The cheeks of the subjects were supported by the investigator’s hands. The subjects were advised to breathe quietly from a mouth piece while wearing a nose-clip during the measurements. The IO measurement was performed in the sitting position. Quartile time fractions numbered 1 e 4 were set and added to the label as respiratory phase 1 to 4 and phase 5 to 8, respectively (figure 1). After the comma-separated value data files were opened on our personal computers, every impulse data point was labelled as inspiratory or expiratory phase based on the spontaneous airflow direction. The comma- separated value data files showing the serial values of each component by every 0.2 s were extracted after the measurements. The R5 (Rrs at 5 Hz), R20 (Rrs at 20 Hz), X5 (Xrs at 5 Hz) and Fres (resonant frequency of Xrs) were provided as real-time data. The respiratory system impedance (Zrs), ratio of the mouth pressure to airflow according to the impulses, the Rrs, its real component, and the Xrs, its imaginary component, were automatically calculated using the attached software including fast Fourier transform analysis (LAB manager, CareFusion version 4.65). The impulse pressure was produced with alternate changes in two directions (positive and negative). Continuous impulses (pyra- midal-form pulses, 5 pulses/s), which contained sinusoidal waves of a broad-frequency spectrum, were applied as the forced oscillation. The oscillatory mechanics were assessed using a commercially available IO device (Master Screen-IOS CareFusion, Germany). The present study was approved by the Tohoku University Medical Ethics Committee, and informed consent was obtained prior to inclusion in this study. Five healthy never-smokers were also recruited. All patients were ex-smokers and under medical management at Tohoku University Hospital and had been clinically stable for at least 1 month. 12 Based on the guidelines, the COPD severity was estimated and classified into four groups. They were diagnosed by an author (KH or OH) as having COPD based on the global initiative for COPD guidelines. Thirty-nine patients with COPD were recruited at Tohoku University Hospital. This approach may reveal technical advances in IO and establish an approach for evaluating the physiological features in respiration in patients with COPD. Using this approach, we studied the relationship between the within-breath behaviour and the COPD severity.
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