and Sterile Techniques

Fig. 3.1

Cleaning of all visible soil and contaminant should be done before any disinfection

The Spaulding Classification scheme categorizes medical equipment decontamination based on the risk of infection that is posed to a patient. The categories of medical instruments are classified as critical, semi-critical, and noncritical, as described below:

Critical: device exposed to or enters normally sterile tissue
Semi-critical: transducer exposed to mucous membranes or non-intact skin
Noncritical: transducer in contact with only intact skin [13]

Critical items are associated with the highest risk of infection and require sterilization. This is usually reserved for intraoperative ultrasound use and does not generally apply to the use of point-of-care ultrasound. In situations where an ultrasound probe is needed, a sterile sheath should be used, and the probe should be sterilized between each patient. If sterilization of the ultrasound probe is not possible, then high-level disinfection should be performed along with the use of a sterile probe cover [1].

A semi-critical exam results in the exposure of the transducer to mucous membranes, body fluids, or non-intact tissue. This necessitates that the transducer undergoes high-level disinfection (HLD). HLD is a disinfecting process that will eliminate all microorganisms with the exception of microbial spores. It is also recommended that the probe be covered with either a sterile sheath or appropriate nonsterile barrier during semi-critical exams. For example, a commercial sterile ultrasound transducer sheath should be used for procedures such as central venous line insertions, while a properly disinfected endocavitary transducer would be covered with a commercial nonsterile condom-like cover for a diagnostic scan in the absence of a sterile field. One study on decontamination practices of transvaginal ultrasound probes found that 0 of 68 hospitals performed high-level disinfection following their use, although all facilities used some type of probe cover [14]. The use of a probe cover has been interpreted by some as changing the Spaulding device Classification, but in general, the examination type and risks involved should determine the level of decontamination. Little is known about potential liability from contaminated ultrasound transducers in point-of-care settings. However, the fact that great variability exists among point-of-care ultrasound providers in disinfection standards adherence coupled with persistence of pathogens on probe surfaces in studies even after intermediate disinfection measures suggests risk potential [15, 16]. Experience in other areas such as endoscopy and transesophageal echocardiography (TEE) laboratories suggests that large-scale infective agent exposure is possible in some cases and carries considerable liability both civilly and in terms of public relations for an institution. All of these factors should lead providers to adhere to hospital and published standards for all ultrasound examinations. One of the few point-of-care medical societies to release guidelines to date, American College of Emergency Physicians, has released guidelines covering the breadth of current point-of-care ultrasound practice and relevant transducer care and decontamination [17].

If a transducer only comes into contact with intact skin, then only cleaning and low-level disinfection are needed. Intact skin will serve as a barrier against microorganism transmission which decreases the risk of transmission. Low-level disinfection means that most bacteria, some viruses, and some fungi be removed, but bacterial spores and some resistant microorganisms may remain (Table 3.1) [13].

Table 3.1

Spaulding classification in point-of-care ultrasound

Device classification	Risks involved	POCUS examples	Level of decontamination
Critical	Invasive sterile tissue/vascular system	Intraoperative liver biopsy	Sterilization
Semi-critical	Contact with mucous membranes, body fluids, or non-intact tissue	Transesophageal echocardiography, transvaginal ultrasound, intraoral ultrasound	High-level disinfection
Noncritical	Transfer of microorganisms on intact skin	Transabdominal/transthoracic ultrasound	Low-level disinfection

Disinfectants and Methods for Transducer Decontamination

When choosing a disinfectant, there are several factors to take into account, with the most important being device classification. In the USA, the Food and Drug Administration (FDA) has not cleared disinfectant wipes and topical spray products for high-level disinfection [18]. There is a broad range of disinfectants available commercially, with all high-level disinfectants (HLD) requiring the probe to be immersed in a chemical solution. When choosing a disinfectant, there are several other considerations that should be taken into account as well:

