A novel ventilation system for hospital hybrid operating rooms.Prototype design and experimental campaign to measure system's performances

Facoltà di ingegneria industriale e dell’informazione

Corso di Laurea in

Ingegneria Energetica

A novel ventilation system for hospital hybrid operating rooms.

Prototype design and experimental campaign to measure system’s

performances.

Relatore: Prof. Cesare Maria JOPPOLO

Correlatori: Ing. Francesco ROMANO

Ismo GRӦNVALL

Tesi di Laurea di:

Andrea SARLI

Matr. 817765

I Hospital Operating Rooms (ORs) have very special requirements for indoor environmental design, mostly concerning the necessity of a sterile air flow to guarantee the infection control, and the need of providing high cooling capacity for handling heat gains coming from medical equipment, personnel, lights, etc. Consequences of poor ventilation design in operating rooms can be very critical, leading to higher infection rates among patients, reducing the performance and affecting the health of the surgical staff. Surgical Site infections (SSIs) are proven to be one of the leading complications in surgeries [1]. These infections occur in the wound of the patient created by an invasive surgical procedure. The factors affecting the SSIs risks are several, such as: cleanness of surgical instruments, adequacy and timing of antimicrobial prophylaxis, air contamination, immune state of the patient, surgeon’s skill, insertion of foreign material, etc.. A way to reduce the incidence of SSs is to reduce the deposition of bacteria into the open wound and this can be obtained with a better ventilation system. An evidence of this relation could be found in research conducted by Lidwell et al. [6] Stacey et al. [7] and Charnley [8]. The quality of air ventilation system and the aseptic condition of the indoor environment in the OR, due also to the growing impact of antibiotic resistance, are considered a key factor for preventing surgical infections. For these reasons, all over the world rules and standards are related to design of adequate OR ventilation system; many of them consider OR as a “clean environment” in which must be applied the requirements of cleanrooms, such as the ISO 14644-1 [9] for airborne particle contamination and the ISO 14698 [10] for microbiological contamination. To reduce the problems related to SSIs several ventilation systems have been proposed over the years. The basic idea behind them is to “clean” as much as possible the air in the OR; this goal is pursued with the provision and diffusion of clean air filtered with High Efficiency Particulate Air filters (HEPA) or Ultra Low Penetration Air filters (ULPA). In the light of all the above issues, it becomes clear that designing an OR’s HVAC system it’s a complex procedure that needs adequate methods and accurate performance calculations. The system’s performances calculation methods require studies and validation procedure in prototype or in a mock-up OR. In this thesis, the goal has been to deepen the knowledge of a novel air diffusion system developed by Halton, intended for use in ORs and clean environments. The specific aims of this work have been to size the ventilation system for a Hybrid OR, to build a mock-up OR and to plan, to perform and to analyze the experimental measurements which could lead to verify that the system works correctly. Hybrid ORs are intended to be Operating Room able to combine advaced imaging capability using devices such as C-Arms, CT (computed tomography) scabbers or MRI (Magnetic resonance imaging) with a fully functioning operating suite. The Hybrid OR configuration allows surgeons to combine a diagnostic procedure (by using real-time image guidance) with a thearapeutic one, thus giving better surgical outcomes and at the same time being less traumatic for the patient. After a review and analysis of standards and regulations about Operating Room’s ventilation systems and an appraisal of characteristics of Hybrid ORs, a real OR case has been chosen.; For this case, the design and sizing were conducted with the help of Computational Fluid Dynamic (CFD) technology. A 1:1 scale mock-up has been realized in the experimental hub of Halton, and the solution has been tested and evaluated. Description

The main steps that were taken during this work are the following:

1) Analysis of the main national Standards in the area of Healthcare facilities and ORs; 2) Analysis of the major issues related with Hybrid ORs;

3) Sizing of main parameters of the HVAC system (airflows, temperatures, diffusers, etc.) and use of Computational Fluid Dynamic (CFD) simulations;

4) Validation of the model on a 1:1 scale mock-up with a measuring campaign of main indoor parameters such as: air temperature, air velocity and airborne particle concentration.

II The OR study-case is equipped with a C-arm, a ceiling mounted monoplane X-ray imaging device for 3D angiography. The area of the OR is about 44,4 m2 _{and the volume about 128,6 m}3_{. Additional medical} devices are present such as: respiratory machine, surgical lamp and instrument tables. One big component is the monitor for the visualization of imaging outputs. (see figure 1) This monitor, like for the C-Arm, is ceiling mounted, dedicated rails allows its handling. This monitor dissipates 800 W heat while C-Arm maximum heating emission is 1280 W (see tab. 01). The presence of imaging devices and accessories represent a big issue for air diffusion since obstacles can disrupt the expected airflow paths, moreover the heat generation may create plumes that interfere with the airflow. As Zoon et al. reports in their experiments [63] the presence of internal thermal loads in the OR play a significant role in the performance of unidirectional systems by changing the temprature gradient in the room. Several national Standards on Healthcare and Operating Rooms were evaluated. The analysis was done to find out guidelines on the design of ORs and to check if a qualification system able to fit the characteristics of the hybrid OR exists. The Swedish technical specification, SIS-TS 39:2015 [38] was used to calculate the airflow-rate, based on a microbial cleanliness principle. A starting value of 1,4 of airflow rate was used. Air temperature and air humidity design values were chosen according several standards, in particular values imposed by Italian legislation were used, temperature set point: , relative humidity . ISO 14644-3 [9] and dutch VCCN RL 7 nov2014 [44] were used to set up tests to asses to the effectiveness of the ventilation system, two test were performed: smoke visualization test and recovery time. Other Standards were found to be useless for this application. Many European standards such as German DIN 1946-4 (2008),Swedish technical specification, SIS-TS 39:2015 [38], Dutch VCCN RL 7 NOV2014 [44] require that the concentration inside the protected area is about 2 or more order of magnitude lower than the outside. This result can’t be obtained for a diffusion system different than a Unidirectional AirFlow (UDF). In mixing ventilation system it can’t be defined a precise boundary for the protected area, this is because the concentrationof particles is decreased mainly for dilution effect. Comparing the concentration in the periphery of the OR and the one in the middle of the surgical table may give not the expected results and the concentration may appear similar. For all thesereasons it was chosen not to follow a standard qualification procedure. The ventilation system on the other side was tested by providing a simulated operational conditions as much similar as possible to the real one. Measurements were done mainly to prove the effectiveness of the CFD model.

