Special Departmental HVAC Issues: Operating Rooms
Two principles for air-conditioning operating rooms are that air should be supplied at the ceiling, in a unidirectional or laminar air pattern, and that higher air change rates result in lower bacterial counts within the room. However, these principles are applied along a wide spectrum, and ongoing research is being conducted to optimize air distribution airflow patterns and quantities.
Standard 170 requires that air in an OR be introduced at the ceiling and exhausted at low wall grilles. A minimum of two low grilles, 8 inches above the floor, shall be provided. Research by Memarzadeh and Manning (2002) and Memarzadeh and Jiang (2004) has indicated appropriate air patterns even with some return/exhaust grilles placed high on the walls.
Laminar flow diffuser arrays are required becaufse this minimizes any secondary air patterns or mixing of air currents within the room. The latest research in air flow patterns in ORs is for the average fvelocity of the diffusers to be 25 to 35 cfm/ft2. The diffusers shall be concentrated to provide an airflow pattern over the patient and surgical team. The area of the primary supply diffuser array shall extend a minimum of 12 inches beyond the footprint of the surgical table on each side. No more than 30 percent of the primary supply diffuser array area shall be use for nondiffuser uses such as lights or gas columns. Because the greatest amount of the bacteria found in the OR comes from the surgical team and their activities during surgery, turbulent air patterns within an operating room are to be avoided. To provide a unidirectional air pattern, laminar flow diffuser arrays (Group E), sized to introduce air into the room at low velocities of maximum face velocities of 20 to 35 fpm, are used. The velocity is low enough to prevent secondary entrainment of room air into the supply pattern.
Two filter banks for air handling serving ORs are required. Filter bank number 1, MERV 7, is located prior to any cooling coils, while filter bank number 2, MERV 14, is located downstream of any fan, coil, or drain pan. All filter efficiencies shall be in accordance with ASHRAE Standard 52.2.
Current design practices for air distribution in operating rooms rely heavily on research by Memarzadeh and Manning. Early research in the late 1960s by Kenneth Goddard started the industry dialog about total air changes needed in operating rooms to minimize post-operative infection rates. Goddard experimentally derived curves that quantify the relationship between air change rates and bacterial count (see Figure 5-11). Note on the curves that increasing air changes per hour from 20 to 25 reduces bacteria colonies per cubic foot of room air from 3.8 to 2.5; increasing the air supply to 40 air changes per hour further reduces the bacterial colonies to 1.5 per cubic foot. However, the curves approach a limit, so that approximately 0.5 bacteria colonies per cubic foot is the lowest measured. These curves have been used as one reason for supplying up to 40 air changes per hour in heart and orthopedic operating rooms.
There have been conflicting opinions about the relative benefits of increasing airflow rate and the use of high air change, laminar flow in operating suites. Because of other variables, it is impossible to directly correlate infection rates with total air changes or laminar flow. The latest guidelines allow 15 air changes in operating rooms. It has been found that the internal loads from people and equipment justify 20 to 25 air changes without regard to infection control. The curves produced by Kenneth Goddard show reduced bacteria at even higher (40 air changes) flow rates. Although the Goddard data is very dated, it is presented to indicate the early research for air distribution in operating rooms.
An air supply rate of 30 to 40 air changes per hour is about the maximum practical air supply available using a conventional hospital air-conditioning (A/C) system. However, some manufacturers offer packaged laminar flow modules, which are installed within a space and used to supplement the building A/C system, particularly if the building A/C system was not designed to provide either a unidirectional air pattern or a high air change rate. These manufactured units can be ceiling or wall-mounted and usually consist of a large perforated face diffuser, a supply plenum, an internal circulating fan, and HEPA filters (sometimes a cooling coil is included). These units will recirculate air within the room, filter it, and discharge it at a low velocity in a laminar airflow pattern. In organ transplant and chemotherapy patient rooms, wall-mounted units are used to introduce air at the head of the bed and return it from the foot of the bed. In operating rooms, the units are either wall- or ceiling-mounted. Packaged laminar flow units are typically seen only in teaching hospitals or where directly requested by orthopedic or cardiac surgeons.
ASHRAE Applications 2008 recommends MERV 17 HEPA filters in orthopedic ORs, bone marrow transplant ORs, and organ transplant ORs. While Standard 170 does not address these ORs, many feel HEPA filters are warranted. Placing terminal filters at grilles in series with HEPA filters at the air-handling unit is unnecessary and wastes fan energy while increasing maintenance. While standard ORs now require minimum of 20 ACHs total supply, high air quantities of 40 ACHs have historically been used in orthopedic and open heart ORs. Some still feel this is warranted; however, current research is still inconclusive on the effect higher air changes can have on reducing surgical site contamination. The current research on air velocity is based on the theory of a small thermal plume radiating up from an open surgical site. Ongoing ASHRAE research is investigating the effect of high air flow on temperatures of 80°F. Many hospitals are installing ORs with built-in imaging equipment, such as CT or fluoroscope machines or, in some cases, even MRIs. These hybrid ORs can be as large as 1,000 ft2, requiring high air flows. A 1,000 ft2 room with a 10-foot ceiling requires 3,300 cfm to maintain 20 ACHs. This air quantity is more than sufficient to handle the increase in cooling load due to the imaging equipment.
Some manufacturers of air distribution equipment have tested and now recommend an alternate air distribution system for ORs. This method is an air curtain system consisting of a laminar array above the operating table with a four-sided linear slot diffuser outside the perimeter of the surgical area. Typically between 65 and 75 percent of the air is supplied through the perimeter slot diffusers; the remaining air is supplied through the laminar diffusers. This method of air distribution is particularly advantageous for the larger ORs with high air volumes necessary to maintain 20 ACHs.
Coordinating the large number of laminar flow diffusers required with other services in the ceiling of an operating room is a real challenge. Medical gas columns and hoses, imaging equipment, and surgical lights all compete for placement with the ceiling diffusers. Constructing the ceilings in the operating rooms is especially challenging. Air distribution manufacturers offer integrating ceilings to reduce total installed cost and the time to completely finish the ceiling in an operating room.
Excerpt from: Mechanical Systems Handbook for Health Care Facilities
J. Robbin Barrick, PE, and Ronald G. Holdaway, PE
ASHE copyright 2014. Available at the ASHE Store.