A prospective physiologic study, in participants with COVID-19-related pneumonia requiring supplemental oxygen (standard oxygen therapy or high-flow nasal cannula (HFNC)) less than 24 hours. The investigators assessed the effect of prone position on ventilation inhomogeneity and ventilation/perfusion mismatch by electrical impedance tomography (EIT).

Once enrolled, an EIT dedicated belt containing 16 electrodes was placed around the participant's chest at the fifth or sixth intercostal space and connected it to an EIT monitor (PulmoVista 500; Dräger Medical GmbH, Lübeck, Germany).

Baseline data were collected during supine position (timepoint SP1), including demographic and anthropometric data, a baseline arterial blood gas measurement, and ventilation parameters including type of supplemental oxygen, respiratory rate, fractional concentration of oxygen in inspired air (FiO2). The participants received instructions of end expiratory occlusion lasting at least 10 seconds and, 1 seconds after the start, a bolus of 10 mL of 5% NaCl solution was injected via the central venous catheter. Subsequently, each participant was helped into the prone position and data collection，end expiratory occlusion and 10% NaCl solution injection were preformed again after approximately 30 min (timepoint PP1). The participant was then encouraged to maintain the prone position for at least 3 h before being helped back into the supine position. Clinical data collection, end expiratory occlusion and injection of a bolus of 10 mL of 5% NaCl solution were repeated again 1 h after resupination (timepoint SP2).

Inclusion Criteria:

1. aged 18 to75 years
2. admitted to intensive care unit with a confirmed diagnosis of COVID-19-related pneumonia
3. requiring supplemental oxygen (standard oxygen therapy or high-flow nasal cannula (HFNC)) less than 24 hours,
4. gave written or witnessed verbal informed consent.

Exclusion Criteria:

1. uncollaborative or had an altered mental status,
2. New York Heart Association class above II
3. history of severe chronic obstructive pulmonary disease
4. Contraindications to the use of EIT (e.g., presence of pacemaker or chest surgical wounds dressing) or prone position (as decided by the attending physician)
5. Impending intubation (on the basis of clinical judgment, including clinical and physiological parameters).