Weig ht [Kg] Dimensions HxLxD [cm] Heat Generation [W] Sound Genera tion [dB(A)] 1.1 - Patient Table 430 90x300x60 60 46-60 1.2 - C-Arm 1085 - 1280 40-60 1.3 - 2 Monitor 58’’ 275 81x137x20 800 - 1.4 - Surgical lamp - 80 50 - 1.5 - Respiratory machine - 120x60x60 300 40-60 1.6- Instruments tables - 90x170x90 - - 1.7- Closets - various - - 1.8 - Operators - - 75 -

Tab. 01 – OR furniture specifications

III The studied HVAC system is the Halton Vita OR Space produced and delivered by the Finnish company Halton Oy. It can be classified as a solution in the middle between a completely mixed/turbulent air flow and a Uni-Directional Flow, UDF (also called Laminar Air Flow, LAF) system since the cleaning effect is not given only by a dilution effect but the fresh air is intended to reach directly the critical areas (patient table, instrument table etc.) and then move toward the exhausts. On the other hand a highly-uniform “piston flow”, like for UDF systems, is not obtainable and some recirculation of dirty air and air sacs occur. A basic representation of the theoretical air patterns can be found in the picture below (picture 2). The diffusion system is made of a square ring of DHN diffusers. These diffusers (picture 3) are designed specifically for clean environments, made of galvanized steel, they supply air through 81 independent and steerable nozzles. Each diffuser is equipped with a plenum for locating the HEPA filters.

Through the use of CFD software it was possible to assess whether the air flow reaches the protected area and if there are any air sacs or eddies in which the dirty air may be collected. Main parameters used are listed below:

 Software used: CFD modeling: ANSYS CFX 18.1; 3D geometry: SolidWorks©.

 Mesh used: 2893919 nodes/11065955 elements, tetrahedral dominant

 Steady state simulation

 Fluid and particle: Fluid: Air Ideal Gas; DEHS: Particle Transport Solid, 0.5 µm diameter

 Full buoyancy model

 Shear Stress Transport (SST) turbulence model

 Particle source: Uniform injection of particles with 500000 positions

 Particle mass flow rate: 1.02e-9 kg/s

 Particle transport model: Lagrangian Particle Tracking

Figure 3 – DHN diffuser produced by Halton

Figure 4 – Halton Test Operating Room, 3D SolidWorks Reproduction

IV Two cases were taken into consideration by varying the total amount of airflow, first case 1,4 (as comes out from the sizing method by Swedish technical specification, SIS-TS 39:2015 [38]) second case using . Results show that by using a higher flow-rate it’s possible to achieve a better air distribution. For case clean air reaches the surgical table and the instruments table from the top; dirty air is, on the other side, directed toward the periphery area and then to the exhaust grilles. For this second case air assumes a unidirectional trend above the surgical table, and the re-circulations of air and eddies are not preset, differently for the 1,4 case (red and black circles in figures 5 and 6). Further studies about particle distribution were made with to assess if the system is able to clean areas including surgical table and instruments tables. These particles are light enough that can be considered no to exert an influence of airflow, they are carried by airflow path, in fact is possible to see a close similarity between air velocity and particle concentration profiles (figure 7, 8 and 11, 12). The calculation procedure follows these steps:

- Compute the field of distribution of fluid velocity, temperature and turbulent parameter; - Specify the source locations where the particles are released. Particles are released constantly

till steady state conditions are reached;

- Perform the computational analysis to calculate the trajectory for each particle from the generation to the extraction in the exhaust grilles. Maximum tracking time is set to 500seconds. The output of analysis includes the concentration of particles in the spatial environment.

Figure 5 - Velocity vectors 1,4 m3/s airflow, Re-circulation areas and eddies in red circles

Figure 7 - Velocity vectors 2,0 m3/s airflow Figure 8 - Velocity vectors 2,0 m3/s airflow

Figure 6 – Velocity vectors 1,4 m3/s airflow, Re-circulation areas and eddies in black circle

V The main assumptions made during this simulation are:

1) Particles are generated from the operators, which are modeled like 1,3m height cylinder (0,35 m diameter). Particles are uniformly emitted through 4 holes 10 cm diameter positioned at the top of the dummies. Such choices are made to have a comparison with experimental data;

2) Only 0,5 µm particles are here considered.

3) The total particle emission is 1,02E9 particles/min divided through six dummies; 4) The scenario is steady-state, particles do not collect somewhere. Approximately 99% of the

particles exited from the room though exhaust, for the few particles that still remains in the room a lifetime of 500 sec. and a maximum tracking distance of 200 m was set;

5) Particles are made of DEHS, characteristics of the synthetic oil used are described in table 02. These assumptions may lead to some limitations due to the not exact correspondence with operational conditions. For example the dissimilarity of geometrical shapes (e.g. the dummies) between real conditions and simulated. However, this study is an important first step to achieve a correspondence for validating a very complex system that involves a great computational and practical effort. Moreover, one of big targets of this work is also to achieve a method to study HVAC systems for ORs though the only use of Computational models, in which is very easy to set boundary conditions. Fixing these boundary conditions like particle emission and reproducing exactly the same conditions in a mock-up room requires some simplifications that are the main reason beyond the choices carried out so far.

DEHS

Chemical designation Sebacic acid-bis(2-ethylhexyl) ester

Triavial name Diethyhexylsebacyte

Density [kg/m3_{] 912}

Metling point [K] 225

Boiling point [K] 529

Flash point [K] >473

Vapour pressure at 293 K [µPa] 1,9

Dynamic viscosity [kg/m s] 0,022 to 0,024

Tab. 02 – DEHS specifications

Figure 9 - particle concentration distribution, cutting plane at 1,2 m, 2 m3/s airflow

Figure 10 - particle concentration distribution, 3D streamlines visualization 2, 2 m3/s airflow

VI CFD output gives good result in terms of particle distribution. Surgical table seems not to be affected by particles emitted by the operators. The system is able to create a kind of barrier to confine the contaminants in the periphery. However the mixing system creates some eddies and re-circulation areas in which the concentration reaches its peaks. It’s possible that this recirculation may cause some particles to be deviated into the inner area before to get carried away to the exhaust units. This effect is clearly visible in cutting plane 1 and cutting plane 2 (figure 4.28 and 4.29). Here, in the area occupied by operators, the concentrations levels are very high, and one possible issue may be that a portion of these particles can reach the instrument tables, where potentially sterile instruments are deposited. This undesirable effect is really difficult to avoid, even changing the airflow and the nozzle distribution. One possible idea is to use a mobile unit equipped with HEPA filter to provide ultraclean air where it’s needed. These systems are able to extract air from the space, treat it with a pre-filter and a HEPA filter, and deliver it as a constant and stable laminar airflow directly to the interested area.

Qualification procedures

The validation of the designed system is intended to prove that the air ventilation system is able to guarantee the expected requirements and to demonstrate that the CFD analysis is reliable in predicting the physical characteristics of airflow. In the real projects is not always possible to have a CFD simulation of the system or to arrange mock-ups, nevertheless a reference operating room solution can be studied and then adapted to similar layouts, being aware of respecting the principles of fluid mechanics similitude. Validation procedures were carried out after the construction of a 1:1 scale prototype (mock-up) of the actual OR (figure 13). The model has been set up in a full-scale OR located in Halton laboratories in Kausala, Finland. The mock-up was built in such a way to reproduce operating conditions similar to the real designed OR. Two important aspects were considered in order to make an adequate mock-up:

1) Geometrical and spatial arrangement of furnishings and personnel 2) Heat load generation reproduction

Three environmental parameters have been compared in this work: air velocity, air temperature and particle concentration. The positions of the measurements points have been chosen following different criteria. The temperature and velocity points are more and distributed into a thicker mesh, this is because these measurements require less time and the values are more stable, the standard deviation of each measurement is very low. Most of the points are positioned in the surgical table area, two on the

Figure 13 - Mock-up of the designed Hybrid OR in Halton Laboratories located in Kausala, Finland

VII 1,00E+02 1,00E+03 1,00E+04 1,00E+05 1,00E+06 1,00E+07 1,00E+08 0 20 40 60 80 100 120 140 [pp/m3] Minutes

Particle concentration in exhaust plenum [pp ≥ 0,5 µm/m3]

instrument table and two points in the positions of the anesthetic personnel. The intent for this campaign is also to check if proper comfort conditions for the operators are provided. The contaminant concentration points are less and located in “strategic” points (see figure 14). ,Some points, for example those on the surgical and instrument table, are mostly important because here the hygiene requirements are more restrictive. Some points are in the periphery area and in front of the exhaust aimed to check if the simulation works properly.

