Sterilizers, Part I: Heat Sterilization

The sterilizer has a single, simple, humble function—to destroy all forms of life on medical supplies, wrapped packs, and loose instruments, and sometimes even in bottled fluids, thus rendering the item free of viable microbes. Theoretically, sterilization is easy. An open flame will sterilize anything that it contacts. Long before recorded history, fire was used to cook food, help heal wounds, and renew fields for next year's crops. In modern healthcare facilities, however, fire can damage or destroy the item being sterilized. That makes things just a bit trickier.Items requiring sterilization must be decontaminated and physically cleaned before the sterilization process begins, as most sterilizing agents will not effectively kill micro-organisms protected by dried organic matter. Therefore, all solid matter must be removed via a more conventional cleaning using detergent or a mechanical or ultrasonic scrub, followed by a distilled or demineralized water rinse. Only then can items truly be sterilized.There are four basic methods of sterilization. Heat and chemical are most commonly used in hospitals. Filtration and radiation are used primarily for industrial applications. The first installment of this two-part Fundamentals Of column will explore heat sterilization and some general issues common to all sterilizers. The second installment, which will appear in the Jan./Feb. 2009 issue, will focus on chemical sterilization. If you're looking for more information on filtration and radiation, please visit www.aami.org/publications/BIT.An open flame obviously produces heat. It is cheap, easy, and simple to apply, but as noted previously, is less than ideal for most healthcare applications. There are exceptions. For example, an open flame sterilizer can be found in the laboratory. Before inoculating (streaking) a culture media plate, the laboratory technician sterilizes (or flames) the metal inoculation loop using an open flame or electrical heating element. This ensures that no infectious agent, aside from those contained in the patient sample, is placed on the media for incubation. The lab technician also uses this same technique to sterilize needles, culture tubes, and glass pipette tips.Beyond open flames, one common heat sterilization method is to use controlled dry heat in an enclosure—the dry heat sterilizer. Heating instruments to 325°F for two hours in a still air oven, or to 375°F for six to 12 minutes in a forced air oven, will kill all pathogens. Dry heat will sterilize dry powders and other items that may be damaged or compromised by moisture (steel items, for example). Disadvantages include the length of time required for sterilization, especially for wrapped packs; weakening and dulling of edged tools; and damage to modern plastics.The preferred method for loose instruments, wrapped packs, and other items that are not heat-sensitive and moisture liable is steam sterilization, which is quicker, more energy efficient, and less damaging than either open flame or dry heat sterilization. A steam sterilizer (also called an autoclave) uses pressurized dry steam to generate temperatures high enough to destroy viable microbes in wrapped packs, loose instruments, implants, liquid loads, etc. Over the past 30 years or so, steam sterilizers have evolved from simple and effective to complex and virtually foolproof.The sterilizer size relates to the capacity of the chamber and the size of the load it can process. Small sterilizers are usually table-top units or those having round chambers up to about 15″ in diameter by 29″ deep. Small sterilizers installed in the operating room (OR) vestibule near or between rooms for flash sterilizing instruments and implants do not have a water reservoir and operate directly from the building steam supply or a dedicated steam generator. Small sterilizers are also distinguished from their larger siblings by being hand loaded. Items are individually placed inside the chamber or onto trays placed in the chamber. All but the smallest sterilizers have some form of recording method to provide cycle information (such as chamber temperature, pressures, sterilization time, etc.) to validate the load's sterility. This information is vital in the event of a post-surgical nosocomial infection.Medium-size sterilizers (around 16″ wide × 16″ high × 26″ deep) are commonly found in the central supply area of small and medium hospitals, while large sterilizers (at least 36″ wide × 48″ high × 60″ deep) are found in the largest hospitals, medical centers, and teaching hospital, as well as manufacturing firms that supply the medical profession with prepackaged, sterile disposables. These larger units are the workhorses of the facility because their daily use supports the ORs and other areas requiring sterile instruments, instrument trays, and packs.The “gravity-type” steam sterilizer is commonly found in small standalone medical and dental clinics and in strategic locations throughout a hospital where a few small items or wrapped packs are routinely sterilized. In this device, hot steam rises to the top and forces the relatively cooler air to the bottom of the chamber. Gravity then allows the air to escape through a drain at the bottom of the chamber (hence the name “gravity sterilizer”) and a temperature-sensitive