DESIGN OF OPERATING THEATRE SUITES The after-vacuum in a high vacuum machine is drawn either with a suitable pump or by condensing the steam from the chamber, or both. Since the chamber contains nothing but steam, its condensation can produce a high approximation to a complete vacuum, e.g. within about two inches of mercury of zero pressure. This is sufficient to induce evaporation of such a high percentage of moisture from the load that further drying is not necessary. The vacuum is broken, with proper precautions to filter the entering air, the door is opened and the load is removed. It should be not only dry but partly cooled by the virtue of the heat expended in evaporation. If the dressings are not dry, something is wrong. Cracking the door When no after-vacuum is provided, at the end of the holding period when the steam has escaped to zero pressure, the door is " cracked ". That is to say, it is opened a little and left in that position long enough (an arbitrary period determined by experiment) for the heat of the jacket to dry the load. This practice is mentioned only to be condemned. It is evidence of an unsatisfactory state of affairs. The venturi pump should be made to produce the best possible vacuum and this should be held for as long as is necessary to produce acceptable drying of the load. The vacuum should then be broken (with appropriate filtration of the air admitted). The door is then opened and the load removed. The partial after-vacuum drawn by a steam venturi pump in older models goes some way towards drying the load and the period of drying with the door cracked may thus be eliminated. Drying in a stream of air, drawn through the load by continuous suction after opening the air-vent, is generally to be condemned. The filters provided on the sort of autoclave likely to be used in such a fashion cannot be depended upon to prevent recontamination of the load by unclean air. Gloves are almost certain to be damaged by this undesirable process. The deleterious effect of drying, for even a short time, upon fabrics and especially upon gloves is discussed later. The time of the sterilizing cycle A large capacity, downward displacement or partial vacuum autoclave may, in practice, take as long as 90 minutes or more to complete its cycle (of which only a small fraction is sterilizing time). The shortest reported cycle for a high vacuum autoclave in daily use is under 10 minutes (Wells and Whitwell, 1960)-pre-vac. 3 mins., penetration 1 min. holding 3 mins., after vac. 2 mins., break vac.4 min. It is worthwhile noting that the autoclave that consistently achieved this performance was of a little more than three cubic feet capacity and that it did 95 per cent. of the sterilizing for a busy general hospital of 600 beds. 278 THE DESIGN OF OPERATING THEATRE SUITES This remarkable output was maintained in spite of the fact that the chamber was cylindrical, which is sometimes thought to be inconvenient, and that drums that defeat economical loading were used throughout. The inference may reasonably be drawn that a small efficient cylindrical autoclave has much to recommend it. The circular chamber gives maximum capacity for surface area, is easier to manufacture than the rectangular and is the least likely to give trouble from a leaking door. Because this cylindrical chamber is inconvenient for the loading of drums and rectangular packages, many hospitals prefer the rectangular shape. Power-operation may overcome the tendency of rectangular doors to leak. The need for power-operation is an index of this tendency. The general use of individual packs should facilitate the loading of cylindrical chambers that are undoubtedly the more efficient engineering article. Pressures, temperatures and holding times It is usual to operate at 30/33 lb. to the square inch pressure. At 32.5 lb. a temperature of 134°C. should be recorded in the discharge line. The sterilizing time is three minutes. At 17/20 lb. to the square inch (recommended for gloves in the older sterilizers) the temperature should read 124/1260 C. and the effective sterilizing time is then 10 minutes. The holding time is determined by adding a margin up to 50 p-r cent. for safety. Timing the cycle In the case of older, downward displacement autoclaves, hand operation from stage to stage is the rule. With high vacuum machines, manual operation is also possible but, to secure the best results, automatic control is the usual practice. Under automatic control, the pre-vacuum pump should continue in action to within at most 20 mm. Hg of absolute vacuum. Steam is then admitted and the pre-determined holding time begins when the correct temperature is reached. The after-vacuum continues until the appropriate negative pressure is attained and is then automatically broken. The integrator If an integrator is included in the automatic timing device, this takes account of slight variations in temperature above or below the predicted level during the holding period and adjusts the length of this period accordingly. It is an expensive addition to the equipment. It allows for variation in the quality of steam admitted to the chamber and for the admission by leakage of air during the short time for which the pre279 THE DESIGN OF OPERATING THEATRE SUITES vacuum