In the experiments, steady-state conditions have been reached before starting the measurements. For temperature and velocity measurements the heat loads were maintained-on till the temperature was stabilized; this time took an average of 1,5 hours. For particle measuring an Optical Particle Counter was located in the exhaust plenum to monitor the outlet concentration. The real-time connection provided with an Ethernet cable makes possible to check if a steady-state condition is reached at the air exhaust. As can be seen in graph. 01 after the beginning of particle emission at time zero, the system takes around 60min to reach steady-state conditions, after that the concentrations remains around 800000 pp>0,5µm/m3_{. A delay time of 10min was set on the OPC to not influence the measures; this time was} found to be enough to avoid presence of particles carried by operators during measures.

Figure 14 - Contaminant concentration measurement points layout

VIII The velocity measurements are made with a spherical omnidirectional air speed sensor and temperature compensation sensor. The speed sensor operates in a constant temperature anemometer bridge (CTA) with automatic temperature compensation. Air speed probe is individually calibrated in wind tunnel. Accuracy is ±0.02 m/s ±1% of readings, automatic temperature compensation: < than ±0.1%/K. The temperature is measured by a probe using Resistance Temperature Detector (RTD) sensors, each sensor is individually calibrated. The accuracy is ±0.1 ºC and measurement range: -10…50 ºC. Particle concentrations measurements were performed with an TSI AeroTrak® 9350 Optical Particle Counter (OPC), maximum concentration @ 10% coincidence loss, size rage 0,3-25 µm, suction flow rate: 50 l/min accuracy, counting efficiency: 50% at 0,3 µm, 100% for particles > 0,45 µm. Temperature and velocity values are taken in four different heights: 1,7m, 1,2m, 0,5m, 0,1m (figure 15). Particle concentration measuring points are taken at surgical and instrument table heights. Points 4,5,6,7 (figure 14) are at 1,2m height from the floor, and exhaust points are taken in front of the grilles. The measured values of air temperature and velocity were elaborated with the spreadsheet software, Microsoft Excel. For each point, the median measured value was used because data are typically not normally distributed. To compare velocity and temperature values a scaled independent index, purposed by J. Hyndman and A. Koehler in 2006 [91], was used. The scaled error is given by the following formula:

( ) ∑ ̅̅̅

(0.1)

N: number of points

̅̅̅: mean of measured values

Formula 0.1 compare the error with the mean of actual values. By using this scaled error is possible to calculate the Mean Absolute Scaled Error (MASE) [91]

(| |) (0.2)

When MASE <1 means that the prediction is accurate, values higher than 1 means that the prediction is bad. In the following tables are given for MASE:

0 1 2 3

PROBE 1 PROBE 2 PROBE 3 PROBE 4

MASE values

CASE 1 - TEMPERATURE CASE 1 - VELOCITY

CASE 2 - TEMPERATURE CASE 2 - VELOCITY

Figure 15 – Probe location during measurements Graph 02 – MASE values for air temperature and velocity comparison

IX Particle concentration measurements were performed by the injection of particles (produced by an aerosol generator) trough the dummies. With a view of obtaining the same particle emission from each dummy and a total emission into certain values a distribution system was built. A simple scheme of the system is depicted in figure 16. The particle generator’s flow is passed through a 6mm inner diameter hose, a special anti-static PVC pipe built with an internal smooth surface. About one meter after the particle generator a tee allows diverting part of the aerosol out of the OR, in this way it was possible to build up a kind of dilution system, by acting on the ball valve placed after the tee. The remaining aerosol is collected into a cylindrical steel concentrator, from which 6 hoses equally distanced spread the contaminant into the dummies. These last hoses are smaller, 5,5 mm inner diameter and made in antistatic Polyethylene. Each hose, same length of others, is positioned inside the cylinder of the dummies in a way that the pipe is oriented downward with the extremity positioned at half of dummy’s height. Experimental measuring revealed that the whole aerosol flow is carried upward by the air moved by convective forces. Each dummy is in fact heated with an internal resistance that move a certain quantity of air which comes out from the holes positioned in the upper part of the dummy.

The arrangement here described makes possible to achieve a constant and equal emission of particles from dummies, this was verified with a series of measures about the concentration out of each dummy before to start with the measuring campaign. The source strength value is equal to 1,02E9 particles /min, and it’s the same used for the CFD calculation. Once become aware of the particle source strength the measurements in the mock-up OR were performed. For each point 10 samples lasting one minutes each have been done, a delay time of 10 minutes was set. The delay time is the time elapsed between the initiation command on the OPC and the actual sampling start time. Error bars for particle concentration measurements represent the maximum and minimum value sampled.

Graph 04 – Comparison between particle concentrations measured in the mock-up room and forecasted values through CFD software

1,00E+02 1,00E+03 1,00E+04 1,00E+05 1,00E+06 1,00E+07 1,00E+08 1 2 3 4 5 6 7 8 9 10 11 12 13 [pp/m3]

Particle concentration in measurement points [pp>0,5µ/m3]

Measured CFD

Figure 16 – Scheme of particle distribution system

X Conclusions

The test performed lead to the conclusions here summarized. Air temperature and velocity comparison, described by MASE indicate a good prediction for probe 1 and 2 (graph 02). It can be noticed that MASE increases from Probe 1 to probe 4. It should be recalled that higher values of MASE indicate a less accurate prediction of the model. This means that at the lowest heights (0,2 m and 0,5 m) the accuracy is lower. For temperature values, this is probably due to the fact that (at those heights) the probes were frequently positioned under the surgical table. The measurements were performed using surgical drapes above the table, which flowed-out from the border of the bed creating a sort of barrier for the air. Particle concentration comparison in graph 04, shows a quite good agreement between the CFD calculated values and experimental results. Measured data are characterized by a more uniform distribution; on the other hand, CFD results reveals peaks and troughs, but the average values of particle concentration is quite similar both in measurements and in CFD. This effect could be explained by a difficulty of CFD sin simulating a large number of injections positions (memory reasons). This calculation requires a lot of memory and computational capability. Moreover, the system studied is in a steady-state condition where the fluctuation in the flow filed may be underestimated. Even if, in principle, a well-defined transient model and a steady-state CFD’s should converge toward the same solution over time, this case remains higly complex and some differences could be acceptable. The trend obtained (both from CFD and experiments) seems to show a reasonable reduction in the surgical table concentrations (points 1,2,3 graph 04). The peaks measured corresponds to the peaks value of CFD: periphery points 4,5 and points 9,10,11: instrument table, and the two-exhaust close to the dummies. Points that both the measurements and CFD reveal to be critic are the points nr. 8 and 9; different solutions could be envisaged, e.g. to use a local mobile ventilation unit equipped with HEPA filter. Let us highlight that also in case of use of unidirectional flow systems, to handle a situation like this one, where the instrument table are located far from the surgical table, could be difficult, because the ventilation system is designed to clean only a specific (critical) area with clearly defined boundaries and placed under the laminar ceiling.

