Boiling and autoclaving

Sterilization is the total destruction of all forms of life, including bacterial spores. It is best done with heat, either dry heat in an oven, or steam under pressure in an autoclave. Processes (usually chemical) which do not destroy spores are termed ''disinfection'. Some of the most important agents to be removed by disinfection are HIV and HBV (hepatitis virus B). All the disinfectants described in Section 2.5 will do this if used as directed. The most suitable one for many purposes is hypochlorite, which is described in Chapter 28a.

The basis of aseptic surgery is to kill all micro-organisms on all instruments and dressings, preferably by exposure to steam under pressure. If this is impractical, immersion in boiling water for 10 minutes at sea level will kill all viruses and all vegetative bacteria, but not spores, particularly those of tetanus and gas gangrene. A boiling water ''sterilizer' is therefore badly named. At a height of 3000 metres water boils at 90[de]C and is much less effective.

Steam is the gaseous form of water. If it is to sterilize effectively, which means killing all spores: (1) It must be at an appropriate temperature (which implies an appropriate pressure). (2) It must be saturated with water. (3) It must not be mixed with air, so it must displace all the air in the chamber of the autoclave. And, (4) it must reach all parts of the load. If it contains droplets of water, it will soak into porous materials. If, on the other hand, it is superheated and therefore too dry, it will be less effective as a sterilizing agent. If air is mixed with steam: (1) the temperature of the mixture at a given pressure will be lower, (2) it will penetrate less well into porous materials, (3) the air may separate as a lower, cooler layer in the bottom of the chamber, so that the contents are not sterilized. If no air is discharged, the bottom of the chamber may be much cooler than the top.

As soon as the chamber of an autoclave is full of steam at the desired temperature and pressure, it must be held there for a critical time[md]the holding time. The standard holding time is 15 minutes, at 121[de]C, but you will need to vary it as described below. This temperature is reached at a pressure of about 1 kg/cm['2] (15 pounds per square inch). An easy minimum figure to remember is ''1 kg per square cm for 15 minutes' (''15 pounds for 15 minutes'). If your autoclave is rated to 1.3 kgcm['2], you can shorten the sterilizing time to 10 minutes. Here we only discuss the simpler forms of autoclave; high vacuum autoclaves are beyond the scope of this manual.

Single walled autoclaves are strong metal chambers with water in the bottom, like large pressure cookers. They have several disadvantages: (1) The air in the chamber is removed by steam rising from the bottom. This is inefficient, so that an undesirable quantity of air remains. (2) They don't have thermometers at the bottom of the chamber, so you never know what the temperature there is. (3) The load remains moist after sterilization, which can be dangerous, because bacteria can more easily enter through moist wrappings.

Double walled autoclaves can be vertical, but are much better horizontal. They should either have an effective prevacuum, or a pulsing system (neither described here), or rely entirely on gravity to displace the air. A partial prevacuum at the start of the sterilizing cycle (which used to be the practice in some older autoclaves) causes turbulence when air is admitted, so that the gravity displacement of air cannot take place satisfactorily.

Steam is generated in, or admitted to, a jacket round the chamber, rather than in the chamber itself. This jacket keeps the walls of the chamber hot, which prevents condensation and helps to dry the load. Steam enters the chamber through a pipe at the top and displaces the air it contains. Air, condensate, and excess steam escape through a pipe at the bottom. This pipe has a thermometer in it to record the temperature in the bottom of the autoclave. In some autoclaves a water pump, which works on the same principle as an ordinary laboratory water pump, sucks out some of the steam afterwards (postvacuum). There is also a means of admitting sterile air to break the vacuum at the end of the cycle.

The drain at the bottom of the chamber should have a ''near- to-steam trap', which will alllow the discharge of condensate and air, and will close automatically when they have been discharged, and the trap meets live steam, thus avoiding the need to close valve 13 in Fig 2-7 manually, which could spoil sterilization.

The thermometer records the temperature in the chamber drain, which is the coolest part of the autoclave. When this reaches the operating temperature, the timing of sterilization can begin.

More sophisticated autoclaves have better pumps, a recording thermometer, a thermocouple to measure the temperature of the load, and an automatic control system.

