Lung diffusing capacities for nitric oxide and carbon monoxide at rest and post-walking in long COVID

BACKGROUND: Approximately one-third of long coronavirus disease 2019 (long COVID) patients report breathlessness and fatigue even during activities of daily living. We hypothesised that abnormalities of combined diffusing capacity of the lung for nitric oxide (D(LNO)) and carbon monoxide (D(LCO)) at rest or after mild exercise are associated with breathlessness in patients with long COVID. METHODS: Single-breath combined D(LNO) and D(LCO) were measured at rest and immediately after a short bout of treadmill exercise simulating ordinary walking in 32 Caucasian patients with long COVID and dyspnoea at rest. 20 subjects served as a control group. RESULTS: At rest, combined D(LNO), D(LCO) and alveolar volume (V(A)) were significantly lower in long COVID than in controls, with D(LNO) and D(LCO) being below the limits of normal in 69% and 41% of cases, respectively. Mean values of D(LNO)/V(A) and D(LCO)/V(A) in long COVID patients were less than controls, yet, in only 22% and 12% of long COVID patients were the values of D(LNO)/V(A) and D(LCO)/V(A) below the limits of normal. After treadmill exercise, D(LNO), D(LNO)/D(LCO), V(A) and heart rate increased significantly without differences between groups. D(LNO) remained below the limit of normal in 47% of long COVID patients. CONCLUSION: These data suggest localised discrete loss of lung units in approximately half of long COVID patients, not completely explained by loss of V(A) or of alveolar–capillary recruitment during exercise.