Highlights
- •Modern air handling systems for operating rooms protect the patient and surgical team.
- •Concentrations of air particles are greater along walls than at the instrument table.
- •Fewer particles are found at the center of the operating room.
- •Stepping 2 m away from patient would not protect personnel in operating rooms.
Background
During the coronavirus disease 2019 (COVID-19) pandemic, recommendations have included that personnel not involved in procedures releasing airborne contaminants reduce their exposure by moving >2 m away. We tested whether air particle concentrations in operating rooms (ORs) are greater in the periphery, downstream from the supply airflow.
Methods
We analyzed data from 15 mock surgical procedures performed in 3 ORs. Two ORs were modern, one with a single large diffuser system above the surgical table, and the other using a multiple diffuser array design. An air particle counting unit was located on the instrument table, another adjacent to an air return grille.
Results
Concentrations of air particles were greater at return grille than instrument table for the single large diffuser at 26 air exchanges per hour, and the multiple diffuser array at both 26 and 20 air exchanges per hour (all P ≤ .0044), including during electrocautery (all P ≤ .0072). The ratios of concentrations, return grille versus instrument table, were greater during electrocautery for 0.5 to 1.0-micron particles and 1.0 to 5.0-micron particles (both P < .0001).
Conclusions
Modern OR airflow systems are so effective at protecting the surgical field and team from airborne particles emitted during surgery that concentrations of particles released at the OR table are greater at the OR walls than near the center of the room.
Key Words
Introduction
During the coronavirus disease 2019 (COVID-19) pandemic, there have been recommendations of safe distances from other people (eg, 2 m), including within operating rooms (ORs).
1
Distancing is a theme of public conversation,2
including during tracheal intubation and/or extubation, and other aerosol generating procedures, when personnel cannot safely or practically leave the room.3
, 4
, 5
, 6
, 7
(See Google search at https://FDshort.com/WagnerAirParticles). Inside ORs, is exposure to pathologic airborne particles (eg, potentially SARS-CoV-2) less when people stand or sit farther from the patient?- Kaushal D
- Nair NP
- Soni K
- Goyal A
- Choudhury B
- Rajan N
Endoscopy in otorhinolaryngology during corona outbreak: a proposal for safe practice [e-pub ahead of print].
Indian J Otolaryngol Head Neck Surg. 2020; (Accessed October 17, 2020)https://doi.org/10.1007/s12070-020-02048-9
3
, 4
, 5
, 6
, 7
When an anesthesiologist intubates or extubates the patient, or surgical smoke is produced,- Kaushal D
- Nair NP
- Soni K
- Goyal A
- Choudhury B
- Rajan N
Endoscopy in otorhinolaryngology during corona outbreak: a proposal for safe practice [e-pub ahead of print].
Indian J Otolaryngol Head Neck Surg. 2020; (Accessed October 17, 2020)https://doi.org/10.1007/s12070-020-02048-9
8
would others in the room be safer by moving away (eg, complete their charting at a computer along a wall)?OR air handling systems are designed to move particles away from the surgical field. These are particles that were generated (eg, surgical smoke) or exhaled or shed by patients, surgeons, anesthesiologists, etc.
9
Some ORs have a single large diffuser system in the ceiling above the surgical field.10
Particle tracing studies show that single large diffusers help to prevent particles from settling into the wound by sweeping them downward and away toward the periphery of the OR.10
(See Supplementary Fig 1 at https://FDshort.com/WagnerAirParticles.) Thus, moving physically in the OR away from the surgical table toward the walls may increase personnel exposure to higher concentrations of airborne contaminants. That would be a consequence of how contemporary ORs are designed to prevent surgical site infection.9
We are unaware of studies testing this relationship for airborne particles; the hypothesis, that moving to a corner of the OR furthest away from the patient would result in greater exposure to airborne contaminates, is based on airflow.
11
, 12
, 13
However, the authors recognized that experimental data from 2018 could be reanalyzed to estimate relative concentrations of airborne particles between the surgical field and periphery of the operating room at air registers.14
We hypothesized greater concentrations in the periphery than at their source at the OR table.Methods
In January 2018, mock surgical procedures were performed in 3 fully functional ORs regularly used.
