Page 177 - Assurance of Sterility for Sensitive Combination Products and Materials

Preventing health care associated infections 159

sporicidal and rapidly heats and penetrates fabrics. Moist heat does have
some deleterious effects on metal, e.g., corrosion and certain polymers. The
goal of moist heat sterilization is to expose all surfaces of a medical device
to direct steam at the required temperature and pressure for a specified
time. Air can prevent steam from contacting the surface of a medical device.
Therefore, it is critical to remove air from the sterilizer chamber for steril-
ization to occur.
Steam quality and steam purity are also important to the sterilization
process. Steam quality is a characteristic of steam reflecting the dryness frac-
tion of the steam and the presence of noncondensable gases in the steam.
A steam quality of 0 indicates 100% liquid while a steam quality of 100
indicates 100% steam with no entrained water. Depending on the type of
product being sterilized steam quality should be at least 90%–97% [34].
Steam purity is the degree to which steam is free of dissolved and suspended
particles, water treatment chemicals, and other contaminants [34]. If steam
quality is poor, less than 90%–97% saturated (depending on the product
being sterilized), it will be difficult to achieve sterilization temperature on
all surfaces of the devices being sterilized. If steam purity is poor, the de-
vice may be contaminated by the process and stains may occur on device
surfaces.
There are two basic types of steam sterilizers: dynamic air removal and
gravity displacement. Dynamic air removal can be accomplished in two
ways, by pulling a vacuum or by a process called steam-flush pressure-pulse
(SFPP). The equipment differs in how it removes air from the chamber. In
gravity displacement, steam is injected into the chamber at the top. As the
steam builds up in the chamber it forces the air out through a drain in the
bottom of the chamber. This process is slow and inefficient. Device design
can result in pockets of air being trapped, preventing steam from contacting
all device surfaces. Devices must be positioned correctly in a gravity dis-
placement steam sterilizer. Devices with lumens cannot be sterilized in a
gravity displacement steam sterilizer.
In a pre-vacuum steam sterilizer air is removed from the sterilization
chamber by using mechanical means to draw a vacuum through the drain at
the bottom of the sterilizer. Steam is then injected at the top of the cham-
ber. This process is usually repeated 2–3 times for a total of 3–4 pulses. After
the last pulse steam is injected into the chamber and the sterilizer enters the
exposure phase of the cycle.
SFPP sterilizers pulse steam into the chamber and use positive pressure
to force air out of the drain of the sterilizer. Usually 3–4 steam pulses are