Relationship between concavity of the flow-volume loop and small airway measures in smokers with normal spirometry.

BACKGROUND: There is increasing evidence of small airway abnormalities in smokers despite normal spirometry. The concavity in the descending limb of the maximum expiratory flow curve (MEFV) is a recognised feature of obstruction and can provide information beyond FEV1, and potentially early smoking-related damage. We aimed to evaluate concavity measures compared to known small airway measurements. METHODS: Eighty smokers with normal spirometry had small airway function assessed: multiple breath nitrogen washout (MBNW) from which ventilation heterogeneity in the diffusion-dependent acinar (Sacin) and convection-dependent conductive (Scond) airways were assessed, and impulse oscillometry system (IOS) from which respiratory resistance and reactance at 5 Hz (R5 and X5) were measured. Concavity measures were calculated from the MEFV, partitioned into global and peripheral concavity. RESULTS: We found abnormal peripheral and global concavity as well as acinar ventilation heterogeneity are common in "normal" smokers. Concavity measures were not related to either MBNW or IOS measurements. CONCLUSION: Abnormalities in concavity indices and MBNW or oscillometry parameters are common in smokers despite normal spirometry. However, these measures likely reflect different mechanisms of peripheral airway dysfunction.

Bronchodilator Responses in Respiratory Impedance, Hyperinflation and Gas Trapping in COPD.

Hyperinflation, gas trapping and their responses to long-acting bronchodilator are clinically important in COPD. The forced oscillation technique (FOT) measures of respiratory system resistance and reactance are sensitive markers of bronchodilator response in COPD. The relationships between changes in resistance and reactance, and changes in hyperinflation and gas trapping, following long-acting bronchodilator (LA-BD) have not been studied. 15 subjects with mild-moderate COPD underwent FOT, spirometry then body plethysmography, before and 2 hours after a single 150 microg dose of the LA-BD indacaterol. Hyperinflation was quantified as the inspiratory capacity to total lung capacity ratio (IC/TLC), and gas trapping as residual volume to TLC ratio (RV/TLC). At baseline, FOT parameters were moderately correlated with IC/TLC (|r| 0.53-0.73, p < 0.05). At 2 hours post-LA-BD, there were moderate correlations between change in FOT and change in RV/TLC (|r| 0.60-0.82, p < 0.05). Baseline FOT parameters also correlated with the subsequent post-LA-BD change in both IC/TLC (|r| 0.54-0.62, p < 0.05) and RV/TLC (|r| 0.57-0.76, p < 0.05). FOT impedance reflects hyperinflation and gas trapping in COPD, and the potential for long-acting bronchodilator responsiveness. These results provide us with further insight into the physiological mechanisms of action of long-acting bronchodilator treatment, and may be clinically useful for predicting treatment responses.