14
Two of the ORs were constructed at the same time in 2017. One OR had a single large diffuser10
system in the ceiling above the surgical table. The other new OR used a multiple diffuser array design10
above the OR table. The 55 m2 ORs were the same except for the configuration of air delivery.14
Both rooms had positive pressure, 26 air changes per hour, and high-efficiency particulate air filtration using 4 low-wall return grilles.14
The single large diffuser was constructed with 9 diffusers coincident to one another, 2.35 m by 2.95 m total dimension.14
The multiple diffuser array design had 6 diffusers each 1.17 m by 0.575 m, separated by hard ceiling surfaces and ceiling-mounted equipment.14
The original study was conducted to compare performance of different airflow designs at protecting the surgical field.
14
The 2 new ORs were compared to a third, older, room constructed in 1992, with a 4-way throw diffuser system.14
Each of the four diffusers was 0.109 m2.14
The room was 44.3 m2 in dimension.14
There were 2 low wall return grilles. There was no air distribution directly over the surgical table and sterile field as required by current ASHRAE 170 Guidelines.10
.The mock surgical procedure was designed for analyzing multiple environmental quality indicators within ORs.
15
An air particle counting unit was located at the instrument table along the foot of the surgical table.15
That unit was 2.5 m from the midpoint of the head of the bed, 7.9 m from the return grille in the 2 new ORs, and 7.1 m from the return grille in the older OR. The Aerotrak particle counter model 9500 (TSI Incorporated) sampled at 100 L of air per minute. A second stationary particle counter was located on a pedestal in front of one of the return air grilles.14
The script and timeline were displayed on monitors showing responsibilities in 4-minute increments for each person in the OR during a 1-hour mock procedure.15
The script included gowning, gloving, draping, passing instruments, personnel entering and leaving the room, and the use of electrocautery on an uncooked steak to generate particulate surgical smoke.15
The steaks were positioned 1.6 m from the air particle counting units on the instrument tables, 6.4 m from the units at the return grilles in the 2 modern ORs, and 6.1 m from the units at the return grille of the older OR. For diagrams of the ORs, see Supplementary Figs 2-4 at https://FDshort.com/WagnerAirParticles.Study personnel wore standard hospital-issued scrub attire, head covers, surgical masks, shoe covers, and scrubbed for the procedure per protocol.
15
There were 10 people in the ORs including surgeon, 4 surgical nurses, script timekeeper, microbiologist, industrial hygienist, and 2 indoor environmental engineers.14
The script included the multiple measurements to be made including airflow, humidity, temperature, pressure, air velocity, particle, and bacterial counts, all reported in the original paper.14
Electrocautery, patient warming device, all computers and monitors, lights, and insufflator all were “on” throughout the mock procedures to result in realistic airflows. In addition, besides the circulating nurse and the “runner” exchanging petri dishes for bacterial sampling, other personnel remained in stationary positions while performing the simulated procedure, including passing of instruments, movement of light booms, etc.The 2 stationary particle counters recorded particle counts over each 2-minute increment. Particle sizes recorded were ≥0.5 microns, ≥1.0, ≥5.0, and ≥10.0 micron-sized particles per cubic meter. There were 28 consecutive measurements at 2-minute increments starting at the second minute of the mock surgical case.
The OR with the single large diffuser had 2 mock surgical cases per day for 3 consecutive days, one case with 26 air exchanges per hour and the other case with 20 air exchanges per hour. The same 6 cases were done in the room with the multiple diffuser arrays. Finally, the older OR with 4-way throw diffuser system had 3 cases on 3 consecutive days, all with 26 air exchanges per hour. Each day, there was a random sequence of the 5 combinations of ORs and air changes.
Statistical methods
This analysis was unplanned when the study was conducted. The sample size was finite, experiments already completed.
14
,15
Therefore, we treated P < .01 as the criterion for statistically significant differences, used nonparametric analyses, and interpreted the results conservatively in the Discussion and Conclusions, limiting statements to “greater.” The principal issue for managerial decision-making is different, specifically whether concentrations of airborne particles are comparable at the periphery of the operating room.Longitudinal analyses of airborne particulate concentrations generally are studied using 2-parameter log-normal distributions.
