Sterile Technique, Infection Control, and Transducer Disinfection

Section 8.3: Sterile Technique, Infection Control, and Transducer Disinfection

Quick Answer: Interventional ultrasound requires aseptic technique and a sterile field where a 1-inch outer drape margin is considered non-sterile. Sterile sheaths must be applied over the transducer with all air bubbles smoothed out to prevent acoustic impedance reflection. Standard precautions mandate treating all bodily fluids as infectious. Transducer processing follows the Spaulding Classification: low-level disinfection (LLD) is used for intact skin contact, whereas high-level disinfection (HLD) via chemical soak or automated nebulizer is mandatory for mucous membrane or non-intact skin exposure.

Sterile Field Setup and Maintenance

Ultrasound is frequently utilized to guide interventional procedures such as paracentesis, thoracentesis, liver biopsies, and renal biopsies. A core responsibility of the sonographer is establishing and maintaining a sterile field to prevent healthcare-associated infections (HAIs).

Principles of a Sterile Field

• The Sterile Boundary: The sterile field is a designated area prepared using sterile drapes where sterile instruments and supplies are placed. A key rule is that a 1-inch (2.5 cm) margin around the outer edge of the sterile drape is considered non-sterile. Anything that touches this 1-inch border is contaminated.
• Field Integrity and Monitoring:
  • The sterile field must be kept in constant line of sight. Never turn your back on a sterile field or leave it unattended, as contamination cannot be ruled out if the field is out of view.
  • All sterile objects must be kept above waist level. Anything that falls below the waist is considered contaminated due to the increased risk of contact with non-sterile clothing or unobserved objects.
  • Sterile personnel must pass each other back-to-back or front-to-front to avoid accidental contamination.
• Handling Sterile Supplies:
  • When opening sterile packages, the wrapper is opened away from the body first, then the sides, and finally the flap closest to the body.
  • Sterile items must be "dropped" onto the sterile field from a safe height (at least 6 inches) without the non-sterile packaging touching the sterile surface.
  • Only sterile personnel wearing sterile gloves may touch items within the sterile field.

Transducer Preparation for Interventions

When a transducer is introduced into a sterile field, it must be covered with a sterile sheath. The protocol is as follows:

1. Apply a sufficient amount of non-sterile ultrasound gel directly to the face of the transducer. This gel acts as a coupling agent between the transducer and the inside of the sheath.
2. The sonographer (or sterile physician) pulls the sterile sheath over the transducer, taking care not to touch the exterior of the sheath with unsterile hands.
3. Elimination of Air Bubbles: The sheath must be pulled tight across the transducer face, and any trapped air bubbles must be smoothed out. Air bubbles create a massive acoustic impedance mismatch, reflecting 100% of the ultrasound beam and causing shadowing or severe image degradation.
4. Secure the sheath with sterile rubber bands or clips.
5. Apply sterile gel (which is packaged in individual, single-use sterile packets) to the exterior of the sterile sheath before placing it on the patient's prepped skin.

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Aseptic Technique for Ultrasound-Guided Interventions

Aseptic technique refers to practices designed to keep an environment as free from pathogens as possible during invasive procedures.

Patient Skin Preparation

Prior to needle insertion, the patient's skin must be disinfected to reduce transient and resident microflora.

• Agents: Chlorhexidine gluconate (e.g., ChloraPrep) is the preferred agent due to its rapid action, broad-spectrum efficacy, and residual antimicrobial activity on the skin. Povidone-iodine (Betadine) is an alternative.
• Application Method: The antiseptic is applied using a friction rub. If using povidone-iodine, it is applied in a circular motion starting at the planned puncture site and moving outward in concentric circles. This ensures that pathogens are pushed away from the sterile entry point. The skin must be allowed to air dry completely before the procedure begins; blowing on the skin or wiping it dry contaminates the site.

Hand Hygiene and PPE

Sonographers must follow hand hygiene guidelines (washing hands with soap and water or using an alcohol-based hand rub before and after patient contact). For sterile procedures, the sonographer may need to perform a surgical hand scrub and wear:

• Mask and eye protection: To protect against splattering of body fluids.
• Sterile gloves: Must be donned using the open-glove or closed-glove technique without touching the outer sterile surfaces.
• Sterile gown: If assisting in high-volume fluid drainages (e.g., large-volume paracentesis).

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Infection Control and Standard Precautions

Standard Precautions dictate that all patient blood, body fluids, secretions, excretions (except sweat), non-intact skin, and mucous membranes must be treated as potentially infectious.

Personal Protective Equipment (PPE) Selection

• Gloves: Worn for any contact with blood, body fluids, mucous membranes, or non-intact skin.
• Gowns/Aprons: Worn during procedures where splashes or sprays of blood/body fluids are anticipated (e.g., drainage of a large abscess).
• Masks and Face Shields: Worn if there is a risk of aerosol generation or splashing (e.g., arterial punctures, high-pressure line placements).

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Transducer Cleaning and Disinfection Protocols

Ultrasound transducers require rigorous disinfection between patients to prevent cross-contamination. The Spaulding Classification system determines the level of disinfection required based on the transducer's contact with the patient.

Low-Level Disinfection (LLD)

• Indication: Appropriate for transducers that come into contact only with intact skin (e.g., standard abdominal scans on healthy skin).
• Efficacy: LLD destroys vegetative bacteria, most fungi, and medium-to-lipid-containing viruses (e.g., Hepatitis B/C, HIV), but it does not destroy bacterial spores or mycobacteria (such as Mycobacterium tuberculosis).
• Agents: Quaternary ammonium compounds (wipes or sprays) are standard. Isopropyl alcohol (70%) should be used with caution, as repeated exposure can degrade the transducer's acoustic lens, housing material, and bonding glues.

High-Level Disinfection (HLD)

• Indication: Mandatory for transducers that contact non-intact skin (e.g., open wounds, skin tears, rashes), mucous membranes (e.g., transvaginal or transrectal probes), or during sterile interventional procedures where there is a risk of sterile sheath puncture or contact with blood.
• Efficacy: HLD destroys all vegetative microorganisms, mycobacteria, fungi, and all viruses. Under extended exposure times, it may destroy some bacterial spores, though it is not equivalent to sterilization.
• Pre-cleaning Requirement: The transducer must be physically cleaned of all ultrasound gel and organic debris (using soap and water or an enzymatic cleaner) before undergoing HLD. Residual gel or biological matter acts as a physical barrier, shielding microorganisms from the disinfectant chemical and rendering the HLD process ineffective.
• HLD Methods:
  • Chemical Soak: Submersing the transducer shaft (excluding the connector plug, which must never be wet) in chemical solutions such as glutaraldehyde (e.g., Cidex) or ortho-phthalaldehyde (OPA). Minimum effective concentration (MEC) of the solution must be tested with test strips daily or before each use.
  • Automated Systems (e.g., Trophon): An automated device that uses a high-frequency ultrasonic nebulizer to generate a fine mist of vaporized hydrogen peroxide. This is a rapid, closed system that provides validated HLD without exposing staff to toxic chemical fumes.

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Spaulding Classification and Disinfection Summary