is held. An autoclave in perfect order does not need an integrator. Opinions differ regarding this desirability. It is likely that a simpler device will supersede the integrator in the near future. Special loads Gloves: In an efficient high vacuum sterilizer, gloves may be included in the standard load at 30 lb. pressure and may be taken up to as many as 16 runs and still be reliable in use. In any other type of sterilizer gloves deteriorate rapidly. They probably suffer most damage in the drying period. It is evident that the sterilizing time is much the same whatever type of autoclave is used and it is obvious that the process is lengthened by the greater time necessary for penetration and for drying. Whilst drying is in progress, air at a high temperature is in contact with the gloves. It is probable that oxidation as well as heat plays a part in the damaging of rubber. It is quite illogical, but because of the rapid deterioration of gloves in old-fashioned autoclaves they are often treated at a reduced pressure (17 to 20 lb.) for a minimal holding time, which may be doubtfully adequate. Even lower pressures should not be employed. It must be emphasized that in an efficient high vacuum autoclave there is no need for a special run for gloves. Although disposable gloves are sterilized only once, care is still necessary as they can easily be damaged even in one run. Glove powder: Small packages of absorbable powder sterilize well in an ordinary cycle (Kelsey, 1962). Talc is difficult or impossible to sterilize by moist heat and should not be used. Instruments and bowls, if packaged and wrapped, e.g. in C.S.S.D., will be wet unless sterilized under the best conditions, preferably with high pre- and post-vacuum. Even then, surfaces on which, and containers in which, moisture can collect, should be avoided by suitable packaging and loading. Unwrapped instruments, sterilized at the site of operation, whether in the small " flash " sterilizer or otherwise, need no pre-vacuum and no vacuum for drying, but after-vacuum, if any, has a cooling effect. Surfaces on which, and containers in which, moisture can collect should be loaded so as to minimize the risk of this nuisance arising. Mackintosh sheets should not be used. The pockets of air in a folded mackintosh sheet cannot be satisfactorily dispelled by downward displacement and their sterilization is at all times subject to doubt. If used, they should be interleaved with fabric, and be vertically disposed when in the autoclave chamber. Even in a high vacuum machine it is found in practice (Wells and Whitwell, 1960) that large mackintosh sheets are more difficult to sterilize 280 THE DESIGN OF OPERATING THEATRE SUITES than other soft goods and that they need to be given additional holding time. (See also under " Drums " below.) Paper sheets are a convenient alternative. Fluids: Fluids cannot be sterilized in a high vacuum cycle. They boil explosively. They need to be cooled relatively slowly. A special cycle therefore needs to be set. An intermittent, slow letting-down of the pressure works very well. Cooling devices are available in certain autoclaves made especially for this purpose. These offer a quicker cycle. Drums: For many reasons, drums are open to criticism and their use is to be condemned. However (Darmady et al., 1959), many thousands are still in use and it is therefore important that their correct handling should be understood. Recontamination: Drums must be kept in good repair to avoid gross soiling through ill-fitting lids and shutters. Even so, when they are cooling, after being removed from the chamber, unclean air is sucked in through one channel or another and it is therefore necessary to have an efficient lining. Fabric is usually used for this purpose, but paper does at least as well. Packing and loading: In a high vacuum autoclave, tight packing is theoretically no bar to sterilization, because the whole load should be almost completely exhausted of air. In downward displacement autoclaves, however, loose packing is essential and relatively impermeable materials should be vertically disposed, so that the downward stream of steam may find its way into all the interstices and displace all the air therefrom. Mackintosh sheets are virtually unsterilizable by steam in a normal sterilizing time by downward displacement, but they may become sterile by dry heat by virtue of prolonged periods of penetration and drying. Their use should be abandoned. When a drum is placed in a downward displacement autoclave, the open vents should lie vertically above one another so as to allow steam to flow freely downwards through them and so as to permit the escape of air from the lowest level of each drum. This may involve placing the drum on its side and this, in turn, may defeat the intended vertical disposition, of folded, impermeable or semi-permeable goods inside the drum. It is necessary that precautions be taken to avoid such errors when drums (or, indeed, packs) are loaded into the chamber of a downward displacement autoclave. If drums are loaded into the chamber in such a way that the lowest part of the drum forms a pool of undrained air, efficient sterilization by steam (which is the object of the procedure) is impossible. (See also under " Mackintosh sheets "-above.) 