Inadequate sterilization is an important cause of wound sepsis in poorly maintained theatres. There are many pitfalls. Start by inspecting your equipment and taking an interest in it. Read the maker's instructions carefully, and make sure: (1) That it has been properly fitted and tested. For example, if a water ejector pump is fitted, it is likely to need a water pressure of 1.5 kg/cm['2]. (2) That all the staff who use it understand how it works, and how to use it effectively. They must realize the importance of packing the drums loosely, the need to discharge the air, and the correct holding time.

Most of the hospitals for which we write don't have piped steam supplies. If so, you will will have to use a vertical autoclave. You will probably have electicity, but it is likely to be unreliable, so you will want one which can be heated by electricity, or by putting a kerosine pressure heater or a charcoal stove under it. Unfortunately, we have not been able to find such an ''all-fuels' autoclave, but Jacob White (WHI, see Appendix B), will consider making one if the demand is high enough (write to them). If you raise an ordinary vertical electric autoclave on bricks so that you can get a pressure heater under it, you are likely to harm the electric cables to the elements. Unfortunately, there is no WHO specification for a district hospital autoclave, as there is for an X-ray machine (1.13).

AUTOCLAVE, horizontal, downward displacement with near-to- steam trap in the chamber drain, post vacuum, six spare gaskets, three spare bellows for the steam trap, and a triple set of other spares, one outfit only. If you have a steam supply, this is the autoclave you need. Horizontal autoclaves are easier to use, but are more expensive. You will need a standby, in case the electricity fails, so you should have an autoclave that can be heated by kerosene or gas somewhere in the hospital (see below).

Alternatively, AUTOCLAVE, vertical, downward displacement, 350 mm, 2[1/2] drum, electric, 6 kw, state voltage, manual operation, with six spare elements, six spare gaskets, and a triple set of other spares as necessary, one outfit only.

AUTOCLAVE, vertical, 350 mm, 2[1/2] drum, for heating by kerosene or gas, manual operation, with six spare gaskets, and a triple set of spares as necessary, one outfit only. This is for use in emergency, see above.

AUTOCLAVE, vertical, ''pressure cooker', 47 l, UNICEF, as (WIS) No.1941, one only (optional). This is UNICEF's large autoclave which can be heated on a stove and has a machined lid so that it needs no gaskets. It is large enough for 5 litres of intravenous solution, or one laparotomy pack. It is a useful standby. It has an air exhaust tube which leads from the exhaust port to the bottom of the sterilizer. If you use it, you can start timing as soon as steam comes from the exhaust.

HEATER, kerosene, pressure, ''Primus' pattern or equivalent, four burners, with 20 spare jets, prickers and washers, two only. Use this to heat the autoclave, either regularly or in an emergency.

TUBES, Browne's, for testing autoclaves, Type one (black spot), for use with ordinary steam sterilizers below 126[de]C, 100 tubes only. These change colour on the basis of time and temperature, and are reliable, provided that there is not a long drying cycle, when prolonged heat in a jacketed sterilizer could change their colour.

Alternatively, CARDS, autoclave testing, ATI ''Steam-clox' 25 rolls only. This brand of tape changes colour on the basis of moisture and temperature, to indicate that something has been autoclaved. Most other brands of autoclave tape are only suitable for high pre-vacuum autoclaves, not for the downward displacement ones described here. Another alternative is ''Diack Controls', a pellet in a glass tube which melts at 121 or 126[de]C.

DRUMS, deep, 340[mu]230 mm, ten only. This is the standard size of drum.

DRUMS, shallow, 340[mu]120 mm, ten only. These are half- size drums. You may have difficulty getting drums because they are no longer used in the developed world. If you are short of drums, sterilize your equipment in packs, covered by two layers of towelling and preferably an outer layer of paper. If you are sterilizing without paper, use all equipment warm straight from the autoclave.

DRESSING BOXES, stainless steel, with hinged lid and perforated sliding shutters at front and back, 250[mu]200[mu]150,,,,mm, three only. Use these for sterilizing gloves and dressings.

TRAYS, dressing, without lids, stainless steel, 275[mu]320[mu]50 mm, 20 only. Use these to prepare sterile sets for the wards. Boil a tray and the instruments, lay a sterile towel on the tray, put the instruments on it and fold it over them. Better, autoclave the tray.

