Frequency dependence of pulmonary compliance and resistance in patients with obstructive lung disease

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Abstract
The frequency dependence of pulmonary compliance and resistance was investigated in 27 patients with obstructive lung disease. Compliance and resistance were determined either by the conventional zero crossing (Cdyn) and isovolume (RL) technique or by a modified Fourier analysis following a smoothing procedure (auto- and cross-correlation function) yielding an effective compliance and resistance, .cxa.CL and .cxa.RL. The latter technique was used to calculate .cxa.CL and .cxa.RL from the fundamental and 3rd and 4th harmonics present in the flow and transpulmonary pressure signals. Three breathing frequencies were investigated: 0.5, 1 and 2 Hz. Both Cdyn and .cxa.CL, calculated from the fundamental component, decreased progressively with frequency. Cdyn showed less frequency dependence than .cxa.CL. .cxa.CL calculated from the harmonics was significantly smaller than .cxa.CL from the fundamental at the same breathing frequency. RL, as well as .cxa.RL calculated from the fundamental, tended to increase with frequency. A decline of resistance with frequency became apparent, however, when .cxa.RL from the fundamental was compared with .cxa.RL obtained from the corresponding higher order harmonics. The frequency dependence of resistance can be masked by the usual procedure of breathing at several frequencies. Instead the measurements should be performed at a single frequency, for instance spontaneous breathing, by computing resistance from the higher order harmonics present in the breathing signals.