Describe the autoclave with its principle, types, uses and sterilisation control.

Principle

Water boils at 100°C at atmospheric pressure → Increasing the pressure raises the boiling point → At 15 lb/in² (1 atmosphere above ambient), steam reaches 121°C → Steam condenses on the cooler load, releasing a large amount of latent heat and wetting the surface → Moisture and heat together denature and coagulate protein → Holding for 15 minutes at 121°C kills all organisms including spores

Types of Autoclave

• Simple (pressure cooker) type — downward displacement, in which steam entering at the top pushes the denser air out through a discharge tap at the bottom; suitable for unwrapped solid instruments and fluids
• Gravity displacement — the standard hospital jacketed autoclave, working on the same principle
• Pre-vacuum (high-vacuum) type — a pump evacuates the chamber before steam is admitted, so air removal is virtually complete and penetration into porous loads is immediate. It allows shorter cycles at 134°C and is essential for wrapped packs, dressings, linen and porous loads

Uses and Limitations

• Uses — surgical instruments, dressings, gowns and linen, gloves, rubber and suitable plastics, culture media, aqueous solutions, and the decontamination of infectious laboratory waste before disposal
• Not suitable for — oils, greases and powders (steam cannot penetrate them, so a hot air oven is used); heat-labile plastics, endoscopes and prosthetic materials (ethylene oxide or hydrogen peroxide plasma); and sharp cutting edges, which are blunted
• Practical requirements — do not overload; leave space between packs; open the taps or lids of containers so steam can enter; and begin timing only when the whole load has reached temperature, not when the gauge does
• Fluids must be cooled slowly to avoid flash boiling and bottle explosion when the door is opened

Sterilisation Control

• Physical — thermometer, pressure gauge, timer and a printed cycle chart; these confirm the machine's conditions but not the centre of the load
• Chemical — autoclave tape (shows exposure only, not sterility), BROWNE tubes (red to green), and the BOWIE-DICK test for air removal, done as the first cycle of each day
• Biological — the definitive control: spore strips of Geobacillus stearothermophilus, which are the most heat-resistant organisms known and germinate at 55 to 60°C if the cycle failed. If these spores are dead, everything else is
• Records must be kept for every cycle, with the load identified, so that a failed cycle can be traced and the instruments recalled

Applied Aspects

• Autoclave tape proves exposure, not sterility — relying on it alone is the commonest false reassurance in a small hospital or clinic
• Run a biological indicator regularly (weekly, and with every implant load), and act on a failure by recalling and reprocessing
• Clean instruments before autoclaving; blood and tissue shield organisms from steam and may bake on permanently
• Autoclave infectious laboratory and hospital waste before disposal, in line with the Biomedical Waste Management Rules
• Do not use a "flash" cycle as routine practice for convenience; it is intended for an urgently needed unwrapped instrument, and packs sterilised this way cannot be stored
• Where autoclaves are unreliable or power is intermittent, as in some peripheral Indian facilities, this must be recognised and addressed rather than assumed away — unsterile instruments transmit hepatitis B, C and HIV