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RespiratoryNov 4, 2021

Research published on infection risks of using CPAP and HFNO with patients with Covid-19

Concerns about the infection risks linked to using non-invasive breathing techniques when treating patients with Covid-19 may be unfounded, according to two papers that were published online today (4 November) in Thorax.

Researchers argue that neither continuous positive airways pressure (CPAP) nor high-flow nasal oxygen (HFNO) produces any more air and surface viral contamination than using simple oxygen therapy.

In fact, they found that coughing is responsible for producing far more aerosol than CPAP or HFNO – both of which are currently categorised as ‘aerosol generating procedures’.

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The risks from coughing patients with early infections may have been 'underplayed', says editorial

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Though CPAP and HFNO are not defined as ‘invasive’, there have been concerns that they generated viral particles that could contaminate the air and nearby surfaces, thus requiring added infection control procedures. 

One option was to segregate patients while in another, healthcare workers used high-grade FFP3 masks to curb the risk of aerosol transmission – with both options having implications for healthcare costs and capacity.

Some researchers have shown that environmental contamination occurs with SARS-CoV2, but few have evaluated the impact of CPAP and/or HFNO in moderate to severe cases of Covid-19 – or found viable (infectious) virus, confirming a transmission risk to healthcare workers.

Patients divided into three groups

The researchers in the first Thorax paper divided 30 hospital patients with moderately severe Covid-19 – none of whom required mechanical ventilation – into three groups of 10. They were given either supplemental oxygen, CPAP, or HFNO. 

Each patient was swabbed for SARS-CoV2, and had three air and three surface samples collected from the immediate vicinity. Positive samples and those suspected of being positive were then cultured for biological viability.

Overall, 21 patients (70 per cent) tested positive for SARS-CoV-2 by PCR (polymerase chain reaction) swab. But only four air samples out of 90 (4 per cent) proved to be PCR positive.

Clinical surfaces were more contaminated than the air samples, and nearly half (14 or 4 per cent) of the patients had at least one positive or suspected-positive sample for viral particles from one or more of the three surface samples collected.

In total, 6 out of 90 (7 per cent) surface swabs tested positive for the virus: 5 out of 30 (17 per cent) floor samples (another four suspected); no table surface samples (another three suspected) and only one high-object surface sample (three more suspected).

Neither the use of CPAP nor HFNO nor coughing were associated with significantly more environmental contamination than supplemental oxygen use. Of the total of 51 PCR positive or suspected positive samples, only one from the nose and throat of an HFNO patient was biologically viable in cell culture.

Time to reassess relevant infection control measures

Evolving evidence ... should prompt an evidence-based reassessment of infection prevention and control measures for non-invasive respiratory support treatments [Rebecca L Winslow et al.]

The researchers, led by Rebecca L Winslow from the Department of Infectious Diseases and Tropical Medicine, University Hospitals Birmingham NHS Foundation Trust and the Epidemiology and Public Health Group at the University of Manchester, state that larger studies are needed to reliably inform pragmatic infection prevention control measures relating to the use of CPAP/ HFNO.

They add: ‘The evolving evidence from hospitalised patients with SARS-CoV-2 infection and the risks of occupational/nosocomial exposure should prompt an evidence-based reassessment of infection prevention and control measures for non-invasive respiratory support treatments that are currently considered “aerosol generating procedures”.’ 

'Over cautious measures' have been implemented

In a linked editorial, which has accepted for publication in the journal, researchers from the University of Bristol and North Bristol NHS Trust, point out: ‘This study adds to the mounting evidence that [the aerosol generating procedure] classification is unhelpful in defining risks of transmission. 

‘It has resulted in over cautious measures for certain settings, mandating full PPE for all intubations and preventing relatives visiting the sickest patients, whilst underplaying risk in others, such as coughing patients with early infection in admissions units or on general wards.'

Second study published in Thorax

A second study published in the journal also questions whether CPAP and HFNO merit categorisation as aerosol generating procedures. 

The researchers compared aerosol production from different oxygen delivery systems among 25 healthy volunteers and eight hospital patients who were infected with Covid-19. Aerosol production was measured while breathing, speaking, and coughing room air and then again while receiving CPAP and HFNO in an ultra-clean operating theatre.  

The amount of aerosol produced was highest while coughing; a single cough generated at least 10 times more aerosol than speaking or breathing. Aerosol production wasn’t increased in those receiving CPAP and any aerosols generated during HFNO came from the machine rather than the individual concerned. 

The researchers study conclude: ‘In summary, our data ... suggest that risk of SARS-CoV-2 infection is not due to CPAP or HFNO generating infective aerosols. This has implications for infection and prevention control policy since aerosol generation appears greatest from patients with Covid-19 who are coughing.’ 

CPAP and HFNO: what are they?

  • CPAP delivers a steady level of pressurised air and oxygen through a hose and mask to assist breathing
  • HFNO pumps oxygen at a high flow rate through two small tubes in the nose
  • 17 per cent of people admitted to hospital as emergencies due to Covid-19 needed non-invasive respiratory support or mechanical ventilation (based on an ‘estimate’ relating to UK data gathered in 2020)

To see the full version of SARS-CoV-2 environmental contamination from hospitalised patients with COVID-19 receiving aerosol-generating procedures, visit: https://thorax.bmj.com/content/early/2021/11/01/thoraxjnl-2021-218035

To see the full version of Aerosol emission from the respiratory tract: an analysis of aerosol generation from oxygen delivery systems, visit: 

A final version of Coughs and sneezes spread diseases – but do “aerosol generating” procedures? A final edited version of this editorial will appear on the journal's homepage in due course.

Author: Ian A McMillan
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