The system works better by increasing the air velocity. This study has been made using only two different airflow values; due to the large incidence of airflow both on energy consumptions and on air cleanliness obtained, more in-depth studies on the influence of airflow rate should be added to this first assessment. The present study has not attempted any measurement of microbiological contaminants and viable bacteria. Let us highlight that even if the main and ultimate goal of the OR ventilation is to limit airborne microbiological contamination, measuring the airborne particles is considered a practical and sensitive method for assessing the performance of OR mechanical ventilation systems. In effect, even if microbiological contaminants are a fraction of total airborne particles, there is no evidence of a constant ratio between the microbiological viable particles and all the airborne particles. Moreover, due to the many concurring factors, there is no scientific consensus on a “simple” relationship between airborne particle concentration, ventilation system conceptual design and performance on one side and SSIs [93] and [94] in different surgeries on the other side. Therefore, further studies should encompass a measuring campaign based on the count of bacteria along with particle concentration. It should be noted that viable bacteria counts can be significantly obtained only in real surgery operations and not in simulated surgeries and that, in real cases, the conditions need to be very well defined in order to have useful results.

The results of the study show that the new Halton OR ventilation system could offer some advantages: - Energy consumptions could be reduced in comparison to UDF unidirectional systems (even if

with some possible reduction in particle concentration performances). In order to quantify this reduction, let consider that according to DIN Standard [41] and German Robert Koch Institute [95], with reference to a 3,2m x 3,2m plenum and to a minimum supply air velocity of 0,3 m/s (but the Italian standard limits is >0,3 m/s) the total supply airflow for UDF system is 3,072

XI m3/sec, i.e. approximately 1/3 larger than the airflow usable with the Halton work. Moreover using UDF covering a smaller OR area, doesn’t seem to be a practicable way of reducing UDF air flows (and energy consumptions) since such small UDF, as shown by the studies conducted by Diab-Elschahawi et al. [80], have no good performance in terms of air cleanliness.

- The system represents an improvement in terms of air cleanliness respect to pure mixing (turbulent) ventilation systems since the air patterns are better controlled and specifically directed to the “critical” area ensuring lesser particle concentrations. This is obviously not happening in turbulent systems where the only principle leading the air cleanliness is the dilution principle. On the other hand, a bigger airflow is required by this new ventilation system. In order to quantify the energy effect, consider that a pure mixing (turbulent) system could require a flow rate of 1,4 m3/s;

- Temperature differences are reduced since the air is better mixed into the room, differently from UDF systems, where the supply delivers the whole quantity of fresh air directly above the patient and surgeons. This leads to higher comfort for operators and for the patient;

- The new system is characterized by a quite good flexibility and adaptation to the position of both equipment and personnel in the OR, making possible to better combine the air diffusion to the room layout and medical device presence. This can be done thanks to the nozzle set-up that can be modified also for already built situations. The system is also flexible in terms of airflow rate change with quite wide margins (air velocity limited by noise generations and HEPA filters’ face velocity). UDF systems, on the other hand, are less adaptable to room layout and they protect a well-defined (and not changing) critical area. Small plenums, which are sometimes used, hardly can obtain good results and could be adapted to changing layout [80].

The air diffusion system presented in this work could represent a possible alternative to ultraclean UDF systems especially for large and challenging ORs such as the hybrid ORs. Designers could consider this new system especially in situations where energy saving is a leading factor, considering together the advantages and disadvantages in terms of adaptation to the OR configuration and of environmental cleanliness (particles and microbiological contaminants).

XIII Extended Summary ... I Index ... XIII Summary (English Version) ... XVII Sommario (Versione Italiana) ... XIX

0. Introduction ... 1

0.1 Background ... 1

0.2 Purpose ... 3

0.3 Method ... 3

1. ORs ventilation systems ... 7

1.1 Ventilation systems... 7

1.2 HVAC design main parameters ... 10

1.2.1 Temperature ... 10

1.2.2 Humidity ... 10

1.2.1 Air filtration ... 11

1.2.1 Pressurization ... 12

1.2.1 Volumetric Flow Rate ... 12

1.2.1 Noise Control ... 13

2. Standard and Regulations ... 15

2.1 Introduction ... 15 2.2 UNI EN ISO 14644 ... 15 2.3 Italian regulations ... 22 2.3.1 UNI 11425:2011 ... 23 2.3.1.1 Structure ... 24 2.3.1.2 Definitions ... 24 2.3.1.3 General Requirements ... 24

XIV

2.3.1.4 Design, installation and commissioning of the plant ... 27

2.3.1.5 Qualification procedures ... 27

2.3.1.6 Management ... 28

2.3.1 Ex ISPESL Guidelines about Standards of Security and Work Hygiene in Operating department: 2009 ... 29

2.4 The Swedish technical specifications: SIS-TS 39:2015 ... 30

2.5 German DIN 1946-4 (2008) ... 32

2.6 Dutch VCCN RL 7 NOV2014 ... 38

2.7 Discussion ... 44

3. Hybrid Operating Rooms ... 47

3.1 Definition ... 47

3.2 Description ... 47

3.3 Sterility Demand ... 48

3.4 Planning Process ... 49

3.5 Design of the Hybrid Operating Room ... 51

3.6 Imaging system configurations ... 53

3.6.1 Magnetic based Imaging Devices ... 54

3.6.1.1 MRI scanner (Magnetic Resonance Imaging) ... 54

3.6.1.1 Magnetic Resonance Angiography (MRA) ... 56

3.6.2 X-Ray based Imaging Devices ... 57

3.6.2.1 Radiography Plain X-rays ... 58

3.6.2.2 Computed Tomography (CT) ... 59

3.6.2.3 Angiography ... 60

3.6.2.4 Fluoroscopy ... 60

3.6.2.4 Main issues related to X-rays based imaging techniques . 62 3.6 Ceiling and floor mounted systems ... 64

XV 4. Design and sizing of a new Hybrid Operating Room Ventilation System

mock-up ... 67

4.1 Introduction ... 67

4.2 Case of Study... 67

4.3 Internal conditions of OR ... 71

4.4 Considerations about the choice of air diffusion system ... 71

4.5 Description of the new Halton OR Ventilation System ... 73

4.6 Air Flow sizing ... 74

4.6.1 Diffusers selection ... 79 4.6.2 Diffusers positioning ... 81 4.6.2 Nozzles positioning ... 86 4.7 Exhaust Air ... 87 4.8 Airflow Dampers ... 90 4.9 Control System ... 91 4.9.1 Airflow control ... 92 4.9.2 Temperature control ... 92 4.9.3 Humidity control ... 93 4.9.4 Alarms ... 93 4.10 CFD Modeling ... 94 4.10.1 CFD Results ... 95 4.10.2 Contaminant distribution ... 95

