Note: Descriptions are shown in the official language in which they were submitted.
~.Z~ 35
DISPOSABLE _REVACUUM STEAM STERILIZER TEST DEVICE
This invention relates generally to the sterilization
of medical and surgical products and more specifically to a dis-
posable test pack adapted to be processed through a conventional
sterilization cycle in a prevacuum steam sterilizer to check the
proper operation of the sterilizer.
In the sterilization of medical dressigns and instru-
ments by steam, such dressigns or instruments are conventionally
assembled in bundles or packs and placed in a steam sterilization
chamber which is sequenced through an appropriate sterilization
cycle. Individual packages may contain stacks of towels, dres-
sings, sponges and similar materials or may contain desired com-
pliments of surgical instruments or eqùipment for specific sur-
gical procedures.
A current method of steam sterilization practice involves
the placement of such packs in a sterilizer, the evacuation of
air from the sterilizer and the introduction of saturated steam
at a desired pressure to produce a desired temperature for a
selected period of time. Prevacuum sterilizers operating in this
manner have frequently replaced the downward displacement or
gravity air discharge sterilizers previously used The significant
advantage of the prevacuum method is that removal of air before
introduction of steam permits the rapid penetration of steam
throughout the surgical pack. Elimination of air is necessary in
all steam sterilizers since air trapped in the packages would
prevent sterilization of the portion of the packs interior where
it collected. With the prevacuum method, the time required for
steam penetration in a typical sterilization cycle is greatly
reduced and prevacuum sterilizers currently operate on a rela-
tively short cycle (with an exposure time in the order of 3-4
minutes at 134C.). The operation of such sterilizers is well
described in, John J. Perkins, Principles and Methods of Sterilization,
3~3S
in the Health Sciences published by Charles C. Thomas, Springfield,
Illinois, Chapter VI, "Prevacuum High Temperature Sterilization."
The ability to sterilize instruments in a shortened
time, however, is dependent on the assumption that air is prop-
erly evacuated from the sterilizer. This includes the expec-
tation that the vacuum system is functioning properly to evacuate
the chamber initially and that there are no air leaks in the
sterilizer or the vacuum lines which would permit introduction of
air after the vacuum is drawn. During routine use, wear on
certain sterilizer parts will eventually result in air leaks and
there is a need to test the sterilizer on a periodic (daily)
basis to ascertain that the vacuum system is functioning properly.
In 1961, a test procedure was proposed by J. Dick et
al. and discribed by J. H. Bowie, et al. of the Department of
Microbiology, Royal Infirmary, Edenborugh, Scotland in an article
appearing in The Lancet, March 16, 1963, pp. 58~-587, which sug-
gested a protocol for determining that the sterilizer was in
proper working order, and that the vacuum system was operating
properly. As indicated in the article, residual air in the sys-
tem at the time steam is introduced will be swept by the steam
pressure into the pack, usually to the pack center. Trapped air
in the pack inhibits proper steam penetration.
According to the Bowie and Dick protocol, sterilization
indicating tape was used in combination with a stack of surgical
towels to test the working order of the sterilizer on a daily
basis. Sterilizer indicating tape is an adhesive tape having
printed on its top surface stripes of a sterilization indicator
material which has the property of changing coloe, for example
from white to black, upon exposure to steam at an appropriate
temperature for an appropriate period of time. According to the
Bowie and Dick protocol, such tape was placed on a fabric sheet
in a cross configuration and the fabric sheet was placed within
Z~ 9~
a stack of folded surgical towels. Thç entire assembly was
placed within the sterilizer. The sterilizer was run through its
usual cycle with an exposure time of three and one-half minutes
at 133-134C, after which the tape cross was examined to deter-
mine whether steam had completely penetrated the towel stacks
effectively. A uniform color change was indication of a pass and
the presence of incompletely changed indicator color was a failure.
This protocol is currently in widespread use and is
described in the Association For the Advancement Of Medical In-
strumentation ~AAMI) Recommended Practice: Good Hospital Prac-
tice for Steam Sterilization and Sterility Assurance published by
the AAMI, 1901 North Ft. Myer Drive, Suite 602, Arlington, Vir-
ginia 22209. As indicated in Paragraph 7.8 of that publication
entitled "Prevacuum Sterilizer Residual Air Test" the test in-
volves the use of 100% cotton huckaback towels, freshly laundered
but not ironed (in view of the fact that excess dryness may affect
the test results) folded in a 9" x 12" configuration and piled
10" - 11" high. The details of the Bowie and Dick procedure are
described in the above-referenced Recommended Practice.
