Note: Descriptions are shown in the official language in which they were submitted.
CA 02298165 2000-02-11
METHOD FOR STERILIZING AN ENDOSCOPE
Field of the Invention
The invention relates to a method of sterilizing endoscopes and in particular
hollow
endoscopes.
Background of the Invention
The development of endoscopic instruments has greatly advanced the ability of
the
medical profession to diagnose and treat diseases in relatively inaccessible
regions of the
body. The first examinations that could be considered "endoscopic" in the
modern sense
were probably the rectal inspections conducted in the 18th century. The
physician peered
through a rigid tube inserted into the patient's rectum and relied on candles
or gas lamps to
illuminate the interior. By today's standards the physician saw very little;
however,
endoscopic examinations continued and endoscopes of various designs were
invented and
have revolutionized many medical procedures.
As is well known to one skilled in the art, endoscope tubes often include
mechanisms for turning the tip in four directions, up, down and from side to
side, to
facilitate passage of the instrument around angles and allow visualization of
all surfaces.
An additional viewing channel, coupled to a separate eye piece, for
simultaneous direct
viewing by a second observer is also available. In addition, the tubes contain
channels for
air insufflation and water instillation, so that lenses can be cleaned during
a procedure, and
to allow passage of biopsy instruments and fulguration instruments. Tube
channels may
also be provided for passage of light from a laser for ablation/photo dynamic
therapy,
spray catheters, polypectomy snare wires etc.
A typical endoscope, as found in a physician's office or hospital, is used
repeatedly.
Thus, it is imperative that endoscopic tubes must be completely sterilized
between each
use to avoid the transmission of diseases, such as AIDS, Hepatitis, etc.
Typically, a
sterilization fluid is passed through the water and air ducts of the
instrument to sterilize the
internal surfaces. This sterilization fluid is at times supplied from a bottle
which
temporarily replaces the water bottle used with the endoscope. As is evident
to one skilled
in the art, conduits (internal surfaces) within the endoscope are the most
difficult parts of
the endoscope to sterilize. Endoscopic materials are also often heat sensitive
and do not
lend themselves to heat sterilization techniques
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In light of the ubiquitous use of the endoscope by the medical profession and
intricate internal surfaces of the endoscope, an efficient method for
sterilizing the
endoscope at ambient temperatures is required.
United States Patent 4,862,872 issued to Yabe et al. on September 5, 1989
discloses an endoscope and a washing apparatus for an endoscope. The endoscope
comprises an elongate insertable part having an observation window and
illuminating
window in the tip part, an observation means for observing an object by
receiving a
returning light from the object which enters through the observing window, an
illuminating light output means emitting an illuminating light from the
illuminating
window and a memorizing means capable of memorizing the information on
washing.
The washing apparatus is provided with a read-out means for reading out the
information
memorized by the memorizing means of the above mentioned endoscope and a
control
means controlling the conditions of washing the above mentioned endoscope by
the
information read out by this read-out means. This endoscope washing apparatus
is
relatively complex and likely expensive to manufacture.
United States Patent 5,297,537 issued to Savitt et al. on March 29, 1994
discloses a
disposable liquid supply kit for use with an endoscope that comprises a closed
liquid
container for connection to an endoscope prior to use. With a disposable unit
of this type,
a fresh supply of sterile water is installed in the endoscope between each use
with a
patient. The liquid supply system is preferably sealed at the factory to
insure complete
sterilization. Obviously, a medical facility using such a kit requires
inventory and incurs
expense as a result of maintaining and monitoring the inventory.
United States Patent 5,534,221 issued to Hillebrenner et al. on July 9, 1996
entitled
"Device and system for sterilizing objects" discloses a hollow cassette for
holding an item
to be sterilized, for example an endoscope. The cassette is a sealable
cassette in which an
endoscope or other medical device is placed. The cassette has input and output
fluid
sealing ports for the introduction and removal of a sterilizing fluid. The
cannula of the
endoscope is coupled either to the input or output port. The cassette is
formed of two
identical halves which are placed in superimposed sealable relationship with
each other to
form a hollow chamber. A latch is placed on one or more handles on the
cassette to create
a presealing condition to allow a vacuum to be introduced at the outlet port.
The cassette is
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then placed in an outer oven-like container or warming chamber where the
temperature is
properly maintained. Connections are made to open the input and output ports
on the
cassette such that the sterilizing agent may be introduced through a first
port to bathe the
outside of the medical endoscope, while one end of the endoscope is coupled to
the output
port where a vacuum is supplied external to the cassette to pull the
sterilization agent into
the cassette and through the interior passageways of the endoscope. When the
sterilization
process is completed, the warming chamber is opened and the sterilizing
cassette is simply
removed from the chamber with the input and output ports being uncoupled from
their
respective sources. A tight seal is maintained and the object remains in the
sterilized
interior of the cassette until the cassette is opened or the device is to be
used. This
represents a sterilization system which is relatively costly to produce and
use.
