Summary of accomplishments

Transcription

1 To design and produce prototype hazard control equipment to ensure proper protection of health care personnel involved with the transport, treatment and caring of patients with respiratory infections including SARS

2 Summary of accomplishments With support for the OSHC, we were able to put ideas to work in the direction of affording better protection for health care workers. Specifically, we were able to adapt industrially proven hazard control concepts to address hazards posed by respiratory infectious agents in the clinical setting. Two mobile local exhaust units with capturing and air cleaning capabilities were assembled with OSHC Funds. One unit was tailor-made for conducting bronchoscopy operations while the other unit was designed to contain and eliminate bioaerosols at the breathing zone of patients during transit in ambulances. Our collaborators included respiratory physicians in the Haven of Hope Hospital and officers and consulting medical directors of the Fire Services Department. While design and construction of the prototype equipment required innovative engineering, promotion of the concept demanded culture-changing efforts to convince health care workers that the use of engineering controls is more effective than personal protective equipment. Throughout the project, technical advancement was complemented with seminars and lectures to introduce the concepts. Our work has furthered the goal of providing better protection for workers in the healthcare setting.

3 1. Concept When managing patients with respiratory illnesses including SARS, it is necessary to protect healthcare workers from inhaling infectious bioaerosols. With their extensive experience in controlling workplace hazards, industrial hygienists have developed some basic principles for addressing airborne hazards. The first such principle is that airborne hazards are best controlled and mitigated at the source by local exhaust ventilation systems. This approach limits the impact of the hazard and eliminates the need to further tackle problems downstream, which is generally less effective. The second principle is that priority must be given to the use of engineering controls such as enclosures and ventilation because they are generally more effective. The third principle is that the use of personal protective equipment such as facemasks, while important in some situations, should be considered the last line of defense. In contrast, the approach practiced in most healthcare settings relies on two basic strategies: 1. general ventilation and 2. the use of personal protective equipment (PPE). This practice allows airborne infectious aerosols to linger in the vicinity of patients and to contaminate the environment and pose a hazard for those who are not using PPE and those not using PPE properly. It is believed that the introduction of the concept of source control can be a valuable addition to controlling airborne hazards in healthcare settings. This project sought to develop engineering controls in the form of ventilated enclosures for isolating the sources of infectious aerosols - the patients suffering from respiratory illness - and, therefore, to provide healthcare workers with improved protection. As the project was oriented towards field applications, trials at hospitals and clinics was conducted before decision was to be made on the final design. The principles such as source control and local exhaust ventilation are well known and well established for hazard control in the industrial environment. The application of these principles to infection control in the healthcare setting is a novel idea. These principles are not adequately addressed in standard infection control references. On the commercial market, there are no products available that combine the idea of isolation, source control, air-filtration and portability for applications in the healthcare setting. Therefore, healthcare

4 professionals are understandably skeptical about the practicability of these occupational hygiene principles. This project had therefore to include substantial efforts in promoting these ideas to healthcare professionals. Exchange of ideas between occupational hygienists, infection control professionals and frontline healthcare workers was essential. 2. Basic Design The basic design employs the ideas of local exhaust ventilation to create an enclosure for isolating the source of the bioaerosols, the breathing zone of the patient with the respiratory disease. The extracted air volume is then passed through a High Efficiency Particulate Arrestance (HEPA) filter, which has a minimum efficiency of 99.97% for removing airborne particulates. The enclosure must allow treatment, for example bronchoscopy, resuscitation, etc. to be performed. The units should also be portable or mobile so they can be quickly moved and installed at places where they are needed. The enclosure relies on air velocity to capture the bioaerosols. The air velocity required for capturing light particulate matters suspended in the air is well established in occupational hygiene practice. Reference is made to the Industrial Ventilation Manual, 24 th edition, published by the American Conference of Governmental Industrial Hygienists (ACGIH). According to the manual, the airflow required in this situation should be 0.4 to 0.7 m/s. The units are designed to have a face velocity of at least 0.7 m/s across all openings. HEPA Filter A throw-away extended/pleated medium dry-type filter with rigid casing enclosing the full depth of the pleats. Minimum particulate arrestance of 99.97% for thermally generated DOP smoke particles or equivalent with a diameter of 0.3 micron

