High-Flow Nasal Cannula in Severe COVID-19 With Acute Hypoxemic Respiratory Failure.

Acute hypoxemic respiratory failure is a condition in which rapid-onset impairment in gas exchange between the lungs and the blood, lead to hypoxemia with or without hypercapnia. Usual management of this condition relies on oxygen supplementation throughout different respiratory support modalities (i.e., low flow oxygen devices, high-flow nasal cannulas, mechanical ventilation, ECMO, etc.) aiming to restore gas exchange and to support respiratory effort. In most cases, initial management of acute hypoxemic respiratory failure might be provided through low-flow oxygen systems, but more severe cases will require more advanced life-supporting strategies. Respiratory compromise by SARS-CoV-2 infection widely varies between subjects. Thus, respiratory system elastance, intrapulmonary shunt, pulmonary perfusion/ventilation inequalities and lung weight can range between almost normal up to very high values. Consequently, modalities to provide initial support in acute severe hypoxemic respiratory failure in SARS-CoV-2 infection should not be limited to immediate invasive mechanical ventilation support as such respiratory support modalities should be adapted to individual requirements. Unfortunately, similar values of initial PaO2/FiO2 ratios (especially when PaO2/FiO2 ratio is \< 200) are not clearly related with more or less severe lung affectation, high or low respiratory system elastance patterns, high or low adaptive hypoxic vasoconstriction, and far beyond clinical signs, there are no widely available methods able to rapidly decide which patients would be more benefit from a relative "conservative" management or on the contrary, which patient would benefit from immediate invasive respiratory support. Use of high-flow nasal cannulas (HFNC) in acute hypoxemic respiratory failure from different etiologies has rapidly increased during the last years. Certainly, randomized controlled trials suggest that HFNC might prevent intubation and the need for invasive mechanical ventilation in patients with moderate and severe hypoxemia. Nevertheless, impact of HFNC on mortality rates and other important clinical outcomes in this condition remains controversial. Initial observational reports of patients with severe SARS-CoV-2 infection subjected to invasive mechanical ventilation showed a consistent and very high mortality. Indeed, some experts claimed for using such non-invasive respiratory support devices in patients with moderate or even high pulmonary shunt values arguing about possible harm induced by mechanical ventilation especially in patients with a relative normal respiratory system elastance. Nevertheless, others recommended against HFNC use because safety concerns for health care workers, which clearly limited its use at the initial phases of the pandemic. Thus, the impact and safety of using HFNC at very early stages of acute hypoxemic respiratory failure induced by severe SARS-CoV-2 infection remain to be elucidated. This is how the HiFlo-COVID trial propose to assess the impact HFNC vs. conventional oxygen therapy on the need for intubation / invasive mechanical ventilation support and the clinical status (at days-14 and -28) as assessed by a modified 7-point ordinal scale in patients with moderate / severe hypoxemic respiratory failure secondary to SARS-CoV-2 infection.