It would be advantageous to provide a method for endoscopic sterilization that
is
inexpensive and operatively simple. It would also be advantageous to provide a
method
that can be used with a sterilizing chamber that is used to sterilize many
different articles
and not just for endoscopic sterilization per se. It has now been found that
by applying
principles of pressure equilibration it is possible for an endoscope's
conduits) and
external surfaces to be sterilized.
Conventional sterilization procedures for medical instruments involve high
temperature (such as steam and dry heat units) or toxic chemicals (such as
ethylene oxide
gas, Et0). Steam pressure sterilization has been the time-honoured method of
sterilization. It is fast and cost effective. However, the autoclave destroys
heat-sensitive
instruments such as arthroscopes and endoscopes.
Ethylene oxide sterilization is used to cold-sterilize heat-sensitive
instruments and
can be used as a sterilization gas. However, it has been deemed by national
health and
safety organizations to be carcinogenic and neurotoxic, and requires long
sterilization and
aeration periods.
Ozone is a more efficient, safer, and less expensive sterilization agent and
is easily
generated from oxygen, preferably hospital grade oxygen. Ozone is widely used
in
industry as an oxidising agent to bleach paper pulp, treat drinking water, and
sterilize
sewage water and food products. Ozone generally acts on chemical compounds in
two
ways. Either by direct reaction or through hydroxyl radical species formed
during the
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decomposition of ozone (Encyclopaedia Of Chemical Technology, Vol. 17, Ozone
page 953 to 964). The amounts (concentrations) of ozone required in the
sterilization
gas for water purification are low, generally less than 36 mg /1 (milligram
per litre).
Significantly higher concentrations are required to make ozone gas an
effective sterilant
of micro-organisms, and those high concentrations of ozone gas are generally
combined
with critical levels of humidity during the entire sterilization cycle to
improve
sterilization efficiency. The activity of ozone increases rapidly with
increased relative
humidity. The resistance of spores to ozone varies from strain to strain, but
the
differences become comparatively small at high relative humidity (Ishizaki et
al., 1986.
Inactivation of the Silas spores by gaseous ozone, J. Appl. Bacterial, 60:67-
72), a high
relative humidity is required for the ozone to penetrate the protective shells
of micro-
organisms. The presence of water often accelerates ozone reactions with
organic
substances (Langlais et al., (EDS), 1991, Ozone in Water Treatment,
Application and
Engineering. Louis Publishers: Chelsea, Michigan, 569 pages). Thus, ozone
containing
gas can also be used as a sterilizing agent for endoscopes, especially when
combined
with high humidity.
Water evaporates at 100 °C at atmospheric pressure (1013 mbar). Thus,
various
prior patents (see Faddis et al., U.S. Patents No. 5,266,275; 5,334,355; and
5,334,622)
teach sterilization systems wherein water is heated to above the boiling point
(100 °C at
1013 mbar) to evaporate the water for injection into the ozone-containing gas
produced
by an ozone generator. The steam is heated to 120 °C prior to injection
into the ozone-
containing gas. However, since the decomposition of ozone increases
exponentially
with temperature in the range of 20 to 300 °C, injecting the water
vapour at a
temperature of about 120 °C leads to premature ozone decomposition. A
more efficient
and effective sterilization apparatus for the sterilization of ozone at a
relative humidity
above at least 95% is disclosed in Canadian Patent Application Serial No.
2,270,512.
Summary of the Invention
It is an object of the present invention to provide a method for the
sterilization
of a hollow endoscope.
It is another object of the invention to provide a sterilization method for an
endoscope in which the sterilization is carned out with a sterilizing gas, for
example an
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ozone-containing gas, preferably humidified ozone-containing gas, having a
temperature
of 20 °C to 35 °C.
Endoscopes are available that do not have passages through which a fluid may
pass. Nevertheless, the majority of endoscopes have at least one passage
through the
endoscope. The present invention provides a method of sterilizing endoscopes
with
internal passages, i.e., hollow endoscopes.
As one skilled in the art will appreciate, it is possible to disassemble an
endoscope
into sections. Methods for sterilizing an "endoscope" as disclosed herein are
applicable
to both assembled endoscopes and sections thereof. Hence, the term "endoscope"
as
used herein is to be construed as encompassing hollow endoscope sections.
In accordance with the present invention there is provided a method for
sterilizing
within a sterilization chamber a hollow endoscope having an internal passage
having an
internal volume comprising the steps of:
(i) sealing the endoscope to a sealed vessel for fluid communication between
the passage and the vessel to form an endoscope-vessel combination, said
vessel having an internal volume;
(ii) disposing the endoscope-vessel combination within the sterilization
chamber;
(iii) supplying a sterilization gas;
(iv) effecting a pressure difference between the internal volume of the vessel
and the sterilizing chamber, the pressure in the sterilizing chamber and the
internal volume of the vessel selected such that sterilizing gas supplied to
the sterilizing chamber flows through the internal passage of the endoscope
into the vessel.
