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TECHNOLOGY, COMPUTING, AND SIMULATION

A Computer Evaluation of Ventilation Performance in a Negative-Pressure Operating Theater

Tin-tai Chow, PhD^*, Anne Kwan, FANZCA, Zhang Lin, PhD^*, and Wei Bai, MSc^*

From the *Division of Building Science and Technology, City University of Hong Kong; and Department of Anaesthesiology, United Christian Hospital, Hong Kong Special Administrative Region, China.

Address correspondence and reprint requests to Tin-tai Chow, PhD, Division of Building Science and Technology, City University of Hong Kong, Hong Kong Special Administrative Region, China. Address e-mail to bsttchow{at}cityu.edu.hk.

	Abstract

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BACKGROUND: A negative-pressure operating theater is required to limit the spread of respiratory diseases in patients with severe acute respiratory syndrome, tuberculosis, avian influenza, or similar infectious diseases. In Hong Kong, we converted a conventional operating theater into a negative-pressure operating theater that has been in service for more than a year. In this article, we introduce its ventilation design and evaluate the airflow performance in relation to different combinations of medical lamp configurations and modes of launching infectious particles into the room air.

METHODS: We used a computational fluid dynamics technique for the numerical analysis.

RESULTS: Our analyses showed that the airflow performance in the negative-pressure operating theater was satisfactory and comparable to the original positive-pressure design. The airflow pattern effectively controlled the dispersion of infectious particles. Our calculations demonstrated that the airflow contained the dispersion of infectious particles released from the patient sufficiently to protect the surgical team, and vice versa.

CONCLUSIONS: Computational fluid dynamics can be used to assess airflow in a negative-pressure operating room and model the dispersion of infectious particles from the patient.

	Introduction

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Severe acute respiratory syndrome (SARS) recently posed an enormous threat to the international community. In Hong Kong, health care workers were infected by ill patients, resulting in several fatalities. SARS spreads mainly through close person-to-person contact. The SARS coronavirus transmission is via respiratory droplets (droplet spread) produced when an infected person coughs or sneezes. It also spreads through the air (air-borne spread) (1,2). Critical exposure to the SARS coronavirus occurred when an unprotected health care worker was within 0.91 m (3 feet) of an infected patient. Although personal protective equipment, as listed in Table 1, was able to protect the health care workers (3), the risk remained because of the possibility of face seal leaks or other unpredictable/ mishandling events.

There were 1755 known cases of SARS in Hong Kong during the outbreak in 2003. More than 400 of these were among health care workers. The United Christian Hospital admitted 184 of these patients (4). Those who contracted the more severe form of SARS required mechanical ventilation and received treatment in intensive care units. When operative procedures were required to be performed in the operating theater, the negative-pressure environment, once used in the early development of operating theaters, was then considered more suitable because this could stop the spread of virus to the outside. A temporary negative-pressure operating theater was set up during the period, within which three confirmed SARS patients had operations. When the crisis was over, a permanent negative-pressure operating theater was created to prepare for SARS re-emergence, and for caring for patients with severe influenza, tuberculosis, or similar airborne infections. Accordingly, we converted an ordinary operating theater to a negative-pressure operating theater which has been in service since July 2004. We introduce its ventilation design and evaluate the airflow performance in relation to different combinations of medical lamp positions and modes of launching infectious particles into the room air.

	METHODS

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Alternation in Air-Conditioning Provisions
The original floor layout plan, air distribution pattern, and transfer flow of the operating theater suite were described in the United Kingdom Operating Theater Standards (5). One primary air unit was to serve two operating theaters as well as their supporting units such as anesthetic rooms, preparation rooms, and corridors. Outdoor air was first treated by a prefilter and then by a HEPA filter before it was let into the primary air unit. The operating theater temperature set point was adjustable at the theater control panel, in the range of 20–26°C (±0.5°C), and relative humidity at 55% ± 5%. The supply airflow of the operating theater was normally established at 27 air changes per hour. Facilities were provided to start, setback, and stop the air conditioning system when the corresponding operating theaters were not in service. The neighboring areas of the operating theaters were also under positive-pressure, with pressure gradients maintained to provide continuous transfer airflows through zones in the order of decreasing sterility requirements. In each operating theater, the supply air was introduced through the perforated ceiling diffuser located directly above the surgical zone. As a routine check for sterility, agar plates were placed periodically inside the operating theater at various locations for biological air sampling. A colony count of <30 cfu/m³ for the trypticase soy (TSA) agar and <3 cfu/m³ for the Sabouraud agar was considered acceptable.

