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Letter Re: Sterilizing Medical Instruments with a Pressure Cooker
Hi Jim...
I noticed and read the letter from "NotDave" on, among other things,
the use of a pressure cooker as an expedient sterilizer (autoclave). I would
like to expound on what he wrote.
First of all I would like to state that he is correct in his statements of
time, temp, and pressure.
To achieve steam-driven sterility you need to satisfy two requirements...time
and temperature. In the medical, and lab/scientific field steam is the prevalent
sterilant medium. This is due to the ability of steam to penetrate that which
is being sterilized. At 20 psi [(g)auge
(psig)]
the temp is 250.3 degrees F. (There are other sterilants but suffice it to
say
that would be another dissertation.)
The point I want to make is in the form of a tip or two for all the good folks
in blog land. It is important to select a pressure cooker that can
maintain a pressure of 20 psi. although most only have valves for 15 psi. Select
one that has the "twist the lid in place" and screw down fasteners
rather that the lollypop shaped lid that twists in place only. The former is
far safer than the latter. Bob at Ready
Made
Resources provided me an "All
American" pressure
cooker that doubles as a sterilizer nicely. It is very well built and is a
gasketless design. Needless to say stock up on gaskets
if you select a pressure cooker that has one.
In the course of sterilizing anything in a pressure cooker it
is important to ensure that those items are placed above the water
that is boiling. The goal is to expose the item to pure steam. If any or all
of the items are immersed in the water then complete sterilization
may not or will not occur. This is due to the fact that the item will
be exposed to the temperature of
the water not the steam. The pure steam will give off it's BTUs
on contact with the item while water may not and will not do so as efficiently
because
water is an insulator as well as a medium for storing those BTUs. Even though
the water is superheated due to the pressure (15-20 psi [(g)auge]) you would
have to re-calculate and increase the exposure time to ensure the item has
been sterilized. There also may be
some air trapped in the cooker. Air is an insulator and no friend to sterilizing.
Because most pressure cookers have the pressure gauge, not the vent, located
at the highest point of the lid some air may remain near the top so. Do
not place item in the uppermost region of the lid.
There is a technique for sterilizing in a pressure cooker and it is easy to
do. A perforated or wire mesh, without handles, stainless steel deep frying/"Fry-o-lator"-type
basket, of the appropriate size, placed open end down inside the p-cooker will
act as the "shelf" for the item(s)
providing the item(s) to be sterilized will fit in the pressure cooker with
the basket. It must be tall enough to raise the item far enough above the anticipated
water level to prevent being splashed by the roiling water surface. Chose a
basket that has free swinging handles if possible so it can be used for par
boiling things when not acting as a shelf. If free floating handles are not
available just cut off the fixed handle and file the stubs smooth ("...cut
to size, file to fit..."). A smaller perforated basket placed on top of
the inverted one is handy for holding the items and convenient for handling
that being sterilized but not necessary given the availability of space. Just
be certain to use a perforated basket (or whatever) versus something solid
such as a pan. Air will get trapped inside and, because the water is boiling,
may get tipped over dropping the instrument (item) into the water.
In general terms, a few guidelines:
1) The boiling point of water drops approximately 2 deg. F. (1.1C) per 1,000
feet rise in elevation above sea level therefore the pressure of the steam
needs to increase to cause the steam temperature to rise to 250 Deg. F.
2) When using a pressure cooker to sterilize something it is imperative that
the pressure control valve/weight is left off to allow the
air trapped inside to be driven off. A good rule is to allow 10 minutes to
pass after steam is
seen flowing out of the vent. Be certain to add sufficient water. Dry heat
is a poor sterilizing medium and needs to be about 320 deg. F. for 2 hours
minimum
to effectively sterilize something such as a towel clamp.
3) 15.3 psi (g)auge) equals (approximately) 250 deg. F. This is the low temperature
sterilization point used in hospitals, labs and in commercial
applications.
4) 10.3 psi (gauge) equals approximately 240 deg. F. This is used in labs and
in commercial application where the items are temp sensitive.
5) The lower the temperature, the longer the exposure time required.
6) The minimum exposure time at 250 Degrees F (15.3 psig) should be 30 minutes.
7) The minimum exposure time at 240 degrees F. (10.3 psig)
should be 1 hour (well, actually 45 minutes, but be safe and err for the longer
time
as there are too many variables and at the lower temp. it is too risky).
Above all, proper sterilization requires items be very clean prior
to sterilizing. Any "debris" in an instrument will act as an insulator
and prevent that zone from reaching temperature. The critters you are trying
to kill may not die beneath that zone and replicate. If you are putting the
item
in
storage there will be more than a sufficient amount of time to re-contaminate
the item. If you use that improperly sterilized item in a surgical procedure
then you may contaminate the op-site. In a SHTF condition
a nasty infection in a surgical site is something you don't want to deal with.
Just ask John O the
M.D.
Thank you for your patience and thank you for reading this. I hope it helps.
- J. at East
Tennessee Sterilizer Service