5. Use of Halton Vita Space for different types of Hybrid ORs ... 103

5.1 Assumptions ... 103

5.2 Floor mounted Hybrid OR ... 104

5.3 Bi-plane system ... 106

6. Mock-up and experimental qualification phase ... 107

6.1 Introduction ... 107

XVI 6.2.1 C-ARM ... 111 6.2.2 2 Monitors 58’’ ... 113 6.2.3 Respiratory machine ... 114 6.2.4 Surgical Table ... 114 6.2.5 Dummies ... 115

6.2.6 Tables and closets ... 115

6.2.7 Guiderails ... 116

6.2.8 Ventilation adjustment ... 116

6.3 Measuring campaign ... 118

6.3.1 Measuring equipment ... 119

6.3.2 Contaminant generation equipment ... 122

6.3.3 Temperature and velocity measurement set-up ... 123

6.3.4 Velocity and temperature results ... 124

6.3.5 Contaminant concentration measurements set-up ... 134

6.4 Flow visualization test ... 139

6.5 Recovery time ... 141 7. Conclusions ... 143 7.1 General ... 143 7.2 Discussion ... 143 List of figures ... 147 List of graphs ... 151 List of tables... 153 Acronyms ... 155 Bibliography ... 157

XVII

Summary (English version)

Hospital Operating Rooms (ORs) have very special requirements for indoor environmental design, mostly concerning the necessity of a sterile air flow to guarantee the infection control. Consequences of poor ventilation design in operating rooms can be very critical, leading to higher infection rates among patients, reducing the performance and affecting the health of the surgical staff. Surgical Site infections (SSIs) are proven to be one of the leading complications in surgeries. These infections occur in the wound of the patient created by an invasive surgical procedure. A way to reduce the incidence of SSIs is to reduce the deposition of bacteria into the open wound and this can be obtained with a better ventilation system. The quality of air ventilation system and the aseptic condition of the indoor environment in the OR, due also to the growing impact of antibiotic resistance, are considered a key factor for preventing surgical infections. To reduce the problems related to SSIs several ventilation systems have been proposed over the years. The basic idea behind them is to “clean” as much as possible the air in the OR; this goal is pursued with the provision and diffusion of clean air filtered with High Efficiency Particulate Air filters (HEPA). In the light of all the above issues, it becomes clear that designing an OR’s Heating, Ventilation and Air Conditioning (HVAC) system it’s a complex procedure that needs adequate methods and accurate performance calculations. The system’s performances calculation methods require studies and validation procedure in prototype or in a mock-up OR. In this thesis, the goal has been to deepen the knowledge of a novel air diffusion system developed by Halton, intended for use in ORs and clean environments. The specific aims of this work have been to size the ventilation system for a Hybrid OR, to build a mock-up OR and to plan, to perform and to analyze the experimental measurements which could lead to verify that the system works correctly. Hybrid ORs are intended to be Operating Room able to combine advaced imaging capability using devices such as C-Arms, CT (computed tomography) scabbers or MRI (Magnetic resonance imaging) with a fully functioning operating suite. The Hybrid OR configuration allows surgeons to combine a diagnostic procedure (by using real-time image guidance) with a thearapeutic one, thus giving better surgical outcomes and at the same time being less traumatic for the patient. After a review and analysis of standards and regulations about Operating Room’s ventilation systems and an appraisal of characteristics of Hybrid ORs, a real OR case has been chosen. For this case, the design and sizing were conducted with the help of Computational Fluid Dynamic (CFD) technology. Thereafter a 1:1 scale mock-up has been realized in the experimental hub of Halton, and the solution has been tested and evaluated.

Keywords: Hybrid Operating Room, CFD, Operating Room HVAC system, airborne

XIX

Sommario (versione Italiana)

Le sale operatorie ospedaliere richiedono particolari requisiti relativi alla progettazione dei sisitemi di ventilazione. Le conseguenze di un sistema non correttamente progettato possono essere molto critiche, causando un tasso più elevato di infezioni tra i pazienti e riducendo le performance dello staff chirurgico. Le infezioni del Sito Chirurgico sono una delle principali complicazioni nelle operazioni chirurgiche. Queste infezioni hanno luogo nelle ferite dei pazienti causate da operazioni chirurgiche invasive. Un modo per ridurre le infezioni è quello di ridurre la frequenza con cui i batteri si depositano nelle ferite aperte e ciò può essere ottenuto con un miglior sistema di ventilazione. Per ridurre i problemi relativi alle infezioni diversi sistemi di ventilazione sono stati proposti negli anni. L’idea alla base di ognuno di essi è di pulire il più possibile l’aria nella sala operatoria; questo obiettivo è perseguito con la diffusione di aria pulita, filtrata con filtri a ad alta efficienza. Alla luce di tutte le problematiche indicate, risulta chiaro che la progettazione di un sistema di ventilazione per una sala operatoria è una procedura complessa che necessita metodi adeguati e calcoli precisi delle prestazioni. In questa tesi, l'obiettivo è di approfondire un nuovo sistema di diffusione dell'aria sviluppato dalla società finlandese Halton e destinato ad essere utilizzato in Sale operatorie e ambienti puliti. I principali aspetti su cui si articola il lavoro sono: dimensionamento del sistema di ventilazione per una sala operatoria ibrida, costruzione di un modello mock-up e realizzazione di una campagna di misure con lo scopo di determinare l’efficacia del sistema. Le sale operatorie ibride sono sale in grado di combinare avanzate tecniche diagnostiche, effettuate con strumenti quali bracci a C per angiografie o tomografie, o attrezzature per la risonanza magnetica, con una suite operativa completamente funzionante. La configurazione delle sale operatorie consente ai chirurghi di combinare una procedura diagnostica (utilizzando immagini ottenuto in tempo reale) con un effetto tearapeutico, con migliori risultati e allo stesso tempo con una procedura meno traumatica per il paziente. Dopo una revisione e un’analisi delle norme e dei regolamenti riguardanti i sistemi di ventilazione per le sale operatorie e una valutazione delle caratteristiche delle sale operatorie ibride è stato scelto un caso reale; Per questo caso, la progettazione e il dimensionamento sono stati condotti con l'aiuto della fluidodinamica computazione nota anche come Computational Fluid Dynamic (CFD). Successivamente è stato realizzato un modello di scala 1: 1 nei laboratori della società Halton, ove la soluzione è stata testata e valutata.

Parole chiave: Sale operatorie ibride, CFD, ventilazione delle sale operatorie, controllo del contaminante areotrasportato in sala operatoria.

1

0.1

Background

Hospital Operating Rooms (ORs) have very special requirements for indoor environmental design, mostly concerning the necessity of a sterile air flow to guarantee the infection control, and the need of providing high cooling load for handling heat gains coming from medical equipment, personnel, lights, etc. The consequences of poor ventilation design in operating rooms can be very critical, leading to higher infection rates among patients and reducing the performance of the surgical staff. The correct design of hospital operating room ventilation systems is consequently critical to both the comfort of the surgical staff, and more importantly to the health of occupants and outcomes of surgical procedures.

