Description:

The operating rooms are one of the most critical links in the structure of a hospital building in terms of the importance of the surgical process, as well as providing special conditions microclimate necessary for its successful implementation and completion. Here, the source of release of bacterial particles is mainly medical personnel capable of generating particles and isolating microorganisms when moving around the room.

Hospital operating rooms
Air flow control

Over the past decades, in our country and abroad, there has been an increase in purulent-inflammatory diseases caused by infections, which, according to the definition of the World Health Organization (WHO), are called nosocomial (nosocomial) infections. An analysis of diseases caused by nosocomial infections shows that their frequency and duration are directly dependent on the state of the air environment in hospital premises. To ensure the required microclimate parameters in operating rooms (and industrial clean rooms), unidirectional air diffusers are used. The results of the air environment control and analysis of the movement of air flows showed that the operation of such distributors provides the required microclimate parameters, but often worsens the bacteriological purity of the air. To protect the critical area, it is necessary that the air flow leaving the device remains straight and does not lose the shape of its borders, that is, the flow should not expand or contract over the protected area where the surgical

The operating rooms are one of the most critical links in the structure of a hospital building in terms of the importance of the surgical process, as well as providing the special microclimate conditions necessary for its successful implementation and completion. Here, the source of release of bacterial particles is mainly medical personnel capable of generating particles and isolating microorganisms when moving around the room. The intensity of particles entering the room air depends on the degree of mobility of people, temperature and air velocity in the room. The HBI tends to move around the operating room with air currents, and there is always a risk of its penetration into the unprotected wound cavity of the operated patient. It is clear from observation that it is wrong organized work ventilation systems leads to an intensive accumulation of infection to levels exceeding the permissible.

For several decades, specialists from different countries have been developing system solutions to ensure the conditions of the air environment in operating rooms. The air flow supplied to the room must not only assimilate various hazards (heat, humidity, odors, harmful substances), maintain the required microclimate parameters, but also protect strictly established zones from infections, that is, the necessary cleanliness of indoor air. The area where invasive interventions are carried out (penetration into the human body) can be called the operating area or "critical". The standard defines such a zone as an “operating sanitary protection zone” and means by it the space where the operating table is located, auxiliary tables for instruments and materials, equipment, as well as medical personnel in sterile clothing. In there is the concept of "technological core", referring to the area of production processes under sterile conditions, which in meaning can be correlated with the operating area.

To prevent the penetration of bacterial contaminants into the most critical areas, screening methods through the use of a displacement air stream have become widely used. Laminar air flow diffusers of various designs were created, subsequently the term "laminar" was changed to "unidirectional" flow. Currently, you can find a variety of names for cleanroom air distribution devices, such as "laminar", "laminar ceiling", "operating ceiling", "clean air operating system", etc., which does not change their essence. The air diffuser is built into the ceiling structure above the protection zone of the room and can be of various sizes depending on the air flow. The recommended optimal area of ​​such a ceiling should be at least 9 m 2 in order to completely cover the operating area with tables, equipment and personnel. The displacing air flow at low speeds enters from top to bottom, like a curtain, cutting off both the aseptic field of the surgical intervention zone and the sterile material transfer zone from environment. Air is removed from the lower and upper zones of the room at the same time. HEPA filters (class H according to ) are built into the ceiling structure, through which the supply air passes. Filters trap but do not decontaminate living particles.

Currently, much attention is paid all over the world to the issues of air disinfection in hospitals and other institutions where there are sources of bacterial contamination. The documents set out the requirements for the need to decontaminate the air of operating rooms with a particle inactivation efficiency of at least 95%, as well as air ducts and equipment for climate systems. Bacterial particles emitted by surgical personnel continuously enter the room air and accumulate in it. To ensure that the concentration of particles in the indoor air does not reach the maximum permissible levels, it is necessary to control the air environment. Such control must be carried out after the installation of climate systems, maintenance or repair, that is, in the mode of an operated clean room.

The use of unidirectional flow air terminals with built-in ceiling-type ultra-fine filters in operating rooms has become commonplace for designers. Air flows of large volumes go down the premises at low speeds, cutting off the protected area from the environment. However, many specialists are unaware that these solutions are not enough to maintain the proper level of air disinfection during surgical operations.

The fact is that there are a lot of designs of air distribution devices, each of which has its own scope. The clean rooms of operating rooms within their "clean" class are divided into classes according to the degree of cleanliness, depending on the purpose. For example, general surgical operating rooms, cardiac surgery or orthopedic, etc. Each specific case has its own requirements for ensuring cleanliness.

