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The Benefits of Ultrasonic Cleaning
By Lawrence F. Muscarella, PhD
Growthin minimally invasive surgery has spawned the development of complex endoscopicinstruments designed to accommodate the physician's surgical technique. However,to the frustration and dismay of many reprocessing staff and infection controlpractitioners, these complex instruments typically are not designed tofacilitate cleaning and sterilization. Some feature delicate fiberoptic andplastic components readily damaged by heat, precluding steam sterilization.Examples include flexible endoscopes. Whereas steam is likely to conduct throughthe surfaces of surgical instruments and upon condensation release energy thatdestroys even the most resistant and inaccessible microorganisms,low-temperature gases, vapors, and liquid sterilants require direct contact tobe effective.1 Moreover, the physical design of some of these complexinstruments include narrow lumens and orifices that hinder the delivery of theselow-temperature chemical agents to every contaminated surface, as required toachieve sterilization.1
In addition to flexible endoscopes, biopsy forceps are an example of acomplex instrument that does not facilitate cleaning of all of its potentiallycontaminated surfaces, particularly its hinge mechanism that controls theopening and closing of its cups. Although complex and designed with an internalwire and lumen that can become contaminated with patient debris,2reusable biopsy forceps are designed using materials like stainless steel thatcan withstand the rigors and stresses of steam sterilization. Reports of steamautoclaved biopsy forceps transmitting disease have not been documented.2Only in instances when the biopsy forceps were inadequately cleaned or alow-temperature biocidal agent, such as 2% glutaraldehyde, was used in lieu ofpressurized steam has cross-infection been reported.3
Cleaning is an integral component of virtually all instrument reprocessingguidelines. Several endoscopy and infection control organizations have publishedguidelines for the proper cleaning and sterilization of flexible endoscopes,biopsy forceps and other types of endoscopic instruments.4,5 Iflabeled for reuse, instruments require manual pre-cleaning, using a brush anddetergent solution, to remove gross patient debris. Because complex endoscopicinstruments may remain contaminated with patient debris even after manualbrushing, most reprocessing guidelines recommend also using ultrasonic energy toremove fine debris that might otherwise be inaccessible and remain on theinstrument. Unless the cleaning process effectively removes microorganisms andorganic debris from even the most inaccessible surfaces of a contaminatedinstrument, the sterilization process is likely to fail. Developing moreadvanced cleaning technologies that can adapt to even the most complex surgicalinstruments and clean their most inaccessible internal surfaces is crucial tothe prevention of patient infection.
Ultrasonic energy: What is it and how is it produced?
Ultrasonic energy is an effective technology routinely used by healthcarefacilities to clean surgical and dental instruments prior to terminalsterilization.6,7 Different types of cleaning devices, designed bothfor bench (or table) tops or for the floor, use ultrasonic energy to optimizetheir cleaning effectiveness. This energy is produced by transducers mounted onthe outside of the cleaning device's processing basin, which is typicallyconstructed of stainless steel. When powered by an electronic generator, thesetransducers expand and contract at a very high frequency, converting electricenergy to ultrasonic waves of energy. These high-intensity sound waves commonlytravel at frequencies between 20 and 120 kHz (1 kHz equals 1000 Hz, oroscillations per second) throughout the processing basin. To enhance theefficient transmission of these waves of energy, immersing the soiledinstruments in an appropriate liquid medium, such as a detergent solution, isessential. Although ultrasonic waves are inaudible, harmonics of the ultrasoniccleaner's primary, or fundamental, frequency may produce audible sound.
As they propagate through the detergent solution, these ultrasonic wavesproduce alternating tensile and compressive forces that oscillate at the samefrequency as the transducers that produced them. These oscillating forces causemillions of microscopically-sized cavities to form in the detergent solution.Once they reach a critical threshold, these cavities violently collapse, orimplode, causing submicroscopic voids to form by a process known as cavitation.These voids induce the formation of high-energy hydraulic shock waves thatproduce a powerful suction-effect. These shock waves, which may reachtemperatures as high as 10,000Â°F and hydrodynamic pressures as low as 10,000pounds-per-square inch (PSI),8 physically loosen and removemicroorganisms and other adhering debris from even the most inaccessiblesurfaces of a soiled instrument.9,10
How effective is ultrasonic cleaning?
