HypErpolarised Xenon Magnetic Resonance PuLmonary Imaging in PAtIeNts with Long-COVID (EXPLAIN): Lung diffusion findings

<bold>Introduction:</bold> EXPLAIN is a prospective, multi-centre, observational cohort study investigating the aetiology of breathlessness in never-hospitalised LC patients (NHLC). <bold>Method:</bold> NHLC participants were recruited from dedicated post-COVID clinics. Controls comprised NHLC participants without dyspnoea and asymptomatic participants with prior SARS-CoV2. Low-dose CT, PFT and HPX-MRI were performed. HPX-MRI was performed on either a 1.5T or 3T scanner after inhalation of 1L hyperpolarized 129Xe. Dissolved-phase xenon imaging sequences were captured and the global mean ratio of RBC:Membrane (RBC:M) and Gas:RBC calculated. CT scans were scored for abnormality. Dyspnoea-12, 1-min sit-to-stand test were performed. <bold>Results:</bold> 217 participants were recruited (dyspnoeic=118, non-dyspnoeic=50, controls=49). Two-thirds of the study participants were female. The mean age was 46, 47 and 45; median time from infection was 607, 375 and 232 days in the dyspnoeic, non-dyspnoeic and control groups, respectively. Analysis of HPX-MRI found no significant difference in the RBC:M between groups (mean RBC:M z-score was −0.5958, −0.4844 and −0.4096 (p=0.63) in dyspnoeic, non-dyspnoeic and control groups, respectively). The Gas:RBC ratio did not differ significantly between groups (p=0.25). DLCO correlated with RBC:M (r=0.36, p<0.001) but was similar between groups. All CT scans were all scored as 0-4 (normal/near normal). Dyspnoea-12 and sit-to-stand mBORG score did not correlate with any of the above parameters. <bold>Conclusion:</bold> Although breathlessness is a common symptom, there is no evidence from this study that it is caused by functional deficits at the level of the alveolar-capillary membrane.