bellows-operated valve called a thermostatic steam trap. Modern tabletop sterilizers have a water reservoir; many older or low-end models do not. The reservoir holds water and, upon opening a valve, fills the chamber to the appropriate level. If there is no reservoir, the operator must pour water directly into the chamber. The operator closes and latches the door, then initiates the sterilization cycle. An immersion-type heating element in the chamber heats the water to boiling. At this point, the steam begins expelling air from the chamber. Once the chamber is completely filled with steam, the thermostatic steam trap closes and chamber pressure begins to build. When the set temperature (anywhere from 250°F to 275°F) is reached, a pressure switch begins cycling the heating element on and off, maintaining the steam pressure and corresponding temperature. Simultaneously, a timer is started. At the end of the set time (up to 30 minutes for wrapped packs sterilized at 250°F), the steam is exhausted through the reservoir (if so equipped) to cool the steam and recapture the condensed water before venting it to the room. If there is no reservoir, the steam is vented either to the room or through a copper line to a safe area.The most advanced systems alternate between a partial vacuum and injecting pulses of steam. This method, termed “preconditioning” of the load, removes air quickly and more effectively than gravity alone and can provide higher temperatures for shorter cycle times. Further improvements found in medium and large sterilizers include steam-filled metal jacket wraps, which help maintain the chamber's interior temperature while providing a condensate-free steam source. The second, unsuitable for liquid loads, employs a large, water-sealed vacuum pump to draw an almost complete vacuum inside the chamber before introducing the steam. Sterilizers using this technology are often termed “prevac” or “high-vacuum sterilizers.” The high vacuum, held for several minutes, removes most of the air from even the deepest recesses of wrapped packs. While this vacuum is being attained, steam is simultaneously injected into the chamber to aid in air removal and to warm the load. Once the preconditioning or prevac cycle is complete, steam is injected into the chamber and the cycle continues as in the gravity sterilization process.A vacuum is not drawn on liquids undergoing sterilization. Although most facilities purchase prepackaged sterile solutions for irrigation and lavage, some prepare their own sterile solutions using reusable bottles. These bottles use special rubber stoppers that allow air to escape during the cycle, then self-seal after the air is released. Drawing a full vacuum during the prevacuum stage can actually cause the liquid to boil away; therefore, that stage is bypassed when running a liquid load and the sterilizer operates in a gravity sterilizer mode. Additionally, quickly exhausting the 30 or so pounds of steam pressure can easily pop the rubber stoppers off the bottles. To prevent this, a slow exhaust feature is used to allow the steam pressure to slowly bleed down to zero.Modern medium and large steam sterilizers feature electrically controlled valves connected to one or more printed circuit boards containing an integrated central processing unit (CPU) with the operating program embedded in firmware. The sterilizing cycle of older steam sterilizers, often still in use in developing nations, is controlled by one of several means. The first is the simplest design, yet the most complicated to use. It requires the operator to manually open and close (in the correct sequence) a number of steam, water, and drain valves located in various places on the sterilizer. An improvement on this design consolidates all the valves in one general location and actuates them sequentially by manually operating a rotary cam connected to the bank. Rotating the cam steps the sterilizer through each portion of the cycle and actuates the appropriate valve at the correct time. A third style is similar to the manually operated rotary cam, but employs a motorized cam with added microswitches and adjustments to control the motors' operation.All sterilizers should be maintained by biomedical equipment technicians (BMETs), who must keep a full maintenance history that incorporates a detailed synopsis of all repairs; records of preventive maintenance procedures performed (date, by whom, what, etc.); and any modifications made to the device. This process should be performed by BMETs because of its role to render medical equipment and supplies free of viable microbes. It is part of a complex system used to prevent nosocomial infections. Proper maintenance is essential for proper performance, and this is critical to keeping patients safe. Further, documentation of such performance can be crucial during defense of lawsuits over hospital-acquired infections. While biological indicators are used to ensure that the items processed are in fact sterile, false-positive results can occur even when the sterilizer is functioning properly and the load is sterile. Maintenance records can be vital in determining the fault of either the indicator or the sterilizer.There are a plethora of standards and guidelines covering various aspects of sterilization and sterilizers. Most sterilizers made and/or used