Two-sided P value shows result for the opposite of the hypothesized relationship that there would be greater concentration at air return grilles than at the instrument table (ie, at the surgical table).
Two-sided P value is for opposite of hypothesized relationship that there would be greater concentration at air return grilles than at the instrument table (ie, at the surgical table).
16
Rumburg et al compared fits of 7 probability distributions to airborne particulate concentrations.17
From their Table 4, none of the other 5 probability distributions consistently performed better.17
We calculated ratios of pairwise measurements of particle concentrations, adjacent to return grille/instrument table. As expected for log-normal distributions, we too found that probability distributions of ratios were more symmetric than differences, and thus analyzed ratios. However, the logarithms of ratios were not normally distributed due to substantive kurtosis (Shapiro-Wilk P < .0001). Therefore, in Tables 1 and 2, we used Wilcoxon signed-rank tests for pairwise comparisons (ie, ratios differing than 1.0). We report medians and interquartile ranges, with the percentiles calculated using the STATA summarize function (STATA 16.1, College Station, TX). In the Results, we also use the Wilcoxon-Mann-Whitney test to compare ratios during versus not during electrocautery. All P values were 2-sided and exact.Table 1Ratios of air particles’ concentrations between return grille and instrument table: median (25th percentile, 75th percentile) and results of Wilcoxon signed-rank test
| Room type | ≥0.5 microns, <1 micron | ≥1.0 micron, <5 microns | ≥5.0 microns, <10.0 microns | ≥10.0 microns |
|---|---|---|---|---|
| Multiple diffuser array, 26 air exchanges per hour, N = 84, 2-minute periods | 1.42 (0.93, 4.52), P < .0001 | 1.44 (1.00, 3.20), P < .0001 | 1.67 (1.29, 2.05), P < .0001 | 1.62 (1.31, 1.93), P < .0001 |
| Single large diffuser, 26 air exchanges per hour, N = 84, 2-minute periods | 1.16 (0.84, 1.54), P = .0044 | 1.18 (0.90, 1.49), P < .0001 | 1.55 (1.31, 1.78), P < .0001 | 1.70 (1.40, 2.05), P < .0001 |
| Multiple diffuser array, 20 air exchanges per hour, N = 84, 2-minute periods | 1.13 (0.90, 2.07), P < .0001 | 1.16 (0.95, 1.77), P < .0001 | 1.42 (1.15, 1.58), P < .0001 | 1.43 (1.22, 1.68), P < .0001 |
| Single large diffuser, 20 air exchanges per hour, N = 84, 2-minute periods | 1.05 (0.87, 1.25), P = .022 | 1.03 (0.92, 1.17), P = .068 | 1.18 (1.05, 1.44), P < .0001 | 1.36 (1.16, 1.61), P < .0001 |
| Older, 26 air exchange per hour, N = 84, 2-minute periods | 0.89 (0.76, 1.10), P = .0016 | 0.91 (0.77, 1.05), P = .0021 | 1.15 (0.94, 1.33), P < .0001 | 1.20 (1.03, 1.40), P < .0001 |
Table 2Ratios of air particles’ concentrations between return grille and instrument table, limited to periods with use of electrocautery, with same format as Table 1
| Room type | ≥0.5 microns, <1 micron | ≥1.0 micron, <5 microns | ≥5.0 microns, <10.0 microns | ≥10.0 microns |
|---|---|---|---|---|
| Multiple diffuser array, 26 air exchanges per hour, N = 24, 2-minute periods | 4.46 (2.01, 6.47), P < .0001 | 2.86 (1.79, 4.03), P < .0001 | 1.95 (1.52, 2.16), P < .0001 | 1.55 (1.32, 1.92), P < .0001 |
| Single large diffuser, 26 air exchanges per hour, N = 24, 2-minute periods | 1.30 (0.89, 1.78), P = .0072 | 1.25 (0.88, 1.68), P = 0.0087 | 1.50 (1.35, 1.69), P < .0001 | 1.72 (1.40, 2.00), P < .0001 |
| Multiple diffuser array, 20 air exchanges per hour, N = 24, 2-minute periods | 2.89 (1.75, 5.12), P < .0001 | 2.21 (1.61, 3.10), P < .0001 | 1.59 (1.47, 1.93), P < .0001 | 1.43 (1.23, 1.68), P < .0001 |
