Sterilization and Disinfection

Sterilization is the destruction of all microorganisms (bacteria, viruses, spores) on an item. It usually refers to objects (e.g., instruments, drapes, catheters, needles) that come in contact with tissue or enter the vascular system. Disinfection is the destruction of most pathogenic microorganisms on inanimate (nonliving) objects, whereas antisepsis is the destruction of most pathogenic microorganisms on animate (living) objects. Neither procedure claims to kill or inactivate all microorganisms, even when used properly. Antiseptics are used to kill microorganisms during patient skin preparation and surgical scrubbing (see Chapters 5 and 6); however, the skin is not sterilized. Cleaning is generally restricted in meaning to the physical removal of surface contaminants, usually with detergents or soap and water, ultrasound, or other methods. Although cleaning does remove soils and bacteria, it does not kill or inactivate viruses or bacteria.

Disinfection

Disinfection usually involves the use of liquid compounds, such as phenol or its derivatives, alcohols, halides, aldehydes, quaternary ammonium compounds, chloroform, ethylene oxide (EtO), heavy metal ions, or dyes. Selection of the appropriate disinfectant depends on the desired result; some disinfectants are effective at destroying a limited number of microorganisms; others are effective at killing all organisms, including spores. Common disinfectants, their uses, and necessary precautions are listed in Table 2-1.

TABLE 2-1

Common Disinfectants Used in Veterinary Practice

Sterilization

Any equipment or supplies that come in contact with body tissues or blood must be sterile. Methods of sterilizing surgical instruments or other equipment include steam, chemicals, plasma, and ionizing radiation. The reliability of any sterilization method depends on the number, type, and inherent resistance of microorganisms on the items to be sterilized and whether other materials (e.g., soil, oil) are present on the items that may shield against or inactivate the sterilizing agent. Commonly used sterilization processes have a variety of advantages and disadvantages. For example, the steam autoclave, a 200-year-old sterilization technology, is an effective sterilization process, but its high temperature and moisture make it unusable for many of today’s devices. Likewise, dry-heat sterilization has process temperatures that cannot be tolerated by most devices. More recently, low-temperature sterilization systems (e.g., hydrogen peroxide gas plasma, peracetic acid immersion, ozone) have been developed and are being used to sterilize medical devices (Rutula and Weber, 2008).

Low-temperature, low-moisture processes, such as sterilization by EtO gas, hydrogen peroxide gas plasma, or peracetic acid, must be used for many medical devices (see pp. 13-15). Increasingly, operating room (OR) personnel are being asked to sterilize equipment more quickly and efficiently and at lower cost. Advanced sterilization systems that enable more rapid availability of wrapped, sterile devices and instruments may result in more rapid turnover of the OR suite and less “down time” between procedures. Swift and efficient sterilization of expensive heat- and moisture-sensitive medical and surgical devices (e.g., cameras, fiber-optic cables, rigid endoscopes) is particularly advantageous when costs of such equipment may limit their duplication in most veterinary practices. A low-temperature hydrogen peroxide gas plasma sterilization system that provides terminal sterilization of sophisticated instruments in 55 minutes is useful for such devices.

Steam Sterilization

Saturated steam under pressure is a practical and dependable agent for sterilization of heat-tolerant medical supplies and packaging. Steam rapidly destroys all known microorganisms by means of coagulation and cellular protein denaturation. To ensure the destruction of all living microorganisms, the correct relationship between temperature, pressure, and exposure time is critical. If steam is contained in a closed compartment and the pressure is increased, the temperature also increases, provided the volume of the compartment remains the same. If items are exposed long enough to steam at a specified temperature and pressure, they become sterile. The unit used to create this high-temperature, pressurized steam is called an autoclave. Certain types of microorganisms have greater inherent heat resistance than do other organisms. Spores of thermophilic aerobes and anaerobes are the most resistant known forms of life to moist heat. Virus particles are much less tolerant to steam sterilization than are spores.

Sterilization failure may occur if packs are wrapped too tightly or are improperly loaded in the autoclave or the gas sterilizer container. Instrument packs should be positioned vertically (i.e., on edge) and longitudinally in an autoclave. Heavy packs should be placed at the periphery, where steam enters the chamber. A small amount of air space is allowed between packs to facilitate the flow of steam (1 to 2 inches between the packs and away from the surrounding walls). Linen packs are loaded so that the fabric layers are oriented vertically (i.e., on edge). These packs are not stacked because increased thickness reduces penetration of the steam. Close supervision and exact standards for preparing, packaging, and loading of supplies are necessary for effective steam and gas sterilization. Sterilization indicators should be used (see p. 16).

Types of Steam Sterilizers

Gravity displacement sterilizer

The most commonly used steam sterilizer in veterinary practice is the gravity (or “downward”) displacement sterilizer (Figs. 2-1 and 2-2). This sterilizer works on the principle that air is heavier than steam. Supplies to be sterilized are loaded into the inner chamber. A narrow, outer jacket-type chamber surrounds the inner chamber. Pressurized steam from the narrow, outer chamber enters the inner chamber and surrounds the supplies. Air in the inner chamber is pulled downward by gravity to the floor and exits through a temperature-sensitive valve. As steam accumulates and the temperature increases, the steam-release valve closes. Because the function of this sterilizer is based on the ability of air to move to the bottom of the autoclave, careful wrapping (see p. 3) and loading of supplies are critical (see previous discussion). The minimum time and temperature standards for a gravity displacement sterilizer are 10 to 25 minutes at 270° F to 275° F (132° C to 135° C) or 15 to 30 minutes at 250° F (121° C). Table 2-2 shows the recommended sterilization times for commonly sterilized items.

TABLE 2-2

Exposure Periods for Sterilization in Gravity Displacement Sterilizers

ITEM	MINIMUM TIME REQUIRED, min, 250° F–254° F (121° C–123° C)
Scrub brushes (in dispensers, cans, individually wrapped)	30
Dressings (wrapped in muslin or paper)	30
Glassware (empty, inverted)	15
Instruments (wrapped in double-thickness muslin)	30
Instruments combined with suture, tubing, porous materials (wrapped in muslin or paper)	30
Metal instruments only (unwrapped)	15
Linen—maximum size 12 × 12 × 20 inches (6 kg wrapped)	30
Needles (individually packaged in glass vials or paper, lumens moist)	30
Needles (unwrapped, lumens moist)	15
Rubber catheters, drains, tubing (wrapped in muslin or paper, lumens moist)	30
Rubber catheters, drains, tubing (unwrapped, lumens moist)	20
Utensils (wrapped in muslin or paper, on edge)	20
Utensils (unwrapped, on edge)	15
Syringes (unassembled, individually packaged in muslin or paper)	30
Syringes (unassembled, unwrapped)	15
Suture—silk, cotton, nylon (wrapped in paper or muslin)	30
Solutions: 75 to 250 ml	20 (slow exhaust)
500 to 1000 ml	30 (slow exhaust)
1500 to 2000 ml	40 (slow exhaust)