Surgical site infections (SSIs) are infections that occur in the wound of the patient created by an invasive surgical procedure, these infections can occur within 30 days of an operative procedure or within one year if an implant is left in place [1]. SSIs are the second most frequent type of HealthCare-Associated Infections (HCAIs) and is the leading complication in surgery [1]. These infections could be deep or superficial, for superficial ones the treatments are easier to deal with, but the deep ones can be difficult and can lead to complication even to life threatening. The causes of SSIs are several, such as: immune state of the patient, surgeon’s skill, insertion of foreign material, air contamination, cleanness of surgical instruments, adequacy and timing of antimicrobial prophylaxis. The source of viable agents causing SSIs is believed to be originated mainly from the patient’s endogenous flora at the time of surgery [1]. Other authors report that more than half of all infections are caused by the normal skin flora of patients or healthcare workers causes [2]. SSIs is also originated from exogenous sources for example the surgical staff members, the material present in the sterile field during the time of surgery and the environment of operating room [1]. A possible scheme of which are the main routes for the SSIs is given in figure 0.1. A large campaign of studies conducted in the 1960s in the UK showed that great quantities of airborne S. aureus resulted in an increased risk of postoperative S. aureus infections [3]. In particular large risk of infection was found to exist with total bacteria number of 700-1800 CFU/m^3. A low risk of infection was found to exist with total bacteria numbers of 36-72 CFU/m^3, with no more than 1% of this being S. aureus.

2 Figure 0.1 - Routes for Surgical Site Infections [4]

SSIs represent a cost for hospitals, depending on the type of surgery and type of infecting pathogen. The estimation of these costs is huge; about this topic the United States has an estimated cost of between $3 000 and $35 000 in hospital fees for treatment of each patient. This results in an annual cost of $3 billion up to $10 billion in the US [5]

A way to reduce the incidence of SSIs is to reduce the deposition of bacteria into the open wound and this can be obtained with a better ventilation system. An evidence of this relation could be found in the research conducted by Lidwell et al. [6] Stacey et al. [7] and Charnley [8].

Therefore, the quality of air ventilation system and the aseptic condition of the indoor environment in the OR becomes essential for preventing surgical complication. For these reasons many European standards requires an adequate system of ventilation regarding the OR, and many of them consider OR such as clean environment in which must be applied the requirements of cleanrooms, such as the ISO 14644-1 [9] for airborne particle contamination and the ISO 14698 [10] for microbiological contamination.

To overcome the problems related to SSIs several ventilation systems have been purposed over the years. The idea behind them is to “clean” as much as possible the air in the Operating Room, this is obtained with the supply of clean air filtrated with High Efficiency Particulate Air filters (HEPA) or Ultra Low Penetration Air filters (ULPA). Moreover, heating, ventilation and air conditioning (HVAC) systems can impact SSIs by affecting:

- Exposure time (by air change and pressure difference between rooms); - Temperature;

- Humidity;

- Organism viability (by ultraviolet UV treatment); - Airflow patterns.

About this topic, Memarzadeh [11] conducted a series of studies making the evidence that microorganisms embedded in water droplets are affected by temperature, humidity and air velocity.

3

0.2

Purpose

The purpose of this work is to design the ventilation system for a Hybrid OR and to perform the qualification procedures necessary to verify that the system works correctly. Hybrid ORs are intended to be Operating Room able to combine advaced imaging capability using devices such as C-Arms, CT (computed tomography) scabbers or MRI (Magnetic resonance imaging) with a fully functioning operating suite. One strenght of Hybrid ORs is that they are less traumatic for the patient, by using real-time image guidance. On the other side they introduce several issues relating to the ventilation system, for example the obstructions that the imaging devices create to the air pattern, the greater heat loads presence, the greater number of operators etc. Main characteristics, and design issue related to hybrid ORs are going to be treated in Chapter 3. The OR object of the study, comes from a real project developed by the Finnish Company Halton Oy, whose HVAC departement is located in Kausala, Finland. Therefore all the data such as OR’s layout, number of personnel, heat loads, etc. comes from offical data of the real project. The qualification procedures were performed by reproducting the hybrid OR on a 1:1 scale mock-up, and testing the air parameters such as: air velocity, temperature and particle concentration. The mock-up was realized in the expermental Hub of Halton where an already built-in OR is arranged for testing purposes. The validation of the designed system is intended to prove that the air ventilation system is able to guarantee the expected requirements and to demonstrate that data obtained in the design phase are reliable. In particular airflow parameters were predicted by the technical office of Halton using Computational Fluid Dynamic (CFD) technology. The results from the qualification procedure allows to offer on the market a product able to guarantee satisfactory conditions in terms of sterility conditions, environmental comfort, and control of termohygrometric conditions. This system works efficiently also on a radically different OR in terms of layout, heat loads etc. such as a hybrid OR. One more strength point is the lower energy costs due to the lower airflow rate needed respect to more common Unidirectional AirFlow (UDF) system where the airflow involved is much higher.

0.3

Method

The main steps that were taken during this work are the following:

1) Analysis of the main national Standards and Guidelines in the area of Healthcare

facilities and ORs (Chapter 2): A detalied analysis of major national Standards

were performed to find out guidelines on the design of ORs and to check if a qualification system able to fit the characteristics of the hybrid OR exists;

2) Analysis of the major issues related with Hybrid ORs (Chapter 3): Hybrid ORs are lesser-used OR suite in Europe but more and more they are gaining ground in Healthcare field. Hybrid ORs present many issues that the designer must deal

4 with; a large literature has been reviewed in order to find out useful elements to get ready for the design procedures.

3) Sizing of main parameters of the HVAC system (airflows, temperatures,

diffusers, nozzles positioning, etc.) and use Computational Fluid Dynamic (CFD) to have a prevision of the air behavior in the OR (Chapter 4): The design phase

was conducted with the help of operators of the technical office of Halton, in particular during the evaluation of the OR’s air parameters through CFD software. The system chosen is the Halton Vita OR Space produced and delivered by Halton. This system can be classified as a solution in the middle between a completely mixed/turbulent air flow and a unidirectional system since the clean effect is not given only by a dilution effect but the fresh air is able to reach directly the critical areas (patient table, instrument table etc.) and then move toward the exhausts. This is actually the principle on which Unidirectional AirFlow (UDF) systems are based. On the other hand a uniform “piston flow”, like for UDF systems, is not provided and some recirculation of dirty air and air sacs occur. A basic representation of the theoretical air patterns can be found in the figure below (picture 0.2). The diffusion system is made of a square ring of nozzle equipped diffusers, named DHN. These diffusers (picture 0.3) supply air through 81 independent and steerable nozzles making possible to achieve a multitude of airflow patterns.