The testing of prevacuum sterilizers according to the
Bowie and Dick protocol involves a number of important short-
comings. Firstly, the test is subject to individual execution by
the sterilization section of the hospital on a daily basis and
the various requirements of the Bowie and Dick protocol, namely
the type of towels or other fabrics used, their condition, age
and the like (all of which affect the significance of the test
result) may very widely from day to day and from institution to
institution. Secondly, the performance of the Bowie and Dick
protocol is relatively inconvenient and costly in that the expense
of laundering towels (which cannot be thereafter used wi~-hout
relaundering) assembling the test arrangement and the like in-
volve costly hospital labor. Additionally, certain hospitals
~.23~ 5
have elected to eliminate laundry facilities entirely, utilizing
only single use disposable fabrics for their procedures making
the proper conduct of the Bowie and Dick protocol more incon-
venient.
It is an object of the present invention to provide a
test pack for use in prevacuum steam sterilizers to determine
whether the sterilizer is functioning in accordance with proper
standards. A related object of the invention is to provide a
test pack which suitably simulates air evacuation and steam
penetration conditions of the conventional pack described in the
Bowie and Dick protocol so as to define an appropriate challenge
for a residual air test in a prevacuum sterilizer.
A further object of the invention is to create a test
pack for prevacuum sterilizers which provides a repeatable and
consistent standard for testing the working order of the
sterilizer from day to day and sterilizer to sterilizer. It is a
further object of the invention to provide such a test pack which
is inexpensive and disposable after a single use, and does not
require assembly by hospital personnel, nor the use of hospital
linen or laundries.
In accomplishing these and other objects in accordance
with the present invention, a disposable test pack is composed of
a sterilization test sheet having defined areas adapted to change
color in response to the presence of steam under selected
exposure conditions, surrounded by a set of disposable nonwoven
porous sheets of material arranged in overlying relation above
and below the test sheet. The innermost sheets of material form
an inner core region around the test sheet and the remaining
sheets form an outer shell region, with the porosity and bulk of
the inner core region and the porosity of the outer shell region
being selected so as to define a desirable challenge to the
sterilizer.
3.~
The test pack is designed to be used according to the
accepted Bowie and Dick protocol and to be placed in an otherwise
empty sterilization chamber, sequenced through a predetermined
cycle and removed. The pack is then opened and the interior test
sheet examined for evidence of inadequate steam penetration, air
bubbles and like defects. The presence of such defects indicates
faults in the vacuum or other systems within the sterilization
unit which require evaluation and repair. The test is intended
to be performed on a daily basis with the interior test sheet
forming a permanent record of such testing. The nonwoven sheet
material and the remainder of the pack are disposed of after a
single use.
Further objects, features and advantages of the present
invention will be more fully appreciated by reference to the
following detailed description of a presently preferred embodi-
ment in accordance with the invention, when taken in conjunction
with the appended drawings, wherein:
Figure 1 is a perspective view, partially cut away, of
a test pack in accordance with the present invention;
Figure 2 is a partial sectional view on line 2-2 in
Figure l;
Figures 3 and 4 are graphic showings of the relation-
ship between test results conducted with applicant's test pack
and test results conducted in accordance with the conventional
test pack of the Bowie and Dick protocol.
Applicant's test pack 10 includes a sterilization test
sheet 12 of a type known in the art. This sheet is generally a
rectangular sheet of relatively porous paper which has printed on
its top surface a steam sensitive ink in a test pattern designed
to cover a substantial portion of the sheet surface. An ap-
propriate test sheet for this purpose is the Once-A-Day test
sheet manufactured by Propper Manufacturing Co., Inc. ox 36-04
Skillman Avenue, Long Island City, New York. This test sheet is
shown in U.S. Patent No. D-222,516.
~.23~ S
The ink areas of this sheet are adapted to change color
from white to black upon exposure to steam at a desired
temperature for a desired period. The color change from white to
black occurs over a period of time so that insufficiency of steam
exposure may result in only partial development of the ink from
white to black. This partial change may result in white or brown
or lightened areas, visible on the test sheet. Test sheets of
this type are generally known in the art and have been used in
place of the cross sterilization tape strips described in the
initial Bowie and Dick test procedure.
Applicant's new test pack 10 is designed to permit the
use of a test sheet such as the Once-A-Day sheet without the use
of the conventional stacks of cotton towels in accordance with
the Bowie and Dick protocol. As indicated on the literature on
the Bowie and Dick protocol, various types of defects are most
frequently found in sterilization equipment. Principal among
these are (1) inadequacy of initial vacuum, leaving residual air
within the packs and (2) air leaks within the chamber or vacuum
system which permit the re-entrainment of air after a vacuum has
been drawn. In designing a test pack to evaluate both these
types of flaws, as well as others, it was discovered that there
are a variety of different constraints operating.