| Single large diffuser, 20 air exchanges per hour, N = 20, 2-minute periods | 1.25 (1.06, 1.76), P = .0020 | 1.13 (0.94, 1.25), P = .070 | 1.18 (1.01, 1.39), P = .0083 | 1.46 (1.11, 1.65), P = .0001 |
| Older, 26 air exchange per hour, N = 16, 2-minute periods | 0.92 (0.65, 1.14), P = .23 | 0.89 (0.71, 1.01), P = .058 | 1.09 (1.00, 1.29), P = .034 | 1.19 (1.00, 1.38), P = .0042 |
There could be a time lag between particle measurements between instrument table and return air grille. For both <1.0-micron and ≥10-micron particles, the observed Kendall's τb and Pearson r correlations were greater for no lag versus lag 1 (ie, 2 minutes) or lag 2 (ie, 4 minutes). Therefore, the ratios of particle concentrations at the 2 fixed room locations were calculated using the same 2-minute periods, as above using 28 observations per case.
Results
The concentration of air particles was greater at the return grille than instrument table for the single large diffuser at 26 air exchanges per hour, and the multiple diffuser arrays at both 26 and 20 air exchanges per hour (Table 1, all P ≤ .0044). The single large diffuser at 20 air exchanges per hour and the older room's 4-way throw diffuser systems had greater concentrations at the return grille for larger particles (≥5.0 microns).
Electrocautery was important to understand the study results because the location of emission was known to be the OR table, unlike for larger particles, shed from people throughout the ORs. The ratios of concentrations, return grille versus instrument table, were greater during electrocautery for the 0.5 to 1.0-micron particles (N = 108 during electrocautery median [25th percentile, 75th percentile] of 1.61 [1.03, 3.80] vs N = 312 without 1.03 [0.82-1.24], P < .0001) and 1.0 to 5.0-micron particles (during 1.32 [0.99, 2.35] vs 1.05 [0.88, 1.28], P < .0001). There were greater concentrations of air particles during electrocautery at the return grille than instrument table for the single large diffuser at 26 air exchanges per hour and the multiple diffuser arrays at both 26 and 20 air exchanges per hour (Table 2, all P ≤ .0072).
Discussion
Increasing air exchanges per hour reduces time for clearance of air particles (eg, SARS-CoV-2 or cautery).
18
Our results show that benefit is achieved with a consequent greater relative concentration at the periphery versus at the OR table, because the airflow is from above the OR table down and then out toward the return air grilles along the walls. Our results for the periods with electrocautery highlight how effective modern OR airflow designs can be at reducing the risk of surgical site infection and exposure of the surgical team. Specifically, even though the “surgery” on the uncooked steak was done at the OR table, the increase in particle concentration was greater at the particle counter along the walls of the ORs. Thus, by design, particles have been carried away from the patient.9
Comparisons with previous studies
Previous studies have shown the bacterial colony forming units in air samples of (actual) surgical cases being greater when there were more people in ORs (ie, people are the source, with a Spearman rank correlation coefficient of 0.45).
19
Previous studies have also shown airborne concentrations of bacteria being greater outside versus inside the sterile field.9
,14
,20
Our results for multiple particle sizes indicate strong chance that infectious aerosols, including viruses generated by patients, would pose as much a risk to health care workers at the periphery of the room than adjacent to the patient.We are aware of 1 previous study with data analogous to ours permitting some comparison.