4) Validation of the model on a 1:1 scale mock-up with a measuring campaign of

main indoor parameters such as: air temperature, air velocity and particle concentration (Chapter 5): The validation of the designed system is intended to

prove that the air ventilation system is able to guarantee the expected requirements and to demonstrate that the CFD analysis is reliable in predicting the physical characteristics of airflow. In the real projects is not always possible to have a CFD simulation of the system or to arrange mock-ups, nevertheless a reference operating room solution can be studied and then adapted to similar layouts, being aware of respecting the principles of fluid mechanics similitude. The Figure 0.2 – Halton Vita OR space operating principle Figure 0.3 – DHN diffusers produced by

5 mock-up was built in such a way to reproduce the same operating conditions of the real designed OR. Two important points were considered in order to make an accurate model:

1) Geometrical and spatial arrangement of furnishings and personnel 2) Heat load generation reproduction

Three parameters have been compared in this work: air velocity, air temperature and particle concentration. In particular particle concentration measurements were performed trough the injection of a particle load trough the dummies, used to simulate the presence of personnel in the operating room. The particle load was generated by a particle generator and then distributed to the dummies through a system of pipes that equally subdivide the particle flux toward the dummies. The same amount of particle was simulated in the CFD software and then a comparison has been made to assess the ability of the system to maintain clean from human produced particles the most critical areas.

5) Conclusions (Chapter 7): Finally experimental and numerical results obtained are compared and discussed. Results are commented in the light of the limitations of the work; possible future developments are also discussed. Thanks to this work is possible to take in considerations an innovative system for ORs’ ventilation, totally different from the existing ones, which strengths are related to the flexibility, the energy efficiency and comfort.

6 Figure 0.4 – Flow chart of logical steps of this work

Chapter 00: Introduction and scope of work Chapter 01: ORs ventilation overview Chapter 02: Standard and Regulations Chapter 03: Hybrid ORs issues

Chapter 04: Design of HVAC system Chapter 04: CFD Numerical study Chapter 06: Experiental full scale study

7

1.1

Ventilation systems

For Operating Rooms (ORs) there are four major types of air ventilation systems in use: - Turbulent airflow;

- Unidirectional airflow; - Mixing airflow;

- Displacement airflow.

Turbulent systems are designed with the aim of diluting contaminants concentration; the dilution principle gives a lower contaminant concentration in case of higher ventilation rate and of higher ventilation efficiency (i.e. obtaining good mixing and avoiding short-circuit flow). The supply air is thrown through ceiling diffusers and the extraction takes place through grilles in the lower and/or upper part of walls. When a good mixing is obtained, temperature and air quality are almost at the same conditions everywhere in the room.

Nowadays mixing turbulent flow ventilation systems should commonly be adopted for general surgery OR, which do not require highest aseptic levels. For ORs requiring highest hygiene level, adopting vertical unidirectional air flow is the indication commonly endorsed by literature and standards (e.g. by the American Society of Heating Refrigerating and Air Conditioning Engineers, ASHRAE, [4]). However in the scientific literature there are also some completely different positions, not agreeing on the better effectiveness of UDF in controlling SSI risks (see paragraph 4.4).

Figure 1.1– Turbulent air diffusion system

The unidirectional systems, commonly named UDF – Unidirectional Air Flow or LAF – Laminar AirFlow, are designed to remove contaminants by creating a unidirectional piston flow that delivers clean air to critical areas before it mixes with contaminated

8 surrounding air. Air flows following an ordered path which sweeps away the airborne particles from the table to the return grilles. The airflow needed is quite high with respect to other systems because the supply surface is very large and the air velocity is lower but not very low. The recommended minimum velocity at the wound site should be around 0,2 m/s; at this velocity Chow et. al. found out that the air stream is effective in washing away the particles from the wound [12]. The theoretical model behind unidirectional system is complex having to take into account many phenomena. The process of diffusion of contaminants, however, is complicated by other elements, such as for example, the possible presence of thermal energy sources, such as equipment, lighting, cooling systems of the electro-medical equipment and people, which determine the onset of thermal draft phenomena (plume) causing upwards convective flows. In practical applications, the non iso-thermal conditions and the interaction between natural and forced convective flows have an incidence strongly variable from case to case. It’s worth noting that also reverse motion phenomena could arise from the specific situations and they could be able to compromise the unidirectional flow principle.

In order to save energy (and related costs), Partial UniDirectional Flow systems (also called mixed systems) have been developed and are often applied. The partial UDF combines a vertical unidirectional flow in the so called “protected” (critical) area and mixing turbulent (induced) airflow in the periphery. The picture below (Figure 1.2) helps to visualize the airflow patterns. In this case, the airflow is quite higher than that of a conventional (turbulent) OR, but results in a reduction in the number of bacteria-carrying particles to between half and one third [13].

Figure 1.2 – Mixed air diffusion system, unidirectional in the protected area and turbulent in the periphery

The displacement ventilation system consists in the supplying of cold air at floor level and evacuation at the ceiling (Figure 1.3). The displacement effect is created by the supply of a certain amount of low velocity air inside the room at few degrees lower temperature than the one existing in the room. The activity inside the room generates heat that warms air which goes up for buoyancy forces. This creates a zone of polluted air in the upper level of the room that is finally extracted. Displacement system is not much used, in particular some studies such as the one conducted by Friberg et al. [14] have shown that the displacement ventilation results in “higher counts of bacteria then the conventional system both in the air and on surfaces”.

9 Other diffusion systems have also been proposed such as the diagonal airflow system proposed by Y. Yamada (Figure 1.4) [15]. This system could be suitable in Hybrid ORs where the presence of many obstacles can cause problems for unidirectional flow. Yamada proposes to change the clean airflow direction from down flow to diagonal by changing the position of supply and return grilles. The study has been conducted with CFD software and shows that the diagonal clean flow when obstacles are present could make the operating field cleaner than the ordinary vertical down flow.

Figure 1.4 – Diagonal air diffusion system Figure 1.3 – Displacement air diffusion system

10

1.2

HVAC design main parameters

Criteria for HVAC design of OR involve indoor temperature and humidity, room pressure, filtration stages, total and fresh air change rates. In addition, economic factors for maintenance and operation, heating and cooling loads etc. must be considered. The choice of the mentioned design parameters is based on thermal comfort and infection control, at the same time the local standard and regulations must be taken into account. Below is given a detailed description of each parameter and which are the main issue related to the HVAC system.

1.2.1

Temperature

Room temperature is an important design parameter since it affects the thermal comfort of both hospital staff and patients. Particularly, the surgical staff wearing protective clothes working under highly radiant lighting can be affected easily in terms of thermal comfort. This uncomfortable feeling affects the surgeon’s ability to focus; consequently, the result of the activity being held in the room can be affected negatively. Surgeons typically feel more comfortable at lower temperatures while nurses and anesthesia specialists feel comfortable at higher temperatures because of the minor physical activity they are supposed to have. Generally, temperatures between 24-26 ºC are suitable for the thermal comfort of patient while temperatures below 21 ºC increase the risk of hypothermia. However, the thermal comfort of surgical staff is greatly reduced with the room temperatures higher than 23 ºC [16].

Not only the thermal comfort is taken into account to determine design temperature, but the type of operation must be defined since different types of operations require different room temperatures. Some examples are the following [4]:

• 32 ºC with a low relative humidity level found beneficial for treating certain kinds of arthritis.

