Background: The pulmonary capillary rarefication and the consequent drop in lung blood volume (Vc) have been identified as major pathogenetic factors underlying SARS.CoV-2 pneumonia, resulting the respiratory gas transport affected at variable degree.

Aim: To investigate over fifty-two-week the duration of troubles in respiratory gas transport occurring in patients radiologically (CT) recovered from COVID-19 pneumonia, but still complaining persistent dyspnea.

Methods: This secondary longitudinal analysis included 30 untreated adults previously hospitalized for COVID-19 pneumonia. Spirometry, DL_CO, simultaneous single-breath diffusing capacities for nitric oxide and carbon monoxide (sDL_NO and sDL_CO), sDL_NO/sDL_CO ratio, pulmonary capillary blood volume (Vc), oxygen saturation, and the mMRC dyspnea score were assessed at baseline and after 4, 12, 24, and 52 weeks. Continuous outcomes were analyzed by linear mixed-effects models while the mMRC scores by mixed-effects ordered logistic regression.

Results: Lung volumes and DL_CO remained broadly stable over 52 weeks. Vc, that was markedly reduced at baseline (39.5% predicted), remained unchanged after 52 weeks (38.9%; global time effect p = 0.165). The sDL_NO/sDL_CO ratio remained abnormally high despite a slight decrease over time from 131.4% to 127.4% predicted (p = 0.008). After 52 weeks, the mMRC score slightly improved in 43.3% of patients, while remained unchanged in 43.3%, and further worsened in 13.3%.

Conclusion: Abnormalities in respiratory gas transport persist essentially unchanged for at least 52 weeks after CT recovery from COVID-19 pneumonia, being the symptomatic improvement quite limited. These findings confirm the long lasting impairment of pulmonary capillary function in respiratory Long COVID.

Keywords: Long COVID; Pulmonary Capillary Blood Volume; Dyspnoea; Nitric Oxide

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