aerosol generating procedures (AGPs)

Introduction

for the purposes of this topic, “airborne transmission” refers to transmission from aerosols and aerosols are regarded as being fluid droplets less than 10 micron in size and thus are able remain suspended in air for some time without being affected by gravity and which could potentially travel via inhaled air to the lower respiratory tract. Larger droplets are more affected by gravity and fall to the ground a short distance from the source and do not travel to the lower respiratory tract unless under artificial high flow situations.
respiratory tract viral infections can be grouped into 3 categories:
- obligate airborne transmission
  - these are only spread by aerosols and not by droplets
  - eg. TB, perhaps MERS
- preferential airborne transmission
  - these are mainly spread through aerosols but may have other routes
  - eg. measles, chickenpox
- opportunistic airborne transmission
  - these are not usually spread by aerosols but may do so if AGPs are used (in which case the risk is probably confined mainly to those within 1m for respiratory viruses) or if aerosol viral load accumulates in a poorly ventilated space
  - if aerosol transmission does occur it could correlate with a more severe lower respiratory tract infection
  - eg. SARS, Covid-19, influenza
  - perhaps Ebola as aerosolization of blood or bodily fluids with high titres of virus is a risk
smaller sized droplets can become aerosols and hang in the air and circulate through the room or into other rooms for a number of hours vastly increasing the range of infective transmissibility but whether opportunistic viruses can survive in these to cause infection in hosts varies greatly with virus and even viral strains, and with environmental conditions such as relative humidity
the act of normal breathing generates aerosols from the alveoli and over a 24 hour period the amount of aerosol exhaled is likely to exceed the amount from coughing throughout the day
when considering the concept of AGPs, we are really trying to define:
- what procedures may increase risk to healthcare workers from pathogen transmission
- how to reduce the risk and what PPE should be used.

Aerosol generating procedures (AGPs)

an AGP is a medical procedure that creates aerosols in addition to those that the patient creates from breathing, talking, singing, crying, coughing, sneezing, and vomiting.
the usual definition is: “aerosol generating procedures are considered to have a greater likelihood of producing aerosols compared to coughing.”
in addition, flushing a toilet generates an aerosol into the room and may spread pathogens
the scientific evidence for the creation of aerosols associated with these procedures, the burden of potential viable microbes within the created aerosols, and the mechanism of transmission to the host have not been well studied!
- Open Forum Infectious Diseases 2017: Defining Aerosol Generating Procedures and Pathogen Transmission Risks in Healthcare Settings failed to show any increase in pathogens in a small study of various “AGPs”
despite the lack of high quality studies, the current evidence-based guidelines recommend that additional precautionary measures be taken for specified aerosol-generating procedures performed on patients with suspected respiratory infection¹⁾
- Very low-quality evidence suggests that some procedures potentially capable of generating aerosols have been associated with increased risk of SARS transmission of SARS-CoV from infected patients to HCWs, with the most consistent association across several studies being with tracheal intubation.²⁾³⁾
studying risk of transmission of infection to healthcare workers is complicated as:
- patients having “AGPs” tend to have higher viral loads and may have higher underlying aerosol production rates due to respiratory distress
- staff generally need to get closer to the patient
- the urgency of the procedure may reduce complete application of PPE
- staff often perform more than one type of “AGP” on the patient in the session
- studying just the amount of aerosol may be misleading as:
  - that aerosol may not have pathogen contamination (eg. nebuliser therapy)
  - the change in amount of aerosol may not actually be significant when compared to underlying levels of baseline aerosol production which may be high
  - the method of aerosol detection may not detect certain particle sizes
there is no firm evidence that viruses in an aerosol-sized particle are able to survive the journey from the mouth to a susceptible host to cause an infection ⁴⁾

what generates an aerosol?

aerosols require air movement over the top of a liquid surface to be produced
thus any procedure which induces coughing or which pushes air into the airways (eg. BVM ventilation) can theoretically create an aerosol
simply sliding an endotracheal tube into the trachea in a paralyzed patient would NOT of itself generate an aerosol⁵⁾
some “AGPs” potentially increase risk of infection by increasing the dispersion distance of the exhaled aerosol
see also: aerosols

examples of what are considered AGPs

bag-valve-mask ventilation (BVM)
- but presumably most of risk is likely to be removed by using two-handed technique with good seal and a viral filter in place
CPR chest compressions - forcibly increases air flow in the lungs
non-invasive ventilation such as BiPAP or CPAP
- air tends to leak around the mask and this may allow aerosols to escape although evidence suggests only droplets larger than 10 microns escape ⁶⁾⁷⁾
- it is likely this may be reduced by use of helmet style set ups if they have a good seal around the neck
high flow oxygen
- may theoretically disperse exhaled aerosol further
- HFNC may increase air flows through the nose and back out through the mouth
airway suctioning
bronchoscopy
surgical tracheostomy
dental and orthopaedic work - high frequency devices and suction used which could also aerosolize blood although there is little evidence to suggest such aerosolized HIV or hepatitis virus particles are able to cause infection

probably NOT biohazard AGPs

nebulizers
- this is debatable as what is aerosolized is the nebulizer solution which of itself poses no risk to bystanders and studies show that when saline is used, this reaches the alveoli and it actually reduces the amount of aerosolization of alveolar fluid layer and increases droplet size
- if a particle in the aerosol coalesces with a contaminated mucous membrane, it will cease to be airborne and therefore will not be part of an aerosol ⁸⁾
- to be safe, it is probably better to use MDI's with spacers instead where possible
AGP-informed elective intubation technique
- had been placed as a potential AGP due to presumed increased risk via pre-oxygenation, suction or BVM
- a UK Oct 2020 study showed that intubations produce less aerosol than a volitional cough, the concentration of aerosol generated is several orders of magnitude less than a single cough and is only very modestly above background levels of circulating particles in an ultraclean theatre. There were no increases in aerosolised particles above the patient's face during anaesthesia, facemask ventilation, airway suction and, on occasion, several repeated attempts at intubation ⁹⁾
extubation
- induces coughing and higher aerosol generation than intubation but these coughs generate only a quarter of the aerosol of volitional coughs and only aerosols were transient lasting 5 seconds according to a a UK Oct 2020 study ¹⁰⁾