Through the years many studies that demonstrate the reliability andeffectiveness of ultrasonic cleaning have been published. Some of these studiesdemonstrate the effectiveness of ultrasonic cleaning in standardizing thecleaning process and removing dried serum, whole blood, and viruses fromcontaminated instruments.9,11,12 Other studies have found thatultrasonic cleaners are significantly more effective and efficient than manualscrubbing,11,13 which is difficult to standardize and can vary ineffectiveness from person-to-person.12 In contrast to manualscrubbing, ultrasonic cleaners are automated and standardized and designed toclean surfaces that might otherwise be inaccessible.
While manual cleaning is intended to remove gross debris from theinstrument's surfaces, ultrasonic cleaners are designed to remove microorganismsand other fine debris from less accessible surfaces. Some reports suggest thatultrasonic cleaning, preceded by manual scrubbing, results in an even greaterreduction in patient debris than achieved by either alone.8 One studydemonstrated that as few as three minutes of ultrasonic exposure was sufficientto remove more than 99.9% of blood on contaminated instruments.10Although data demonstrating its effectiveness in narrow lumens and channels ofsome complex instruments is limited, ultrasonic cleaners are recommended toincrease cleaning efficiency,14 particularly for surgicalinstruments, like biopsy forceps, that have complicated joints, hinges and otherinternal surfaces that are difficult, if not impossible, to clean manually.15
Despite all of its benefits, ultrasonic cleaning, like any decontaminationprocess, has its limitations, and understanding each permits its safe harnessingand effective application.7 For example, as a result of itsaggressive scrubbing action, ultrasonic energy is not indicated for all medicalinstruments. Although the reprocessing instructions of most surgical instrumentsrecommend ultrasonic cleaning as an integral step in their preparation forterminal sterilization, some instruments may be constructed of delicatematerials damaged by its power, precluding the use of ultrasonic energy.Materials, such as quartz, silicon, and carbon steel may erode or become etchedafter prolonged exposure to ultrasonic cavitation.7 Erosion caused byultrasonic energy can be minimized, if not eliminated, however, by reducing theultrasonic cleaner's power and cleaning time. Review of each instrument'sinstructional manual to determine whether ultrasonic cleaning is contraindicatedby its manufacturer is recommended.
Ultrasonic cleaning is usually one in a multi-step process that begins withmanual cleaning to remove gross debris. This step is performed immediately afterthe instrument's use to prevent patient soil from drying. Once manually cleaned,the instrument is then placed in the ultrasonic cleaner. This cleaning step isparticularly important for removing fine debris that may not have been removedduring manual cleaning. Some ultrasonic cleaners may automatically injectdetergent into the instrument's processing basin, as well as lubricate theinstrument to prevent corrosion prior to terminal sterilization. Some may alsobe equipped with channel adapters that flush a detergent solution thorough thelumens of cannulated instruments.
In general, ultrasonic cleaners feature a timer and temperature control toadjust the cleaning time and to increase the temperature of the detergentsolution, respectively. They may also be equipped with controls that permitadjustment of their power output (Watts) and frequency (kHz). Covers that reduceexposure of personnel to potentially harmful contaminants and aerosols duringcleaning, as well as instrument trays, holders and baskets, may be standard oroptional.
Factors that can enhance or limit cleaning effectiveness
Several factors can enhance or limit the cleaning effectiveness of anultrasonic cleaner. None is as significant as the physical properties of thecleaning solution (or other liquid medium) through which the ultrasonic wavespropagate. Briefly, the amplitude of the ultrasonic waves is directlyproportional to the electrical power applied to the transducers. Cavitationcannot occur unless the amplitude of these waves, and therefore the electricalpower, exceeds a minimum threshold value. The properties of the cleaningsolution, which include its temperature, viscosity, density, vapor pressure, andsurface tension, cause this threshold value to vary such that changes in any oneof these properties is likely to affect cleaning effectiveness.
