Exploring a controversial procedure in treating pandemic.
Novel coronavirus disease of 2019 (COVID-19), caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), can precipitate a spectrum of respiratory compromise from mild illness to severe respiratory failure. [1,2]
The use of high flow nasal cannula (HFNC) or non-invasive positive pressure ventilation (NIV) in the care of these patients is controversial for a few reasons: the increased risk of aerosolization and potential exposure of healthcare workers, [3,4] the scarcity of data to support an optimal mode of treatment and the concern for patient self-induced lung injury. 
Care of these patients, including the choice of non-invasive support and the decision on when to intubate in the emergency department, depends on several factors: available resources, clinical trajectory and knowledge of both short- and long-term benefits and risks. This review focuses on relevant pre-COVID-19 evidence and current expert guidelines on the use of simple oxygen, HFNC and NIV in COVID-19.
High flow nasal cannula
In COVID-19, conventional oxygen devices (i.e. nasal cannula, face masks, etc.) remain the first line treatment for hypoxemia to target an oxygen saturation 90-96%.  If intubation is not immediately indicated and conventional oxygen fails to maintain an oxygen saturation of > 90% or if increased respiratory effort persists, HFNC is the recommended next step. HFNC is recommended over NIV in the Surviving Sepsis Campaign and National Institutes of Health COVID-19 guidelines. [6,7]
HFNC has been used successfully in other forms of pneumonia and acute respiratory distress syndrome (ARDS) 8,9 and may reduce the need for intubation when compared to conventional oxygen. [10–13]
The benefit of HFNC over conventional oxygen devices is from the flow of up to 60 L/min. Respiratory effort is decreased through nasopharyngeal dead space washout, reliably delivers a high fraction of inspired oxygen (FiO2), generates low levels of positive end expiratory pressure (PEEP), and improves comfort with warmth and humidification. 
HFNC in the ED was reviewed in a prior issue of EP Monthly.  While HFNC might increase the risk of viral transmission through aerosolization, studies have not shown an increased risk over standard oxygen devices. 
If HFNC is used in COVID-19 a surgical mask covering the face along with airborne precautions for staff is recommended. [16–18] Awake self-proning has the potential to increase oxygenation and may be trialed with HFNC, but the long term benefits are unclear. [19–22] As with any patient with respiratory failure, close monitoring for deterioration is necessary as a delay in intubation is associated with worse outcomes. 
Figure 1. An initial approach to respiratory failure in COVID-19
NIV for respiratory failure in chronic obstructive pulmonary disease (COPD) and cardiogenic pulmonary edema decrease both intubation and mortality rates.[24–26] The benefit of NIV in ARDS, pneumonia and other forms of acute hypoxemic respiratory failure remains controversial and COVID-19 specific evidence remains limited.
One retrospective study suggests that NIV use in severe hypoxemic respiratory failure (i.e. PaO2:FiO2 ratio <150) from ARDS is associated with a higher ICU mortality rate (36.2%) than those that were placed on invasive mechanical ventilation (24.7%).
In a 2019 retrospective study in ARDS, NIV success was associated with a survival benefit, while NIV failure did not show an increased mortality when compared to invasive ventilation, suggesting a trial of NIV is safe. A recent meta-analysis in patients with hypoxemic respiratory failure, prior to COVID-19, found a decreased rate of intubation with HFNC and NIV when compared to standard oxygen therapy.
Helmet and facemask NIV were associated with a lower risk of death, but the association of facemask NIV with lower mortality was no longer statistically significant with more severe hypoxemia (PaO2: FiO2 ratio ≤200). Overall, higher NIV failure rates have been demonstrated in patients with pneumonia, cirrhosis, sepsis, severe hypoxemia and persistent large tidal volumes (e.g. >9.5 mL/kg). [13,28,29]
One patient population that may benefit from NIV are the immunosuppressed with hypoxemic respiratory failure as early NIV use has been shown to improve mortality and decrease intubation rates in these patients. NIV may also be used, if permitted, in those whose goals of care do not include intubation.
While NIV has the potential to improve alveolar recruitment and oxygenation, it may increase the risk of patient self-inflicted lung injury (PSILI). PSILI is the theory that lung injury can be self-generated from large spontaneous tidal volumes and large changes in transpulmonary pressure from an increased respiratory drive.
The application of NIV with an added inspiratory positive pressure may potentiate this lung injury — with the negative effects seen long after they leave the ED. [5,31] Another important consideration is the aerosolization of viral particles and staff exposure with NIV. [3,4,32,33] Use of NIV in COVID-19 is subject to local infection prevention guidelines and ideally should be conducted in a negative pressure environment with adequate staff protective equipment.
In patients with hypoxic respiratory failure from COVID-19 who do not improve with conventional oxygen therapy, HFNC is recommended. NIV may be trialed cautiously in those patients with less severe hypoxemia (e.g. PaO2:FiO2 ratios >150-200) or in those who are immunosuppressed. Severe hypoxemic failure, pneumonia and persistent large tidal volumes are associated with NIV failure.
There are no specific criteria for intubation in COVID-19, instead the decision to intubate relies on patient trajectory, response to therapy, and clinical judgement at the bedside. However, if the patient shows ongoing signs of respiratory failure (e.g. high respiratory rate, accessory muscle use, worsening hypoxemia, etc.) or is rapidly deteriorating despite optimized HFNC or NIV, then intubation with a lung protective, low tidal volume ventilation strategy should be initiated.
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