Shunt estimation in pneumonia of different aetiologies: a non-invasive physiological assessment

COVID-19 has challenged traditional models of respiratory failure, and several studies have suggested that gas exchange impairment in COVID-19 pneumonia may involve mechanisms beyond anatomical shunt. This has renewed interest in non-invasive physiological tools to explore gas exchange abnormalities, including the evaluation of shunt, dead space and ventilation/perfusion mismatch.

This prospective cross-sectional study was conducted between October 2020 and January 2021 at Papa Giovanni XXIII Hospital (Bergamo, Italy), enrolling adults with acute respiratory failure due to COVID-19 or other pneumonias. Pulmonary shunt fraction was estimated non-invasively via the BEACON system using peripheral oxigen saturation (SpO₂)/fraction of inspired oxygen responses during incremental oxygen steps. Chest CT scans were quantitatively analysed for pathological patterns. The primary outcome was the comparison of BEACON-estimated shunt fraction between COVID-19 and non-COVID-19 groups; the secondary outcome included correlations with CT findings.

A total of 51 patients were enrolled, including 36 with COVID-19 and 15 with non-COVID-19 pneumonia. COVID-19 patients showed significantly higher pulmonary shunt fractions (18.2% vs 12.5%, p=0.022). In non-COVID-19 cases, shunt fraction correlated positively with the extent of CT consolidations (ρ=0.567, p=0.035) and negatively with ground-glass opacities (ρ =-0.565, p=0.035). No significant correlations between shunt and CT findings were observed in COVID-19 patients.

Our findings demonstrate increased estimated shunt in COVID-19 pneumonia despite comparable radiological severity to non-COVID-19 pneumonias, reinforcing the concept of distinct gas exchange pathophysiology across different pneumonia aetiologies.