The present results, based on quantitative, temporal, and visual assessments, showed that an aerosol box in a depressurized room significantly decreased physician, medical staff, and environmental aerosol exposure compared with and without an aerosol box in a pressurized room during intubation and extubation. However, an aerosol box in a depressurized room did not completely prevent aerosol dispersal during intubation and extubation.
Of particular importance, aerosol exposure of the physician was not prevented during aerosol box removal in a depressurized room and a pressurized room with all settings.
Why an aerosol box did not completely prevent physician aerosol exposure?
Particle counting showed that an aerosol box did not completely prevent physician aerosol exposure. The videography showed that the aerosolized particles spread and moved on the external roof of the aerosol box to the physician’s face. An aerosol box could change air flow direction and speed and the cough vector and re-spread the aerosol, which is why an aerosol box did not completely prevent physician aerosol exposure. The combined particle counting and three-dimensional visual assessment of the present study could explain this.
Difference in aerosol dispersal between pressurized and depressurized rooms
The differences in aerosol exposure without an aerosol box at three locations between a pressurized room and a depressurized room during intubation and extubation were not significant. On the other hand, the aerosol exposures of the physician and medical staff with an aerosol box were significantly lower in a depressurized room than in a pressurized room. The particle counts (especially for particle diameters >1.0 µm) were decreased at environmental locations 2 m from the mouth of the mannequin with or without an aerosol box in a depressurized room during intubation and extubation. The results showed that the depressurized room enhanced the efficacy of the aerosol box, and the medical staff and instruments must be more than 2 m from the patient’s mouth during intubation and extubation in a depressurized room. In a pressurized room, a distance of 2 m was insufficient to decrease aerosol exposure even with an aerosol box.
In our hospital, the most important difference between a depressurized room and a pressurized room is whether there is an airflow pathway. The results suggest that the airflow pathway is an important factor to spread aerosolized particles. The airflow in a depressurized room can pass into the duct immediately so that the airflow speed can be kept high enough for aerosolized particles (especially >1.0 µm) to drop. However, the airflow in a pressurized room (in our operating room, the initial airflow speeds in both rooms are the same) could not maintain enough speed for aerosolized particles to drop, because there is no airflow exit in a pressurized room. The association between aerosol dispersal and the airflow pathway is considered important not only in the operating room, but also on patient wards, in emergency rooms, and in intensive care units.
Difference between the present study and other studies
Although one study has provided visual evidence of two-dimensional aerosol dispersal of a human cough with a face mask, the present study is the first to demonstrate visual aerosol dispersal during intubation and extubation in an operating room under clinical settings. Moreover, the present study provides the first evidence of three-dimensional visual aerosol dispersal during intubation and extubation. Some studies used a strong laser sheet to assess aerosol dispersal and droplets, because a strong laser sheet is useful to dramatically improve the visualization of aerosols compared with other light sources.[6,7] However, visual assessment of aerosol dispersal with a strong laser sheet is not enough to assess spatial aerosol dispersal, because a laser sheet can detect particles only along the laser pathway, providing a two-dimensional assessment of aerosol dispersal. In addition, RIKEN (Kobe, Japan) showed that a supercomputer (Fugaku, RIKEN, Kobe, Japan) can predict spatial aerosol dispersal after cough. However, even supercomputers, which are only available in a few countries, cannot include all clinical and environmental settings and the movements of physicians and medical staff in the prediction. The present experiment required only an ACLS mannequin, cake flour, a projector, and a PC, which are common items. The present study showed a method of assessing aerosol dispersal independently with common items, which is useful.
Particle counting during intubation and coughing is useful for objective, numerical assessment of aerosol dispersal.[4,9] However, particle counters can count only particles that enter the device inlet. Furthermore, most particle counters cannot measure the full range of aerosol particle sizes simultaneously.[4,9] Although Simpson et al. showed particle numbers with small variation during experimental intubation models, their results were measured in a powerfully depressurized room (pressure -10 Pa, 18 room air changes per hour). Their results apply only under extremely limited conditions. Most critically ill patients must be intubated not in an operating room or intensive care unit, but in an emergency room or general ward during a pandemic.
The present complex assessment that consisted of three-dimensional assessment and particle counting appears necessary and useful to analyse aerosol dispersal.