Theater 1, at the far end of the main operating suite, was chosen for the conversion into a negative-pressure operating theater. Figure 1 shows this part of the floor plan layout marked with the key design pressure requirements. The main feature of the negative-pressure design was the strong low-level exhaust system without flow recirculation, which could support a rapid air exchange rate, and the removal of the heavier anesthetic gases. An anteroom was added to the clean corridor outside Operating Theater 1 to maintain the proper pressure differentials. Computer simulation was used to assess the room air distribution. To investigate the risk of cross infection between the surgical staff and the patient, two different configurations of medical lamps were considered in the evaluation. Our previous study showed that the positions of the medical lamps during a surgery can have a significant effect on the dispersion of air-borne infectious particles (6).

View larger version (18K): [in this window] [in a new window]   Figure 1. Design pressure and part plan of the negative pressure operating theater at the operating theater suite.

Airflow Simulation Cases
Figure 2 shows the two negative-pressure operating theater simulation models used in our computer analysis. The dimension of the numerical flow field was 6.3 m (L) x 5.9 m (W) x 3.1 m (H). Above the slide doors at the front wall were two pressure stabilizers, through which the transfer air streams (equivalent to 15% of total supply flow) entered. Eighty-five percentage of the ventilating air was from the perforated diffuser. Two exhaust grills were at the same side wall. Seven surgical staffs (labeled A–G) were in their upright stationary positions surrounding the patient on the operating table, with each person releasing a heat flux of 100 W. We used the standard k– turbulence model of the commercial software FLUENT (7). Wall functions were used to describe the turbulent flow properties in the near wall region. All heat and contaminant sources were assumed to be uniform at the emitting surfaces. The bacteria particles were considered air-borne without any direct impact of gravity. Only steady-state conditions were simulated.

View larger version (39K): [in this window] [in a new window]   Figure 2. Simulation models of negative pressure operating theater. (a) Isometric view of Case I: medical lamps directly above operating table. (b) Plan view of Case II: medical lamps at two sides of operating table.

The following two cases were investigated:

Case I:

The main medical lamp was set above the patient’s feet and the satellite lamp above the patient’s face.

Case II:

The two medical lamps were at the opposite sides of the operating table.

Though Case I represents one typical configuration of medical lamps that may have an adverse effect on bacteria dispersion, Case II represents a favorable situation.

The infectious particle concentration in the room space was evaluated based on each of the following three situations:

1. Each staff member released infectious particles at a rate of 100 cfu/min;
   
2. A bacterial release rate of 400 cfu/min (including SARS coronavirus) from the patient’s wound; and
   
3. A bacterial release (including SARS coronavirus) at a velocity of 5 m/s from the mouth of the patient, and at an elevation angle of 45°.
   

A rate of 100 cfu/min per staff was based on the estimation of 1,000 particles/min released per person and 10% of them carrying bacteria. A change of the estimated release rate either from the staff or the patient will change the results of concentration proportionally in this case study. More descriptions about the related simulation methodology and validation work can be found in the literature (6,8–11).

	RESULTS AND DISCUSSION

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In our two simulation models, the medical lamp configuration for Case I was expected to be more representative of operating theaters, but also less effective for maintaining a uniform and direct down-flow of air to protect the patient. Although the discharge velocity at the diffuser was only 0.1 m/s, the resulting air stream was still sufficient to maintain the necessary sequestering effect over the patient’s body. The vertical temperature gradient was <3°C from head to ankle at the occupied zone. In general, the overall thermal and air distribution performance was slightly better for Case II than for Case I, but the differences were not substantial.

Consider first the release of infectious particles from the staff members. The simulation results showed that for both Cases I and II, the concentration levels at the operating plane (above the top surface of the operation table at z = 1.1 m) were <10 cfu/m³. The level of performance was comparable to that found in the ordinary positive-pressure operating theaters. This indicates that the ventilation scheme in this negative-pressure operating theater was able to protect the patient against infection from air-borne contamination from operating theater staff.

Figure 3 shows the concentration contours of the infectious particles (released from the patient’s wound) at the staff breathing plane (at 1.6 m from the floor level). It can be seen that the concentrations close to the face positions of the staff members are not more than 5 cfu/m³, based on an arbitrary source strength of 400 cfu/min. For the entire plane, the maximum concentration is not more than 17 cfu/m³. Therefore, the surgical team has little risk of being infected during an operation. This also indicates that the airflow can wash the infectious particles from the patient’s body toward the floor before being drawn into the exhaust grills.

View larger version (19K): [in this window] [in a new window]   Figure 3. Concentration contours (cfu/m3) of infectious particles (released from patient) at the staff breathing plane (z = 1.6 m).

In normal circumstances, a patient in the operating theater might not cough or sneeze. Nevertheless, we believed it worthwhile to investigate similar human behavior as an alternative transmission route. The velocity of a cough is very high when compared with the velocity of supply air from the ceiling. The velocity of sneezing is much less than coughing. When the infectious particles continuously leave the patient’s mouth at an arbitrary velocity of 5 m/s, the airflow pattern was different from these two situations. Figure 4 shows the concentration distribution pattern cutting along the operating table and passing through the source of release. It can be seen that for both Cases I and II, the plume of infectious particles, which covers a traveling distance of 0.91 m, carries a boundary of around 12.5% normalized concentration (relative to the source). The plume is bending downward, indicating the influence of the down-flow of ventilating air in protecting the staff members. At the 1.6 m breathing plane, the maximum normalized concentration is about 2%. This occurs at a location close to the opposite wall of the exhaust grills (not far from Staff D). At the face positions of the other six staff members, the normalized concentrations for both cases are <1%. These levels are likely insignificant.

