A thread on what studying "AGPs" tells us about 'aerosols and droplets'

Focus on this paper

Done in Sydney
Hats off to @NWilson247 @EuanTovey
My role was microscopic

First: discussion should be 'what is the proportion of each in disease risk: not A vs B
The study builds on work by others esp Bristol AERATOR study which has examined designated 'aerosol generating procedures' in a laminar flow theatre


& largely identified a LACK of aerosol

More to come on other designated AGPs
For balance here are some studies with slightly different conclusions

Another important paper by @nate_gaeckle
Studied NIV/HFNO in a clean isolation chamber

-no more aerosol than other oxygen delivery methods
-massive variation between individuals

Key in all these studies is conduct in ULTRAclean (HEPA filtered) environment

...needed to detect respiratory aerosols (signal) over non-respiratory (noise)

-noise amplitude >> signal
-size distribution differs

(studies in non-clean settings can be ignored!)
...back to the paper at hand

Done in an ultraclean chamber

Key difference from other papers
-massive head sized cone
-high sampling rates
to try to capture ALL exhaled aerosol
Expired particles sampled

During respiratory ACTIVITIES
-force expiration

With respiratory PROCEDURES
-surgical mask
First key finding.
A modest increase in aerosols with respiratory THERAPIES during quiet breathing

BUT (big but) this disappeared & emissions fell with respiratory therapy during exertional respiratory activities
This fall in exhaled aerosols with NIV/CPAP/HFNO compared to without their use shows they are NOT AGPs

The fall likely due to
-PEEP reducing airway collapse and open/close cycling
-positive inward flow reducing egress
You may have picked it up already....
Yes coughing, exercise & forced expiration (each mimicking breathing patterns in illness) produce many-fold more aerosols than quiet breathing (log scale)

Respiratory ACTIVITY should be focus of concern
NOT respiratory PROCEDURES
For those who want to dive deep here are the particle size distributions (generally >95% <5micron) & the % of all particle volume in these small aerosols
Again - as per Gaeckle and others - there was a massive inter-individual variation
So what do we learn

1 NIV, CPAP, HFNO are not AGPs
2 Exertional activities (cough, fast deep breathing) generate up to 370-fold more aerosol than quiet breathing
3 This aerosol is respirable
4 It constitutes a significant volume of all expired volume
5 We need to rethink risk
We know some superspreader events create massive infection & can only be explained by aerosols

These likely combination of
-high viral load (disease time course)
-high risk person (high emitter/spreader)
-high risk activity (sing, shout etc)
-poor ventilation
-prolonged exposure
While droplets may be a high risk at short distance...

...the high production of respirable aerosols by exertional respiratory activities (common in illness) point to a prominent role of aerosol transmission at short distances

(& in some settings over long distances)
The other point I should’ve made. Even more important than an ultraclean setting..

....collaboration between aerosol/environment scientists & clinicians

Key to all the papers in the thread (& missing from many others)

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More from @doctimcook

12 Apr
I am interested (as an anaesthetist/intensivist) in the claims that N95/FFP masks could
-raise CO2
-decreased oxygen uptake in pregnant patients

It is not a trivial claim
& is stated here by a WHO IPC expert group

..among a long list of downsides

The paper quoted regarding CO2 elevation is this one

It studied
10 nurses
All white
9 female
9 elevated BMI, 5 BMI >30 kg/m2

2x 12 hour shifts
N95 +/- surgical mask over it
Unclear whether expiratory valve

compliance comfort & physiology

The compliance and comfort evaluations showed
-lots of minor discomfort
-but rather well tolerated
-most removals at shift end or to drink
-compliance on day 2 better than day 1

Read 15 tweets
20 Mar
A longish thread on RESTARTING ELECTIVE SURGERY for a Saturday morning

First up it is very clear we need to do it

Some patients have life threatening or painful conditions that need addressing.

Provision of healthcare is a must for any civilised society

The volume of missed surgery is huge

We've probably lost at least 2 million cases in 2020 and we now have a waiting list of about 5 million

Different sources have broadly similar numbers between 2 and 3 million additional cases on the waiting list

The @RCoANews ACCC Track survey has examined the impact of the pandemic on surgical services
In early December pre the big surge)

Many hospitals are unable to undertake elective surgery (red) or struggling (orange)

Space/staff issues most prominent

Read 18 tweets
13 Mar
Strong article @By_CJewett on AEROSOLS, COUGH, the complex discussion around risk for HEALTHCARE WORKERS and FFP3/N95 masks

With @drjulesbrown me and others

Here's a bit of the evidence behind it

First the archetypal metanalysis by Tran which seemingly underpins the early approach by many public health organisations

Evidence base...
"We identified 5 case-control and 5 retrospective cohort studies which evaluated transmission of SARS to HCWs"

As I often quote the excellent paper from @mugecevik shows important differences in viral dynamics between SARS and SARS-CoV-2

Infectivity from SARS-CoV-2 starts and finishes earlier
Hugely important for infection control strategies

Read 19 tweets
23 Feb
A thread on why a slower lockdown release makes sense for the wide and younger community (& so for all)

ICU pressures will not fall for up to 8 weeks after similar falls in deaths

The reasons can be illustrated by the median ages of patients in the 3 groups affected by COVID
-patients who died (median age 83) @ONS
-hospital admissions (age 73) @ISARIC1
-ICU admissions (age 61) @ICNARC

Impact of vaccination is much slower in the younger groups

There's been evidence vaccination is impacting deaths in the older groups for some time

John adds to the tweet below

Read 15 tweets
12 Feb
Nice to see this published
Working with @john_actuary from @COVID19actuary we’ve modelled impact of vaccination on
-hospital admissions
-ICU admissions

Vaccinating just by age would have this impact on the three measures

The lag in the last two is because the groups differ.

Median ages
-deaths 83
-hospitalised 73
-ICU 61

So the cohort who might get to ICU have to wait for vaccination
If the graphs are adjusted to account for
-gp2 health/social care workers
-gp4 extremely clin vulnerable
-gp6 high risk
They look like this with lag slightly reduced (and the health service staff protected)

Vaccinating 15% of popln
-huge impact on deaths
-modest impact on ICU
Read 7 tweets
10 Feb
Round 3 of ACCC TRACK. Exploring impact of COVID on anaesthetic departments and surgical activity out now
@emirakur @jas_soar @HSRCNews @RCoANews

Only about a third of anaesthetic departments operating well in December- worse then October and before the new year surge hit Image
All but 14% of hospitals in a surge setting and half not able to meet ICU demand as normal Image
Read 11 tweets

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