Frequency spectra of flow and volume events for forced vital capacity

We determined the frequency content of forced expiratory flow-time (FT) and volume-time (VT) curves by a fast Fourier transform to determine the dynamic responses required of devices to reproduce these curves. We analyzed 304 FT and VT curves from 15 normal (N) subjects (ages 8-34 yr), 15 cystic fibrosis (CF) patients (ages 7-36 yr), and 9 smokers (S, ages 20-63 yr). The highest frequency (HF) with significant amplitude content, defined as the highest frequency with 5% or more amplitude of the fundamental, for FT curves in N (mean +/- SD, 5.06 +/- 0.7 Hz) was significantly (P less than 0.05) lower than in CF (6.43 +/- 0.9 Hz) and S (6.49 +/- 1.8 Hz). These differences were due predominantly to flow transients at high lung volumes in CF and S. No significant differences in HF of VT curves were noted among N (3.0 +/- 1.4 Hz), CF (2.9 +/- 1.4 Hz), and S (1.7 +/- 1.5 Hz). All subjects had HF for FT of 12 Hz or less and for VT of 6 Hz or less. For spirometric recordings our results suggest that the amplitude response of flow- and volume-measuring devices should be flat (+/- 5%) up to 12 Hz for flow signals and up to 6 Hz for volume signals.