in the United States are considered pressure vessels and must conform to the requirements of the American Society of Mechanical Engineers' Code for Boiler and Pressure Vessels. There are also a number of joint AAMI and American National Standards Institute (ANSI) standards and guidelines that cover recommended sterilization practices and chemical and biological indicators for sterilizer monitoring. Other countries such as Australia, Canada, and England, as well as the European Economic Community, have laws, standards, and guidelines covering all phases of sterilizer design, operation, and use.The broadest risk involving sterilizers is that of nosocomial infection. An improperly sterilized load—one that contains live pathogens—can unnecessarily expose both hospital patients and staff to infections from relatively common cold and flu viruses to Methicillin-Resistant Staphylococcus aureus (MRSA). Improper sterilization in research facilities can expose staff to even more dangerous bacteria and viruses.The greatest risk, which can be mitigated by good maintenance practices, involves the temperature of the pressurized jacket and chamber. While idle, the jacket is pressurized to around 30 psi (about 207 kPa) with steam. This results in a relatively high surface temperature of the jacket and associated piping, typically approaching that of boiling water. The steam pressure equates to very high temperature (about 250°F to 270°F) steam, so even a small steam leak will cause almost immediate second-and third-degree burns.The last issue, which holds true for all sterilizers, is knowing how to properly operate the unit and being aware that sterilizers may present hotter-than-expected operating temperatures.Trained BMETs should maintain sterilizers. Steam sterilizers habitually require dry steam and periodic replacement of parts exposed to high heat and pressure, especially door gaskets. Likewise, if the water supply is considered hard or scale and rust are in the lines, water valves will require more frequent rebuilding or replacement.Other equipment problems appear as a result of false-positive biological indicators; these can occur even when the sterilizer appears to be operating normally. For example, poor rack loading (over loading or packing a shelf too tightly), a worn door gasket, or mechanical problems with the vacuum system can cause inadequate air removal from vacuum steam sterilizers, which will result in a false-positive. All modern sterilizers contain an event recorder that makes a permanent recording of sterilizer parameters during each load, which provides additional evidence to aid in diagnosing a problem. The BMET must be able to determine the most likely cause, or whether there is a totally different problem—a defective lot of biological indicators or the accidental use of an incorrect biological indicator (such as one for a gas sterilizer). Even in cases where the biological indicator properly shows the load is sterile, improper, incomplete, and ineffective instrument cleaning before pack preparation can result in unsterile instruments within the wrapped and sterilized pack.Following the manufacturer's recommended maintenance procedures, especially the scheduled replacement or rebuilding of certain valves, is important in achieving high uptime. The parts time-phased replacement schedule is based upon the average operation of hundreds of similar units. Even though there may appear to be nothing wrong with a valve diaphragm or steam trap, experience has shown the part is about to fail. Proactively scheduling downtime for its replacement is far superior to waiting until there is a failure. Likewise, due to the pressures involved, maintenance of both steam and gas sterilizers must be performed in strict accordance with the manufacturer's instructions using quality repair parts and best maintenance practices. Replacement of a leaking safety pop-off valve with a pipe plug, for example, was the documented cause of a sterilizer-related explosion.Maintenance of most sterilizers is fairly straightforward. Experience in pneumatics, pressurized steam and water systems, and basic electrical circuitry is helpful, as is a good understanding of general mechanical principles. Minimal electronics experience is required since any printed circuit board used in the sterilizer is replaced as a single part. Schematics and board components are generally not available as repair parts. Knowledge of their operating sequence of events and a good bit of common sense are the main tools needed to maintain these units. In cases where a number of sterilizers are maintained in-house, a max register thermometer (one that holds the maximum temperature reading obtained until it is reset) or an in-drain recording thermometer and an accurate high temperature, steam and water-compatible pressure gauge are essential service aids.Steam sterilizers have been around hospitals for more than a century and their basic design is quite stable and proven. Over the past 40 years, incremental improvements have occurred in air removal, monitors, and the use of microprocessor-based cycle controls. It is believed that these incremental improvements will continue since no other sterilization method has been proven as cost-effective at providing sterile supplies.

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