Sterilizer function and components
Let’s have a look at the basic parts of a typical steam sterilizer:
Basic Parts of a Steam Sterilizer
Chamber
The chamber is the area in which the sterilizing process occurs. The walls of the chamber are extremely strong to withstand the pressure changes of sterilization. Chambers vary in size from small table top models to large floor loading units.
Door
Doors can be self-closing or manual closing; some open outward and some slide up and down. All steam sterilizer doors are locked during sterilization and form part of the chamber. Therefore they must be very strong and be able to prevent steam from escaping during the cycle. Most have some form of gasket around the inside to prevent steam escape. Once the sterilizer door has been locked and the cycle is started it cannot be unlocked until pressure in the chamber has returned to normal.
Jacket
The jacket surrounds the outer wall of the chamber. It holds steam at the correct temperature for sterilization so there is a ready supply when the sterilizer is turned on.
Having steam circulating in the jacket also keeps the chamber walls warm. This prevents condensation, which can occur when hot steam hits a cold steel wall. Too much condensation in a steam sterilizer can cause wet loads – packages are too wet to dry properly at the end of a cycle
Baffle
The baffle is a metal plate that sits over the steam inlet in the chamber and disperses the steam as it enters. The baffle protects packages in the load from the rush of incoming steam. Without it packages close to the steam inlet would be very wet.
Drain and Steam Trap (Thermostatic Trap)
Air is removed from the chamber through the drain. Air is cooler than steam. The steam trap remains open as long as it senses the cooler air. Once the trap senses steam at the correct temperature it closes and seals the chamber. Now pressure can build up and the sterilization temperature can be reached. The drain area is often the coolest place in a sterilizer and the most difficult area in the chamber to reach sterilization temperature. That is why we always place our load monitoring test packages over the drain. This area offers the greatest challenge for our tests.
– air removal
Air must be removed from the chamber of a steam sterilizer before sterilization can begin. If air pockets remain in the chamber or in the packages to be sterilized the steam will not contact all surfaces and these areas cannot be considered sterile. As well, too much air in a chamber can prevent the increase in pressure that is necessary to reach the correct sterilizing temperature. Air is removed from the sterilizer through the drain. This is done in one of two ways:
Gravity Displacement Air Removal
- Also called “downward displacement”
- Steam is lighter than air. When steam enters the chamber it sits on top of the air. As more steam is injected it pushes down on the air, and eventually pushes it into the drain.
- It takes quite a long time to push all the air out of a large chamber. Exposure times for these loads have to be longer to ensure that steam contacts all surfaces.
- Smaller sterilizers, such as those used for Flash steam sterilization, work well with gravity air removal because there is less air to remove. Sterilization times can be shorter in these small machines.
- Items to be sterilized must be carefully placed so that no air is trapped in cupped surfaces, such as bowls and med cups. Trapped air = not sterile!
Vacuum Air Removal or Pre-Vac (or High-Vac)
- Air is quickly removed though the drain by a vacuum pump.
- Steam is able to flow quickly to all areas of the sterilizer resulting in better exposure of steam to all surfaces, and in a shorter cycle.
- The vacuum pump also removes steam at the end of the cycle and assists in drying the load.
- The ability of the vacuum pump to remove air is tested daily with a Bowie Dick or DART test (Dynamic Air Removal test).
Phases of the steam cycle:
Phase 1 – Condition, or Conditioning
· Steam is injected and air is removed. Most sterilizers that we work with use a pre-vacuum system where the air is removed in pulses. Four vacuum pulses are common in a steam cycle
· As more air is removed and more steam enters, the temperature and pressure in the sterilizer chamber increases.
· Once all air is removed the thermostatic trap in the drain closes, affectively sealing off the chamber. Steam is injected until a set temperature is reached – at least 132ºC (270ºF) Temperatures of 134ºC (273ºF), or 135ºC (275ºF) are common as well.
