Coughs and sneezes disperse a large number of droplets of varying size into the environment, and they transmit respiratory viral infections by direct or indirect physical contact, by droplets or inhalation of fine particulate droplet nuclei. Larger droplets in the cloud produced by coughing and sneezing settle quickly, and the force with which they are expelled determines how far they are dispersed. The respiratory droplets evaporate to form smaller droplet nuclei that carry infectious agents, remain suspended in air, and susceptible individuals farther away from the source could inhale them. The particle size distribution within the multi-phase cloud produced by coughs/sneezes changes with time and distance from the source depending on several environmental factors. After inhalation, larger respiratory droplets are filtered by the nose or deposit in the oropharynx, whereas smaller droplet nuclei are carried by the airstream into the lungs where their site of deposition depends on their mass, size and shape and is governed by various mechanisms. Airborne particles could also be produced by various aerosol generating procedures, such as suctioning or tracheal intubation, and by aerosol generators, especially jet nebulizers. Prevention of respiratory viral infections depends upon whether they are carried in respiratory droplets or as fine droplet nuclei (airborne or aerosol transmission). The SARS-CoV-2 virus that causes COVID-19 is mainly transmitted by respiratory droplets or by contact. Airborne transmission of this virus has not been established, but is possible under special circumstances. Appropriate protective measures are necessary to prevent transmission of SARS-CoV-2 virus in various settings. This article is open access and distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives License 4.0 (http://creativecommons.org/licenses/by-nc-nd/4.0/)

Human-to-human SARS-COV2 transmission has been established with more than 3300 clinicians reported to be infected in China and more than 1116 clinicians infected in Italy where 13 882 cases were confirmed by 13 March 2020. Room surfaces in the vicinity of COVID-19 symptomatic patients and clinician's protection equipment were found to be contaminated [1]. The primary strategy for COVID-19 patients is supportive care, including oxygen therapy for hypoxemic patients, in which high-flow nasal cannula (HFNC) was reported as effective in improving oxygenation. Among patients with acute hypoxemic respiratory failure, HFNC was proven to avoid intubation compared to conventional oxygen device [2, 3].

Rationale: Defining lung recruitability is needed for safe positive end-expiratory pressure (PEEP) selection in mechanically ventilated patients. However, there is no simple bedside method including both assessment of recruitability and risks of overdistension as well as personalized PEEP titration. Objectives: To describe the range of recruitability using electrical impedance tomography (EIT), effects of PEEP on recruitability, respiratory mechanics and gas exchange, and a method to select optimal EIT-based PEEP. Methods: This is the analysis of patients with coronavirus disease (COVID-19) from an ongoing multicenter prospective physiological study including patients with moderate-severe acute respiratory distress syndrome of different causes. EIT, ventilator data, hemodynamics, and arterial blood gases were obtained during PEEP titration maneuvers. EIT-based optimal PEEP was defined as the crossing point of the overdistension and collapse curves during a decremental PEEP trial. Recruitability was defined as the amount of modifiable collapse when increasing PEEP from 6 to 24 cm H2O (ΔCollapse24–6). Patients were classified as low, medium, or high recruiters on the basis of tertiles of ΔCollapse24–6. Measurements and Main Results: In 108 patients with COVID-19, recruitability varied from 0.3% to 66.9% and was unrelated to acute respiratory distress syndrome severity. Median EIT-based PEEP differed between groups: 10 versus 13.5 versus 15.5 cm H2O for low versus medium versus high recruitability (P < 0.05). This approach assigned a different PEEP level from the highest compliance approach in 81% of patients. The protocol was well tolerated; in four patients, the PEEP level did not reach 24 cm H2O because of hemodynamic instability. Conclusions: Recruitability varies widely among patients with COVID-19. EIT allows personalizing PEEP setting as a compromise between recruitability and overdistension. Clinical trial registered with www.clinicaltrials.gov (NCT04460859).