21
Alsved et al used computational fluid dynamics to model airflow velocities.21
Laminar airflow had relatively high velocity straight down from the ceiling above the OR table, then out along the floor to the walls, where the flow extended upward with turbulent flow.21
Temperature-controlled airflow had lower velocity over the table and less turbulent flow up the walls.21
The designs of the simulated rooms would be expected to have greater concentration of air particles at the lower walls, as we observed. They measured colony-forming units of viable airborne bacteria (≥5 microns).21
The bacteria were not released deliberately and personnel were not preferentially at the OR table (ie, the authors appropriately did not examine associations between OR locations appropriately, and one area of the OR may not have caused observation at another area).21
Nevertheless, for both of the modern designed systems, laminar flow and temperature-controlled airflow, median concentrations at the periphery of the room were greater than at the wound or instrument tray.21
However, for the older design with substantial resulting turbulent flow, that relationship was disrupted.21
These findings qualitatively match our results.Limitations
A limitation of our study is that particle concentrations were measured above air return grilles, not at the height of a circulating nurse sitting in a chair at a computer. However, based on airflow, we doubt that our results would differ. First, air does not all exit the air register grilles; some travels up walls.
21
This is shown by particle-tracing studies of single large diffusers and computational fluid dynamic simulations of laminar or temperature-controlled airflows.10
,21
Second, nonuniformity and greater rates of airflow near the return grilles and corresponding air particle counting units would not affect results because particle counting was analyzed as a density, particles per volume, thus normalizing the values for flow.Another limitation is that the particle sizes were appropriate for pathogenic bacteria (eg, 10 microns for Staphylococcus aureus clusters or skin cells carrying bacteria) but larger than some viruses (eg, the minimum studied size of 0.5 microns exceeds the diameter of SARS-CoV-2).
22
However, aerosolized particles in ORs apply to all types of particles (eg, surgical smoke8
) and our findings qualitatively applied to all sizes studied (Tables 1 and 2).Applications
Our paper applies generally to airborne transmission of a pathogen in the OR, whether SARS-CoV-2
3
, 4
, 5
released by surgical or airway manipulation, or to other airborne contaminants such as electrocautery smoke and tuberculosis. However, maintaining distances from released airborne particles has permeated colloquial considerations since the onset of the COVID-19 pandemic (See Google search at https://FDshort.com/WagnerAirParticles).2
We recommend personnel awareness that in an OR it may be a counter-productive strategy to move away from the patient toward a wall (ie, where air return grilles are located). During robotic surgery, considerations for placement of the surgical console traditionally have included being outside the sterile field and at a location that facilitates line of site to the operative field and easy communication with the assistant surgeon and scrub nurse.23
With COVID-19, there is the additional consideration of exposure of the surgeon at the console to the patient.World Laparoscopy Hospital. OR configuration in Da Vinci robotic surgery. Available at: https://www.laparoscopyhospital.com/OR-Configuration-in-da-Vinci-surgery.html. Accessed June 30, 2020.
6
Conclusions
Our results show the value of education that the seated surgeon should expect to be exposed to a higher concentration of particles than the anesthesia team, assistant surgeon, or scrub nurse. Modern OR airflow systems are so effective at protecting the surgical field and team from airborne particles emitted during surgery that concentrations of particles released at the OR table are greater at the OR walls than near the center of the room.
