Background. Lung diffusion capacity impairment is common after COVID-19. However, whether this impairment is due to alveolar membrane diffusive conductance (DM) or pulmonary capillary blood volume (VC), or both is still debated. Lung diffusion for nitric oxide (DLNO) and for carbon oxide (DLCO) measured together allows to evaluate DM and VC.

Aim. To evaluate the alveolar membrane and pulmonary capillary impairments at one-year after severe COVID-19, assessed by DLNO and DLCO.

Results. 33 patients (70% male), median age 67[IQR 61-71] years, BMI 29.1[ 26.1-33.4] without pre-existing pulmonary disease performed pulmonary function and chest CT scan at one-year after discharge. One patient developed obstructive impairment and 14 had reduced DLCO (<LLN), even median values were normal (z-score -1.30[-2.43-0.75]). Median DLNO were normal (z-score -0.91[-2.03?0.31]), with 40% of patients showing reduced DLNO. However, 10 patients had both reduced DLCO and DLNO. VA was reduced in 12 patients, while FVC in only one patient. Three patients presented a reduced DM, while 19 a reduced VC and no patients had a reduction of both. 13 patients demonstrated an interstitial or indeterminate patter (i.e. DLNO/DLCO<110), while 21 a vascular pattern (i.e DLNO/DLCO?110). Overall lung involvement at CT scan was 11%, without any difference between patients with interstitial/indeterminate and vascular pattern and any significant correlation with DLNO/DLCO ratio (p=0.56). Finally, KCO and hemoglobin resulted significant reduced in vascular pattern (p=0.01 and p=0.02).

Conclusion. In patients without pre-existing pulmonary disease still symptomatic or with chest CT alteration at one-year after COVID-19, gas exchange abnormalities are dominated by vascular component, as confirmed by DLCO and DLNO.