Covid-19 associated Acute Respiratory Distress Syndrome (ARDS) may present with profound hypoxemia not fully explained with pulmonary infiltrates. Accordingly, how prone positioning improves oxygenation in these patients is not fully known. The investigators conducted a study among patients with severe Covid-19 ARDS receiving prone position for at least 16 hours. End Expiratory Lung Volume (EELV) was measured with Nitrogen wash-in/wash-out technique before (Supine Position 1- SP1), during (Prone Position - PP) and after (Supine Position 2 - SP2) prone positioning.

After initiation of invasive mechanical ventilation, initial ventilator settings were performed by the attending physician. These settings include titration of best PEEP to promote oxygenation if Spo2 <92% despite a Fio2 higher than 80% or best compliance that do not jeopardize oxygenation if Fio2 is between 50-80% and Spo2 >92%. After a period of 30 minutes for stabilisation, the first set of measurements were done in the SP1 position. Afterwards, the patient was turned to prone position with the same PEEP (which was unchanged throughout the study period) and measurements were repeated every four hours until the patient was reverted back to SP2. Eventually, there were 7 measurements in total. (SP1, PP0, PP4, PP8, PP12, PP16, SP2). Dead space ventilation and shunt fraction calculations were made based on central venous blood gas measurements and oxygen consumption and carbon dioxide production at the relevant timepoint.

Inclusion Criteria:

• Patients were considered eligible if they met the Berlin definition criteria for ARDS and intubated due to increased work of breathing and/or worsening hypoxemia. All patients had a positive Covid-19 real time Polymerase Chain Reaction test

Exclusion Criteria:

• Exclusion criteria were; age < 18 years, ARDS resulting from other risk factors, presence of chest drainage tubes, tracheostomy, pneumomediastinum, hemodynamic instability (systolic blood pressure < 100 mmHg, lactate > 2 mmol/l, or an increase in lactate concentrations for 20% in two consecutive blood gas analysis within 2 hours interval) and suspicion or confirmed pulmonary emboli.

Adult Patients receiving invasive mechanic ventilation due to Covid-19 ARDS