Surgical spirit

Air distribution in operating theatres must not only ensure suitable indoor air conditions, such as the removal of thermal and material loads, but also guarantee low germ and anaesthetic gas concentrations in the operating zone and at the instrument table. However, when designing a system it is vital to include other factors such as the integration of the operating lighting, which must achieve provide sufficient light to the incision zone, without significantly disrupting the airflow. Normally the standard for an HVAC system requires a minimum air exchange, however in the case of operating theatres a minimum requisite supply air volume is specified. This value is different depending on the air distribution chosen. There are two types of system available: a dilution system and a displacement system.

Dilution system In the dilution system air pollutants in the operating theatre such as germs, particles and gases are distributed evenly to lower their concentration. This is carried out by using a turbulent mixing airflow. The supply air is discharged in turbulent air jets with a high velocity. The high-induction air jets mix the air intensively, which results in an even dilution of pollutants.

Displacement system The preferred method of supplying conditioned air to operating theatres and removing pollutants is by using a laminar displacement system. In this system, the air pollutants are displaced from the sensitive areas of the operating zone and the instrument table. This is carried out with low turbulence laminar airflow. The supply air is discharged at a low momentum and low turbulence over the operating table and flows at low velocity vertically downwards and displaces the polluted air from the operating zone to the floor. From here the air flows into the rest of the room and then to the return air vents where it is collected and removed. The difference in supply air discharge velocities between a dilution system and a displacement system is marked. The supply air in a turbulent mixing flow (dilution system) is discharged at velocities of 2 to 3ms-1, whereas in displacement flow the discharge velocity varies between 0.15 to 0.25ms-1. In a displacement flow system, the supply air volume can be supplied at a lower rate than for the dilution system, which must have a minimum supply air volume flow rate of 666 ls-1. However, when a lower supply rate is used, evidence must be provided to prove the system's effectiveness at removing pollutants. Although recirculated air can be used, there are factors governing its use. For example, the recirculated air must stem from the same room or same group of rooms. It must be cleaned at the same filter level as the outside air and the return air for recirculating air operation must be collected under the ceiling and not in the floor zone. Anaesthetic gases and disinfectant vapours cannot be removed in the filters of a recirculating air system and therefore the minimum outside air volume required to keep the concentration of pollutants within prescribed limits is 0.333m3s-1 per operating theatre. This outside air is also used to provide fresh air to the operating theatre staff and maintain the indoor air humidity, as well as a corresponding positive pressure to the adjoining rooms giving an additional level of containment to the operating theatre.

Filtration To maintain the strictest air hygiene, the supply air must be cleaned in the operating theatre at three separate levels. The first level filter must be at least F5 (a low-quality bag filter) and fitted near the intake point of the outside air. This first filter protects the ductwork from pollutants in the atmosphere. The next level filter, at F7 (high-quality bag filter), is installed at the entry of the air duct with the positive pressure. It protects the air ducts from pollutants either induced into the duct with a negative pressure or emitted in the air-handling unit (from the sound attenuator for example). The third and final stage filter is an H13 HEPA filter. This filter is built into the air outlet of the operating theatre and protects the room from pollutants, which may have been emitted in the supply air ducts, or not removed at the previous two filters. To avoid the build up of mould and bacteria the dew point for a HEPA filter should never be exceeded. Maximum relative supply air humidity should not exceed 95% ideally should be kept to 90% to avoid an undesirable loss in filter pressure.

Laminar outlet In displacement flow, the air pollutants are displaced from the operating zone and the zone itself is flushed with filtered fresh air. This makes for a lower germ concentration in the operating theatre when compared to the alternative dilution system. The air outlet for displacement flow is installed in the ceiling above the operating table and the supply air flows vertically down to the operating table. The lower the turbulence of the supply air jets, the less indoor admixture en route and the cleaner the air flow at the operating table. Air jet turbulence can be significantly influenced by the construction of the air outlet. With outlets such as Krantz Systems' operating theatre outlet (type OP-Z), an almost laminar airflow is generated, with an extremely low germ concentration at the operating table. In the outlet, the air discharge surface is made from a fine mesh sieve. Obviously the finer the mesh of the perforated metal sheet, the lower the airflow turbulence. In this case, the meshing width of the air outlet is well under 1mm and will not clog because the air is cleaned in two HEPA filters built into the casing. The aluminium frame into which the outlet is fastened can also be clipped open, so that the casing can be disinfected from the outside. The HEPA filters can also be cleaned in this way.