For example, to the extent a flaw is present in the
vacuum system which prevents a sufficient vacuum from being
formed in the first instance, a test pack having a low porosity
and/or high bulk is more likely to reveal a flaw of this type
than one having a high porosity and/or lower bulk. As the vacuum
is drawn, the air within the test pack tends to be drawn out of
the pack. Low porosity and/or very bulky material surrounding
the test sheet would tend to increase the difficulty of removal
of such air and increase the likelihood that the test pack would
indicate a flaw.
3.~
On the other hand, flaws caused by leaks involve the
introduction of air into the sterilizer after the vacuum has been
drawn. In this situation, the relationship between the porosity
of the material surrounding the test sheet and sensitivity of the
test is the reverse. The less porous and/or more bulky the
material surrounding the test sheet, the less likely it is that
air introduced into the sterilizer after the vacuum has been
drawn (as in the case of a leak) will re-enter the test packO
Accordingly, with respect to a flaw resulting from air leaks
after a sufficient vacuum has been drawn, low porosity material
surrounding the test sheet would tend to make it more difficult
for air from such leak to enter the pack and decrease the
likelihood that such a test pack would indicate a flaw.
In order to properly test the sterilizer, a test pack
must balance these conflicting needs and provide appropriate
challenge to either common type of flaws. Such a proper test
pack is the currently accepted standard based on 10 - 11 inch
stacks of folded cotton huchaback towels ln accordance with the
Bowie and Dick protocol. It is desirable to design a relatively
small, inexpensive and disposable test pack which achieves a
challenge to the sterilizer comparable to the challenge provided
by the accepted Bowie and Dick protocol.
Applicant has found that a test pack can be fabricated
which provides detection of the common sterilizer flaws in a
manner comparable to the Bowie and Dick protocol by establishing
a package of individùal sheets of disposable material above and
below an appropriate test sheet. Such sheets are assembled to
form an inner core region adjacent the test sheet, and a shell
region above and below the core region, with the thickness and
porosity of the core region and the porosity of the shell region
being selected according to desired parameters.
--7--
The preferred construction of applicant's test pack
shown in Figure 1 includes a conventional test sheet 12 sur-
rounded by stacked sheets of nonwoven material of selected weight
and porosity. In the preferred embodiment two types of material
are used, with the first type forming the inner core region 14
and the second forming the shell region 16.
The entire package is then wrapped with an appropriate
porous wrapping material 18 and taped 20 to form a sealed unit.
The porosity of the wrapping material is such that it does not
affect the test.
In the preferred embodiment, the shell region (16) is
composed of five sheets of filter paper having an appropriate
basis weight of 214 lbs. (per 3,000 square feet) and an ap-
propriate thickness of .0~ inches per sheet. This material has a
Frazier porosity of approximately .5-1.5 cu. ft./sq. ft./min. per
sheet.
Frazier porosity is the measure of air permeabili.v of
sheet material as measured by the Frazier Differential Pressure
Air Permeability Measuring Machine manufactured by Frazier
Precision Instrument Company, Inc. of 210 Oakmont Avenue,
Gaithersburg, Maryland 02760. These measures of porosity are
based on the differential pressure principle as measured by
manometers. The porosity measure is given in cubic feet of air
per square foot per minute at .5 inches of water pressure.
Another common measure of porosity of sheet material is
the Gurley method which provides a measure of the time required
for 100 ml. of air to pass through one square inch area of the
specimen material at a pressure. Obviously, the Frazier
porosities given herein in cubic feet per square foot per minute
could be expressed by other standards of measurement.
The sheets in the core region (14) of the preferred
embodiment of Figure 1 include 11 sheets above and 11 sheets
--8--
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below the test sheet, each sheet comprising filter paper having
an approximate basis weight of 64 lbs. (per 3,000 sq. ft.) and a
thickness of approximately .02 inch. This material has a Frazier
porosity of 75-95 cu. ft./ sq. ft./min.
In the preferred example, the height of this core region
is .22 inch above and .22 inch below the test sheet and the total
porosity of the sheets in the core region on each side of the
test sheet is approximately 6.9-8.7 cu. ft./sq. ft./min. In the
preferred example, the shell region has a thickness of 0.1 inch
on each side of the core region and a Frazier porosity of approxi-
mately 0.1-0.3 cu. ft./sq. ft./min. on each side of the core.
The test pack, sheets and test sheet are preferably
9" x 12", although other dimensions could be employed with
possible alteration in other parameters.
Applicant has found that the porosity of stacked material
of the type herein used is substantially linear so that if an
individual sheet of material has a Frazier porosity of 90 cu. ft./sq.
ft./min. a stack of ten sheets of such material will have a porosity
1/10 that of the individual sheet or 9 cu. ft./sq. ft./min.
