## 1 Introduction  
The coronavirus disease 2019 (COVID-19) appeared for the first time in December 2019, in Wuhan, Hubei Province, China. Within 3 months, COVID-19 had quickly spread worldwide, with incidence of COVID-19 cases increased to be declared a pandemic by the WHO on 11 March 2020 [1]. COVID-19 is quite heterogeneous in its clinical manifestations, ranging from no symptoms to critical illness. Older age, male sex, race, and comorbidities as diabetes, hypertensive cardiopathy, chronic pulmonary disease, chronic kidney disease, cancer, and liver disease have been tightly associated with worse outcomes [2, 3, 4]. Asymptomatic infection or mild respiratory disease is by far the most frequent outcome. About one-third of patients, however, hospitalized progress to interstitial pneumonia, causing acute hypoxemic respiratory failure in a proportion of cases, up to 1 in 10, quite challenging for health care system capacities worldwide [5]. Arterial hypoxemia is the leading feature of severe cases of COVID-19. Consequently, its management should rely on prompt and steady oxygen supplementation, aiming to improve oxygenation and to assist respiratory efforts throughout different support modalities, until intubation with invasive ventilation. High-flow nasal cannula (HFNC) is often used to treat respiratory failure during COVID-19 pneumonia [6; 7; 8; 9]. HFNC combines the possibility of delivering high flows of gases, actively humidified and heated, with the use of a comfortable nasal interface, well tolerated by patients. HFNC decreased the need for intubation or treatment escalation compared with standard oxygen delivery systems in critically ill patients without COVID-19 and with acute hypoxemic respiratory failure, and recent clinical practice guidelines strongly recommended its use; similarly, HFNC performed better than standard oxygen delivery supplies in COVID-19 settings [10]. This beneficial effect is probably due to its ability to match patients' respiratory flow demands more adequately, to reduce inspiratory effort, and decrease the risk of patients' self-inflicted lung injury, which may have important implications for the management of the heterogeneous pulmonary manifestations of patients with COVID-19. Furthermore, heating and humidification by HFNC help to maintain hydration and mobilize secretions, which positively affects secretory imbalances in patients with COVID-19. Use of HFNC in the intensive care gained great attention as a bridge therapy after early extubating, too [11; 12]. Recent studies have shown that HFNC improves respiratory drive and lung mechanics and enhances CO2 removal [13; 14]. Patients with acute hypoxemic respiratory failure, such as in COVID-19 severe pneumonia, are prone to hypokinetic (immobilization) syndrome, defined as the risk of deterioration of the body's systems as a result of prescribed or unavoidable musculoskeletal inactivity. Pulmonary rehabilitation is another setting whereby HFNC may be used in patients with chronic obstructive pulmonary disease and bronchiectasis, as HFNC could immediately increase patients' exercising endurance and decrease dyspnea caused by exercising [15]. Recent studies revealed that a large proportion of patients with severe COVID-19 suffer significant physical deconditioning and impairment of activities of daily living (ADLs) [16]; furthermore, in patients with prolonged hospitalization, bed rest-related damage can cause slow and often incomplete functional recovery, with persistently reduced exercise capacity and impaired cognitive-emotional functioning. The main objectives of early rehabilitation are to counteract the onset of respiratory complications in critically ill patients with COVID-19, restore respiratory and physical independence, prevent subsequent dependence on mechanical ventilation, help recover exercise capacity, and to preserve or recover autonomy in ADLs.