Pre-Vac Steam Steriliser

I hope someone can answer my query for me.

My understanding has always been that where there is air there is no steam contact as air and steam do not mix. In a prevac steam steriliser the air is mechanically removed downwards and out through the drain at the bottom of the chamber.

All hollow ware must be tilted for air removal and drainage, trays that do not have holes must be placed on their sides to ensure full air removal.

I ran some tests a couple of weeks ago using the GKE, class 4 integrators and the 3m Clock. Placing the indicators inside a bowl wrapped in double kimquard and placed one bowl up right so air would be trapped and positioned them on the top rack and the other on the bottom over drain. Ran a full cycle through the pre-vac steam steriliser.

I was very surprised at the result and their was complete colour change on all indicators. From what I have read and studied over the years there should have been air trapped in the bottom of the bowl.

Do these indicators change whether there is air present or not?

I hope some one can clarify this for me

Please e-mail me at

thank you
Judy
Clinical Educator
DASS
South Auckland Health
New Zealand

Hi Judy,

I believe the results you achieved in your experiment are a testimonial to the effectiveness of your pre-vacuum system at removing all the air from the sterilizing chamber before the sterilization portion of the cycle begins. If you run the same experiment with a sterilizer that fails the Bowie-Dick style test, or with a gravity-displacement cycle, I believe your results will be different. A gravity-displacement (sometimes called 'Liquid Load') cycle pumps steam into the chamber until steam exits the drain at the bottom of the chamber. Because steam floats on top of air, when the drain at the bottom of the chamber registers steam instead of air, the sterilizer is theoretically filled with steam and the cycle begins. Any item on the sterilizer cart during a gravity-displacement cycle that is placed so it will hold water outside the sterilizer, can hold or trap air during a gravity-displacement cycle. Therefore we tip basins as if we want all the imaginary water to run out so that it is easier for the steam to reach all the surfaces. Must we also load the sterilizers in this manner in a pre-vacuum system where the air is removed mechanically? The tests that you ran say no, the steam will reach the inside of your basin when placed in your sterilizing chamber in the most challenging position. To me, this result means I have an even greater overkill or confidence factor when I load the basin in the sterilizing chamber in the least challenging way. A pre-vacuum sterilizer is much more forgiving of an improperly loaded sterilizer cart than a gravity-displacement sterilizer is, but I prefer maintaining the same principles for both types of steam cycles - Load the cart so that the steam can easily reach all surfaces.

An answer to your other question: Integrators that use a dye pellet and wick (instead of a pre-printed ink that changes color) are frequently made with a dye where steam acts as a catalyst to lower the melting point of the dye. In this manner it integrates the presence of steam, the temperature, and the time. Without the presence of steam, the temperature must be higher or the time at temperature must be longer for the dye to melt and be visible on the wick.

Regards, Pete Bobb

Dear Judy!

Thanks for your question which highlights the difference between gravity and pre-vac steam sterilizers. With gravity displacement type machines, it is imperative to allow the steam to "wash out" the air. That is the rationale to "tilt" any hollow load items. Sterilizers employing a vacuum pump for an enforced air removal are less prone to such problems. The pressure difference (the vacuum) removes air from the bottom of a cup as described by you. I take it from your note, that you used a "pre-vac" cycle, which was successful to remove the air from the chamber, load and your home-made test pack. Thus the indicators exposed changed colour.

Let me add a word on testing indicators (chemical or biological). This is a delicate issue as one can see from the requirements for test equipment and procedures described in the relevant standards, e.g. ISO 11140 or EN 867 series. To ensure a meaningful and valid result from such tests, any influencing factors or parameters have to be carefully controlled in dedicated test vessels. The technical specifications for such test vessels (CIER = Chemical Indicator Evaluation Resistometer) are much tighter than those for a standard hospital steam sterilizer. The results of any tests performed in standard hospital sterilizers have to be carefully interpreted and analysed against the cycle parameters and conditions used, e.g. steam quality, level of non-condensable gases, etc..

I hope the above is of help. Please do not hesitate to ask if you have any further questions regarding this matter!

Kind regards

Klaus Hahnen
Senior Technical Service Specialist
Sterilization Assurance Products
3M Laboratories (Europe), Neuss, Germany

Dear Mrs Rowson,

as already explained by Pete Bobb and Klaus Hahnen, Cups have to be placed upside down in a gravity displacement steam sterilizer to remove the air inside. You have a pre-vac sterilizer which removes the air prior to steam introduction by a vacuum pump, so that those indicators change in deed the colour, if the pre-vacuum is working.

If you want to test a steam indicator, if he changes only in a steam environment and not in dry heat, take one home and put it in your bakery-oven heat up to 140 °C and leave the indicator inside for 30 minutes. According to the standard EN 867 and ISO 11140 the colour of the indicator is allowed to change, but not to the colour- change in a steam sterilizer. Our gke-indictors for batch monitoring systems change their colour from yellow to black in steam, but to yellow-brown in dry heat.

For more information visit our website: www.gke-mbh.de

For specific technical questions don’t hesitate to email to our application lab:

With Best Regards

gke-mbH

Dr. Ulrich Kaiser
Application Lab

Maybe is my reply a little late, because I can hardly add something to the already given answers but as I had prepared it I give it to you anyway.

If your steriliser has a well developed cycle with an effective air removal by vacuum and steam pulses there is no reason why air should not be removed from a bowl if standing upright. Goods with narrow lumen also have to be sterilised under the same conditions and these are even more difficult for air removal. (See the discussion about the test Helix.) Your test results with the indicators gives no guarantee that the correct sterilisation conditions are reached in time and during the preset sterilisation time. Most indicators are already starting with the colour change during the pre vacuum phase with steam pulses.
These test strips are not developed to test a sterilisation cycle with great accuracy but give a quick and reasonable safe indication that a device or package is sterilised or not.
The best way to verify the process performance is by validation. This means that (tiny and thin - Ø 0,2 mm) thermocouples will be brought in to the load to verify if the temperature distribution within the steriliser chamber and the load meets the standard as described in the EN 285 and EN 554, during a cycle.

A more important reason to place a bowl or other hollow ware on their side is that condensate which will be generated in the bowl should be able to drain off. This to make the drying during post vacuum more easy and secure.

I hope my answer is satisfactory but if you have any questions you can contact me directly on:

Kind Regards,
Henk Ras

Sanamij B.V.
http://www.sanamij.com