Imposed Expiratory Resistance and Pulmonary Function in Young Healthy Volunteers

Abstract

Expiratory flow limitation is a primary characteristic in chronic obstructive pulmonary disease (COPD) and asthma. High airway resistance results from bronchoconstriction, destruction of elastic tissue, and mucus hypersecretion caused by irritation of the epithelium. As a result, the airways become narrowed. Obstruction can lead to dynamic hyperinflation, dyspnea, and exercise intolerance. However, increased airway resistance is just one of many abnormalities in COPD and asthma – COPD in particular is an exceedingly heterogeneous disease. Isolating the effects of expiratory airflow limitation, alone, is therefore challenging. In order to study abnormal lung mechanics in isolation, we aimed to test whether adjustable imposed external expiratory resistance in young healthy volunteers produced deficits in the flow‐volume relationship similar to that of patients with COPD or asthma.PURPOSEMeasure pulmonary function in young healthy volunteers with and without imposed external expiratory resistance.METHODSThirty‐nine healthy adults (25.6 ± 4.7 years, 171.6 ± 10.5 cm, 72 ± 13.7 kg, 23 men, 16 women) completed standard pulmonary function testing according to the ATS/ERS standards. The testing included four conditions in random order: control, imposed expiratory loading of 7 cmH2O, 11 cmH2O, and 20 cmH2O. Resistance was imposed with a threshold inspiratory muscle trainer (Vacumed, Ventura, CA) installed in reverse in the spirometer (Parvo Medics, Sandy, UT). Volunteers with FEV1/FVC<0.70 were excluded from analysis (n=6).RESULTSParticipants had reduced FEV1 (F[1.6, 51.8] = 68.2, p < 0.05) at 7 and 11 cmH2O vs. control (3.28 ± 0.84 and 3.26 ± 0.83 L, respectively, vs. 4.10 ± 1.08 L). FVC was reduced (F[1.4, 45.6] = 71.6, p < 0.05) at 7 and 11 cmH2O vs control (4.15 ± 1.05 and 4.16 ± 1.05 L, respectively, vs. 5.10 ± 1.36 L). FEV1/FVC was reduced (F[1.5, 46.5] = 5.4, p < 0.05) at 7 and 11 cmH2O vs control (77.6 ± 8.48 and 78.7 ± 6.18 %, respectively, vs. 80.7 ± 5.30 %). PEF was reduced (F[1.6, 51.5] = 74.7, p < 0.05) with 7 and 11 cmH2O vs control (6.12 ± 1.73 and 6.07 ± 1.91 L/s, respectively, vs. 8.70 ± 2.87 L/s).Volunteers with additional imposed expiratory loading of 20 cmH2O (n=7) had reduced FEV1 (F[3, 18] = 10.6, p < 0.05) at 11 and 20 cmH2O vs. control (3.07 ± 0.52, and 3.15 ± 0.63 L, respectively, vs. 4.09 ± 0.91 L). FVC was reduced (F[1.2, 6.9] = 16.7, p < 0.05) at 7, 11, and 20 cmH2O vs control (4.01 ± 0.76, 3.93 ± 0.74, and 4.04 ± 0.85 L, respectively, vs. 5.02 ± 1.14 L). FEV1/FVC was reduced (F[3, 18] = 2.7, p = 0.08) at 20 cmH2O vs control (78.5 ± 5.99 % vs. 81.9 ± 5.48 %, p < 0.05). PEF was reduced (F[1.3, 7.6] = 12.9, p < 0.05) with 7 and 20 cmH2O vs control (5.76 ± 1.22 and 5.50 ± 1.78 L/s, respectively, vs. 8.05 ± 2.47 L/s).CONCLUSIONSImposed expiratory resistance reduced key spirometric variables. A concave expiratory flow‐volume relationship was consistently absent – a key limitation for model comparison with pulmonary function in COPD. This is most likely due to the external imposed loading leading to higher airway pressures. This likely resists dynamic airway compression and either maintains the equal pressure point position or possibly moves it proximally.