The first applications of cleanroom air diffusers appeared in the mid-1950s. Since then, it has become traditional to distribute air in cleanrooms in cases where it is required to ensure low concentrations of particles or microorganisms in them, to be carried out through a perforated ceiling. The air flow moves through the entire volume of the room in one direction at a uniform speed, usually equal to 0.3–0.5 m/s. Air is supplied through a group of high-efficiency air filters placed on the ceiling of the clean room. The air supply is organized on the principle of an air piston moving downward through the entire room, while removing pollution. Air is removed through the floor. This air movement pattern helps to remove airborne contaminants from personnel and processes. This organization of ventilation is aimed at ensuring the cleanliness of the air in the room, but requires high air flow and is therefore uneconomical. For clean rooms of class 1000 or class ISO 6 (according to ISO classification), air exchange can be from 70 to 160 times/hour.

In the future, more rational devices of a modular type appeared, much smaller in size with low flow rates, allowing you to choose an air supply device based on the size of the protected area and the required air exchange rates of the room, depending on the purpose of the room.

Analysis of the operation of laminar air diffusers

Laminar devices are used in cleanrooms and are used to distribute large volumes of air, providing for the presence of specially designed ceilings, floor hoods and pressure control in the room. Under these conditions, the operation of laminar flow distributors is guaranteed to provide the required unidirectional flow with parallel flow paths. The high air exchange rate contributes to maintaining close to isothermal conditions in the supply air flow. Ceilings designed for air distribution with large air exchanges, due to the large area, provide a small initial air flow velocity. The operation of floor-level extractors and room pressure control minimize the size of the recirculation zones, and the principle of "one pass and one exit" easily works. Suspended particles are pressed against the floor and removed, so the risk of their recirculation is low.

However, when such air distributors operate in the operating room, the situation changes significantly. In order to maintain acceptable levels of bacteriological purity of air in operating rooms, air exchange values ​​according to the calculation usually average 25 times / h and even less, that is, they are not comparable with the values ​​for industrial premises. To maintain the stability of the movement of air flows between the operating room and adjacent rooms, it is usually maintained at an overpressure. Air is removed through exhaust devices symmetrically installed in the walls of the lower zone of the room. For distribution of smaller air volumes, as a rule, small-area laminar devices are used, which are installed only above the critical zone of the room in the form of an island in the middle of the room, instead of using the entire ceiling.

As observations show, such laminar devices will not always provide unidirectional flow. Since there is almost always a difference between the temperature in the supply jet and the ambient air temperature (5–7 °C), the colder air leaving the air handling unit descends much faster than an isothermal unidirectional flow. For operation of ceiling diffusers used in public institutions, this is a common occurrence. There is an erroneous conventional wisdom that laminars provide stable unidirectional airflow regardless of where or how they are used. In fact, under real-world conditions, the velocity of a low-temperature vertical laminar flow will increase as it approaches the floor. The larger the volume of supply air and the lower its temperature relative to the room air, the greater the acceleration of its flow. The table shows that the use of a laminar system with an area of ​​3 m 2 with a temperature difference of 9 ° C gives an increase in air velocity by a factor of three already at a distance of 1.8 m from the beginning of the path. The air speed at the outlet of the supply unit is 0.15 m/s, and at the level of the operating table it reaches 0.46 m/s. This value exceeds the allowed level. It has long been proven by many studies that at overestimated inlet flow rates it is impossible to maintain its “unidirectionality”. An analysis of air control in operating rooms, carried out by Salvati (1982) and Lewis (Lewis, 1993) in particular, showed that in some cases the use of laminar units with high air velocities leads to an increase in the level of air contamination in the area of ​​the surgical incision with subsequent risk of infection.

When the flow moves, at the starting point the air current lines will be parallel, then the flow boundaries will change, narrowing towards the floor, and it will no longer be able to protect the area defined by the dimensions of the laminar installation. At air speeds of 0.46 m/s, the flow will capture the stagnant air from the room. Since bacterial particles are constantly released in the room, contaminated particles will be mixed into the air flow coming from the supply unit, since the sources of their release are constantly operating in the room. This is facilitated by air recirculation resulting from the overpressure of air in the room. To maintain the cleanliness of the operating rooms, according to the standards, it is required to ensure an imbalance of air due to the excess of inflow over exhaust by 10%. Excess air is moved to adjacent less clean rooms. In modern conditions, airtight sliding doors are often used in operating rooms, there is nowhere for excess air to go, it circulates around the room and is taken back into the supply unit using fans built into it for further cleaning in filters and secondary supply to the room. The circulating air collects all polluted particles from the room air and, moving near the supply air flow, can pollute it. Due to the violation of the boundaries of the flow, air from the surrounding space is mixed into it and pathogenic particles penetrate into the sterile zone, which is considered to be protected.

