Physiological analysis of extended-spectrum oscillometry.

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Using a forced oscillation technique, the resistance and reactance of the respiratory system in a frequency range between 4 and 52 Hz were described in a group of healthy subjects and a group of patients with severe chronic obstructive pulmonary disease (COPD). In normal subjects, resistance values increased at higher frequencies. As compared to the results in normal subjects, resistance values were much higher and decreased with frequency in COPD patients. Reactance values were more negative, resulting in an increase of resonant frequency. Using matrix network topography, these findings were analyzed in a modified Mead's model. Compressibility of alveolar gas was incorporated in the model calculations. Resistance and reactance values slightly decreased by adding gas compliance in the model calculations. Our results support Mead's hypothesis that the shunt compliance is formed by the compliance of intrathoracic airway walls. Input impedance measurement by forced oscillation is therefore an easily implemented, non-invasive method to investigate respiratory mechanics not requiring active cooperation from the subject. Analysis of resistance and reactance over an extended frequency range gives information about the distribution of resistance along the bronchial system and about compliance of the intrathoracic airway walls as expanding structures in parallel with the air spaces.