281 THE DESIGN OF OPERATING THEATRE SUITES Caskets: Recognizing the risks inherent in the use of drums, but attracted by their advantages as containers, some workers favour a metal casket of robust design (B.S. 3281: 1960, rectangular sterilizing boxes) with a permanent filter incorporated behind the vents. They are extremely expensive and, like any other containers that need to be opened and shut many times for the removal of part of the contents, they are vulnerable to recontamination in use. Penetration through caskets may be slow in downward displacement machines. Other metal containers: The practice of placing small articles in a closed metal box without air vents is plainly contrary to the principles of sterilization by steam under pressure and should be discontinued. Packs versus drums and caskets The danger of recontaminating the contents of a drum during cooling and closing has already been amply illustrated. The further likelihood of the contents becoming contaminated during use is also well known and there is a consequent and very general move in favour of individual packs (Medical Research Council, 1959). Two layers of fabric or two or more sheets of special paper are the most popular wrapping materials. Cardboard boxes, specially made and supplied free from bacterial spores, are often used as containers for a number of individual packs. They should not be used for loose goods as a substitute for drums as they are subject to much the same objections. Checking the efficiency of the autoclave On installation the autoclaves should be tested by the manufacturer before handing over. For this purpose a thermocouple should be placed in loads in different positions in the chamber and the temperature read and compared with the discharge line temperature. The capacity of the chamber to hold a vacuum must also be tested on installation and at regular intervals. Monitoring the load. The pressure gauge may have a second series of numbers on its dial indicating the temperature that should be achieved at each level of pressure. These figures are sometimes read as though representing the actual temperature. This is bad practice. The temperature should be read from the steam escaping through the chamber drain to the discharge line. This is generally accepted as being a sufficiently accurate indication of the temperature of the load. Bowie (1961) and Fallon (1961) press the view that the thermocouple method of monitoring, that is used on installation, should be routine. Clearly this would be ideal, but it is questionable whether it is practicable or, indeed, necessary, and at the time of writing it is not recommnended, 282 THE DESIGN OF OPERATING THEATRE SUITES The construction within the chamber itself, at the time of manufacture, of a load-simulating device, from which the temperature can be read, is under consideration. It may be thought to be an unnecessary refinement, an added complication and something more liable to give trouble than the present method. This remains to be seen. Routine temperature/time testing can be carried out with a high degree of significance by the aid of Browne's Tubes. The Type 1, black spot, is suitable for downward displacement autoclaves. The exposure to high temperatures before and after the holding period is so short in high vacuum machines that the Type 2, yellow spot, that changes colour after a shorter exposure time, should be used. In view of the brief periods before and after holding, this remains a sensitive test of the sterilizing time/ temperature complex. Tubes should be put into a variety of positions in packages in different parts of the chamber to ensure valid results. Every load should be tested by at least one tube in this way. The tube should be placed near the centre of the drum or pack, with a ribbon gauze tape attached to it. It may then be extracted and examined before the contents of the drum or pack are used. Heat/time/moisture sensitive tape may be used in much the same way on the outside of every package, and similar material, non-adhesive, may be used inside the package. For this purpose the tape is arranged in the form of a cross as nearly as possible at the centre of the load. Under the combined effect of heat, moisture and time, black bars appear on the tape. The change should be uniform throughout the cross. Paler areas indicate inefficient sterilization and are most likely to be seen at the centre, indicating imperfect steam penetration (Bowie et al., 1963). Bacteriological tests may be carried out in two ways: 1. Commercially available packets containing spore bearing B. stearothermophilus of known heat resistance are placed in various sites and their sterilization confirmed by a competent bacteriologist. 2. Test swabs are placed in various vulnerable positions, for example near a vent in a drum, and cultured after sterilization to exclude recontamination. For extra safety these tests may be carried out at regular intervals of about a month or two or three. Record charts: All modern autoclaves have a writing mechanism that records the time/temperature changes, read from the discharge line, and also a record of the vacuum drawn and the time taken to achieve it. These charts should be studied daily by a member of the staff capable of interpreting their meaning. 