High mobility promotes intensive exfoliation of dead skin particles from unprotected areas of the skin of medical personnel and their entry directly into the surgical incision. On the other hand, it should be noted that the development of infectious diseases in the postoperative period is caused by the patient's hypothermic state, which is aggravated by exposure to cold air flows of increased mobility.

Thus, a laminar flow air diffuser, traditionally used and effectively operated in a clean room, may be detrimental to operations in a conventional operating room.

This conversation is true for laminar devices with an average area of ​​​​about 3 m 2 - optimal for protecting the operating area. According to American requirements, the air flow rate at the outlet of laminar panels should not exceed 0.15 m / s, that is, from 1 foot 2 (0.09 m 2) of the panel area, 14 l / s of air should enter the room. In our case, this will be 466 l / s (1677.6 m 3 / h) or about 17 times / h. According to the normative value of air exchange in operating rooms should be 20 times / h, according to - 25 times / h, therefore 17 times / h is quite consistent with the requirements. It turns out that the value of 20 times / h corresponds to a room with a volume of 64 m 3.

According to today's standards, the area of ​​​​a standard operating room (general surgical profile) should be at least 36 m 2. And for operating rooms for more complex operations (cardiology, orthopedic, etc.), the requirements are much higher, and often the volume of such an operating room can exceed 135–150 m 3. The air distribution system for these cases will require a much larger area and air capacity.

In the case of organizing air flow in larger operating rooms, there is a problem of maintaining the laminar flow from the exit plane to the level of the operating table. Several operating rooms have been used to study the behavior of airflow. In different rooms, laminar panels were installed, which were divided by area into two groups: 1.5–3 m 2 and more than 3 m 3, and experimental air conditioning units were installed, allowing you to change the temperature of the supply air. Multiple measurements of the incoming air flow rate were carried out at various flow rates and temperature drops, the results of which can be seen in the table.

Room cleanliness criteria

The right decisions regarding the organization of air distribution in operating rooms: the choice of a rational size of supply panels, ensuring the normative flow rate and temperature of the supply air - do not guarantee absolute air disinfection in the room. The issue of air disinfection in operating rooms was sharply raised more than 30 years ago, when various anti-epidemiological measures were proposed. And now the goal of the requirements of modern regulatory documents for the design and operation of hospitals is air disinfection, where HVAC systems are presented as the main way to prevent the spread and accumulation of infections.

For example, the standard considers decontamination to be the main goal of its requirements, noting: "a properly designed HVAC system minimizes the airborne transmission of viruses, bacteria, fungal spores and other biological contaminants", HVAC systems are given a major role in the control of infections and other harmful factors. The requirement for air conditioning systems in operating rooms is highlighted: "The air supply system must be designed in such a way as to minimize the penetration of bacteria into sterile areas along with air, and also maintain the maximum level of cleanliness in the rest of the operating room."

However, regulatory documents do not contain direct requirements for determining and monitoring the effectiveness of disinfection for various ventilation methods, and designers often have to engage in search activities, which takes a lot of time and distracts from their main work.

In our country, there are quite a lot of different regulatory literature on the design of HVAC systems for hospital buildings, and everywhere there are voiced requirements for air disinfection, which, for a variety of objective reasons, are practically difficult for designers to implement. This requires not only knowledge of modern disinfection equipment and the correct use of it, but, most importantly, further timely epidemiological control of the indoor air environment, which gives an idea of ​​the quality of HVAC systems, but, unfortunately, is not always carried out. If the cleanliness of clean industrial premises is assessed by the presence of particles (for example, dust particles) in it, then the indicator of air purity in clean rooms of medical buildings is live bacterial or colony-forming particles, the permissible levels of which are given in. To maintain these levels, it is necessary to regularly monitor the air environment for microbiological indicators, for which it is necessary to be able to count them. The methodology for collecting and counting microorganisms to assess the purity of the air has not yet been given in any of the regulatory documents. It is important that the counting of microbial particles should be carried out in the operated room, that is, during the operation. But for this, the design and installation of the air distribution system must be ready. The level of disinfection or the effectiveness of the system cannot be established before it starts working in the operating room, this can only be done under the conditions of at least several operational processes. For engineers, this presents great difficulties, since research, although necessary, is contrary to the order of compliance with the anti-epidemic discipline of the hospital.

air curtain

To ensure the required air regime in the operating room, it is important to properly organize the joint work of air inflow and removal. The rational interposition of supply and exhaust devices in the operating room can improve the nature of the movement of air flows.

