Airflow patterns from patients with chronic obstructive airway diseases (COAD) and normal subjects were analyzed using time and frequency domain analysis. Data were recorded during tidal breathing with a pause between the breaths, digitized at 320 samples per second (IO-bit resolution), and processed with a CDC 6600 computer. The appearance of high-frequency components (lo-20 Hz) in the time domain waveform and the spectral curve in the power spectrum were studied. One complete waveform was taken as a reference signal and all subsequent waves were analyzed using the cross-correlation function which was employed via the cross spectrum and the fast Fourier transform akgorithm. The energy content from the averaged spectrum and the root mean square (RMS) value from the filtered waveforms were calculated. Out study indicated that the RMS and the power content estimated from a part of the filtered wave (lo-20 Hz) which included the time interval from the peak of the expiratory Row (tE) to the end of the flow curve (tu) were significantly greater in normal subjects (n = 13; 0.86 -C 0.30 x lo-* I/s: P < 0.00005 for RMS value, and 0.76 +-0.32 Us; P < 0.00005 for the power content) than in patients with chronic airways obstruction (n = 19; 0.40 ? 0.13 x IO-* I/s; for RMS value and 0.35 2 0.16 I/s; for the power content). It is concluded that the RMS and the power values of the filtered flow curve during tidal breathing over the time interval tE-tN can detect chronic airway obstruction. o vxt6Academic Press, Inc. Frequency domain analysis of flow-time breath patterns has been attempted by few investigators (1, 2) in order to study the dynamic accuracy of respiratory flowmeters and spirometers. Lemen et al. (I) analyzed the frequency spectra of flow and volume time curves and suggested that for spirometric recordings, the amplitude response of flow and volume measuring devices should be flat up to 12 Hz for Bow signals and up to 6 Hz for volume signals. McCall et al. (2) determined frequency contents of flow-time curves during 266 OOlO-4NW86 $3.00