Airflow pattern of laminar outlet If cold, laminar supply air jets are discharged vertically downwards and the airflow accelerates over the jet length. As long as low discharge velocities are selected, air velocity increases, but the lack of turbulence ensures there will be no thermal discomfort to the operating theatre staff. The vertical laminar flow is stable for air discharge velocities of 0.15ms-1. Supply air temperature should be 0.5 to 4K below room temperature. With warm supply air jets the flow becomes unstable and does not reach the operation zone in an unmixed condition. Although in the marginal zone of the supply air jet admixture with indoor air is unavoidable, this jet constricts this in the mixed zone. The greater the temperature difference between indoor air and supply air, the greater this constriction. The minimum outlet width of 1.4m however, ensures that the operating zone of the 0.5m wide operating table is flushed with filtered and sterile supply air.

Influence of operation lightings on jet patterns Due to its typical flow pattern, laminar flow is susceptible to disruptions, particularly thermal ones. The low air discharge velocity ensures draught avoidance in the work zone but it also results in a higher airflow sensitivity. A flow disruption causes air circulations and whirls, which in turn result in a higher admixture with the less clean indoor air and an inevitable increase in the particle and germ count in the operating zone. The main thermal disruption variables are operating staff, medical supplies and monitoring apparatus as well as the operating lights; the positioning of the operating staff and lighting can also cause turbulence in the flow zone. The heat emitted by the occupants cannot be reduced. There are, however, effective ways of reducing the heat load from the medical supply and monitoring equipment and the operating lighting and of lessening their disruptive effect on airflow by means of a favourable aerodynamic lay out. Therefore factors that should be taken into account include: ensuring that the surface temperature of any lighting should be as low as possible, the projection surface should be as small as possible and the lighting should have an aerodynamic shape. Krantz Systems rigorously tested this outlet under varying light conditions, with various layouts and occupancies. From the tests, there are a number of conclusions that can be drawn. The first is that, where possible, the operating lights should not be positioned under the laminar outlet. Also, if two operating lights are used, only one should be positioned under the laminar outlet and finally, if both lights have to be positioned under the laminar outlet, the incision zone should not be situated between the lights or under one of them.

Hygienic tests The laminar outlet was subjected to hygienic tests according to DIN 4799 specifications. For this purpose 7 heated human dummies were placed around the operating table and an inoculation gas discharged at chest height. Nitrous oxide (N2O) was used as the inoculation gas. One of the seven human dummies was mobile. An eighth human dummy simulated the patient. The laminar outlet used was 1800mm wide. A dummy simulated an operating light in order to meet the requirements of DIN 4799. The second light was real. With the laminar outlet operational, the concentration of the nitrous oxide in the protection zone over the operating table and the instrument table was measured under steady conditions. The measuring height was 1200 mm. The gas concentration in the return air was also measured. Two-thirds of the return air was collected in the floor zone and one-third under the ceiling. DIN Standard 4799 states that in operating theatres requiring low germ concentrations the permissible tracer gas concentration should be <1. In rooms requiring very low germ concentrations it should be <2/3. In the results of the laminar outlet, local measurements taken above the operating table were between 0.05 and 0.17 with a mean value of 0.12, exceeding both DIN Standards. Tracer gas concentration in the return air amounted to about 500 ppm.

Summary The laminar outlet for operating theatres generates a laminar downflow, produces a very stable air pattern at air discharge velocities of > 0.15ms-1. Airflow tests show that the influence on airflow by operating lights in the flow field is localised. Hygienic tests to DIN 4799 recorded relative airborne germ concentrations that comfortably meet the requirements of DIN 1946 Part 4, even at an air discharge velocity of 0.15 ms-1. In an arena where clean air and low germ concentration are paramount, the laminar outlet and displacement system are valuable weapons in the consultants armoury.

Contact: Richard Belcham or Yvette Worth: tel: +44 (0) 1376 563156 at Quay West Communications