View larger version (21K): [in this window] [in a new window]   Figure 4. Normalized concentration of infectious particles (released from the mouth of the patient at 5 m/s) at the operating table (y = 2.95 m).

Although the actual path lines of SARS coronavirus released from the patient may not be the same as those being considered in the above situations, the simulation results provided a good reference in assessing ventilation performance. Judging from the above findings, the air distribution performance of the negative-pressure operating theater is satisfactory, and in no way worse than the original positive-pressure environment. Moreover, it has the advantage of protecting the health care workers and other patients outside the operating theater.

In this particular case, velocity measurements using a vane anemometer were performed to check the design specifications. Differential pressure measurements (to examine the relative pressures across the rooms) and a smoke test on airflow (to examine airflow pattern) were performed during the commissioning tests. The smoke test confirmed that the infected air inside the negative-pressure operating theater could not contaminate nearby areas. The quality of the ventilation was also confirmed by the periodic microbiological air sampling measurements, which verified that the air standard of an ordinary operating theater had been met. In our routine checks, the actual pressure differentials were less than the design specification. Nevertheless, colony counts of <30 cfu/m³ for the TSA agar and <3 cfu/m³ for the Sabouraud agar were consistently achieved. Listed in Table 2 are the results extracted from a recent air sampling check.

	CONCLUSIONS

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We evaluated the airflow behavior and dispersion of infectious particles released from both surgical staff and patient in a special-made negative-pressure operating theater. Two different configurations of medical lamps were considered. According to the simulation results, the ventilation provision in our negative-pressure operating theater met the intended goals of protecting both the patient and health care workers from the airborne infection. For both cases of medical lamp configuration,

1. air velocity near the patient (i.e., at the operating plane 1.1 m above floor level) was able to provide the necessary sequestering effect;
   
2. the vertical temperature gradient at the occupied zone was satisfactory (i.e., <3°C);
   
3. infectious particles released from the patient should not reach the surgical team in significant quantities, at breathing level 1.6 m above floor level; and
   
4. infectious particles released from the surgical staff would not contaminate the patient on the operating table (with a concentration <10 cfu/m³ at the operating plane).
   

Nevertheless, attention should be paid to the dispersion of infectious particles from medical lamps. During a surgery, dispersion is best avoided by not to set the medical lamps directly above the wound area.

	Footnotes

 
Accepted for publication June 20, 2006.

Supported by the City University of Hong Kong Strategic Research Grant 7001609.

	REFERENCES

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1. Tang JW, Chan RCW. Severe acute respiratory syndrome (SARS) in intensive care units (ICUs): limiting the risk to healthcare workers. Curr Anesth Crit Care 2004;15:143–55.
2. Centers for Disease Control and Prevention. Severe acute respiratory syndrome. Fact sheet. Basic information about SARS, May 3, 2005. Available at http://www.cdc.gov/ncidod/sars/factsheet.htm
3. Seto WH, Tsang D, Yung RW, et al.; Advisors of Expert SARS group of Hospital Authority. Effectiveness of precautions against droplets and contact in prevention of nosocomial transmission of severe acute respiratory syndrome (SARS). Lancet 2003;361: 1519–20.[Web of Science][Medline]
4. Kwan A, Fok WG, Law KI, Lam SH. Tracheostomy in a patient with severe acute respiratory syndrome. Br J Anaesth 2004;92: 280–2.[Abstract/Free Full Text]
5. National Health Service Estates. Health technical memorandum 2025: ventilation in healthcare premises. London: HMSO, 1994.
6. Chow TT, Yang XY. Ventilation performance in the operating theatre against airborne infection: numerical study on an ultra-clean system. J Hosp Infect 2005;59:138–47.[Web of Science][Medline]
7. Fluent Inc. User’s guide for Fluent 6.1, Lebanon, NH, 2003.
8. Buchanan CR, Dunn-Rankin D. Transport of surgically produced aerosols in an operating room. AIHAJ 1998;59:393–402.
9. Chow TT, Yang XY. Performance of ventilation system in a non-standard operating room. Build Environ 2003;38:1401–11.
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11. Woloszyn M, Virgone J, Melen S. Diagonal air-distribution system for operating rooms: experiment and modeling. Build Environ 2004;39:1171–8.

This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a colleague Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Chow, T.-t. Articles by Bai, W. Search for Related Content PubMed PubMed Citation Articles by Chow, T.-t. Articles by Bai, W. Related Collections Operating Rooms Technology