Aerosol Generating Procedures

As more cases of Covid19 present to health care facilities across the world, there seems to be some confusion as to what is an aerosol-generating procedure. Turning up to work is not without risk with a large number of healthcare workers in Italy and Ireland. diagnosed with COVID19. There is a case report of asymptomatic carriage lasting up to 16 days so we need to be careful whether the child in front of us has been diagnosed with COVID19 or not.

A lot of the data we have comes from the 2003 SARS epidemic and the H5N1 influenza outbreaks. There are always going to be a number of confounding variables when looking at these reports – whether the HCW was wearing appropriate PPE (or had access to it), how good their hand-washing was, how close together patients are – but nosocomial infections do occur.

First off,  we are going to take a look at what an aerosol is, then how aerosols and droplets relate to some common, and uncommon, things we do in paediatrics.

Aerosol or droplet?

Let’s define some terms before we get started – not as easy as it sounds, it turns out.

A  respiratory droplet is a fluid bundle of infectious particles that travels from the respiratory tract of the infected individual onto the mucosal surface of another, rather than floating down the respiratory tract. Small droplets are between 5-20μm and tend to hang up around the glottis. Large droplets are > 20μm and are probably too big to follow airflow. They tend to obey the laws of gravity and so settle on nearby surfaces when you sneeze. If you inadvertently touch the same surface then touch your face you can potentially transmit the infection. This is why we wash our hands. In healthcare, droplet precautions include a surgical mask, eyewear, disposable gown, and gloves. The surgical mask acts as a physical barrier to droplets that are too large to be inhaled.

A droplet nucleus is what is left once the liquid rapidly evaporates from a droplet. They are in the order of 10μm in diameter and are in the respirable range. This is generally defined as any particle less than 10μm. The inspirable range is defined as anything between 10 – 100μm in size.

An aerosol is a liquid (or solid) suspended in the air – think mist and fog. These small particles are less than 5μm and so are in the respirable range (rather than the inspirable range like droplets) and can enter the lower respiratory tract. They are affected by diffusion rather than gravity so tend to hang around for a while.  Measles is one such airborne disease. A recent letter in the NEJM suggests that SARS-CoV-2 can remain viable in aerosols for at least 3 hours, though the WHO’s guidance is clear that it should be managed with droplet and contact precautions UNLESS you are performing an aerosolising procedure.

Consider them on the continuum of aerosol -> small droplets -> large droplets -> puddles. Aerosols and small droplets have the ability to travel fair distances, especially if powered by a blast of oxygen or expired air. Larger droplets tend to obey the laws of gravity and settle on surfaces.

Just breathing, coughing and sneezing

But even putting an oxygen mask on the patient may not protect you. Hui et al. (2006) used fancy laser beams and smoke to detect just how far a single breath might travel.  With a standard oxygen mask on the patient and a flow rate of 4l/min, a tidal volume of 500mls, and 12 breaths a minute the smoke plume traveled approximately 0.45m. In most experiments, scientists use smoke as a stand-in for the more nebulous breath of air. Non-biological aerosols will behave differently depending on the airflow and ventilation in the room and have a constant density. Mathematical modelling would suggest that the further from the source a sample is taken then the lower the potential infectivity until a state of equilibrium is reached. Fortunately, the air is exchanged in most hospital rooms on a regular basis.

A patient that is coughing and sneezing can produce large, short-range droplets and small, long-range aerosols. The aerosols produced by coughing are heavier than the smoke used in experiments so hopefully, they may not be able to travel as far. Experimental data will tend to over-estimate the spread of droplets.

Thompson et al (2013) took 99 air samples around presumptive AGPs. 26.1% of them contained viral RNA. But the baseline level of contamination, when no AGPs (as defined by WHO 2009) were performed was 10.5%. Just because a procedure might generate an aerosol, it does not hold true that the aerosol can cause an infection.

Most of the data we have comes from the fast SARS-CoV epidemic in 2002-2003. Tran et al. tried to find all of the papers related to HCW infection and aerosol-generating procedures. They found 10 – 5 non-randomized cohort studies and 5 retrospective cohort studies. They then created pooled estimates of odds ratios.