References
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- Understanding public perception of coronavirus disease 2019 (COVID-19) social distancing on Twitter.Infect Control Hosp Epidemiol. 2021; 42: 131-138
- Asymptomatic COVID-19 infection in a child with nasal foreign body.Int J Pediatr Otorhinolaryngol. 2020; 135110092
- COVID-19: role of ambulatory surgery facilities in this global pandemic.Anesth Analg. 2020; 131: 31-36
- Considerations for pediatric heart programs during COVID-19: recommendations from the congenital cardiac anesthesia society.Anesth Analg. 2020; 131: 403-409
- Robotic surgery in otolaryngology during the Covid-19 pandemic: a safer approach?.Indian J Otolaryngol Head Neck Surg. 2021; 73: 1-4
- Endoscopy in otorhinolaryngology during corona outbreak: a proposal for safe practice [e-pub ahead of print].Indian J Otolaryngol Head Neck Surg. 2020; (Accessed October 17, 2020)https://doi.org/10.1007/s12070-020-02048-9
- Risk of virus contamination through surgical smoke during minimally invasive surgery: a systematic review of literature on a neglected issue revived in the COVID-19 pandemic era.Eur Urol Focus. 2020; 6: 1058-1069
- Hats off: a study of different operating room headgear assessed by environmental quality indicators.J Am Coll Surg. 2017; 225: 573-581
- Improving operating room contamination control.ASHRAE J. 2014; 56: 18-27
- Strategies for daily operating room management of ambulatory surgery centers following resolution of the acute phase of the COVID-19 pandemic.J Clin Anesth. 2020; 64109854
- An evaluation of a partial-walled laminar-flow operating room.J Hyg Camb. 1974; 73: 61-74
- The effect of door opening on positive pressure and airflow in operating rooms.J Am Acad Orthop Surg. 2018; 26 (e105-113)
- Comparison of operating room air distribution systems using the environmental quality indicator method of dynamic simulated surgical procedures.Am J Infect Control. 2019; 47: e1-e6
- Methodology for analyzing environmental quality indicators in a dynamic operating room environment.Am J Infect Control. 2017; 45: 354-359
- Assessing changes to the probability distribution of sulphur dioxide in the UK using a lognormal model.Atmos Environ. 2003; 37: 1461-1474
- Statistical distributions of particulate matter and the error associated with sampling frequency.Atmos Environ. 2001; 31: 2907-2920
- Determination of length of time for “postaerosolause” for patients under investigation or positive for coronavirus disease 2019.Anesth Analg. 2020; 131: e94-e96
- Nine years of microbiological air monitoring in the operating theatres of a university hospital in Southern Italy.Ann Ig. 2019; 31: 1-12
- Covering the instrument table decreases bacterial bioburden: an evaluation of environmental quality indicators.Am J Infect Control. 2018; 46: 1127-1133
- Temperature-controlled airflow ventilation in operating rooms compared with laminar airflow and turbulent mixed airflow.J Hosp Infect. 2018; 98: 181-190
- Science forum: SARS-CoV-2 (COVID-19) by the numbers.Elife. 2020; 9: e57309
World Laparoscopy Hospital. OR configuration in Da Vinci robotic surgery. Available at: https://www.laparoscopyhospital.com/OR-Configuration-in-da-Vinci-surgery.html. Accessed June 30, 2020.
Article Info
Publication History
Published online: October 08, 2020
Footnotes
Funding: This study was funded separately by everyone's organization: University of Iowa , OnSite-LLC , and SLD Technology, Inc .
Conflicts of interest: Dr Wagner receives no funds personally other than her salary, disbursements, and allowable expense reimbursements from OnSite, LLC and Prism Environmental Inc., both of which are consultative firms that advise their clients on the proper application of technologies to support improved infection prevention and control. Neither she nor her family has any financial holdings in any company related to her work. Dr Dexter receives no funds personally other than his salary and allowable expense reimbursements from the University of Iowa and has tenure with no incentive program. He and his family have no financial holdings in any company related to his work, other than indirectly through mutual funds for retirement. Income from the Division's consulting work is used to fund Division research. The Division of Management Consulting of the University of Iowa's Department of Anesthesia provides consultations to companies, including one done previously for SLD Technology. A list of the Division's consults is available at http://www.FranklinDexter.net/Contact_Info.htm. Mr Greeley receives no funds personally other than his salary, disbursements, and allowable expense reimbursements from OnSite, LLC and Global Health Systems Inc., both of which are consultative firms that advise their clients on the proper application of technologies to support improved infection prevention and control. He and his family have no financial holdings in any company related to his work. Mr Schreiber receives no funds personally other than his salary, disbursements, and allowable expense reimbursements from SLD Technology, Inc.
Author contributions: J.A.W. helped with methodology, investigation, resources, and writing review and editing. F.D. helped with conceptualization, methodology, software, validation, formal analysis, writing original draft, writing review and editing, and visualization. D.G.G. helped with validation, data curation, and writing review and editing. K.S. helped with conceptualization, methodology, and writing review and editing.
Identification
Copyright
© 2020 Association for Professionals in Infection Control and Epidemiology, Inc. Published by Elsevier Inc. All rights reserved.