• High relative humidity with 32 ºC is used for burn patients. • Room temperature around 30 ºC is used for pediatric surgery.

• For cardiac surgery, room temperature is set about 15-16ºC and raised up to temperatures around 25 ºC

• Room temperature around 15-16 ºC is used for transplant operations. Room temperature limits are often defined by local standard and regulations.

1.2.2

Humidity

Humidity, as the room temperature, is a factor affecting the thermal comfort of both patients and surgical staff. The key criterion is relative humidity RH rather than absolute humidity, because RH affects the rate of evaporation. Human body is tolerant for a wide range of RH variation (between 35% and 75%), over these extremes there may be serious problems. A high level of relative humidity is a common disturbance element, particularly when combined with low room temperature. Consequently, the concentration of staff may be adversely affected by this disturbance. Humidity control during cooling of the air is

11 very important to prevent this effect. As for temperature, relative humidity level of the room must not act as a strong risk for the patient’s health. Previous studies have shown that average values for relative humidity between 40% and 70% are not suitable for microbial growth and dispersal[17]. In addition to this fact, low levels of relative humidity results with the drying of the mucous covering on special tissues in the upper and lower respiratory tracts which causes the particles in the air to be breathed deeply into the lungs [4]. Other effect of relative humidity ratio of room air is on the patient’s wounds. Low relative humidity ratio results with excessive drying of the wound, especially in surgeries. High relative humidity ratio is needed during eye surgeries or tissue transplant operations for burn wounds where the drying of the wound is not desired. For example, up to 95% relative humidity is used for burn patients. In some cases, low relative humidity levels may be required; such conditions can be experienced in treatment of arthiritis, where the relative humidity levels are maintained at around 35% [4]. Moreover, condensation on surfaces must be avoided because these places may be a support for microbial growth. This can be achieved by ensuring that all surfaces are above the dew-point temperature of adjacent air. The conclusion that can be achieved is that as the temperature, the humidity depends strictly on the type of operation that takes place in the OR, and the mechanical ventilation units must be equipped with system for the humidification, performed with steam, and de-humidification, typically performed with a cooling battery and re-heating of air.

1.2.3

Air Filtration

The air treatment for a OR is provided by mechanical ventilation system which typically handle outdoor fresh air and recirculated air. In order to prevent the increasing of particle concentration in the space, the supply air must be filtered appropriately, typically three stages of filtration are required for OR. Microorganisms are proved to be transported by the particles suspended in the air, in particular microorganisms associated with human disease were usually found on particles in the range of equivalent diameter [18] The particles present in the supply air are not the only source of indoor particle concentration. Along with the particles transported into a sterile space by supply air, particles are also generated in space by the activities of personnel and machines. These particles may also carry microorganisms, this makes the filtration not able to control independently the quantity of microorganisms in OR.

The microorganisms that are present in the air may be bacteria, viruses or originated from molds. The bacteria which are highly infectious and transported via air or air-water mixture are Mycobacterium tuberculosis and Legionella pneumophila (Legionnaire’s disease). Multiple studies found extremely high levels, between and cells for m3 _of

air, of Legionella in health care facilities [19]. Varicella (chicken pox), Rubella (German measles), and Rubeola (regular measles) are the examples or viral infections that are transported by air. It is proved that some molds like Aspergillus can be fatal to advanced leukemia, bone marrow transplant and other seriously immunosuppressed patients. Previous studies have shown that 99.9% of all bacteria present in a hospital are removed by 90-95% efficient filters [4]. The main reason of this is that the bacteria usually travel attached to other particles which dimensions are typically larger than 1 µm. The use of

12 high efficiency particulate air (HEPA) filters having filtering efficiencies of 99.97% in certain areas is recommended. It is proved that many of the airborne viruses are in sub-micron size, thus, there is no exact method to eliminate 100% of the viable viruses from air even if HEPA and/or ultra-low penetration (ULPA) filters offer the greatest efficiency. Ultraviolet Germicidal Irradiation (UVGI) has also been used for lowering the number of environmental bacteria and then lowering the infections rates by killing them. However, this method and also chemical solutions used for healthcare facilities are not proven to be effective as can be deduced from the study conducted by F. Memarzadeh [20].

1.2.4

Pressurization

The aim of the pressurization is to guarantee the cleanliness of the OR by avoiding the entrance of contaminants that may come in with air from the less clean neighboring spaces. The air can flow from an adjacent space through the openings of the room to the sterile ambient. The pressure difference between these spaces is the main factor to specify the airflow direction between them. Positive pressurization means an outwards flow from the room while negative pressurization refers to an inwards flow. The pressurization of OR respect to other ambient is always positive, since the environment of OR is typically the cleanest. This situation may change in case of treating contagious diseases, for which it’s mandatory, for most of the local regulations, to adopt a negative pressurization, to prevent airborne contaminants (e.g., microbial pathogens, chemicals) from drifting to other areas. The pressurization can be controlled using differential pressure measurement device, and is maintained by providing a difference in volumetric flow rates of supplied and extracted air. Adjustment can be done changing the supply airflow rate, the return air flow rate, or both of them. There are some issues to be avoided regarding the noise generation, if the overpressure is too high and the doors are closed, a noise may be generated due to high velocity of air flow through door perimeter gaps.

1.2.5

Volumetric Flow Rate

Total supply air and fresh air change rates are important to maintain the required air quality of the spaces. The supply of fresh air improves the air quality in terms of increasing the oxygen amount and diluting the chemical gases and particles that exist in the room air. The mechanically supplied air can be 100% fresh air or the fresh air can be mixed with filtered return air. The decision about supplying 100% fresh air or mixture of fresh and recycled air depends on various factors such as the activity being held in the room, required hygiene level, energy conservation, operation costs etc. The total amount of supply air is difficult to define, and each OR must be designed singularly, many local regulations impose a minimum value of fresh air, and a limit for particle concentrations. These values may help the designer to calculate the correct airflow that allows to maintain the desired conditions of cleanliness and comfort. Computational Fluid Dynamics (CFD) simulations may help to visualize the airflow and help to decide which value of total supply air flow best fits the actual situation. The designed airflow between the room must be provided 24-hours a day, except during maintenance and conditions requiring shutdown

13 by the building’s fire alarm system. During unoccupied hours, depending on local regulations, air exchange can be reduced as long as positive pressure is maintained in each OR. Complete shutdown of the ventilation system may permit airflow from areas with less clean air into the relatively negative pressure area of the ORs.

1.2.6

Noise Control

Noise control is very important for ORs because of the negative impact of high noise levels on patients and staff. In particular surgical staff must work in the most comfortable situation possible and loud noise may increase stress and cause dangerous irritation and distraction during the performances of critical activities. Following are listed some of noise source from HVAC systems:

 Direct transmission of mechanical and/or medical equipment room noise to adjacent spaces;

 Ductborne noise generated by fans and/or high air velocities in ducts, fittings, terminal equipment, or diffuser and transmitted through ductwork to adjoining occupied spaces;

 Duct rumble, a form of low-frequency breakout noise caused by the acoustical response of ductworks to fan noise – particularly high-aspect-ratio, poorly braced rectangular ductworks;