Hospital Sterilization Methods: Autoclaving and Ethylene Oxide Gas

Autoclave (Moist Heat Sterilization)

The autoclave is a device used in hospitals for moist heat sterilization. It effectively destroys spores, fungi, bacteria, and viruses. It maintains a temperature of 120 ºC for 15 to 30 minutes, depending on the size and nature of the load.

Autoclave Components and Operation

The autoclave features a reservoir with thick stainless steel walls and a tight-fitting lid. Key components include:

• A pressure gauge indicating the internal pressure and temperature.
• A safety valve that opens automatically if the pressure reaches a dangerous level, preventing explosion.
• A bleed valve (or purge valve) through which steam is released.
• A tank containing water up to a certain level, which is heated by electric heat, gas, or other means to generate steam.
• A grid or rack used to place the objects being sterilized.

Phases of Autoclaving

1. Packaging: Place the items on the rack, close the lid, and engage the system with the bleed valve open.
2. Vapor Exposure: Close the bleed valve. Pressure begins to rise. Once the desired pressure is reached, it is maintained for 15 to 30 minutes.
3. Evacuation of Steam: Open the purge valve to release the steam.
4. Drying: The heat retained by the walls and the evacuation of vapor dry the material. When the steam stops coming out and the gauge reads zero, the lid can be opened to remove the sterilized objects.

This method is suitable for materials that do not degrade when exposed to moist heat or high temperatures.

Advantages of Autoclaving

• Fast process time.
• Great penetrating power of steam.
• Low cost.
• Leaves no toxic residues.
• High efficacy and rapid action.
• Ease of control and manageability.

Disadvantages of Autoclaving

• Requires very high temperatures.
• Corrodes metals.
• Degrades thermolabile materials.
• Deteriorates sharp edges.
• Impossible to sterilize oil, grease, and dust.

Sterilization by Ethylene Oxide (EO) Gas

Ethylene Oxide (EO) gas sterilization, along with autoclaving, is one of the two most commonly used methods in hospitals.

Properties of Ethylene Oxide

EO is colorless at room temperature, with a density nine times greater than air. It is toxic by inhalation, flammable, and explosive, so it is typically mixed with other gases to minimize these hazards. (Note: Withdrawal of certain uses was considered around 2015.)

This method removes microorganisms quickly, offers high penetrating power, and does not deteriorate materials. It is typically used at temperatures between 30 ºC and 60 ºC. Once sterilization is complete, the material must be aerated to eliminate the residual toxicity of the gas.

Phases of the Ethylene Oxide Process

1. Air Conditioning: Pre-heating of the chamber, placement of the material, closing the chamber, and removal of air.
2. Exposure to Gas: The chamber is wetted with water vapor, the gas enters, and the exposure time is maintained.
3. Gas Evacuation: The chamber is emptied of gas.
4. Aeration: Filtered air is introduced into the chamber several times to remove residual gas toxicity.