Respiratory impedance is correlated with morphological changes in the lungs on three-dimensional CT in patients with COPD

Abstract

The forced oscillation technique provides information concerning respiratory impedance, which comprises resistance and reactance of the respiratory system. However, its relationship with morphological changes of the lungs in chronic obstructive pulmonary disease (COPD) remains unclear. Respiratory impedance and spirometric data were evaluated in 98 patients with COPD and 49 reference subjects. Wall thickness (WT) and airway intraluminal area (Ai) of third- to sixth-generation bronchi, and percentage low-attenuation area with less than −950 HU (%LAA) of lungs were measured using three-dimensional computed tomography. COPD patients had higher respiratory impedance, decreased Ai, and increased %LAA compared with reference subjects. Indices of respiratory resistance and reactance and forced expiratory volume in 1 second (FEV1) were correlated with Ai, and the association between percent predicted FEV1 and Ai was predominant in distal bronchi. The difference in respiratory resistance between 5 Hz and 20 Hz (R5–R20) and FEV1/forced vital capacity ratio (FEV1/FVC) were correlated with WT. The %LAA was correlated with the FEV1/FVC ratio and respiratory reactance. Airway function measurements with the forced oscillation technique provide complementary information to spirometry in COPD.

Respiratory impedance is correlated with airway narrowing in asthma using three‐dimensional computed tomography

“…The magnitude of the correlations between Ai and resistance differed according to the wave frequency, R5, and R20. This differed from previous studies in patients with COPD that showed no difference between R5 and R20 regarding a correlation with Ai . High‐frequency waves reflect back from large airways, and low‐frequency waves travel deep into the lungs .…”

“…In this study, we validated the theoretical model that respiratory resistance reflects airway narrowing in asthmatic patients. Respiratory resistance (forward airway pressure) increased inversely based on bronchial airway calibre in chronic obstructive pulmonary disease (COPD) in our previous study, 28 which was also seen in asthmatic patients. Respiratory resistance may be useful to assess airway narrowing in pulmonary diseases with airflow obstruction.…”

“…This differed from previous studies in patients with COPD that showed no difference between R5 and R20 regarding a correlation with Ai. 28 High-frequency waves reflect back from large airways, and low-frequency waves travel deep into the lungs. 14 Therefore, it has been hypothesized that R5 denotes total airway resistance and R20 denotes central airway resistance.…”

<p>Relationships Between Forced Oscillatory Impedance and 6-minute Walk Distance After Pulmonary Rehabilitation in COPD</p>

“…As well as having a direct effect on an individual's ability to maintain fitness, ventilatory impairment as measured by FOT could also be a marker of morphological changes and extra-pulmonary manifestations in COPD that could affect exercise capacity, e.g. emphysema severity 30 or reduced muscle function. 31 Baseline X mean , DeltaXrs and FVC, which correlated with the change in 6MWD that occurred between finishing rehabilitation and 3 months later, are lung function measures of ventilatory impairment in COPD.…”

“…31 Baseline X mean , DeltaXrs and FVC, which correlated with the change in 6MWD that occurred between finishing rehabilitation and 3 months later, are lung function measures of ventilatory impairment in COPD. Xrs (and Rrs) correlates strongly with percentage of emphysematous lung, 30 which is known to be poorly ventilated. Furthermore, Xrs correlates strongly with airway closure (as measured by FVC) in asthmatics during induced bronchoconstriction, 32 with closing volume in healthy subjects 33,34 and with the difference between plethysmographic TLC and single breath alveolar volume difference in COPD subjects.…”