5 Mechanisms of Particle Capture Diffusion Inertial impaction Direct interception Particle size and filter efficiency The HEPA filter was originally designed in the 1940s for removing radioactive particles in nuclear facilities. Their extraordinary effectiveness in removing small airborne particulates has found wider use in medical and, more recently, the microelectronics industry, which both require ultra clean air. HEPA filters must have an efficiency of at least 99.97% for removing particles with an aerodynamic diameter of 0.3 micron. HEPA filters have a long history of successful use in biosafety cabinets and for cleaning the air supply to critical areas such as operating theaters in hospitals. The 0.3 micron in the specification is often misunderstood. It is a common misconception that the HEPA can only remove particles larger or equal to 0.3 micron. While bacteria are generally larger than 0.3 micron, most viruses are smaller than this size. This has led some people to doubt the filter s effectiveness against infectious viral particles. In fact, the HEPA filter is least effective (greater or equal to 99.97%) for removing particles at 0.3 micron. It

6 is more effective (greater than 99.97%) for particles greater or smaller than 0.3 micron. The removal of submicron particles relies on several mechanisms. Direct interception and inertial impaction are more effective for larger particles. Diffusion and electrostatic attraction are more effective when the particles are smaller. The boundary where the combination of these mechanisms is least effective is 0.1 to 0.3 micron. 3. Prototype Equipment Variations on the basic design were made to accommodate their application to different areas in the healthcare environment. Since the inception of the project, four prototypes have been developed: The Infection Control at Source (ICAS) device, was designed to be used for high-risk aerosol generating procedures such as intubation, bronchoscopy and tracheotomy. The negative pressure is capable of capturing, at the breathing zone, bioaerosols generated by patients, thereby reducing the risk of medical professionals being infected. The Isotent, is designed for single-bed isolation for isolating individual patients undergoing aerosol-generating medical procedures. It contains and extracts any bioaerosols generated by the patient inside the tent. This protects healthcare workers as well as other patients in the same room. The Mini-isotent is a miniaturized version of the isotent, designed for use in ambulances to protect paramedical professionals from possible infections while resuscitating and transporting patients who present symptoms of respiratory illness. The Isohood is a portable unit with a hood that fits over the patient's head to confine and extract bioaerosols emitted via coughing and sneezing. The unit can be mounted on a wheelchair and can be used for transporting patients through elevators or within buildings.

7 Basic (IsoTent) Unit ICAS

8 IsoTent field trial under Mini-isotent for ambulance use

9 IsoHood for transporting patients 4 Further Development of Prototypes 4.1 An under-bed unit As the prototype units were tested in the Department of Health clinics, modification and refinements were made to adapt the units to meet various needs. For example, in the chest clinic, resuscitation and endotrachael intubation was believed to be one process from which there was a high risk of aerosol exposure. But the bed where the resuscitation is performed is taller than normal and the standard Isotent obstructs access from above the patient, which is necessary in the resuscitation procedure. A modified prototype (Figure 1) with the fan and filter unit installed under the bed was developed to suit this need.

10 Figure 1 Prototype with the fan unit installed under the bed. 4.2 Further development of the ambulance unit The original design of the ambulance unit created several challenges. First of all, space is very limited inside the ambulance cab so the unit needs to be as compact as possible. Furthermore, unlike regular vehicles, the ambulance is loaded with electrical equipment and appliances such that electrical power is in short supply. The local exhaust unit must also minimize the use of electrical power. An early prototype, shown in Figure 2, featured a fan unit mounted on the side of the overhead cabinet in a mock up of the ambulance cab. The hood is hung on rails attached to the bottom of the cabinet. The air is drawn to the filter and fan unit via a flexible duct. Initial tests showed that this arrangement was unable to deliver sufficient air-flow given the limited supply of electricity. The limited space also excluded the use of a larger fan.

11 Figure 2 Early version of the ambulance unit, fan mounted onto and connected to hood via a flexible duct. In view of these limitations, a second prototype, shown in Figure 3, was built. The same fan was used, but it was mounted under the cabinet, which eliminated the need for the flexible duct. The HEPA filter is attached directly on the top of the hood. The entire hood, including the HEPA filter, which is likely to be contaminated after use, can be considered a disposable item. This eliminates the need to disinfect between patients. Inadequate disinfection between patients can cause possible cross infection. The patients anxiety about the cleanliness of this rather small hood is also eliminated if it is thrown away after each use. It is believed that the cost of the thrown-away part, i.e. the flexible hood and the HEPA filter, can be limited to within HK$100.