Applicant has found that by forming a shell area havins
a relatively low porosity as compared with the core area, the
number of sheets and height of the test pack can be significantly
reduced with attendant economies of material and reduction in
cost. Applicant has found that it is preferred that the stacked
sheets of material in the core regicn of the pack on each side of
the test sheet have a combined Frazier porosity of greater than
1.5 cu. ft./sq. ft./min. Applicant has also found that it is
preferred that the stacked sheets of material in the core region
of the pack on each side of the test sheet have a height of at
least 0.03 inch. Applicant has also found that a shell area
having a Frazier porosity of less than .S cu. ft./sq. ft./min is
desirable.
. . ,~,~3~?~,S
As indicated previously, it is desirable that the
disposable test pack respond to faults in the sterilizing
equipment in a manner comparable to cotton towels prepared in
accordance with the Bowie and Dick protocol. Applicant's
preferred embodiment accomplishes this desired objective.
Specifically, tests have been conducted in a specially
modified prevacuum steam sterilizer altered to create conditions
equivalent to sterilizer defects in a controlled and reproducable
manner. The prevacuum steam sterilizer included a manually
controlled vacuum pump, permitting alteration in the degree of
vacuum drawn in the system, and an adjustable volume piston
driven air injector. Incomplete air removal was simulated by
evacuating the chamber up to 60 millimeters of mercury in a
single step and omitting any subsequent evacuation. Air leaks in the
sterilizer were simulated by adjusting the stroke of the injector
piston to deliver a specified amount of air into the chamber
during a normal steam exposure cycle following a satisfactorally
complete evacuation of air.
The test sheet of applicant's preferred test pack is
intended normally to be evaluated visually in the normal manner
for such devices employed in the Bowie and Dick protocol. For
purpose of quantifying applicant's tests, color changes in the
chemical indicator sheet were evaluated by a reflectometer with a
10 millimeter orifice setting (Model XL-20 Gardener/Neotec Instru-
ment Division of Pacific Scientific, Silver Spring, Maryland).
Using a green filter this instrument measured reflectance of
color changes from white to black and was used in accordance with
methods prescribed by The American Society For Testing And
Materials (AST~I 1977). The upper limit of the reflectant scale
was measured at 79 for an unexposed white sheet using the
commercial Propper, Once-A-Day test sheet and 8.5 for a
completely exposed black sheet. Surface area of the incompletely
--10--
3~ S
exposed portion of the test sheet was measured by calculating the
area of the ellipse that best described the perimeter of the
incompletely exposed area.
Applicant's tests revealed that in the range of
reaction of primary interest, the effects of the two common flaws
on the test sheets were substantially the same for applicant's
disposable test pack and a test sheet prepared in accordance with
the conventional Bowie and Dick protocol. Applicant gathered
data with respect to the surface area of the undeveloped portion
of the test sheet and the reflectance at the center of the
indicator sheet within the undeveloped area. Measured data are
as follows:
Applicant's Pack Towel Pack
Partial Vacuum Test Area Color Area Color
55 mm Hg. 0 8.5 t; 8.5
60 mm Hg.1.25 9.4 2.375 9.9
70 mm 1.75 12.2 2.625 14.8
80 mm 4.38 13.8 3.375 25.8
90 mm 6.80 14.7 4.70 25.7
100 mm 72025 42.9 8.44 69.4
Applicant's Pack Towel Pack
Air Injection TestArea Color Area Color
(Moles Of Air
Injected/leter of
sterilizer volume
8.8 x 10-6 1.2 11.4 1.2 10~3
17.5 x 10-6 2.4 12.2 1.7 10,3
38.2 x 10_6 3.2 12.9 2.0 13.8
55.6 x 10-6 5.1 13.5 4.9 11.4
78.2 x 10-6 6.6 16.5 5.85 20.5
101 x 10-6 10.3 19.3 6.9 32.8
l ~3~5
As the above data indicates, the product of the rneasured surface
area of the faults in applicant's tests multiplied by the re-
flectance of the test sheet center for the disposable test pack
is equivalent to the same product for the conventional of Bowie
and Dick protocol. This relationship is shown in Figures 3 and
4. Figure 3 shows reflectance multiplied by area plotted versus
the degree of vacuum (indicated in millimeters of mercury) in
the sterilizer before the introduction of steam. Figure 4 shows
a similar comparison for the data gathered based on simulated
air leaks with the reflectance times area being plotted against the
volume of air injected in Moles per liter if sterilizer volume times 106.
As indicated in the data presentation of Figures 3
and 4 the refelctance multiplied by area measure for applicant's test
pack and the conventional Bowie and Dick protocol closely follow one
another.
The f,oregoing is considered as illustrative only of
the principles of the invention. Further, since numerous modifications
and changes will readily occur to those skilled in the art,
it is not desired to limit the invention to the exact construction
and operation shown and described and accordingly all suitable
modifications and equivalents may be resorted to falling within
the scope of the invention as defined by the following claims.