Preferably the end of the endoscope which is sealed in fluid communication
with
said vessel, protrudes into said vessel.
In accordance with a further preferred embodiment of the present invention
there
is provided a method for sterilizing within a sterilization chamber a hollow
endoscope
having a distal end, a proximal end and an internal passage having an internal
volume
comprising the steps of:
(i) providing a sterilization chamber;
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(ii) sealing the distal end of the endoscope in fluid communication with a
substantially closed vessel to form an endoscope-vessel combination, said
vessel having an internal volume larger than the internal volume of the
endoscope;
(iii) placing the endoscope-vessel combination into the sterilization chamber;
(iv) sealing the sterilization chamber;
(v) applying a vacuum of a preselected vacuum pressure to the sterilization
chamber;
(vi) supplying an amount of water vapour to the sterilization chamber for
humidifying the sterilization chamber;
(vii) supplying ozone to the sterilizing chamber at a higher pressure than
said
pre- selected vacuum pressure, said ozone being sufficient to sterilize the
endoscope and the internal passage of the endoscope; and
(viii) maintaining the endoscope, the internal volume of the endoscope and the
internal volume of the vessel in contact with the ozone for a preselected
treatment period to sterilize the endoscope
Brief Description of the Drawings
The invention will be described in more detail in the following by way of
example
only and with reference to the attached drawings, wherein:
Figure 1 shows an endoscope that is commonly known in the art;
Figure 2a shows a schematic cross-section through a preferred sterilization
arrangement in accordance with the invention;
Figure 2b shows a schematic cross-section through a preferred sterilization
arrangement in accordance with the invention;
Figure 3 shows a schematic illustration of a sterilization apparatus for use
in a
sterilization method in accordance with the invention; and
Figure 4 is a flow diagram of a preferred sterilisation method in accordance
with
the invention.
Detailed Description of the Invention
Typically, in the preferred embodiment of the method according to the present
invention a hollow endoscope is connected to a vessel, the endoscope-vessel
combination
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placed in a sterilization chamber and sterilizing gas is exchanged between the
chamber and
the vessel through the endoscope. The exchange is effected by varying the
pressure
between the vessel and the sterilization chamber. Thus when the pressure in
the
sterilization chamber containing sterilizing gas is greater than the pressure
in the vessel,
the sterilizing gas is caused to pass through the internal passage of the
endoscope into the
vessel, thus permitting an effective sterilization of the internal passage of
the endoscope.
The terms gas or gaseous as used herein are to be construed broadly and
include, for
example, airborne droplets, airborne micro-particles and the like. The term
air extends to
any suitable gas or mixture thereof, for example nitrogen, carbon dioxide etc.
The method makes possible an effective sterilization of a hollow endoscope by
selecting the volume of the vessel and the pressure difference between the
sterilization
chamber and the internal volume of the vessel, such that sterilizing gas
passes through any
internal passage in the endoscope. In a preferred embodiment, the pressure
differential is
between about half atmospheric pressure and a reduced pressure substantially
less than
atmospheric pressure. In this embodiment, the internal volume of the vessel
should be the
same as, or preferably larger than, the volume of the internal passage of the
endoscope.
The larger the difference in the volume between the vessel and the endoscope,
the greater
will be the volume of sterilizing gas passing through the internal passage
when the
pressure difference is effected. For example, in a further preferred
embodiment, the
sterilizing chamber is evacuated down to a pressure of 0.1 mbar and allowed to
stabilize so
that the pressure in the internal volume of the vessel will also be 0.1 mbar.
The chamber is
then filled with sterilizing gas and the pressure is allowed to rise to about
half atmospheric
pressure, thus creating a pressure difference between the internal volume of
the vessel and
the sterilizing chamber. In view of the pressure difference between the inside
of the vessel
and the chamber, a volume of sterilizing gas substantially equal to the
internal volume of
the vessel, will be drawn through the internal passage of the endoscope thus
flushing the
passage with the sterilizing gas.
However, the method may also be effected by using an overpressure of
sterilizing
gas. For example, if the pressure inside the sterilizing chamber is at
atmospheric pressure,
then the sterilizing gas may be introduced at such an overpressure that it
flows through the
internal passage of the endoscope into the vessel.
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If the endoscope has more than one internal passage, then the internal volume
of
the vessel should be at least as great as, and preferably greater than, the
combined volume
of all the internal passages.
It is also possible for the vessel to be connected to more than one endoscope
to
form a multiple endoscope-vessel combination. In this case, the internal
volume of the
vessel should be equal to, or preferably greater than, the combined volume of
all the
internal passages of each endoscope.
The vessel is attached to the endoscope to form a sealed vessel-endoscope
combination. Since the sterilization of the endoscope may be reduced or
ineffective at the
point of contact with the vessel, the vessel should be attached to the outside
of the
endoscope and preferably not too close to the end of the endoscope to ensure
sterilization
of the ends. Further, since endoscopes are usually designed with one end to
enter the
patient, the vessel is preferably attached to the end of the endoscope, which
was not
inserted into the patient.