12 Figure 3 A improved prototype of the ambulance unit with the fan mounted directly above the hood. 5. Field trials To ensure that the equipment is designed and built to meet actual operational needs, prototypes have been sent to hospitals, DH clinics and the Fire Services Department for field trials. Feedback has been received and modifications were made to accommodate specific needs discovered during the trials. These trials are continuing. Initial feedback from the hospitals, clinics and the FSD are positive and encouraging. 6. Development of sterilization procedure 6.1 The need for a decontamination procedure When the units are used in the clinical environment, they will inevitably get contaminated by infectious materials. The contaminated parts include the hood, the upstream plenum, ducting before the HEPA filter and the HEPA filter itself. Assuming the integrity of the HEPA filter, all areas down stream of the HEPA filter, including the fan unit and the silencer, are considered clean. The assessable surfaces such as the inside of the flexible hood can be wiped down with a liquid disinfectant. This disinfection procedure must be carried

13 out between patients. On the other hand, the inaccessible parts starting from the duct inlet to the HEPA cannot be disinfected this way. During normal use, the possible presence of infectious materials in these areas do not present any risk to the patients because these areas are constantly under negative pressure such that anything lodged inside there could not come out in the reverse direction of airflow. Disinfection of these parts is not needed as frequently as the accessible parts and it is not necessary to disinfect these areas between patients. During periodic maintenance work, it may be necessary to disassemble the units and expose the normally inaccessible areas, which are assumed to be contaminated. Regular maintenance includes the HEPA filter integrity test and changing the HEPA filter. To protect the maintenance workers, a suitable disinfection process for these parts must be developed. 6.2 Similarity with Biological Safety Cabinets Our units share many similarities with Biological Safety Cabinets (BSCs) used for handling hazardous biological materials in the laboratory environment. Procedures exist for sterilizing BSCs with a gaseous disinfectant - usually formaldehyde. First, the equipment (BSC or local exhaust unit) is sealed off, and then a suitable amount of formaldehyde gas is introduced into the inside of the equipment and maintained for a suitable contact time (normally >4hr). Afterwards, the formaldehyde is vented off or neutralized with a chemical agent (usually ammonium bicarbonate). 6.3 Problems with Formaldehdye as a fumigant Formaldehyde is a highly irritant gas and it is classified as a confirmed animal carcinogen. To achieve effective sterilization, the concentration used in these processes is very high. Therefore, the process must be carefully controlled. Unlike laboratories where BSCs are normally used, the clinical environment usually does not have exhaust points or fume cupboards that can be used to safely vent off the formaldehyde once sterilization is complete. The formaldehyde must be neutralized. As mentioned above, the usual method is to use ammonia gas generated from ammonium bicarbonate. It is thought that this process is too complicated and it introduces yet another hazardous gas into the process. Therefore, an effort to find an alternative method was attempted.

14 The original procedure, which has been well established and used for decontaminating biosafety cabinets for many years, involves the introduction of formaldehyde gas and subsequent neutralization by ammonia gas. The formaldehyde gas is generated either by heating paraformadehyde (which is a solid at room temperature) or by heating concentrated formalin (37% solution of formaldehyde in water). The ammonia gas is generated by heating the right amount of ammonia carbonate. An attempt was made to simplify this process by eliminating the use of ammonia gas in the neutralization step. Instead of using ammonia gas, a solid-phase adsorbant impregnated with permanganate was used to remove the formaldehyde gas. The initial tests were promising. However, further tests revealed important inconsistencies and deficiencies. In biological safety cabinets, the literature recommended using 11g of paraformaldehyde per cubic meter of cabinet volume. When translated to concentration units, this is roughly 10,000 ppm of formaldehyde gas in air. However, when we measured the concentration of formaldehyde inside the units during the tests, we were unable to achieve this concentration even after boiling off the excess amount of formalin. The highest concentration achieved was only around 1,500 ppm. It was also observed that significant re-condensation appeared in tubing and ducts. We posited that the condensation might have accounted for the shortfall. Alternative methods for generating formaldehyde gas were attempted. Instead of a heater that boils the formalin, an ultrasound misting machine capable of generating micron-sized mists at room temperature was tested. It was hoped that without the abrupt temperature change, the condensation would be reduced. However, the condensation did appear and the concentration of formaldehyde gas did not increase by much. We posited that too much water might have been the problem, so we switched to the use of solid paraformaldehyde but the resulting concentration was still far short of the expected the 10,000 ppm. We identified some flaws in one of our basic assumptions. It turns out that formaldehdye fumigation is not entirely a gas-phase process. When paraformaldehyde is heated to > 200 o C, it turns into a liquid and then boils off as formaldehyde gas. Formaldehyde gas also has a tendency to recondense back into paraformaldehyde. At room temperature, the equilibrium