Phase 2 – Sterilize
· The sterilize or “exposure” time at a temperature of 132ºC (270ºF) is made up of 3 parts:
1. Heat-up
The chamber is filled with steam but the insides of the packages have not been contacted by steam yet. This heat-up phase lasts for about 1 minute and allows the steam to fully penetrate packages.
2. Hold
This is where the actual sterilization of items takes place. The set temperature is held for 2 minutes
3. Safety
This final part of the sterilizing phase is an added safety margin of 1 minute to ensure that sterilization has taken place.
Phase 3 – Exhaust and Drying Time
· The thermostatic trap in the drain line opens and the steam is removed by the vacuum pump.
· The vacuum continues to operate, at about 27 PSIG or more, pulling moisture from the load and drying the packages.
· Most common dry times are 30 – 40 minutes.
· After the set dry time filtered air enters the chamber and the load is ready to be removed.
Principles and procedures
The steam sterilizers we use in MDR use a moist heat process to destroy microorganisms. We start with steam (moist heat) in a sealed chamber; we remove all the air; we can then increase the pressure, which increases the temperature. By increasing the temperature we can shorten the time it takes to destroy microorganisms.
Steam by itself has a temperature of 100°C (212°F). We know that our steam sterilizers run at a much higher temperature, usually between 132°C (270°F) and 135°C (275°F). We increase the temperature of steam by applying pressure. We can apply sufficient pressure because we first remove all the air in the sterilizer chamber and keep the chamber sealed.
We can see that moisture, temperature, pressure, and time are all important components of the steam sterilization process. We call these the parameters of the process. We monitor and test these parameters to ensure that the steam process is working properly.
All these parameters interact with each other. If we increase or decrease one of them we must increase or decrease another. Have a look at the following:
- If you decrease temperature you must increase time
121°C (250°F) for 20 minutes will sterilize
- If you increase temperature, you may decrease time
132°C (270°F) for 4 minutes will also sterilize
Steam sterilization has several advantages over other sterilization methods. It is relatively fast, it is non-toxic, and steam is easy and inexpensive to produce. It is the most common method of sterilization in hospital and clinical settings.
It does have some disadvantages however:
- Steam sterilization cannot be used for anhydrous materials such as oils or powders. Anhydrous means “absence of water”. Because these items are so “dry” the moist heat of steam cannot penetrate and sterilize them.
- Research has proven that wood and cork items cannot be properly sterilized in steam because their cellular structure is dense and steam cannot penetrate to all areas of the item. As well, wood releases lignocellulose resin (lignin) when it is exposed to heat. Lignin can cause serious reactions in the tissues of patients.
- We must be very careful with heat or pressure sensitive items in steam sterilization. Manufacturer’s instructions will tell us what sterilizing method works best for their items. We may have to adjust our sterilize times and temperatures for sensitive items. Most steam sterilizers will have several different cycle settings to accommodate these kinds of changes.
Reading a Printout
· troubleshooting wet loads
Here are some general guidelines for loading a sterilizer:
- Items must be loaded onto the sterilizer rack or cart so that steam can be circulated around and into all packages. Do not crowd items too closely. Allowing at least 5 to 10cm (2 to 4 inches) between large packages and 2.5 to 5cm (1 to 2 inches) between smaller packages is a good rule to follow.
- Do not stack items on top of each other. The only exception to this is some rigid containers that are specifically designed to be stacked during sterilization. You must have specific manufacturer’s instructions for these containers. Not all rigid containers are made to be stacked. Check with your supervisor and/or work instructions before stacking any containers.
- In Steam sterilization many items need to be placed on edge to allow air to be removed easily. Basins, mayo trays and solid metal containers are examples of items that should be placed on edge. These items may also trap water if they are not on edge. Trays and containers with perforated tops and bottoms can be placed flat as air and the sterilizing agent will move through the perforations. Rigid containers are made to be placed flat.
- Peel back pouches should be on edge with paper sides facing plastic sides. If two plastic sides are touching during sterilization condensation will form on them. If two paper sides are facing each other the sterilant may not penetrate the packaging.
- Items should not touch the walls of the sterilizer. In a steam sterilizer any packages that touch the walls may come out wet.
