Reduced carbon monoxide diffusion capacity (DLCO) is common after recovery from severe COVID-19. The extent to which this relates to membrane dysfunction Vs vascular injury is uncertain. Concurrent measurement of nitric oxide diffusion capacity (DLNO) and DLCO can partition gas diffusion into its two components, membrane conductance (DmCO) and capillary blood volume (VC). We sought to evaluate DmCO and VC in the early and later recovery periods after severe COVID-19.
Methods: Patients attended for post-COVID-19 clinical review and lung function testing including DLNO/DLCO. Repeat testing occurred when indicated, and comparisons made using t-tests.
Results: 49 (8 female) subjects (mean±SD age 58±13, BMI 34±8), who had severe COVID-19 pneumonitis, WHO ordinal severity classification of 6±1 and prolonged (19±22 days) hospital stay, were assessed 56±29 days (?2 months) post discharge. DLCOc (z-score -1.70±1.49) was reduced. VC and DmCO were reduced to a similar extent (Z-score -1.19±1.05 and -1.41±1.20, p=0.4). 17 (1 female) patients returned for repeat testing 122±61 days (?4 months) post discharge. In this subgroup with more impaired lung function, DLCO improved but remained below LLN (Z-score -3.15±0.83 Vs -2.39±0.86, p=0.01). DmCO improved (Z-score -2.05±0.89 Vs -1.41±0.78, p=0.01) but VC was unchanged (Z-score -2.51±0.55 Vs -2.29±0.59, p=0.16).
Conclusion: Membrane conductance is abnormal in the earlier recovery phase following severe COVID-19 but significantly improves. In contrast, reduced capillary blood volume persists. This data suggest persisting effects of acute vascular injury contribute to physiological impairmant long after severe COVID-19 pneumonitis.