The range of aerosol diameters was defined as 0.01-100 µm by the Centers for Disease Control and Prevention. Cake flour, submicron oil particles, and titanium dioxide have been used as aerosol models in some studies.[10-12] The size of infective aerosol particles was reported to be >5-10 µm. Airborne transmission could be attributed to some pathogens, including SARS-CoV-2, with sizes ≤5 µm. According to the present experiments, cake flour (2.5–280 mm (D50=25.56 µm)) was suitable as a tracer of aerosolized particles with diameters >2.5 mm. The present results for particles <2.5 µm in size could be unstable and reference values, because an aerosolized particle is not a perfect sphere.
Pattern of aerosol dispersal during intubation and extubation
During both intubation and extubation, the aerosol tended to spread to the right side. This can be explained by the laryngoscope having been inserted from the right side during intubation, and the endotracheal tube, which was fixed at the right corner of the mouth, was removed from the right side. Medical staff should not stand on the patient’s right side during intubation and extubation.
In the present settings, an aerosol box did not protect physicians and medical staff from aerosol exposure during intubation and removal of the aerosol box even in a depressurized room. This is important, because we must intubate infected patients with paralysis during this pandemic. Because full personal protective equipment (PPE) cannot prevent aerosol transmission completely, neuromuscular blockade is essential for intubation. Moreover, medical personnel should pay attention to aerosol dispersal again with the removal of the aerosol box after the procedures.
Efficacy of an aerosol box
Unfortunately, the present results did not show the efficacy of an aerosol box at three locations in a pressurized room. An aerosol box in a depressurized room could decrease most particle counts at three locations, but not significantly. Combined use of an aerosol box and a depressurized room significantly decreased particle counts at three locations during intubation and extubation. However, the quantitative and visual results showed that an aerosol box did not completely prevent aerosol exposure of the physician and medical staff.
A recent study showed that an aerosol box effectively prevented aerosol dispersal. However, this study was conducted in a powerfully depressurized room, but such a powerfully depressurized room is not usually available for physicians to use. In most clinical settings (pressurized rooms), an aerosol box cannot be used to prevent aerosol transmission during intubation and extubation.
Environmental factors including air change rate, fresh air change rate, room size, temperature, humidity, airflow and turbulence, atmospheric pressure in a depressurized room or pressurized pressure room, bed position, patient’s head position during intubation and extubation, and the position and posture of the physician and medical staff could affect aerosol dispersal. All factors could differ among countries, hospitals, and rooms. The present particle visualizing method could allow clinicians to assess aerosol dispersal independently with common items.
Recommendations from the present study to protect from aerosol exposure during intubation and extubation
i) All aerosol-generating procedures must be done leeward. If necessary, the bed position should be moved considering the direction of airflow. The physicians and medical staff must stand windward. It is better to place any instruments windward. Even if airflow speed is high enough to eliminate aerosolized particles, a duct is necessary and can create an airflow pathway, which is important to eliminate aerosolized particles.
ii) During intubation, medical staff should stand on the patient’s left side. During extubation, the medical staff should stand on the patient’s left side if the endotracheal tube was fixed at the right corner of the mouth.
iii) Intubation must be done with neuromuscular blockade, even in a depressurized room and with full PPE and an aerosol box.
iv) Extubation could be done alone as well as possible even in a depressurized room. An aerosol box cannot completely prevent aerosol dispersal during extubation.
v) Independent assessment of aerosol dispersal is essential to protect patients, medical staff, and physicians from aerosol exposure. The present particle visualizing method can be used to assess three-dimensional aerosol dispersal with common items (if video cameras are unavailable, naked eye observation may be sufficient).
vi) If an aerosol box is used during intubation and extubation, attention to aerosol dispersal is again needed when removing it.
As a limitation, the diameter of the cake flour was 5–100 mm, which results in a somewhat artificial human coughing pattern because of the aerodynamics (Supplemental Video 1), since differences in particle diameters and coughing patterns affect aerosol spread. Because cake flour does not include small aerosolized particles (0.01-1 µm), and even high-resolution cameras cannot detect all sizes of aerosolized particles, clinical aerosol exposure could be greater than in the current experiment.