Any suitable means of attachment may be used. Thus the vessel may have a neck
which is clamped to the endoscope or the neck may be made of elastomeric
material with
an aperture slightly smaller than the outside diameter of the endoscope so the
end of the
endoscope can be pushed into the aperture which will naturally grip and seal
with the
endoscope. In one suitable embodiment, the vessel is cylindrical, of diameter
several times
larger than the diameter of the endoscope and having a closed end. The other
end is
formed into an inverted conical shape with the apex of the cone having the
aperture and
being within the cylindrical body of the vessel. Thus the end of the endoscope
is guided
towards the aperture by the inclined shape of the sides of the cone.
In a preferred sterilization procedure, the treatment is repeated. The first
treatment
is referred to as a first half cycle and the second treatment as a second half
cycle or
completion. That is, the first half cycle may involve evacuation, then
introducing
sterilizing gas and allowing the pressure to rise to about half atmospheric
pressure. Then
the second half cycle would involve a repeat of those steps. That is the
chamber and
contents would again be evacuated, fresh sterilizing gas introduced and then
allowing the
pressure to rise to about half atmospheric pressure. Further such repetitions
may be
effected.
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A function of the pressure differential between the internal volume of the
vessel
and the sterilization chamber is to ensure that sterilizing gas will flow into
the internal
passage of the endoscope. Any pressure differential may be used. However, for
economy
and convenience it is preferred to work between a higher pressure of about 500
to 525
mbar (about half atmospheric pressure) and a vacuum of from about 0.1 mbar to
about 10
mbar, preferably 0.5 mbar to about 2 mbar. Thus it is preferred to operate at
a pressure
differential between the reduced pressure and the increased pressure of about
half an
atmo sphere.
The vessel may be equipped with means to indicate the pressure within its
internal
volume or means to indicate the degree of sterilization. If such means
includes visual
indication, then a wall of the vessel, or a portion thereof, may be
transparent , so that the
means may be observed from outside the vessel. The means may also be connected
to a
remote monitoring device so that the pressure or sterilization could be
monitored outside
the sterilization chamber.
If ozone is used as the sterilizing gas, since ozone decomposes more rapidly
at
higher temperatures, it is preferred that the ozone would be at a temperature
in the range of
20 °C to 35 °C, more preferably 20 °C to 30 °C and
particularly at around room
temperature. To ensure effective sterilization, the ozone should also be
adequately
humidified.
The invention will be described with reference to an apparatus for generating
ozone as the sterlizing gas. However, it should be understood that any
sterilizing gas may
be used.
Referring to Figure 1, the prior art endoscope 210 illustrated has a control
section
212 with the air/water valve 214 which, when its valve cover, not shown, is
depressed,
activates water feeding to a distal end 222 of an insertion tube 220. A
suction valve 216,
when its valve cover is depressed, activates suction from the distal end 222
of tube 220
and carbon dioxide, CO2, gas valve 218, when depressed, connects lumens in the
control
section that insufflate noncombustible gas into the body cavity. The insertion
tube 220 is
inserted in the body cavity and the operator, using the control section 212,
controls the
flow of air, water, suction and gas to and from the body cavity. A universal
cord 224
couples the control section 212 to a light guide connector section 225 that
has a light guide
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226 to be connected to a light source, an air pipe 228 and a water container
connector 230
also has a suction connector 232 and other connectors 236 and 238 for such
functions as a
gas tube connector and a vent connector at the proximal end 229 of the
endoscope 210.
The endoscope 210 illustrated is of the flexible type however many different
types of
endoscope are available depending upon medical requirements, for example
endoscopes
with rigid endoscopic tubes. The present invention is not limited to any
particular type of
endoscope and the endoscope 210 may be regarded as a complex conduit(s).
Refernng to Figure 2a, the endoscope sterilization method and arrangement
according to one embodiment of the invention will be described in more detail.
The
hollow endoscope is sealed to an endoscope 210 as shown in a simplified
fashion for ease
of description, i.e., a conduit. The endoscope 210 is sealed in fluid
communication with a
vessel 235 forming an endoscope-vessel combination that is held within a
sealed
sterilisation chamber 10. Pressure within the sterilisation chamber and the
vessel are
indicated by Psc and Pv respectively. The vessel 235 has an opening that is
provided with
a seal allowing ready attachment and detachment of the endoscope. In
operation, pressure
within the sterilisation chamber is reduced, i.e., Psc < Pv. During the
reduction in
pressure, gas is drawn from the vessel 235 via the endoscope 210 conduit into
the
sterilising chamber 10 as indicated by an arrow until pressures within the
sterilising
chamber 10 and the vessel 235 are equilibrated, i.e., Psc = Pv.
