Relationships between spirometry, ventilation inhomogeneity and fast and slow ventilation lung units in COPD

Background: Chronic obstructive pulmonary disease (COPD) is characterized by small airway obstruction and parenchymal lung tissue destruction (emphysema). Aim: To assess the relationships between spirometry and indices of global, peripheral airway and inter-regional ventilation inhomogeneity (VI) in COPD. Subjects and Methods: 35 COPD subjects aged 63 (47; 74) yrs (median; range) with a history of >10 pack-years performed triplets of N 2 MBW using the Exhalyzer D (Eco Medics AG, Duernten, Switzerland) plus spirometry. A global index of VI (lung clearance index; LCI) and an index of peripheral airway VI (Sacin) were calculated. Semi-log plots of exhaled N 2 volume vs. breath number were used to calculate volume of the slow and fast emptying lung compartments and their specific ventilation. "All ages" spirometry normative values (Stanojevic et al 2008) and LCI and Sacin reference equations from 510 healthy controls were used. Results: Slow compartment was 63% (43;73) of FRC (functional residual capacity) and specific ventilation of slow and fast compartments were 4.8% (1.3;9.6) and 23.9% (10.4; 55.30). LCI correlated with slow compartment size (R 2 =0.31, p<0.001) and its degree of underventilation (R 2 =0.47, p<0.001). Sacin correlated similarly with slow compartment size (R 2 =0.31, p<0.001) but less well with its degree of underventilation (R 2 =0.22, p<0.01). Underventilation of the slow compartment correlated with FEV 1 (z-score) (R 2 =0.31 p<0.001) and FEV 1 /FVC (z-score) R 2 =0.50 p<0.001). Conclusion: The magnitude of the slow ventilating lung volume and its specific ventilation are important determinants of ventilatory capacity and overall ventilation distribution in COPD.