Title: Dynamic hyperinflation and ventilatory reserve in long COVID: a cross-sectional study

Introduction: Specific measures of pulmonary function, such as dynamic hyperinflation (DH) and breathing reserve (BR), may increase understanding of exercise mechanisms intolerance in patients with long COVID.

Objective: To investigate dynamic ventilatory responses and their influence on functional exercise capacity in patients with long COVID.

Methods: Sixteen long COVID patients subjected to resting lung function (spirometry and respiratory oscillometry-RO) and cardiopulmonary performance to exercise (Spiropalm®-equipped 6?min walk test-6MWT and cardiopulmonary exercise test-CPX).

Results: Median age and time since diagnosis of acute COVID were 57 (50?59) years and 98 (93?106) days, respectively. At rest, spirometry showed a normal, restrictive and obstructive pattern in 87.5%, 6.2% and 6.2% of participants, respectively. At rest, RO showed increased resonance frequency, increased integrated low-frequency reactance and increased difference between resistance at 4-20 Hz (R4-R20) in 43.7%, 50%, and 31.2% of participants, respectively. The median of 6-min walk distance (6MWD) was 434m (386?478), which corresponds to a value of 83% (78?97%) of predicted. DH and reduced BR were detected in 62.5% and 12.5% of participants, respectively. At CPX, the median peak oxygen uptake (VO2peak) was 19 ml/kg/min (14?37). There was a significant correlation of 6MWD with both R4-R20 (rs=-0.50, p=0.039) and VO2 peak (rs=0.63, p=0.009).

Conclusion: DH and low BR determine poor exercise performance, that is associated with peripheral airway disease. They are achieved with simple, portable ventilatory and metabolic systems.