Referring to Figure 2b, after the reduction in the pressure of the sterilising
chamber
a gaseous sterilising mixture is provided, not shown, to the sterilisation
chamber 10.
The provision of the gaseous sterilising mixture to the sterilising chamber 10
results in an
increase in the pressure within the sterilising chamber 10, Psc > Pv, and the
gaseous
sterilising mixture is drawn into the vessel 235 via the conduit(s)of the
endoscope until
pressures within the sterilising chamber 10 and the vessel 235 are
equilibrated, i.e., Psc =
Pv. Thus by effecting a pressure difference between the internal volume of the
vessel and
the sterilizing chamber, sterilizing gas is caused to flow through the
internal passage of the
endoscope 210. In this manner inner surfaces of the endoscope 210 have been
subjected to
sterilising conditions. Detailed above is a single sterilisation cycle and the
number of
cycles to which the endoscope 210 is subjected is varied according to
requirements and
parameters, namely type of sterilising gas, length of time of sterilising
cycle, concentration
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of sterilising gas, type of endoscopic contamination present etc. As will be
evident to one
skilled in the art, it is possible to vary the order of pressure reduction or
pressure increase
within the chamber to achieve the same result. It is possible for the
sterilising chamber to
be charged at a positive pressure driving a sterilising gas from the
sterilising chamber into
the vessel 235 via the endoscope conduit(s). Nevertheless, a pressure reducing
step is
preferred over that of a step in which pressure within the sterilising chamber
10 is
increased in excess of atmospheric pressure.
The sealing means 240 of the vessel 235 is not at the essence of the invention
and
many means of doing so will be evident to one skilled in the art. In one
embodiment the
vessel is equipped with an elastomeric septum, for example a SubrasealTM,
having a pre-
bored hole of a smaller diameter than the distal end 222 of the endoscope 210
that covers
the opening of the vessel 235. The distal end 222 of the endoscope 210 is
inserted into the
pre-bored hole such that the endoscope 210 is sealed in fluid communication
with the
vessel 235. Alternatively, the vessel is equipped with a screw top having a
flexible gasket
washer through which the distal end 222 of the insertion tube 220 is inserted
and the screw
top tightened such that the endoscope 210 is sealed in fluid communication
with the vessel
235. The distal end 222 is one end of the endoscope and as one skilled in the
art will
recognize it is possible to seal either end of the conduits) in fluid
communication with the
vessel 235 for carrying out the method of the present invention.
Referring again to Figure l, sealing the distal end 222 in fluid communication
with
the vessel 235, to form an endoscope-vessel combination, provides an easier
method of
sterilising a plurality of conduits within the endoscope 210. This is because
sterilising gas
passes through the air pipe 228, the water container connector 230, the
suction connector
232 and the other connectors 236 and 238 at the proximal end 229 of the
endoscope 210.
Nevertheless, it is within the scope of the present invention to attach a
series of pipes to
the air pipe 228, the water container connector 230, the suction connector 232
and the
other connectors 236 and 238 respectively; the pipes being sealed in fluid
connection with
the vessel 235. Obviously, all supply and evacuation conduits leading to the
unattached
end 222 or preferably sealed in fluid connection with the vessel 235. It is
also within the
scope of the invention for different connectors at the proximal end to be
sealed in fluid
connection with more than one vessel 235.
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The vessel 235 needs to be strong enough to withstand the pressures applied
during
the present method of endoscopic sterilisation and maintain the required
volume to effect
sterilization. The vessel 235 is made of any suitable material, for example a
rigid material
such as glass, or flexible or elastomeric materials such as PTFETM
(polytetrafluoroethylene)
and polyethylene.
In a preferred embodiment the method of endoscope sterilisation described
above is
carried out in connection with a sterilising apparatus as described in
Canadian Patent
Application Serial No. 2,270,512 entitled "Method and Apparatus for Ozone
Sterilization".
However, as one skilled in the art will appreciate, the method of endoscope
sterilisation
described above is applicable to many types of sterilisation chambers in which
the pressure
is variable. A suitable sterilisation chamber, is for example, a SterradTM
unit made by
Johnson & JohnsonTM. Other sterlization units are available from TS03TM.
An ozone sterilizer for use with the invention as illustrated schematically in
Figure
3 operates in a relatively simple manner. Medical quality oxygen is subjected
in an ozone-
generating unit 20 to an electrical field, which converts the oxygen into
ozone. The ozone
is then fed into a humidified sterilization chamber 10 where it sterilises
medical devices.
The ozone is subsequently reverted into oxygen using an ozone converting unit
52
containing an ozone converting catalyst. The only residues left at the end of
the sterilization
cycle are oxygen and clean water vapour.
Single cycle sterilization with ozone is more efficient, it provides for a
shorter
sterilization cycle than with Et0 and requires few changes in user habits.
Moreover, the
ozone-based process in accordance with the invention is compatible for use
with current
packaging, such as sterile pouches and rigid containers.