STERILIZER, boiling water, electric: (a) ''Bowl sterilizer', 450[mu]350[mu]380 mm, with counterbalanced lid, 6 kW, with six spare elements, state voltage, one only. (b) Instrument sterilizer, 350[mu]160[mu]120 mm, 1.2 kW, with six spare elements, state voltage, one only. One of these is for trays and bowls, and the other for instruments. Keep them both in the preparation room. Never try to sterilize anything contaminated with faeces with boiling water in a sterilizer[md]it does not destroy spores.

FORCEPS, sterilizer, Cheatle's, 267 mm, UNIPAC 0735200, two only.

FORCEPS, sterilizer, Cheatle's extra large, 279 mm, complete with can of appropriate size for antiseptic fluid, two only. These are useful for bowls and utensils, and will also pick up small objects.

FORCEPS, bowl sterilizing, Harrison's double jawed, complete with can of appropriate size for antiseptic fluid, one only. Autoclave these and Cheatle's forceps and their cans after each day's use, then fill them with fresh antiseptic fluid.

Fig. 2-7 AUTOCLAVES. A, a simple autoclave is a strong metal chamber with water in the bottom, like a large pressure cooker.

B, a jacketed vertical gravity displacement autoclave. This is filled through a tundish (open funnel) (1) and a filling valve (2). On the same pipe there is a safety valve (3) and a pressure gauge (4) to measure the pressure in the jacket. A pressure switch (5) controls the pressure in the jacket and an indicator (6) monitors its water level. A float switch (7) cuts off the power if the water level is too low, and a drain tap (8) lets water out of the jacket. Several heating elements (9) heat it. The chamber is drained through a pipe (10) and a strainer (11). A thermometer (12) and a valve (13) are fitted to the drain pipe (the valve should be an automatic near-to-steam trap, preceded by a non-return valve, to prevent dirty air and some water being sucked up during the vacuum). Steam from the jacket is admitted to the chamber through valve (14). Pressure and vacuum in the chamber are measured by a gauge (15). Air is admitted to the chamber through a valve (16) and an air filter (17). Air and steam are discharged from the chamber through valve (18) by means of the water-operated ejector pump (19) operated by tap (20).

C, a vertical gravity displacement autoclave. Steam is admitted fairly high up the sterilizer. The drain with the thermometer is as near the chamber as possible. There is a near- to-steam trap separated from the drains by a tundish, which prevents dirty water being sucked back up the waste pipe into the autoclave during a vacuum phase.

D, a ''near-to-steam trap' (valve) in the waste line remains open, until steam following the air heats the bellows under the diaphragm and closes the trap automatically. C, and D, kindly contributed by Dr Ronald Fallon.

STERILIZING WITH MOIST HEAT BOILING WATER Microbes are much more easily killed if instruments are clean, so make sure that everything is cleaned before it is sterilized. Remove instruments from boiling water with long- handled Cheatle's forceps which have been in saponated cresol (''Lysol') up to their handles. If you are not wearing sterile gloves, make sure you let the instruments dry. If you use them wet, bacteria from your hands may flow down from your fingers in drops of water.

PACKING [s7]ANY AUTOCLAVE Sterilization is impaired by anything which hinders the removal of air, so: (1) Arrange the contents loosely; a drum which can only be closed with difficulty is grossly overpacked. Place the contents so that air can readily be displaced downwards[md]the principles are the same in horizontal and vertical autoclaves. To avoid air pockets, interleave sheets of macintosh or jaconet with some permeable fabric, so that no two surfaces of the non-permeable material are in contact.

A SIMPLE AUTOCLAVE [s7](or pressure cooker) Make sure there is enough water in the bottom of the autoclave. Insert the drums to be sterilized, and turn on the heater. See that the discharge tap is open, and then screw down the lid. As the water boils the steam will rise and carry away the air in the autoclave.

CAUTION ! Let the air and the steam escape freely until there is no more air in the autoclave, usually about 10 minutes. To test this lead a rubber tube from the discharge tap into a bucket of water. When air no longer bubbles to the surface, there is no more air. After some trials you will learn how long to allow for this to happen.

Close the discharge tap. Let the temperature rise until it reaches 121[de]C. The safety valve will open and allow steam to escape.

Now start to measure the holding period and continue this for 15 minutes. Turn off the heater and allow the autoclave to cool, until the pressure gauge records zero pressure. Then open the discharge tap and allow air to enter the autoclave. Remove the load.

