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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
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.
Filling the chamber
The chamber is filled with steam by one of two physical processes,
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
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
Drying is either a very quick or a very slow process, depending upon
whether the system in operation includes a high vacuum.
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.
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
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
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
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
than other soft goods and that they need to be given additional holding
time. (See also under " Drums " below.) Paper sheets are a convenient
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.)
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
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,
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
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.
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
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.
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
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.
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,
(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.
(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.