An ozone sterilization method requires substantially no aeration or cooling
down of
sterilized instruments so that they can be used immediately following the
sterilization
cycle. This allows hospitals to reduce the cost of maintaining expensive
medical device
inventories. An ozone sterilization method offers several further advantages.
It produces no
toxic waste, does not require the handling of dangerous gas cylinders, and
poses no threat
to the environment or the user's health. Stainless-steel instruments and heat-
sensitive
instruments can be treated simultaneously, which for some users will obviate
the
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need for two separate sterilizers.
A preferred sterilization apparatus suitable for use in accordance with the
invention
is illustrated schematically in Figure 3. The apparatus includes a
sterilization chamber 10
which can be sealed to contain a vacuum. This is achieved with an access door
12, which
can be selectively opened for access into the chamber and which seals the
chamber in the
closed condition. The apparatus further includes an ozone generating unit 20
for
supplying ozone-containing gas to the sterilization chamber, a humidifier
arrangement 30
for supplying water vapour to the sterilization chamber, and a vacuum pump 40
(Trivac~,
model D25BCS PFPE, manufacturer Leybold). The vacuum pump 40 is used for the
application of a sufficient vacuum to the sterilization chamber 10 to increase
the
penetration of the sterilizing gas and to be able to generate water vapour at
a temperature
below the temperature inside the sterilization chamber. The vacuum pump 40 in
the
preferred embodiment is capable of producing a sufficient vacuum in the
sterilization
chamber to lower the boiling point of water in the chamber below the
temperature in the
chamber. In the preferred apparatus, the vacuum pump is capable of producing a
vacuum
of 0.1 mbar. Ozone produced in the ozone-generating unit 20 is destroyed by
the ozone
converting catalyst in ozone converting unit 52 to which ozone-containing gas
is fed either
after passage through the sterilization chamber 10 or directly from the ozone-
generating
unit 20 through valve 29b (optional). An example of an ozone converting
catalyst is
DEST 25, manufacturer TS03. The ozone converting unit 52 is connected in
series after
the vacuum pump 40 to prevent ozone gas escaping to ambient. The ozone
decomposing
material in the preferred catalyst is carulite. For economic and practical
reasons, it is
preferred to use a catalyst for decomposition of the ozone in the
sterilization gas exhausted
from the sterilization chamber 10. The catalyst destroys ozone on contact and
reverts it
back into oxygen with a certain amount of heat being produced. Catalysts of
this type and
their manufacture are well known to a person skilled in the art of ozone
generators and
need not be described in detail herein. Furthermore, other means for
destroying the ozone
contained in the sterilization gas will be readily apparent to a person
skilled in the art. For
example, the gas can be heated for a preselected time to a temperature at
which the ozone
decomposition is accelerated, for example, to 300 °C.
The humidifier arrangement 30 includes a humidifier chamber 32 ( HUM 0.5,
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manufacturer TS03) sealed to ambient and connected to the sterilization
chamber 10
through a conduit and a vapour intake valve 34. The humidifier chamber 32 is
equipped
with a level control to ensure a sufficiently high water level, not shown.
Water is directly
supplied to the humidifier chamber 32 from a drinking or purified water supply
connection. Water is supplied to the humidifier chamber 32 by way of a filter
33, a
pressure regulator 35, and input valve 36. The water vapour produced in the
humidifier
chamber 32 enters the sterilization chamber 10 by way of a vapour intake valve
34.
The ozone-generating unit 20 includes a pair of ozone generators 22 (OZ, model
14a, manufacturer TS03) of the corona discharge type, which are cooled to
decrease the
ozone decomposition rate, which is well known in the art. To achieve a good
lethality rate
in an ozone sterilization process, the amount of ozone supplied to the
sterilization chamber
should be sufficient to obtain a concentration of 48 to 96 milligram per litre
preferably 60
to 72 milligram per litre. At these concentrations, the ozone generation is
associated with
a relatively high energy loss in the form of heat. Generally, about 95% of the
supplied
electrical energy is converted into heat and only 5% is used to produce ozone.
Since heat
accelerates the inverse transformation of ozone into oxygen, it must be
removed as quickly
as possible by cooling the ozone generators 22. The ozone generators in the
above
summarized apparatus are kept at the relatively low temperature of 3 to 6
°C by cooling
system 60 which may be an indirect system with cooling water recirculation, or
a direct
cooling system with a refrigeration unit. The cooling system is preferably
kept at the
temperature of 3 to 6 °C. In the preferred embodiment, the cooling
system is kept at 4 °C
so that the ozone-containing gas generated by unit 20 is at the ambient
temperature around
20 to 35 °C. Thus, the ozone-containing gas entering into the
sterilization chamber for
humidification and sterilization is kept at ambient temperatures of 20 to 35
°C. This
means that ozone decomposition is kept to a minimum and that the sterilization
process is
more efficient. This, provides a significant advantage over the apparatus of
the prior art,
since the temperature and pressure are maintained low throughout the
sterilization cycle.
