Importance of alveolo-arterial oxygen gradient in monitoring moderate/severe forms of COVID-19

Abstract

The value of (A-a)DO2 (alveolar-arterial oxygen gradient) is a simple way to measure alterations of the alveolar-capillary unit and could represent a reference of respiratory worsening. The aim of our study was to establish if the (A-a)DO2 values in patients with moderate/severe forms of COVID-19 could predict the evolution of the patients. We enrolled 41 adult patients with confirmed SARS-CoV-2 infection and respiratory failure, evaluated in the Hospital of Pneumology over three months. Considering only patients with available data from arterial blood gases analysis: three representative samples - between days 1-3, 4-7 and 8-10 on the same FiO2 (fraction of inspired oxygen), the (A-a)DO2 was measured and compared to the expected values for age. From a final population of 24 patients, 12 under RM (reservoir mask) and 12 under NIV (non-invasive ventilation), 9 died (55% under NIV). The gradient curves showed that in the group of the deceased, the (A-a)DO2 average values remained high (RM: 447– 470– 482 mmHg; NIV: 136– 171– 198 mmHg), whereas in the survivors’ group, the values of the gradient described an ascending-decreasing slope (RM: 467– 458– 426 mmHg; NIV: 151– 167– 119 mmHg) that corresponds to the improvement of the pulmonary damage. The (A-a)DO2 resulted to be a good predictor of severity and mortality in patients with SARS-COV-2 infection and also for the need of oxygen therapy and NIV. Therefore we decided to use (A-a)DO2 instead of PaO2 (partial pressure of oxygen) in the surveillance of patients with COVID-19 because of its better capability to underline the presence of ventilatory/perfusion mismatch, mechanism underlying in the pathophysiology of hypoxemia in COVID-19.