CAUTION ! If anything in the load has paper or cloth wrappings, don't allow them to touch anything unsterile, until they have dried, because microbes can penetrate wet paper.

JACKETED AUTOCLAVE Keep the jacket full of steam at 121[de]C all through the working day. Drain the chamber to remove any water that may gather in it.

Load the heated chamber, close the lid, and open valve (13).

STERILIZING. Open valve (14). When the temperature on thermometer (12) has reached the sterilizing temperature (usually 121[de]C), the holding time can start. Close valve (13). If it is letting much steam through, the temperature will not reach 121[de]C, until it is closed. So close it as soon as no further air and condensate come out of the chamber. If you still don't get the temperature you need (usually 121[de]C), open valve (13) for a minute or two and try again (a near-to-steam trap does this automatically). When the temperature has been reached, start timing.

CAUTION ! Don't infer the temperature from the reading of the pressure gauge. This may give you an inaccurate indication of its temperature and is a common cause of sterilization failure.

POSTVACUUM (drying). Open valve (20), then valve (18). Leave them open for 15 or 20 minutes. Close valve (18) then valve (20).

TO BREAK THE VACUUM. Open valve (16).

TESTING AUTOCLAVES If you are using Browne's tubes, put a tube in the centre of the load, with, if possible, one on the outside to show that the autoclave has indeed been switched on!

If you don't have Browne's tubes, put some dry earth in an envelope, autoclave this and then culture it in a bottle or tube of nutrient broth. Spores may be slow to grow, so incubate it for a week. If even this is impossible, some theatre sisters have been known to put an egg in the middle of a drum to see if it is hard boiled!

Fig. 2-7a PACKING AN AUTOCLAVE. A, the orientation of a load to facilitate the escape of air in a gravity displacement sterilizer. Steam enters from the top, flows downwards through the load and displaces the air in it. B, a properly packed glove container. C, a folded glove lined with gauze. D, a pair of gloves packed in a fabric envelope. E, a fabric envelope on edge to show its correct position during sterilization. F, a correctly packed drum with open ports positioned to allow air to be displaced by gravity. G, glove containers in an autoclave turned on edge so that steam can displace air through them. From the Lancet 1959;i:425[nd]435, with kind permission.

PARTICULAR PROCEDURES [s7]FOR AUTOCLAVING The following figures are guidelines only and vary with the type of autoclave and the size of the load. They apply to a sterilizing temperature of 121[de]C.

Empty glassware and unwrapped instruments. Sterilizing time 15 minutes, drying 10 minutes.

Wrapped instruments, rubber gloves, tubes and catheters, and sutures being reautoclaved. A common regime is 0.7 kgcm['2] (10 lb per square inch) for 20 minutes.

Fabrics and dressings. Sterilizing time 20 minutes, drying 15 minutes.

Liquids in flasks and bottles. Sterilize 100 ml bottles for 20 minutes, 300 ml bottles for 30 minutes, 500 ml bottles for 35 minutes, 1000 ml bottles for 40 minutes, and 3000 ml bottles for 50 minutes. Switch off the heat and let the autoclave cool down. Don't open it until the pressure is zero, or the bottles may burst. See also the Appendix A in Primary Anaesthesia.

PREVENTIVE MAINTENANCE Follow the maker's instructions carefully.

DIFFICULTIES [s7]WITH DOWNWARD DISPLACEMENT AUTOCLAVES If the TEMPERATURE FALLS below 121[de]C, while the pressure remains at 1 kgcm['2] (15lb), the outlet from the chamber may be blocked, and the chamber full of air. Check it daily.

If you work at HIGH ALTITUDES, for each 1000 feet you are above sea level, increase the time you immerse things in boiling water by 5 minutes, and increase the pressure of your autoclave by 0.03 kg/cm['2] (0.5 lb/inch['2]). Water above 80[de]C will kill all vegetative organisms and viruses; boiling water is still effective at 12,,,,000 feet.

If dressings are WET after autoclaving, the steam is probably wet, due to: (1) inadequate lagging of the steam supply pipe, or (2) inadequate tapping of condensate.

If you have reason to suspect IMPERFECT STERILIZATION, run the tests above. Also check that: (1) The drums are packed properly. (2) The correct temperature and sterilizing times are used. (3) The chamber drain is not blocked. (4) The drums are not being recontaminated after sterilization.