The ozone-generating unit is preferably supplied with medical quality oxygen.
The
apparatus can be connected to a wall oxygen outlet common in hospitals or to
an oxygen
cylinder or to any other source capable of supplying the required quality and
flow. The
supply of oxygen to the generators 22 takes place across a filter 23, a
pressure regulator
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24, a flow meter 25 and an oxygen shut off valve 26. The generators are
protected against
oxygen overpressure by a safety pressure switch 27. The ozone-oxygen mixture
generated
by the generators 22 is directed to the sterilization chamber 10 by a
regulator valve 28 and
a mixture supply solenoid valve 29a. The mixture can also be directly supplied
to the
ozone converting unit 52 by way of a bypass solenoid valve 29b (optional). In
the
preferred embodiment which includes a sterilization chamber of 125 liters
volume, the
pressure regulator 24 preferably controls the oxygen input at a flow rate of
between about
1.5 and 2 litres per minute. However, it will be readily apparent to the
skilled person that
other flow rates may be used depending on the make and model of the ozone
generators 22
and the size of the sterilization chamber.
Operation
The preferred sterilization method includes the following general steps as
illustrated by the flow chart of Figure 4. Step 1 is to load the sterilization
chamber.
Medical instruments including endoscopes to be sterilized are sealed in
sterile packaging
containers or pouches such as generally used in the hospital environment and
then placed
into the sterilization chamber. Steps 2 and 3 are sealing the sterilization
chamber and
applying a vacuum. The door of the sterilization chamber is closed and locked
and the
preconditioning phase is started by applying a vacuum to the sterilization
chamber. In step
4 water vapour is admitted into the sterilization chamber to humidify the
chamber
contents. In steps 5 and 6 a mixture of ozone and oxygen is supplied to the
chamber and
the chamber maintained sealed for a preselected treatment period. In step 7
the vacuum
application and ozone supply steps are preferably repeated at least once. To
remove all
remaining ozone in the sterilization chamber 10 when the sterilization cycle
is completed,
a ventilation phase begins with step 8 by applying a vacuum and flushing the
chamber
with oxygen in step 9. As shown in step 10, the vacuum and flushing steps 8
and 9 are
repeated twice for a total of three oxygen flushes. The oxygen is allowed to
fill the
chamber 10 to atmospheric pressure each time. After the ventilation phase, the
door is
unlocked and the sterilized material can be taken out of the chamber.
Before the sterilization cycle begins, the humidifier chamber 32 is filled
with water
to an adequate level, which is sufficient to satisfy the requirements for the
whole
sterilization cycle. This is done by temporarily opening the water-input valve
36. Valve
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36 remains closed for the whole remainder of the sterilization cycle. In the
first phase of
the sterilization cycle, oxygen intake valve 18, oxygen shut-off valve 26,
mixture supply
valve 29a, and mixture bypass valve 29b are closed and vapour intake valve 34,
chamber
drainage valve 44, and bypass valve 54 are opened. The sterilization chamber
10 is
evacuated to a vacuum pressure of about 0.1 mbar. Water vapour inlet valve 34
closes
when the absolute pressure in the sterilization chamber falls below 60 mbar.
Once a
pressure of about 1.0 mbar is achieved, the chamber drainage valve 44 closes
and the
vapour intake valve 34 opens to lower the pressure in the humidifier chamber
32 to the
vacuum pressure in the sterilization chamber. That forces the water in the
humidifier
chamber to evaporate and to enter the sterilization chamber 10. Shortly before
the end of
the humidification period, usually about 2 to 6 minutes, the ozone generators
are activated.
The flow of the oxygen/ozone mixture exiting the ozone generator is controlled
at all times
by regulator valve 28 capable of resisting the vacuum and of adjusting the
flow to between
about 1.5 and 2 litres per minute. As an optional feature, the generators can
be started at
the same time as the humidification period begins. This is then achieved with
shut-off
valve 26 and mixture bypass valve 29b. Shut-off valve 26 opens to let oxygen
enter the
generators. The ozone-oxygen mixture produced by the generators is then guided
directly
into the ozone converting unit 50 through mixture bypass valve 29b. After a
humidification period of approximately 30 minutes, the oxygen-ozone mixture is
guided
into the sterilization chamber by opening the mixture supply valve 29a and
closing the
mixture bypass valve 29b. The oxygen-ozone mixture enters the chamber 10 until
an
ozone concentration of 85 milligram per litre in the chamber is achieved. The
time
required for this step is dependent on the flow rate and concentration of the
ozone gas in
the mixture (preferably 10 % to 12 % by weight). At this point in time, the
mixture supply
valve 29a is closed to seal off the sterilization chamber and to maintain the
humidified
ozone/oxygen gas mixture in the chamber under vacuum.
Once the sterilization chamber is filled with the sterilization gas, mixture
of
oxygen and ozone gas, the generators 22 are stopped, the oxygen shut-off valve
26 is
closed, and the ozone is maintained in contact with the articles to be
sterilized for about 15
minutes, for a sterilization chamber of a volume of 125 liters (4 cubic feet).
