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Description
Expiratory flow limitation is a key characteristic in obstructive pulmonary diseases. In order to study abnormal lung mechanics in isolation from the heterogeneities of chronic obstructive disease, we measured pulmonary function with expiratory loading in young healthy adults. METHODS: Forty-three volunteers (26±5 yrs, 171.8±10.2 cm, 72.2±13.3 kg) completed spirometry and body plethysmography according to the ATS/ERS standards under three randomized conditions: control, and expiratory loadings of 7 or 11 cmH2O. The expiratory load was added by installing a threshold inspiratory muscle trainer in reverse. We analyzed the shape of the flow-volume (F-V) relationship with rectangular area ratio (RAR) using custom MATLab software. RESULTS: Airway resistance was increased (F[1.5, 27.5]= 446.0, p<0.05) with 7 and 11 cmH2O loading vs control (9.20±1.02 and 11.76±1.68 vs. 2.53± 0.80 cmH2O/L/s). RAR (F[1.9, 77.9]= 3.71, p>0.05) was reduced with 7 and 11 cmH2O loading vs control (0.45±0.07 and 0.47±0.09L vs. 0.48±0.08). FEV1 was reduced (F[1.6, 60.38.6]= 67.54, p<0.05) with 7 and 11 cmH2O loading vs control (3.26±0.82 and 3.23±0.79 vs. 4.03±1.04 L). FVC was also reduced (F[1.5, 55.31] = 77.71, p<0.05) with 7 and 11 cmH2O loading vs control (4.15±1.01 and 4.17±1.02 vs. 5.05±1.33 L). PEF was reduced (F[1.5, 55.99] = 72.69, p<0.05) with 7 and 11 cmH2O loading vs control (6.02±1.65 and 5.95±1.85 vs. 8.39±2.85 L/s). FEV1/FVC (F[1.9, 69.17] = 5.5, p>0.05), and FRC (F[1.863, 33.53]= 0.93, p<0.05) were not different between resistance conditions or compared to the control. CONCLUSIONS: Expiratory loading reduced FEV1, FVC, PEF and provided a dose response in airway resistance, but there are no clinically meaningful differences in FEV1/FVC or RAR. A concave expiratory F-V relationship was consistently absent—a key limitation for model comparison with pulmonary function in COPD. This is most likely due to the imposed resistance being applied outside of the airways, leading to higher airway pressures. The higher airway pressures are likely to resist dynamic airway compression and either maintain the equal pressure point position or possibly move it proximally in the airway.
