Steriliser types

Steam under pressure kills/inactivates organisms on the surfaces of the reusable medical equipment by using the latent heat of condensation to coagulate protein. The use of steam under pressure is the most reliable method for sterilising cleaned reusable medical devices.

Sterilisers are classified according to the types of cycles they can run. Some sterilisers can run more than one type of cycle.

Class S cycles

Class S cycles are defined as those provided by sterilisers that have an active drying cycle and are capable of sterilising unwrapped solid goods and at least one of the following:

• porous products (limited/no relevance to general practice)
• small porous items (such as gauze swabs; limited relevance to general practice)
• narrow-lumen (hollow A) items: long, thin hollow items – these are not sterilisable by most sterilisers that provide Class S cycles and are generally reprocessed using a Class B cycle
• simple hollow items (hollow B): shorter hollow items such as punch biopsy tips
• single wrapped products
• multiple wrapped products.

The use of Class S cycles is adequate for most general practices. Various steriliser designs can provide Class S cycles.

Gravity (downward displacement) sterilisers are the easiest to monitor and the most cost-efficient.

Table 10.15 Steriliser types

Type	Class	Description	Example of temperature -pressure curve
Gravity (downward displacement) and active drying	N	Traditional type Relatively slow drying phase Air removal by continual release of air/steam mixture via chamber drain line (eg back to internal water reservoir)
Purge under pressure (assisted air removal)	S	Air removal by alternating inflow and outflow of steam
Single vacuum pulse (pre-vacuum, preliminary vacuum, vacuum assisted)	S	Air removal assisted by pump before sterilisation stage Drying time reduced by active (‘post-vacuum’) removal of steam by pump
Multiple vacuum pulse (fractionated)	B	Complete air removal via vacuum pump in multiple vacuum pulses before the sterilisation phase, to ensure complete air removal and steam penetration into complex hollow and porous items. Pulsed vacuum is also used in the drying phase to reduce the time and temperature during drying to ensure the porous / wrapping/ packaging is dry at the end of the cycle

Class B cycles

Class B classes are cycles capable of sterilising:

• all packaged items (single and double wrapped)
• narrow lumen items (Table 10.16 Definitions of hollow medical devices)
• solid and porous items.

Class B cycles are suitable for sterilising very long narrow hollow devices, which are rarely used in general practices. Class B sterilisers are commonly used in dentistry due to the frequent use of long, hollow reusable medical devices and the need for rapid processing of instruments.

In this type of steriliser cycle, air is removed by one or more vacuum stages to vent the air and allow in pressurised steam under pressure (fractionated vacuum system). Chamber pressures vary between types of systems: above and below atmospheric pressure (trans-atmospheric), above atmospheric pressure (supra-atmospheric) or below atmospheric (sub-atmospheric).

Sterilisers with Class B cycles (fractionated vacuum) are the most expensive to buy and operate and may require an onsite supply of deionised water.

Table 10.16 Definitions of hollow medical devices