In operating rooms, it is impossible to use both the area of ​​the entire ceiling for air distribution and the area of ​​the floor for its removal. Floor exhaust units are unhygienic as they get dirty quickly and are difficult to clean. Bulky, complex and expensive systems have not found their application in small-sized operating rooms. For these reasons, the most rational is the "island" arrangement of laminar panels above the critical zone with the installation of exhaust holes in the lower part of the walls. This makes it possible to model airflows similar to an industrial clean room in a cheaper and less cumbersome way. Such a method as the use of air curtains operating on the principle of a protective barrier has successfully proved itself. The air curtain is well combined with the supply air flow in the form of a narrow "shell" of air with a higher velocity, specially organized around the perimeter of the ceiling. The air curtain operates continuously for extraction and prevents the entry of polluted ambient air into the laminar flow.

To understand the operation of an air curtain, one should imagine an operating room with an exhaust fan arranged on all four sides of the room. The supply air coming from the "laminar island" located in the center of the ceiling will only go down, expanding towards the walls as it descends. This solution reduces the recirculation zones, the size of the stagnant areas in which pathogenic microorganisms collect, and also prevents the laminar flow from mixing with the room air, reduces its acceleration and stabilizes the speed, as a result of which the downward flow covers (locks) the entire sterile zone. This helps to remove biological contaminants from the protected area and isolate it from the environment.

On fig. 1 shows a standard design of an air curtain with slots around the perimeter of the room. When organizing the exhaust along the perimeter of the laminar flow, it is stretched, it expands and fills the entire zone inside the curtain, as a result of which the “narrowing” effect is prevented and the required laminar flow rate is stabilized.

From fig. Figure 3 shows the actual (measured) velocity values ​​that occur with a properly designed air curtain, which clearly demonstrate the interaction of a laminar flow with an air curtain, with a laminar flow moving evenly. The air curtain eliminates the need for a cumbersome exhaust system around the entire perimeter of the room, instead installing a traditional hood in the walls, as is customary in operating rooms. The air curtain protects the area immediately around the surgical staff and table, preventing contaminated particles from returning to the primary air stream.

After the design of the air curtain, the question arises of what level of disinfection can be achieved during its operation. A poorly designed air curtain will be no more effective than a traditional laminar system. A high air velocity can be a design error, as such a curtain will "pull" the laminar flow too quickly, i.e. even before it reaches the operating ceiling. The behavior of the flow cannot be controlled and there may be a risk of infiltration of contaminated particles into the operating area from floor level. Likewise, an air curtain with a low suction velocity cannot effectively shield a laminar flow and may be drawn into it. In this case, the air regime of the room will be the same as when using only a laminar supply unit. When designing, it is important to correctly determine the speed range and select the appropriate system. This directly affects the calculation of disinfecting characteristics.

Despite the clear advantages of air curtains, they should not be blindly applied. Sterile airflow generated by air curtains during surgery is not always required. The need to ensure the level of air disinfection should be decided jointly with technologists, who in this case should be surgeons involved in specific operations.

Conclusion

Vertical laminar flow can behave unpredictably depending on its mode of operation. Laminar panels used in cleanrooms generally cannot provide the required level of decontamination in operating rooms. Air curtain systems help to correct the nature of the movement of vertical laminar flows. Air curtains are the optimal solution to the problem of bacteriological control of the air environment in operating rooms, especially during long-term surgical operations and when there are patients with a compromised immune system, for whom airborne infections pose a particular risk.

The article was prepared by A.P. Borisoglebskaya using materials from the ASHRAE journal.

"... laminar air flow: an air flow in which the air velocities along parallel streamlines are the same..."

Source:

"ASEPTIC MANUFACTURE OF MEDICAL PRODUCTS. PART 1. GENERAL REQUIREMENTS. GOST R ISO 13408-1-2000"

(approved by the Decree of the State Standard of the Russian Federation of September 25, 2000 N 232-st)

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"laminar air flow" in books

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