- For steam sterilizer loads, metal rigid containers or other “heavy” items should be placed on the bottom of a load with linen or lighter items on top shelves. These metal or heavy containers can produce condensation during the cycle. Linen or peel back pouches placed below them could become very wet.
Monitoring
All sterilization methods require monitoring and regular testing. We must be able to prove that the items we process in a sterilizer have been exposed to the proper amount of sterilant, for the correct time, and under the right conditions such as temperature and pressure.
Each sterilizing method has its own forms of monitoring. We always monitor the sterilization process, and with steam sterilizers we need to monitor the function of the machine itself. All this testing and monitoring must be recorded and the records kept for many years. This gives us the ability to track down an item and prove to the best of our ability that it was properly processed.
There are four categories of monitoring we need to be aware of: Administrative, Mechanical, Chemical and Biological.
Administrative Monitors
These provide consistency in a department and the assurance that quality service is provided to patients.
o Standards and recommendations developed by external agencies such as the Canadian Standards Association, Health Canada, ISO, and many professional organisations provide the guidelines for SPD management to develop policies that govern our work
o Policies and procedures are written that cover employee health and safety; that standardize such things as dress codes, work instructions, job descriptions and duties.
o Providing staff education and supervision is also a form of Administrative monitoring.
Mechanical Monitoring
Involves the use of devices attached directly to the sterilizer. These devices indicate what is happening in the sterilizer chamber and show us whether or not conditions for sterilization are being, or have been, met. They do not, necessarily, show us what is going on inside the packages in the load. We look at mechanical monitors for the parameters of the steam sterilization process – time, temperature, and pressure
o Gauges on a steam sterilizer will show us the pressure inside the chamber and inside the jacket. Monitoring these pressure readings is important as they can tell us if the machine is ready to use, and if we are reaching the correct pressure, and therefore temperature, in the chamber.
o LED readouts on the front of a sterilizer can give us information on the time remaining in a cycle, as well as the pressures and temperatures in the chamber. We are able to ensure that we have chosen the proper cycle and we can be notified of any errors in the cycle as it progresses by monitoring the LED panel.
o Printout tapes are very important for our processes. They provide a permanent record of the load and the parameters of the sterilization process. We need to be able to read the information on the printout and know whether it is correct according to the parameters we have set for the process. We are asked to sign the printout after we have checked the information on it. You would never sign a contract or legal document without understanding it. Please make sure you understand the printouts you are signing every day. If you are unsure please ask your supervisor or co-workers for help.
- Chemical
Chemical Monitoring
We use chemical indicators inside and outside our packaged items to monitor the sterilization process. We also use chemical indicators to test the efficacy, or strength, of liquid sterilization processes. These indicators use a special chemical to show exposure to a sterilizing process, or to a specific portion of that process. The chemical is embedded on a strip of paper or plastic, or on adhesive tape. A colour change on these indicators does not show that items are sterile. The colour change only shows that an item has been exposed to a process. They are valuable tools for us to show that exposure occurred, but we need to look at all of the monitoring methods we have to ensure that sterilization has occurred.
Chemical indicators (CIs) are grouped into five “classes” according to the parameters of sterilization they measure:
Class 1. External CIs (chemical indicators) or Process Indicators
o Examples of class 1 indicators would be Autoclave Tape, and the indicators on labels for rigid containers.
o They show that an item has been exposed to a process so that we can quickly differentiate between processed and non-processed items.
o They are susceptible to heat and sometimes light and must be stored and used according to their manufactures instructions. Many types of Class 1 CIs have expiry dates, so check that roll of tape before you use it!
Class 2. Specific test Indicators
o The most common example of this class is the Bowie Dick or DART test.
o They are made to monitor a specific portion of a machine cycle.
Class 3. Single Parameter Indicators
o They respond to only one parameter of the sterilization process
o An example of Class 3 would be the CI that is used in each cycle in the Steris System One. It changes colour when exposed to the Peracetic acid.
