Ultrasound is a well-known cleaning method which has stood the test of time and is used in precision mechanical workshops and also for surgical instruments. Indeed in some Anglo-Saxon countries surgical instruments are cleaned in an ultrasonic bath followed by manual or automated disinfection.
Ultrasound waves are mechanical waves that are emitted in liquid media and whose intensity decreases in accordance with the square of the distance. They are produced by ULTRASONIC TRANSDUCERS operating with either magnetic or electrical energy (piezoelectric effect). The ultrasound waves are broken on solid objects and walls, thus giving rise to ultrasonic shadowing. The ultrasonic baths customarily used in medical technology operate in the frequency range between 32 - 50 kHz.
The ultrasonic effect is based on what is know as cavitation, occurring mainly at the boundary surfaces between water and a solid object. Here cavities are formed containing gas at a negative pressure. When these cavities collide with each other SUCTION PRESSURES of up to 100 bar are generated and result in the removal of soil particles from solid surfaces. This effect cannot be generated in the case of soft objects, e.g. materials made of rubber, latex and silicone rubber. Recent investigations have demonstrated that distribution of the energy in a bath can vary greatly, thus producing different cleaning results.
Damage can occur during ultrasonic treatment. Therefore the general guidelines must be observed. To avoid an adverse effect on metallic parts, the frequency range should be complied with. The sonication period should be restricted to around 5 min, but this depends on the energy input. For example for ultrasonic cleaning of biopsy forceps, 30 min are recommended. Under no circumstances should optics, fibre optic cables and multiply coated instruments be cleaned with ultrasound. In the latter case there would be a risk of the coating being detached. In addition, the MANUFACTURER'S INSTRUCTIONS must be observed.
To reduce the surface tension and promote gas removal, a SPECIAL CLEANING AGENT must be added. It is important that the gas bubbles should emerge from the liquid. The cleaning agent should produce little foam, especially if cleaning is followed by automated processing. Otherwise, the item to be sterilised must be rinsed after removal from the ultrasonic bath. Surfactant cleaning agents with or without enzymes should be used preferably. But there are also suitably acidic or alkaline agents that can be used in the ultrasonic bath. On using these agents, their efficacy and the suitability of the material must be checked.
Under no circumstances should chlorine-based cleaning agents be used. It must be borne in mind that normal domestic cleaning agents may contain common salt so as to ensure viscosity. Hence chlorine may be released in the ultrasonic bath, thus damaging instruments and stainless steel baths. Therefore domestic cleaning agents, such as e.g. wash-up agents, should not be used either.
Ultrasound per se is not endowed with a DISINFECTANT EFFECT Instrument disinfectants recommended for immersion baths are often unsuitable for use in an ultrasonic bath. The increase in temperature expedites coagulation processes between proteins, aldehydes and quaternary ammonium compounds. This can result in adherence of solid residues to instruments in which microorganisms can survive, while the bath solutions show only low microbial counts. Appearing on the list of the German Society for Hygiene and Microbiology (DGHM) are only disinfectants intended for use in immersion procedures at 20°C, and which have been tested under the conditions applicable to the immersion bath. There are no certified instrument disinfectants or combined cleaning and disinfectant agents that have been tested as per the DGKH methods under the conditions applicable to the ultrasonic bath. But there are combined cleaning and disinfectant agents specially formulated for use in the ultrasound bath.
To protect personnel it must also be possible to inactivate the hepatitis B virus within the relatively short exposure times. But as a rule this cannot be guaranteed. Therefore, despite the use of a disinfectant product MEASURES FOR PERSONNEL PROTECTION must be taken such as avoidance of unnecessary handling, gloves, protection against splashing, etc. In the case of ultrasonic baths integrated in washer-disinfectors, e.g. in tunnel washers, the addition of suitable cleaning agents is sufficient in any case because this step is followed by thermal disinfection without the sterile items being handled. If aldehyde-based products are used the ultrasonic bath should be fitted with a lid to reduce escape of aerosols into the ambient air and curtail noise.
1 This article focuses primarily on the basic principles for using ultrasound. Loading instructions and experience from the practical setting will be given in the next issue of Central Service.