The new severe acute respiratory syndrome coronavirus 2019 (SARS-CoV-2) causes the illness named COVID-19, which is primarily characterized by pneumonia. As of 27 December, there have been over 79.2 million cases and over 1.7 million deaths reported since the start of the pandemic. In many cases, pneumonia evolves to acute respiratory distress syndrome (ARDS) with the need for mechanical ventilation and patient admission to intensive care unit, determining a marked increase in the need for intensive care beds worldwide.

Pulmonary involvement causes predominantly hypoxemic respiratory failure. Although COVID-19 pneumonia often falls within the diagnostic criteria of ARDS, it differs from it for some peculiar pathophysiological characteristics. In particular, patients with ARDS secondary to COVID-19 often have the compliance of the respiratory system within the normal range. A significant role in the pathophysiology of hypoxemia seems to depend on vascular alterations such as altered pulmonary vascular self-regulation, pulmonary capillary leakage, and microvascular thrombosis in a complex process known as "immunothrombosis". All together they act by altering the relationship between ventilation and perfusion and increasing the dead space, which ultimately results in impaired efficiency of the pulmonary ventilation. Among the various markers associated with the prognosis of patients with COVID-19, D-dimer is linked to both the inflammatory state and thrombotic phenomena and could help to identify patients at greater risk of developing early ventilation-perfusion changes.

This study aims at measuring the ventilatory efficiency, assessed by Ventilatory Ratio, in critically ill, mechanically ventilated, COVID-19 patients and its correlation with plasma D-dimer and quasi-static respiratory compliance.

Inclusion Criteria:

All of the following:

• confirmed SARS-CoV-2 infection by RT-PCR on a nasopharyngeal swab
• severe hypoxemia due to COVID-19 who meets the diagnostic criteria of ARDS (Berlin's definition)
• invasive mechanical ventilation
• patients receiving neuromuscular blocking drugs

Exclusion Criteria:

• history of preexisting severe hypoxemia (i.e. primary pulmonary hypertension, COPD in therapy with O2 supplementation, pulmonary fibrosis, etc.)

• severe haemodynamic instability defined as:

  • Mean arterial pressure < 65 mmHg despite the infusion of norepinephrine, or epinephrine, or dobutamine, or levosimendan
  • severe left ventricular dysfunction with ejection fraction <20%
  • right ventricular failure due to pulmonary embolism

All patients consecutively admitted to ICU with the eligibility criteria will be enrolled in the study. They will be treated according to the standard of care.