Chaotic breathing in post COVID-19 breathlessness: a key feature of dysfunctional breathing can be characterized 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.