In addition to aiding in the removal of patient debris from soiledinstruments, detergents increase cleaning effectiveness by reducing the water'ssurface tension. This effect increases cleaning effectiveness by: (a)facilitating the transmission of the ultrasonic waves through the detergentsolution; (b) lowering the minimum amount of ultrasonic energy necessary forcavitation to occur; and (c) reducing the resistance to flow of the detergentsolution through the instrument's narrow lumens and orifices. Detergentsspecifically formulated for ultrasonics, and known to be compatible with theinstruments to be cleaned, are recommended to increase cleaning effectiveness.10Neutral or alkaline detergents are the most commonly used formulation hospitalsuse with ultrasonic cleaners.
Temperature is also a significant a factor to the physics and effectivenessof cleaning. An increase in the temperature causes a corresponding increase inthe detergent solution's vapor pressure and a reduction in minimum energyrequired for cavitation. Mixing the detergent with warm water is thereforerecommended to enhance the effectiveness of ultrasonic cleaners. Temperaturesbetween 11Â°F and 140Â°F are usually indicated for water-based detergent. Ofcourse, to avoid damaging the surgical instrument, the temperature of the watercannot exceed the instrument's temperature parameters. Also, because reportsindicate that bacteria can proliferate in the ultrasonic cleaner's detergentsolution (and its aerosols) during the course of the day,16 using afresh volume of water for cleaning and rinsing each new batch of soiledinstruments may be advantageous to minimize personnel exposure to potentiallypathogenic microorganisms. Although costly and not required, using deionizedwater may also be advantageous, as, in addition to dissolving detergents moreefficiently, it does not contain minerals that frequently tarnish instruments.
Instrument baskets, trays: The benefits of ultrasonic cleaners cannotreally be appreciated without using specially designed instrument baskets,trays, or cassettes.7 These fixtures are typically constructed ofstainless steel (or other sound-reflecting material) and are often wired, meshedor sieve-like to ensure efficient passage of the ultrasonic waves. Each of thesefixtures is crucial, as it: (a) maximizes exposure of the instruments to theultrasonic waves; (b) minimizes movement of the soiled instruments against oneanother during ultrasonic cleaning, which can result in costly instrumentdamage; and (c) optimizes cleaning effectiveness by preventing the instrumentsfrom contacting the bottom of the cleaner's processing basin (the other side ofwhich the transducers are usually mounted) where they might interfere with theproper operation of the transducers and prevent transmission of the ultrasonicwaves.
Instrument arrangement: The method by which contaminated instrumentsare arranged in the processing chamber can have as much effect on cleaningeffectiveness as the choice of the detergent. Ultrasonic energy is uni-directional,traveling from its source (the transducers) in one direction through thedetergent solution. This potential limitation can be overcome by properlyarranging the contaminated instruments in the processing basket (or tray) tomaximize their exposure to and contact with the ultrasonic waves. Placing thecontaminated instrument's most heavily soiled surface towards the bottom of theultrasonic cleaner's processing basin optimizes cleaning effectiveness. Althoughnot usually necessary, rotating the instrument and repeating the ultrasoniccleaning cycle to expose all of its surfaces to the ultrasonic energy may beindicated if the instrument is heavily-soiled.
Cleaning time: In addition to the type and temperature of thedetergent, the time required to clean a instrument depends on, among otherfactors, the: (a) number and arrangement of contaminated instrument in theprocessing basin; (b) degree of instrument contamination (e.g., lightly-soiled,heavily-soiled); and (c) frequency and power of the ultrasonic cleaner.
Air bubbles: The presence of air bubbles in the cleaning medium alsoaffects cleaning time. Unlike audible sound waves emitted from a stereo speaker,ultrasonic waves require a liquid medium for their efficient transmission.Consequently, the surfaces of instruments that are spotted with air bubblescannot be effectively cleaned by ultrasonic energy. Nor can instruments beeffectively cleaned by ultrasonic energy if pockets of air remain between them.Similarly, detergent solutions that contain air bubbles and other gases arelikely to interfere with the efficient transmission of ultrasonic waves,reducing cleaning effectiveness. Once the ultrasonic cleaning cycle isactivated, however, degassing of--that is, removal of air and other gasesfrom--detergent solution or other liquid medium can be expected.7
Intensity and energy distribution: Most quantitative methods forevaluating the cleaning effectiveness of an ultrasonic cleaner can be verytime-consuming and cumbersome and usually require at least some subjectiveinterpretation of the results. A few methods, however, may be helpful inestimating their cleaning effectiveness.9,10 For example, the"aluminum foil erosion test" evaluates both the intensity anddistribution of the cleaner's ultrasonic energy.10 Several new sheetsof aluminum foil are placed vertically in the middle of cleaner's processingchamber filled with water. (Detergent is not used because this test is intendedto assess the intensity and distribution of the ultrasonic energy--not cleaningeffectiveness.) After several cycles, the sheets are examined for patterns oferosion or damage. The more significant and uniform the damage to the foil, themore powerful and uniform the intensity and distribution of the cavitation.