15 concentration is no more than 2,000 ppm. When 11 grams of paraformaldehyde is boiled off into one cubic meter of air at room temperature, only about 20% can remain in the gaseous form. The rest is recondensed back into the solid form onto surfaces. Therefore, the formaldehyde fumigation process is not entirely a gas-phase process. Previous studies on the subject have established that the amounts of formaldehyde as well as the relative humidity during fumigation are important factors that impact the effectiveness of sterilization. Reducing the amount of formaldehyde or lowering the relative humidity had been shown to reduce the germicidal effects. It appears that the recondensation is probably an important part of the germicidal mechanism. In fact, inside the cabinet or local exhaust units, microorganisms mainly exist on internal surfaces and are not suspended in the air. Therefore, the actual airborne concentration of the fumigant may not be as important as the amount available on the internal surfaces. It is therefore not surprising that conditions that favor. recondensation, such as high humidity and excess amounts of formaldehyde, are of critical importance for effective sterilization. At the end of the fumigation process, a large portion of the formaldehyde introduced would be in solid form (paraformaldehyde) condensed onto surfaces. Purging with fresh air can only remove the gaseous portion; the remaining portion stays inside the cabinet for a much longer time, slowly evaporating back into formaldehyde gas. The residue formaldehyde odour may persist for days post fumigation. 6.4 Possible solutions Formaldehyde had been used successfully as a fumigant for many decades and it is no doubt highly effective. It remains the standard practice to use gaseous formaldehyde in many critical applications. However, because of the health hazards associated with formaldehyde and the complexity of the fumigation process, it is far from ideal. Initially, we attempted to eliminate the neutralization step by using an adsorbent to remove the formaldehyde. Unfortunately, we found out through our experiments that this was not possible because formaldehyde leaves a solid residue on internal surfaces and the adsorbant could only remove the gaseous portion of formaldehyde but leaves the solid residues behind. In recent years, there has been increased interest in the use of hydrogen

16 peroxide vapour as a fumigant. Hydrogen peroxide vapour is a strong oxidant with known germicidal properties and it quickly dissociates into oxygen and water, leaving no toxic residues. Currently, hydrogen peroxide vapour is commonly used in aseptic manufacturing processes in the food and pharmaceutical industries where the presence of any harmful chemical residue is unacceptable. Although formaldehyde is still very much the standard gaseous sterilant used in laboratories and healthcare institutions, hydrogen peroxide vapour has recently found increasing acceptance. Equipment specifically built for this purpose is becoming commercially available. For our prototype local exhaust devices, fumigation with hydrogen peroxide vapour offers many clear advantages. The process is simple, and does not leave harmful residues. We believe it is worthwhile to look into the suitability of this technology for decontaminating local exhaust devices, thereby achieving the goal of avoiding, as much as possible, the use of toxic chemicals. 7 Promotion of idea and cultural change 7.1 Meetings and visits In order to promote the idea of controlling at source to the healthcare profession, and to facilitate exchange of ideas, the project team maintains frequent contact with healthcare professionals through meeting and seminars. In the preceding months, such meetings have been conducted with the top management of the Hospital Authority, hospitals including Heaven of Hope Hospital (HHH), TKO and UCH, as well as officials at the Department of Health. A field visit to HHH, where the prototype units are being tested, was arranged for the Chairman, Council Members and Principal Consultant of the OSHC. Locally, the idea was promoted to more than 200 participants at a seminars organized by the Hong Kong Institute of Occupational and Environmental Hygiene (HKIOEH). The idea was also introduced at local and international scientific meetings: 2003 Prevention and Cure of SARS Seminar, Guangzhou, China 中华预防医学会武汉分会, 首届学术年会 中华预防医学会第十二次全国医院感染学术会议 Occupational hygiene technical conference, HKUST, 7 February 2004

17 Symposium on advanced technology for HealthCare and Hygiene control (4-March 2004), Hong Kong Hong Kong SARS forum and Hospital Authority Convention 2004, on 8-11 May 2004 (Posters and prototypes were also displayed in the exhibition area) America Industrial Hygiene Conference and Exposition, May 8-13, Atlanta, Georgia, USA 7.2 Publications Kwan, Joseph K. and Yu, Samuel C.T. Application of occupational hygiene and hazards control concepts for respiratory infection control. Proceedings of the 2003 Prevention and Cure of SARS Seminar, Guangzhou, China 关继祖俞宗岱. 职业卫生有害物质控制的概念在呼吸道传染性疾病医院感染控制中的应用中华预防医学会第十二次全国医院感染学术会议论文集