F&P Optiflow

Flow Matters (Updated)

Flow Matters Edition 10

29 July 2021

The outbreak and ongoing surges of COVID-19 have impacted healthcare services around the world. Optiflow™ Nasal High Flow (NHF) therapy is being used to treat patients, whilst its association with the risk of transmission continues to be challenged.


“Patients acutely requiring [NHF]... are likely to present a high disease transmission risk due to their propensity to produce aerosols,
but we find no basis for withholding or delaying access to [this therapy]. We conclude instead that exertional respiratory activities
themselves are the primary modes of aerosol generation and represent a greater transmission risk than is widely recognised currently.”
– Wilson et al. Anaesthesia. 2021.1


The following have emerged as dual primary objectives in the clinical management of COVID-19:
  • Improving patient outcomes e.g. by avoiding the need for tracheal intubation.
  • Maintaining health care worker (HCW) safety e.g. by avoiding an increase in widespread nosocomial transmission.

Collectively, evidence based guidelines for COVID-19,2-9 published clinical observations of NHF use10-22 and HCW infections,10-11,18,21-23 investigational research on dispersion of exhaled particles,1,24-42 and expert recommendations43-50 indicate that:
  • NHF is recommended as respiratory support for patients with hypoxemia caused by viral pneumonia, such as COVID-19.10-22
  • NHF is currently not considered to represent an increased risk of HCW infection via contact, droplet or airborne transmission routes: 10-11,18,21-23
    • Advocacy for NHF is called for in recommendations for hospital preparedness.24,43-45
    • The aerosol generating procedure (AGP) paradigm should be discussed in the context of emerging evidence.1,41-49
    • Cough is now considered to be a relatively high risk respiratory activity which puts all forms of respiratory therapy into perspective.1,25,37,40-42,46-47,50
Improving Patient Outcomes

The use of NHF to improve outcomes for COVID-19 patients is well documented in published literature:

Evidence based guidelines
  • World Health Organisation2
  • National Institutes of Health3
  • National Health Commission of the Peoples Republic of China4
  • Surviving Sepsis Campaign5
  • Australia and New Zealand Intensive Care Society6
  • European Respiratory Society7
  • International expert consensus statement8
  • Expert recommendations from a French panel consisting of members from various intensive care societies9

  Observational research on outcomes for COVID-19 patients
As NHF has been used as respiratory support throughout the pandemic, clinical observations on its impact on patient outcomes have been peer reviewed, published and continue to emerge.10-22
NHF use on COVID-19 patients has been observed to:
  • Keep patients off mechanical ventilation10-13,19 and help them stay off.13-16
  • Lower the rate of mortality.10,14
  • Be successfully used outside of ICU settings.16-18
  • Reduce length of stay.19-20

“[NHF] use is associated with a reduction in the rate of invasive mechanical ventilation and overall mortality
in patients with COVID-19 infection.” - Patel et al. 2020.10

Maintaining healthcare worker safety

Collectively, clinical observations,10-11,18,21-23 investigative research1,24-42 and expert opinions43-50 highlight that NHF therapy is not considered to represent an increased risk of infection for HCWs.

Wilson et al. 20211 compared the effect of respiratory activity, noninvasive respiratory support and facemasks on aerosol generation.  This publication is the first to successfully capture data from the entire respiratory plume.  Results from the study are illustrated in the chart below.
Comparison of total particle count for respiratory activities*

*Data collated from Wilson et al. 2021.1
† FEV: Forced expiratory volume manoeuvres. Used as proxies for symptomatic laboured breathing and atelectasis.


Key clinical study

Wilson et al. 2021. Anaesthesia Download PDF

Wilson et al. 2021. Anaesthesia
Download PDF

Further reading:
Pubmed abstract
Expert opinions
Advocacy for NHF

The publication from Wilson et. al1 adds to a body of research from experts24,43-45 advocating for the use of NHF for Covid-19 patients:

“... administrators and policymakers must consider amending protocols to not only allow, but actually advocate for, the use of [NHF] for COVID-19 patients with significant hypoxemia who, without this option, would be placed on [mechanical ventilation].”
- Gershengorn et al. 2020.24

Advocacy for NHF

Publications1,42 and articles41,46-49 question the accuracy and helpfulness of the term AGP with particular emphasis on the classification of respiratory support therapies like NHF as AGPs:

“Recent data have raised questions as to whether procedures currently classified as AGPs actually generate aerosols, including tracheal intubation and extubation, non-invasive ventilation and high-flow nasal oxygen.”
– Cook et al. 2021.47

“We propose an end to the term aerosol generating procedure, as it is [not] accurate (aerosol is not generated above a cough for many of these procedures), implies aerosol emission is only from specific procedures (rather than being generated during normal respiratory events), potentially misidentifies the source of infection risk, and applies a binary definition to a situation that is more complex.” 
– Hamilton et al. 2021.41
Patient-related risks to HCWs

Researchers and experts have considered aerosols generated from COVID-19 patients’ respiratory activities (like cough) and the risk this represents to HCWs:1,25,37,40-42,46-47,50

“We have shown that the emissions per min during common exertional respiratory activities are often one to two orders of magnitude greater than during [NHF] and [NIV], which are currently classified as aerosol-generating procedures.  Importantly, when these therapies were used during exertional respiratory activities that mimic respiratory illness, emissions were reduced compared with activities alone.”
– Wilson et al. 2021.1

“Aerosol emission from the respiratory tract does not appear to be increased by [NHF]. Although direct comparisons are complex, cough appears to generate significant aerosols in a size range compatible with airborne transmission of SARS-CoV-2.  As a consequence, the risk of SARS-CoV-2 aerosolisation is likely to be high in all areas where patients with Covid-19 are coughing.  Guidance on personal protective equipment policy should reflect these updated risks.”
– Hamilton et al. 2021.42
Helpful terms
Matter with physical dimensions such as a water vapor molecule, a pathogen (virus or bacteria), an aerosol or a droplet.

Water vapor molecule:
Gas particle of H2O. Size: < 0.001 microns.

Infectious agent replicating in living cells. Size: 0.017 to 0.3 microns.

Infectious organism. Size: 0.2 to 10 microns.

Very small liquid particle, suspended in the air. Size: up to about 5 microns.
Larger liquid particle, falling to the ground. Size: about 5 microns.

Aerosol or droplet including a suspended pharmaceutical agent such as salbutomol, for delivery to a patient.

Medical particle small enough to be delivered to a patient’s lower airway or lungs.

Aerosol or droplet expelled by a patient during exhalation which includes biological material (e.g. a suspended pathogen).
Very small bio-particle, suspended in the air. Size: up to about 5 microns.

Larger bio-particle, falling to the ground. Size: about 5 microns.

Bio-aerosol generating procedure:
A procedure which includes the type of patient airway interaction known to break fluids into aerosol sized particles.

Bio-aerosol dispersing procedure:
A procedure which doesn’t break fluids into aerosols but may disperse bioaerosols generated by normal airway functions.
Any clinical opinions in this newsletter are the opinions of the contributing authors and are given for information purposes only.
The clinical opinions are not intended as and do not substitute medical advice.
F&P and Optiflow are trademarks of Fisher & Paykel Healthcare Limited. For patent information, see fphcare.com/ip