Frequency spectra of flow and volume events for forced vital capacity

Abstract

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– 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.