An Impairment in Resting and Exertional Breathing Pattern May Occur in Long-COVID Patients with Normal Spirometry and Unexplained Dyspnoea

Background: Long-term sequelae, called Long-COVID (LC), may occur after SARS-CoV-2 infection, with unexplained dyspnoea as the most common symptom. The breathing pattern (BP) analysis, by means of the ratio of the inspiratory time (TI) during the tidal volume (VT) to the total breath duration (TI/TTOT) and by the VT/TI ratio, could further elucidate the underlying mechanisms of the unexplained dyspnoea in LC patients. Therefore, we analysed TI/TTOT and VT/TI at rest and during maximal exercise in LC patients with unexplained dyspnoea, compared to a control group. Methods: In this cross-sectional study, we enrolled LC patients with normal spirometry, who were required to perform a cardio-pulmonary exercise test (CPET) for unexplained dyspnoea, lasting at least 3 months after SARS-CoV-2 infection. As a control group, we recruited healthy age and sex-matched subjects (HS). All subjects performed spirometry and CPET, according to standardized procedures. Results: We found that 42 LC patients (23 females) had lower maximal exercise capacity, both in terms of maximal O2 uptake (VO2peak) and workload, compared to 40 HS (22 females) (p < 0.05). LC patients also showed significantly higher values of TI/TTOT at rest and at peak, and lower values in VT/TI at peak (p < 0.05). In LC patients, values of TI/TTOT at peak were significantly related to ∆PETCO2, i.e., the end-tidal pressure of CO2 at peak minus the one at rest (p < 0.05). When LC patients were categorized by the TI/TTOT 0.38 cut-off value, patients with TI/TTOT > 0.38 showed lower values in VO2peak and maximal workload, and greater values in the ventilation/CO2 linear relationship slope than patients with TI/TTOT ≤ 0.38 (p < 0.05). Conclusions: Our findings show that LC patients with unexplained dyspnoea have resting and exertional BP more prone to diaphragmatic fatigue, and less effective than controls. Pulmonary rehabilitation might be useful to revert this unpleasant condition.