Level of decontamination necessary based on device classification
Compatibility with probe and manufacturer guidelines
Length of processing and disinfection time
Need for ventilation and personal protective equipment
Risk for transducer degradation
Risk of toxic exposure to personnel and the patient
Cost of installation and daily operation

Common high-level disinfectants include glutaraldehyde, hydrogen peroxide, and ortho-phthalaldehyde. Glutaraldehyde (e.g., Cidex^®) is used very commonly in the healthcare setting to provide high-level disinfection. High-level disinfectants should not be used for noncritical devices due to its toxicity and relatively high cost [1]. These disinfectants can take anywhere from 5 minutes to 45 minutes of contact with the probe to achieve high-level disinfection. Caution should be used when selecting a HLD, as they are toxic and can cause respiratory or mucous membrane irritation [19]. Some high-level disinfectants may require a separate cleaning room, ventilation hood, and personal protective equipment to be used [20]. For example, installing a Cidex OPA station without a hood may require an adjustment in the total room air changes per hour in order to meet FDA and OSHA standards. Providers should coordinate such initiatives with their facility biomedical and infection control departments.

Another option to achieve HLD is through an automated, hydrogen peroxide mist device (e.g., Trophon^® EPR). It requires the probe to be manually cleaned and placed in a self-contained unit for disinfection. Benefits of this include limiting user error related to immersion time and decreased exposure of personnel to the disinfectant [19]. This makes it a possible and, in some cases, an ideal point-of-care disinfecting solution. Experience in many training programs seems to carry a common thread; if left up to trainees to clean and care for transducers as well as manage infection control equipment maintenance, proper technique and maintenance quickly fall off. Automated devices, clear chains of responsibility, and involvement of full-time hospital staff for regular maintenance, cleaning, and upkeep are the best methods on ensuring quality and consistency.

Device manufacturers provide a list of compatible disinfectants for their devices. Some disinfectants can cause deterioration of the transducer, making it important to choose a compatible disinfectant. The depth to which a probe can be submerged in a solution varies between device manufacturers; therefore, the device manual should be referenced before submerging a transducer. It is very important to keep in mind that the entire probe is not electrically isolated and immersion of the portion from which the cord emanates can cause electric shock and equipment breakdown (Table 3.2) [18, 20].

Table 3.2

Chemical Disinfectants^a

Sterilants	High-level disinfectants^b	Low-level disinfectants
Ethylene oxide	Glutaraldehyde (e.g., Cidex^®)	Ethanol or isopropyl alcohol
Hydrogen peroxide gas plasma	Hydrogen peroxide solution and mist (e.g., trophon)	Chlorine compounds
Peracetic acid	ortho-phthalaldehyde (e.g., Cidex^® OPA)	Quaternary ammonium compounds
		Phenolics

^aThe device manufacturer chemical compatibility list should also be cross-referenced with your governmental regulatory agency (e.g., FDA) to determine compliance with approved standards

^bSome high-level disinfectants can work as chemical sterilants with increased immersion times

Probe Cover Type: Sterile Sheaths Versus Condoms

The use of a protective cover has become a routine when performing transvaginal exams with the endocavitary probe (Fig. 3.2). One issue that has persisted is what level of decontamination is necessary when performing semi-critical exams if a cover is used. Low-level disinfection is commonly used, but risks of probe cover failure and persistent probe contamination by infectious microorganisms should be taken into account [14]. The use of a probe cover does not guarantee that the probe is protected from contamination by bodily fluids [21]. There is a chance that the probe cover may perforate during use or that bodily fluids can contaminate the probe near the edge of the cover [22]. Several studies have shown a 3% and 21% probe contamination rate by human papillomavirus (HPV) DNA after low-level decontamination despite use of a probe cover [23–25]. Whether the presence of HPV DNA is clinically significant is not known, as persistence of HPV DNA has been detected on other medical equipment despite proper disinfection procedures [24]. There are also no documented reports of disease transmission from a contaminated transvaginal ultrasound probe [26]. It is recommended that high-level disinfection be performed between examinations even when using a probe cover in semi-critical devices.