The length of
this sterilization period varies with the volume of the sterilization chamber.
At this stage,
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the sterilization chamber is still under the effect of a partial vacuum of
about 500 to 525
mbar. In an optional second step, the pressure level is raised to about 900
mbar using
oxygen as a filling gas. This pressure level is maintained for about 20 min.
After the
sterilization period, the vacuum is reapplied, preferably at a pressure of
about 0.1 mbar
again. Once the vacuum reaches 0.1 mbar, the humidification phase is
recommenced,
followed by the renewed injection of an oxygen/ozone sterilization gas
mixture, followed
by the sterilization period. The cycle of applying a vacuum of about 0.1 mbar,
injecting
sterilization gas, humidifying and sterilization period, can be repeated, and
the number of
repeat cycles, mini-cycles, selected to achieve complete sterilization of the
instruments.
The number of repeat cycles used in an experimental set-up of a method and
apparatus of
the type described above including a 125 liters (4 cubic foot) chamber was 2
repeat cycles.
This set-up conformed to the Security Assurance Level standards of the FDA
(SAL 10 -6).
To remove all remaining ozone and humidity in the sterilization chamber 10
after
complete sterilization a ventilation phase is engaged. The ventilation phase
begins after
the last sterilization period. The chamber drainage valve 44 opens and the
vacuum is
applied down to approximately 13 mbar. Vapour intake valve 34 closes when the
pressure
reaches 60 mbar to evacuate the remaining ozone in the humidifier. Once the
vacuum
pressure of 13 mbar is obtained, drainage valve 44 closes and the oxygen
intake valve 18
opens, admitting oxygen into the sterilization chamber 10. Once atmospheric
pressure is
reached, the oxygen intake valve 18 is closed, the sterilization chamber
drainage valve 44
opened, and vacuum reapplied until a pressure of 13 mbar is reached. The
ventilation
cycle is then repeated twice. Once the atmospheric pressure is reached after
the last cycle,
the door mechanism of the sterilization chamber is activated to permit access
to the
contents of the sterilization chamber. This ventilation phase has two
functions. First, to
remove all ozone residues in the sterilization chamber before opening the
access door and,
second, to dry the sterilized material by evaporation when the vacuum pressure
is applied.
The ozone-containing gas evacuated from the sterilization chamber 10 is passed
over the ozone decomposing catalyst of the ozone converting unit 52 prior to
exhausting
the gas to the atmosphere to ensure a complete decomposition of the ozone in
the
sterilization gas. The ozone converting unit 52 is used during only two
portions of the
sterilization cycle, the activation of the generators 22, with optional valves
26 and 29b,
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and the evacuation of the sterilization chamber 10. During the start up phase
of the
generators 22, the mixture bypass valve 29b is opened and the ozone is guided
across the
catalyst 52. Once the start-up phase of the generators 22 is complete, the
bypass valve 29b
closes. During evacuation of the sterilization chamber 10, the sterilization
chamber
drainage valve 44 is opened and the ozone containing sterilization waste gas
guided to the
catalyst 52. Once the evacuation of the sterilization chamber 10 is completed,
the drainage
valve 44 is closed. The circulation of ozone is ensured by the vacuum pump 40,
which
operates during the whole sterilization cycle including all repeat cycles.
Oxygen/ozone-containing sterilization gas is injected into the humidified
sterilization chamber at ambient temperature. The ozone-containing gas is not
heated. For
optimum operation of a sterilizer having a 125 liter chamber, a system is
preferably used
which is capable of generating an ozone flow of between about 1.5 and 2 litres
per minute
containing about 85 mg/1 of ozone to obtain at least at total of 9000 mg of
ozone for each
of the fillings of the sterilization chamber.
In another preferred process, humidification of the sterilization chamber is
carried
out by a pair of atomizers. The water is supplied to each of the atomizers
from a water
tank hooked up to the drinking water supply or a purified water supply. Ozone
is supplied
to the atomizers from an ozone accumulation tank. The atomizers are made of
ozone
oxidation resistant material, and are installed directly in the sterilization
chamber. When
the vacuum level is reached in the sterilization chamber, the atomizers
release water and
ozone. The ozone is moistened inside the atomizer. The ozone/atomized water
mixture
penetrates the sterilization chamber. Injecting the water into the
sterilization chamber
under vacuum has the immediate effect of evaporating the water. The
sterilization
chamber operating temperature is 20 to 35 °C, a temperature at which
water evaporates at
pressures of 23.3 to 56.3 mbar. Thus, the water becomes vapour due to the
vacuum
created by the vacuum pump. The resulting ozone/water vapour mixture
penetrates the
material to be sterilized.
Changes and modifications in the specifically described embodiments can be
carned out without departing from the scope of the invention which is intended
to be
limited only by the scope of the appended claims.
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