Judson and Munster usefully categorized AGPs into those that mechanically create and disperse aerosols and those that make the patient wriggle and cough. Or you could think of them, as suggested by Brewster et al. (2020) as those procedures that require gas flow and those that require no extrinsic gas flow.

Bag-valve-mask ventilation and CPR

High risk 

A paediatric cardiac arrest is uncommon. When it occurs your first move* should be to open the airway and provide rescue breaths. In this time of COVID19, I doubt anyone is going to be doing mouth-to-mouth/nose ventilation. They are going to reach for an appropriately sized bag-valve-mask. Just like when placing a standard oxygen mask, there is a transverse movement of droplets even with a reasonable seal. The addition of an HME filter does appear to attenuate some of this, as demonstrated by Chan et al.(2018).

Adult CPR guidelines are advocating for chest compression-only CPR in the community and rapid intubation pre-compressions if circumstances allow. There has been little guidance on paediatric CPR from the ALSG but a number of enterprising teams are looking at it.

Possible cases of SARS transmission by CPR have been reported (Christian et al. 2004) but BVM ventilation took place during the cases and this may be the most important factor for possible viral transmission.

Intubation

High risk 

Anything, where the clinician is inches away from the respiratory tract of the patient, is going to be a high-risk procedure. There have been huge collaborative efforts worldwide creating COVID intubation algorithms. They share a lot of commonalities.

• The most experienced operator performs the procedure – this is not a time for learning
• No bag-valve-mask ventilation prior to intubation
• Use of videolaryngoscopy to maximize the distance between intubator and patient
• Minimum number of staff present

This is my favourite paediatric intubation resource from Queensland Children’s Hospital.

Nebulizing a medication

High risk / Unclear evidence

There are few indications for nebulizing medication. Bronchodilators are best delivered by MDI and spacer when possible but in cases of severe asthma or perhaps, more commonly, in croup, a nebulizer chamber may be the way to go. The UK guidelines do not consider the delivery of nebulized medications as an AGP. The rationale behind this is that the aerosol is derived from a non-patient source. Even if they do have the disease the medication sticks to the mucus membranes and so will not get released into the general environs. There seems to be a lack of global consensus on this.

Nebulizers generate small particles, between 1-5microns in diameter, in order to get down into the bronchioles and not just be deposited in the oropharynx. Viable COVID19 viral RNA has been detected in aerosol form 3 hours after delivery by nebulizer in experimental conditions but this does not prove infectivity, just infectious potential.

In 2009 O’Neill et al. performed air sampling studies for common patient activities, including making the bed and providing nebulized therapy, as well as some more invasive treatments (bronchoscopy and suctioning). Although small numbers they found an increase in influenza particle numbers (from baseline) of up to 70,000/cm³.

High Flow Nasal Cannula

High risk 

In adult practice, high flow oxygen delivery is anything over 6l/min. In paediatrics, it is 2l/kg/min up to the adult maximum of 60l/min. In one of my favourite studies to date (and certainly in keeping with the DFTB ethos) five anaesthetists gargled 10mls of red food dye, inhaled to their vital capacity and then coughed. They then repeated the experiment using blue food dye and HFNC at 60l/min and compared the distance traveled. They showed a baseline cough distance of 2.48m increasing up to 2.91m with high flow. Of course, children have a much smaller vital capacity.

This is in contradiction to the data from Hui et al. (2019). They used a human-patient-simulator (as opposed to humans in the above study), smoke and lasers. With a properly fitted mask flow forward flow was increased to ~26 cm with 5cm of CPAP and to around 33cm with 20cm of CPAP. With HFNC the exhalation distance increased from 6.5cm (10l/min) to ~17cm (60l/min). When the mask became loose or disconnected smoke was detected up to ~62cm laterally.  So why the big difference in the studies? It is the cough that causes the problem.

This video from Sick Kids in Toronto says more than any words ever could.

Whether you believe in the benefits of high-flow or not, pushing oxygen through the nose at 2l/kg/min and out through the mouth can create an aerosol spread of snot and virus. We would advise that it is only be used in cases where low flow oxygen therapy has failed. It also makes sense then, that it should only be started in the place where the patient is going to end up. It would not be wise to start a patient on HFNCO₂ then wheel them through the hospital leaving a cloud of viral particles in their wake like some overactive Bisto Kid. And if you are going to do it with a coughing patient then it would be sensible to put a standard face mask on first.

