Chaotic breathing in post-COVID-19 breathlessness: a key feature of dysfunctional breathing can be characterised objectively by approximate entropy

Exertional breathlessness is highly prevalent in individuals with the post-COVID syndrome, reporting delayed recovery following SARS CoV-2 infection [1]. The pathophysiology underlying breathlessness in this setting remains unclear, however the use of cardiopulmonary exercise testing (CPET) has provided valuable insight, highlighting abnormalities in peripheral oxygen extraction, energy utilisation and autonomic dysfunction [2–5]. In addition, several studies have highlighted the presence of dysfunctional breathing and breathing pattern disorders (BPD) in this context [6, 7]. There remains however a lack of validated physiological metrics to characterise breathing pattern, in this context. Footnotes This manuscript has recently been accepted for publication in the ERJ Open Research . It is published here in its accepted form prior to copyediting and typesetting by our production team. After these production processes are complete and the authors have approved the resulting proofs, the article will move to the latest issue of the ERJOR online. Please open or download the PDF to view this article.