Viable virus aerosol propagation by PAP circuit leak and mitigation with a ventilated patient hood – a model for improving health care worker safety in the COVID-19 pandemic
Authors: Landry Shane A , Barr Jeremy J , MacDonald Martin I , Subedi Dinesh , Mansfield Darren , Hamilton Garun S , Edwards Bradley A. , Joosten Simon A 
Published: September 2020

Background

Nosocomial transmission of SARS-CoV-2 has been a major cause of morbidity and mortality in the COVID-19 pandemic. Emerging evidence suggests patients auto-emit aerosols containing viable respiratory viruses. These aerosols could be further propagated when patients undergo certain treatments including continuous positive airway pressure (PAP) therapy. This study aimed to assess the degree of viable virus propagated from mask leak in a PAP circuit and the mitigation of virus propagation by an air filter combined with a plastic canopy.

Methods

Bacteriophage PhiX174 (108copies/mL) was nebulised into a custom PAP circuit within a non-vented clinical room. Mask leak was systematically varied to allow 0, 7, 21, 28 and 42 L/min at the mask interface. Plates containingEscherichia colihost assessed the degree of viable virus (via plaque forming unit) settling on surfaces around the room. In order to contain virus spread, the efficacy of a simple, low-cost ventilated headboard, created from a plastic tarpaulin hood and a high efficiency particulate air (HEPA) filter was tested.

Findings

Increasing mask leak was associated with virus contamination in a dose response manner (χ2= 58.24, df = 4, p< 0.001). Clinically relevant levels of leak (≥21 L/min) were associated with virus counts equivalent to using PAP with a standard vented mask. The highest frequency of viruses was detected on surfaces 1m from the leak source, however, viable viruses were recorded on all plates (up to 3.86m from source). A plastic hood with HEPA filtration significantly reduced viable viruses on all plates. HEPA exchange rates of 170 and 470m3/hr eradicated all evidence of virus contamination.

Interpretation

Mask leak from PAP circuits may be a major source of environmental contamination and nosocomial spread of infectious respiratory diseases. Subclinical levels of leak should be treated as an infectious risk. Cheap and low-cost patient hoods with HEPA filtration are an effective countermeasure.

Funding

National Health and Medical Research Council of Australia (1139745).

RESEARCH IN CONTEXT

Evidence before this study

Nosocomial spread of SARS-CoV-2 results in increased infection rates among healthcare workers compared to the general population. Those workers involved in the delivery of non-invasive ventilation are at higher risk based on evidence from previous SARS outbreaks. However, little is known about virus aerosol spread and environmental contamination from respiratory interventions like non-invasive ventilation, which is one of few life-saving treatments for COVID-19 patients. We therefore searched through PubMed with no language restrictions from inception to August 21, 2020 using the search terms ([NIV] or [non-invasive ventilation] or [noninvasive ventilation] or [CPAP] or [continuous positive airways pressure] or [PAP] or [positive airways pressure]) and ([aerosol spread] or [aerosol dispersion] or [aerosol generation]). The search returned 130 publications of which 28 related to the generation or spread of aerosols. Of the 28 related papers, 17 were consensus or opinion papers, 4 were reviews and 7 were original research papers. All previous studies investigating aerosol propagation with respiratory interventions utilised particle sizers or smoke visualisation techniques. These methodological limitations mean that particles are counted or visualised close to the aerosol source and reveal little about wider aerosol spread. Furthermore, they ignore the inherent biological aspects of viral aerosol dispersion in that the aerosol needs to contain viable virus in order to be infectious. It has not been directly established that clinical respiratory interventions are capable of propagating viable virus aerosol and no attempt has been made to systematically quantify the degree of environmental contamination from viable virus aerosol escaping from non-invasive ventilation circuits. There are no current studies informing us as to the effectiveness of air filtration interventions at mitigating environmental contamination with viable virus aerosol escaping from non-invasive ventilation circuits.

Added value of this study

Our study quantifies the degree of viable virus aerosol spread from clinically relevant levels of noninvasive ventilator circuit mask leak, and demonstrates a risk mitigation strategy using a hood and air-purifier at completely eliminating viable virus aerosol environmental contamination. We developed a viable virus aerosol model utilising bacteriophage PhiX174 which is similar in size to SARS-CoV-2 and is harmless to humans. Through nebulising a solution of PhiX174 into a custom ventilation circuit with controllable mask leak settings, we were able to demonstrate that increasing circuit leak was associated with environmental virus contamination in a dose response manner (p< 0.001). Even sub-clinically apparent levels of circuit leak (< 7L/min) were associated with detectable virus propagation up to 3.86 metres from the leak source. Deployment of a hood and air-purifier setup as described by the United States Centres for Disease Control and Prevention, completely eliminated environmental virus contamination from viral aerosol dispersion.

Implications of all the available evidence

Non-invasive ventilator circuit mask leak can propagate live virus containing aerosol and can lead to extensive environmental contamination up to 3.86 metres from the leak source, even at levels of leak that would be difficult to detect clinically. This raises important safety considerations for open wards delivering non-invasive ventilatory support and could explain the noted increased risk of nosocomial SARS infections in healthcare workers delivering non-invasive ventilation treatment. Point of emission air filtration with simple hood and air-purifier completely eliminates environmental contamination with viable virus and could be readily deployed to protect health care workers in the COVID-19 pandemic.