Non-invasive ventilation (CPAP or BiPAP)

High risk 

High flow nasal cannula seems to have superseded non-invasive ventilation in many cases, though CPAP is regularly used in neonatal practice. There is very little evidence for maternal transmission of COVID19 and one might suppose that full PPE is then not warranted. However, you need to consider where the baby has come from.

Open suctioning and chest physiotherapy

High risk 

Removal of nasal foreign body

Medium to high risk

There are lots of ways to remove a nasal foreign body but all of them will generate snot. The old standby – the mother’s kiss – is, realistically, no more dangerous for the parent than living in close proximity. If your pre-encounter probability of infection with SARS-CoV-2 is low, i.e. there is little community transmission, then the risk to the provider is probably low.

Nitrous oxide

Medium to high risk 

Respiratory illness is a contra-indication to nitrous sedation but given that there is a degree of asymptomatic carriage it is not impossible that we might need to use it. With children not going to school and being told to stay away from their friends, there is going to be a spike in trampoline and bunk-bed related injuries. Again consideration should be made as to the possibility of community transmission. Logically holding a continuous flow mask on an uncooperative toddler would expose a HCW to higher risk than being a room Sith a cooperative patient using a demand system with appropriately attached to suction.

Examining the throat

Medium to high risk 

In normal times, no paediatric examination is complete without looking in the ears, nose, and throat, no matter how hard it might be. You can argue that looking at tonsils might not be overly helpful, given that the inter-rate variability is pretty high but there are other things to look for too – emerging teeth, Koplik spots, ulcers. But does a look in the throat put us at risk?

The Royal College of Paediatric and Child Health concurs, and in a statement put out on the 24th of March suggest that we only look in the throat if it is essential. If we have to do it we should be wearing appropriate protection (glove, gown, surgical face mask). If a child is at particularly high risk then they recommend empiric antibiotics.

Even ENT experts, like Eric Levi, recognize the unique risks that fiddling around near the upper respiratory tract hold.

Inserting a nasogastric tube

Medium to high risk

The combined Colleges of Surgeons of Great Britain and Ireland suggest that insertion of a nasogastric tube in an adult is an AGP, probably as it may induce coughing.

Taking a nasopharyngeal swab

Low to moderate risk 

The CDC state that collecting a nasopharyngeal swab doesn’t need to take place in an isolation room but should at least be performed in a single room with a closed door. The health care practitioner should wear an N95 mask or equivalent, coupled with eye protection, gloves, and gown. Given how far the swab has to travel up the nasopharynx nobody should be surprised that it might make someone sneeze.

The current Australian guidance contains slightly different advice.

We can also add things like IV access, suprapubic aspiration and performance of a lumbar puncture to this list of LOW-risk procedures.

And let’s not forget our surgical and dental colleagues

Surgical procedures

Clearly, some surgical procedures are more dangerous than others. Eric Levi. advocates for a risk assessment before any procedure takes place, starting with ‘Does it need to be done now?” Take a look at his post on how he is modifying his operative technique in order to reduce risk to himself and his colleagues.

On the 25th of March, the combined Colleges of Surgeons of Great Britain and Ireland recommended against laparoscopic surgery due to the potential for aerosol formation. Endoscopy, at either end, also has the potential for the creation of fomites and aerosolizing droplets and so should be carried out with extreme caution.

Dental procedures

There are very few dental procedures that need to be performed as an emergency but given that high-speed drills can lead to aerosolization have a care for our dental colleagues that may also be exposed in the course of duty.

The guidance for these procedures is common sense. Don’t perform them if you don’t have to. This is not the time for some minor dental procedures. If they have to be carried out then it should happen in the appropriate space with the appropriate staff. This means in a single room (ideally) with the minimum number of staff wearing appropriate PPE.

These are our thoughts, based on the current evidence, and we’d love you to persuade us otherwise in the comments below.

*Clearly the first step of the algorithm is D for Danger. That means putting on your PPE.