Power of cavitation: The ultrasonic cleaner's power of cavitation canbe evaluated by placing samples of a smooth material, such as gypsum, in theprocessing chamber filled with water.10 The samples are weighed bothbefore and after exposure to the cleaner's ultrasonic energy, and changes in theweights of the samples indicate mechanical erosion caused by cavitation. Anincrease in the power of the ultrasonic energy will usually cause an increase incavitation activity, and therefore an increase in the samples' weight loss.Cavitation activity can also be visually estimated by the examining the samples'surface for erosion. (A detergent is not used because this test is intended toassess ultrasonic power, not cleaning effectiveness.)
Cleaning effectiveness: Several tests have been suggested to evaluatethe cleaning effectiveness of ultrasonic cleaners. Visual observation of theextent to which patient soil is removed from a contaminated instrument, althoughsubjective, can be a reliable measure of cleaning effectiveness. Other morequantitative tests may assess cleaning effectiveness by measuring and comparingthe levels of a radioactively-tagged material, such as blood, before and afterultrasonic cleaning.8,10 The more significant the difference betweenthese two levels, the greater the expected effectiveness of the ultrasoniccleaner. The use of optical density and micro-assay techniques to measure theamount of protein (e.g., blood) removed from an instrument by anultrasonic cleaner has also been reported.11,12 In general,ultrasonic cleaners are expected to reduce at least 99.9% of patient soil on acontaminated instrument after only a few minutes of exposure. (These tests areusually performed using a detergent solution.)
In addition to increasing the productivity of reprocessing staff andminimizing the staff's exposure to contaminated instruments, ultrasonic cleanershave been shown to be more effective and efficient than manual scrubbing, whichis often laborious and whose results are often incomplete and unpredictable.7Other less obvious benefits of ultrasonic energy include its reportedenhancement of the sporicidal properties of liquid chemical sterilants. Onestudy found that ultrasonic energy reduced the time needed for a solution ofglutaraldehyde to destroy bacterial endospores from 3.5 hours to 30 minutes.17At a time when the popularity of low-temperature sterilization processes isgrowing, more emphasis and importance must be place on optimizing theeffectiveness of the cleaning process to compensate for the lower sterilityassurance levels of low-temperature sterilization processes compared to thermalsterilization.1 The development of instrument designs that facilitatecleaning and are not damaged by the rigors of cavitation is recommended toreduce the risk of cross-infection.
Lawrence F Muscarella, PhD, is the Director, Research and DevelopmentChief, Infection Control Editor-in-Chief of Q-Net Monthly Custom Ultrasonics,Inc.
For a complete list of references, visit www.infectioncontroltoday.com
The following list of questions may be helpful in thepurchase of an ultrasonic cleaner:
1. What are the dimensions of the ultrasonic cleaner? Is the size of itsprocessing chamber sufficient to accommodate the widths and lengths of all ofthe facility's soiled instruments?
2. How much power does the ultrasonic cleaner produce? Does the cleanerfeature one power setting, or it is equipped with different power settings topermit processing of lightly- and heavily-soiled instruments, as well asdelicate instruments?
3. What is the ultrasonic cleaner's frequency setting? Can it be adjusted?
4. What is the ultrasonic cleaner's standard cleaning time? Can this time beadjusted to permit extended cleaning for big loads and heavily-soiledinstruments?
5. Is the ultrasonic cleaner labeled for only a few types of instruments?Does the ultrasonic cleaner's manufacturer have any data to support its cleaningeffectiveness? What types of instruments if any are contraindicated?
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