283 THE DESIGN OF OPERATING THEATRE SUITES The filter: If the filter is inefficient, unclean air is drawn into the chamber and recontamination of the sterilized load is almost certain if unlined drums are used. Since the air admitted penetrates all parts of the load to dispel the vacuum it is obvious that efficient filtration is important. In some old autoclaves, a glass cup containing a wad of cotton wool is fitted to the door and is the only filter provided. The cotton wool must be dry to be effective and a fresh, sterilized piece should be inserted every day. There are cases on record where the filter material had not been changed for 14 years. In modern instruments, if such a cup is still in use, it is merely a first filter to remove gross dirt before the air-stream is drawn through the filter proper. Probably the best material for the filtration of air is fibre-glass paper, and specially constructed filters good for 12 months' or more use may be bought commercially (Howie, 1961). Metal filings and ceramic and paper filters are also popular. The type of filter supplied by the manufacturer should be known and its maintenance understood and cared for by the hospital engineer. The importance of reliable filtration must be recognized by everyone. The small load phenomenon A disproportionate amount of importance has been attached to this phenomenon, the significance of which is simply that the high vacuum autoclave works best with a full load. The phenomenon is as follows. A single small package is placed in a disproportionately large chamber, in a high vacuum autoclave. The pre-vacuum is drawn and steam admitted. Now it is known that the pre-vacuum is almost but not quite absolute. It is also known that when steam is admitted, it condenses on the load. This, in the case of a particularly small load, means that as the steam is admitted to the chamber as a whole it tends to stream towards the load and, as it does so, it entrains with it whatever air has been left in the chamber. This is enough to form a bubble of air at the centre of the package where sterilization does not take place. In order to guard against this theoretical danger and at the same time to add an extra margin of safety to the whole process, it has been suggested that whilst the pre-vacuum is being drawn a small trickle of steam should be admitted to the chamber to replace the modicum of air that is otherwise left behind. Alternatively, the pre-vacuum may be drawn twice with the admission of steam in between. These procedures may, at some future date, become standard practice but are not so at present. 284 THE DESIGN OF OPERATING THEATRE SUITES Installation and maintenance A good, efficient and economical sterilizing performance demands in the first place: 1. The selection of suitable apparatus. 2. Its correct installation. 3. Full testing on the site by the manufacturer with special attention to the drawing and holding of a vacuum within 20 mm. Hg of absolute. The maintenance of a good performance involves a number of functions and a number of individuals, namely: 1. For ensuring a good and steady supply of steam-the engineer. 2. For repairs and replacements-the manufacturers. 3. For daily operating and loading-a technician or a nurse, or both. 4. For packing the loads-the theatre superintendent or chief of C.S.S.D. and possibly ward sisters and nurses. 5. For the checking of sterility-the bacteriologist. If, as too often is the case, these individuals act independently of one another, efficiency is certain to suffer. They have a joint responsibility that can be exercised only if they meet together at regular stated times. There should be added to their number a surgeon who is prepared to make it his business to become knowledgeable in these matters so that he can share the responsibility for the maintenance of proper standards and advise his colleagues. The help of Professor J. W. Howie and his associates in compiling these notes is acknowledged with gratitude. References BOWIE, J. H. (1961) in Pharmaceutical Society (etc.). Recent developments in the sterilisation of surgical materials. See below. KELSEY, J. C., and THOMPSON, G. R. (1963) The Bowie and Dick autoclave tape test. Lancet, 1, 576. DARMADY, E. M., HUGHES, K. E. A., JONES, J. D., and VERNON, P. F. (1959) Failure of sterility in hospital ward practice. Lancet, 1, 622. FALLON, R. J. (1961) Steam penetration into containers for surgical dressings. Lancet, 2,41. (1961) Monitoring sterilisation of dressings in high-vacuum pressuresteam sterilisers. J. clin. Path. 14, 666. HOWIE, J. W. (1961) The surgeon's autoclave. J. clin. Path. 14,49. KELSEY, J. C. (1962) Sterilisation of glove powder by autoclaving. Mth. Bull. Minist. Hlth. (Lond.) 21, 17. MEDICAL RESEARCH COUNCIL (1959) Report by the working party on pressure-steam sterilisers. Lancet, 1, 425. (1960) A second report by the working party on pressure steam sterilisers. Lancet, 2, 1243. PHARMACEUTICAL SOCIETY OF GREAT BRITAIN, and SMITH AND NEPHEW RESEARCH, LTD. (1961) Recent developments in the sterilisation of surgical materials. Report of a symposium. London, Pharmaceutical Press. WELLS, C., and WHITWELL, F. R. (1960) A small autoclave in a large hospital. Lancet, 2, 643. 285