Commentary: Role of sterile air in ophthalmic surgery operation theater (2024)

Sterile air is an indispensable requirement for the smooth functioning of an ophthalmic operation theatre (OT). Unsterile air has been believed to be a major cause of outbreaks of post-surgical infections. Outbreaks of fungal and even bacterial endophthalmitis have been reported where contaminated air was believed to be the major cause.[1,2] Unattended air conditioning and humidifying units are often culprits. Positive pressure filtered air should be delivered vertically down to the OT table.[3] Sterile air can be obtained with the help of high efficiency particulate air (HEPA) filters, laminar air flow system (LAF), air curtains and UV irradiation.[4] HEPA filters trap 99.97% of particulate of size 0.3 micron (μ). They achieve this by various mechanisms like direct impaction, sieving, interception and diffusion. Most microbes are in range of 0.5 to 5 μ in size. LAF provide unidirectional air flow towards OT table with around 300 air changes per hour. LAF avoids any turbulent air flow or swirls which deposits particulate dirt in a random manner. Proper positioning of surgical equipment, tables, etc. should be done for optimal functioning.[5]

Large setups can have proper Heating, Ventilation and Air-Conditioning system (HVAC). The quality of air is maintained in OT as per ISO class 6 standards.[6] It controls temperature, humidity, particulate count, and pressure gradient between different OT zones. The extensive setup consists of the air handling unit (AHU), inflow and outflow ducts, air-conditioning compressor, air blower, pre-HEPA filters (3 and 5 μ filters), and laminar air flow (LAF) plenum with terminal 0.3 μ HEPA filter. It should be given due importance at the time of OT designing. It should be functional throughout the day. Air filters in AHU can be cleaned weekly and fogged with bactericidal solution. HEPA pre filters can be changed after every three months. AC filters should be cleaned once in a week with detergent and sun dried. AC repairing and servicing needs to be done once in every three months.[4] Other factors like number and movement of OT personnel, exposure of skin and hairs can lead to accumulation of particulate biological matter. Wet mopping of floors and walls should be done instead of dry mopping. Sliding doors should be preferred at entrances.

US Public Health Service guidelines state temperature of OT should be maintained between 18°C and 24°C, humidity of 50% to 55%, 25 air changes per hour and up to 80% recirculation of air is allowed with effective filtering.[4] The present study has concluded similar infection rates with or without the use of HEPA and LAF but OT air quality must be ensured at all times with available resources.[7] Latest NABH guidelines have recommended use of terminal HEPA filters and AHU in ophthalmic OTs with adherence of Class 1000/ISO class 6 air quality standards with positive pressure ventilation. Window and split ACs are not recommended.[8]

Microbiological surveillance of OT air can be ensured by air sampling (using slit air sampler), swabs or settle plate method. Settle plate method is commonly used where duration of exposure should be minimum as that of time taken for the shortest surgery. In HVAC systems, particulate count of 0.3 and 0.5 μ size can be additionally done.[6]

For instrument sterilization, pre vacuum based autoclaves are now preferred as they ensure complete removal of any air pockets in sterilisation packets/trays. Sterile air should always be injected by the surgeon for any intracameral or intravitreal indications. Disposable gas filters should be used. Use of direct injection of room air should be discouraged. Thus, by following adequate precautions, air can be maintained pathogen free which will help in maintaining asepsis during surgery.