Class 4. Multi-parameter Indicators
o They respond to two or more of the critical parameters in a sterilization process, such as sterilant and time; or temperature and pressure.
o Examples of Class 4 would be the CIs that we regularly place into wrapped packages.
Class 5. Integrators
o They will respond only when all the critical parameters of a sterilization cycle have been achieved.
o An example would be the SPS Steam integrator packs that Fraser Health uses for each steam load.
o They are often referred to as a “Biological Equivalent” because they react to all the parameters of the sterilization cycle that are necessary to sterilize a biological test. However, they cannot be used as a substitute for regular biological tests, because they do not show biological kill. On the other hand, they do give us a very good indication that the sterilizer is able to reach all the required parameters for successful sterilization.
Class 6 Emulators have been used in Europe for several years, but are not commonly used in North American SPDs at this time.
o They are made to respond to all the critical parameters of sterilization just like a Class 5 Integrator, but they are specific to certain cycles. They are most often specific to the time of the cycle; say a 4 minute steam sterilization, 30 minute dry time cycle. Because an Emulator is specific to one cycle it cannot be used on any other cycle, for example you can’t use a 4 minute Emulator on a 10 minute sterilize cycle. They do give a very definite assurance that the cycle has achieved all parameters, so we may find that in the near future we will be adding Class 6 Emulators to our monitoring methods.
- Biological
Biological Monitoring
The most reliable way to test our sterilizing processes is to actually kill some microorganisms. All of the sterilizing processes we use have their own biological tests that are exposed to the sterilizing cycle, placed in a warm incubator for a specific time, and “read” for any changes. All types of biological indicators use the most difficult micro organisms to kill. If a sterilizing process can kill these difficult microorganisms then we can assume that it will kill all others. We can then make the statement that our sterilizing processes are working correctly.
Most of the biological tests we use in SPD contain the microorganism Geobacillus Stearothermophilus. It is a very tough microorganism and is used in steam, Sterrad, and Steris System One biological tests. The microorganisms in these biological test may be tough but they are not harmful to us. The liquid growth medium in the test vials is not harmful either and is simply a soy broth mixture. There are small glass vials contained in the tests that we must be careful of. Never crush the test vial with your fingers as the glass could puncture the plastic casing and injure you. We dispose of biological tests in an approved sharps/biohazard container because of the glass vials in the tests.
All biological tests should be placed in the most challenging area of the sterilizer; the hardest place for the sterilant to reach. In steam sterilizers it is over the drain.
Biological indicators come in many shapes and sizes. Some have caps that need to be closed after sterilization; some have filters in the cap and do not require closing. They all work on the same basic principle:
o A strip of paper or other fabric is coated with the most difficult to kill microorganisms. This spore strip is kept away from the “growth media” (the food that the microorganisms like).
o The spore strip is exposed to the sterilizing process. For most sterilizing processes the biological indicator is placed in the most difficult area for the sterilant to reach.
o The sterilized spore strip is then exposed to the growth media and placed in a warm incubator.
In a self-contained indicator, such as the one below, the growth media is sealed in a glass ampule (little glass tube) while the spore strip sits outside of the ampule, usually at the bottom of the vial. After sterilizing and before incubating, the glass ampule is broken, usually by crushing, and the spore strip is flooded with the growth media. If the vial is the type that requires the cap to be closed we must make sure this is done as soon as possible after sterilization, according to manufacturers instructions.
Rapid Read indicators are very similar to the self contained biological indicators. The difference is the time that is required to obtain a reading of the indicator growth results. When the microorganisms in a biological indicator grow they give off an enzyme.
The Rapid Read systems use a growth media that reacts to this enzyme and causes the indicator to glow under ultra violet light. These Rapid Read systems need a special incubator that shines an ultra violet light through the vial and can detect the glow. Most of these systems can give a result in 3-4 hours, long before our eyes could detect growth.
Some of the incubators that read these types of indicators must be calibrated every day, so that the incubators know what type of glow to look for.
Make sure that you understand the calibration procedure if your SPD uses this type of Rapid Read system. If the incubator reads the biological indicator for you then you are using a Rapid Read type system.