ABSTRACT
Introduction/Aim: Reduced carbon monoxide diffusing capacity (DL CO) is common after recovery from severe COVID-19 and cohort studies have found it to be more abnormal than either VC or TLC. There is no specific evidence that this relates to membrane disfunction or vascular injury. Concurrent measurement of nitric oxide diffusing capacity (DL NO) and DL CO can be used to partition gas diffusion into its two components - membrane conductance (D m CO) and capillary blood volume (V C). In this study, we sought to evaluate D m CO and V C in the early and later recovery periods after severe COVID-19. Method(s): Patients attended for post-COVID outpatient clinical review and complex lung function testing including DL NO /DL CO (Hyp'Air;Medisoft, Leeds). Further appointments and repeat testing occurred when indicated. Lung function comparisons were made using t-tests. Result(s): 46 (8 female) subjects (mean+/-SD age 58+/-13, BMI 34+/-8), who had severe COVID pneumonitis, WHO ordinal severity classification of 6+/-1 and prolonged (19+/-22 days) length of hospital stay, were assessed 51+/-29 days post discharge. Mean TLC [z-score -1.64+/-1.31] and D L CO [z-score -1.60+/-1.48] were both reduced. V C and D m CO were reduced to a similar extent (Z-score -1.36+/-1.19 and -1.14+/-1.06, p=0.4). 14 (1 female) patients returned for testing 70+/-35 days later. In this subgroup, D L CO improved but remained below LLN (Z-score -2.98+/-0.73 [Visit 1] Vs -2.17+/-0.69 [Visit 2], p=0.01). D m CO improved (Z-score -1.99+/-0.91 Vs -1.25+/-1.17, p=0.01) but V C was unchanged (Z-score -2.33+/-0.53 Vs -2.03+/-0.76, p=0.17). Conclusion(s): Gas exchange is persistently abnormal after severe COVID. Membrane conductance is abnormal in the earlier recovery phase but improves to a significant extent. In contrast, reduced capillary blood volume persists. Repeat testing at longer intervals after recovery from acute illness is still required but these data raise the possibility that persisting effects of acute vascular injury will contribute to physiological impairment long after severe COVID pneumonitis.
ABSTRACT
Introduction/Aim: Reduced carbon monoxide diffusing capacity (DL CO) is the most prevalent lung function abnormality post COVID-19 infection. Two studies suggested this relates to alveolar unit loss with preserved capillary blood volume. The measurement of nitric oxide diffusing capacity (D L NO) concurrently with D L CO allows for the quantitation of the membrane component of gas diffusion (D m CO) and capillary blood volume (V C). We sought to monitor D m CO and V C in the recovery period of patients hospitalised for severe COVID-19. Method(s): Patients attended outpatient clinical review and lung function testing including DL NO /DL CO (Hyp'Air;Medisoft, Leeds), with further appointment if indicated. Lung function comparisons were made using t-tests and clinical associations using Pearson correlation. Result(s): 46 (8 female) patients (mean+/-SD) (age 58+/-13, BMI 34+/-8), were assessed 51+/-29 days post discharge. WHO ordinal severity classification was 6+/-1, suggesting severe disease, with prolonged (19+/-22 days) length of admission. V C and D m CO were similarly reduced (Z-score -1.36+/-1.19 Vs -1.14+/-1.06, p = 0.4). V C was negatively correlated with length of stay (r=-0.42, p < 0.01). TLC (Z-score -1.64+/-1.31) and D L CO (-1.60+/-1.48) were significantly reduced and negatively correlated with length of stay (r>-0.41, p<=0.02). WHO severity negatively correlated with TLC only (r=-0.45, p < 0.01). Demographic and biochemical data did not correlate with lung function.14 (1 female) patients returned for repeat testing 70+/-35 days later. D m CO improved (Z-score -1.99+/-0.91 Vs -1.25+/-1.17, p = 0.01). V C was unchanged (Z-score -2.33+/-0.53 Vs -2.03+/-0.76, p = 0.17). D L CO improved but remained below LLN (Z-score -2.98+/-0.73 Vs -2.17+/-0.69, p = 0.01). Conclusion(s): Similar reductions in D m CO and V C following hospitalisation for COVID-19 were identified. In those who returned for repeat testing, D m CO values normalised, but V C did not improve. Abnormal lung function related to increasing severity and length of stay. These findings suggests vascular injury may play a more important role rather than alveolar unit loss as the primary contributor to gas exchange impairment following COVID-19.