A Fan-Based, Low-Frequent, Forced Oscillation Technique Apparatus

The forced oscillation technique (FOT) is a noninvasive method to characterize the respiratory impedance (Z). Z is defined as the frequency-dependent ratio between pressure and flow. The FOT determines Z by superimposing small amplitude (in the order of 0.1 kPa) pressure oscillations on the normal breathing. It has been shown that a lot of useful information is contained in the frequency range of spontaneous breathing (0.1-1 Hz). In the current state-of-the-art methods, patient cooperation by means of voluntary apnea or mechanical ventilation is required to obtain the respiratory impedance at low frequencies. This article proposes a fan-based setup driven by a microcontroller. The setup allows to excite the respiratory mechanics at frequencies around the spontaneous breathing rate without requiring any patient effort. However, the (nonlinear) dynamic behavior of the setup and the pressure perturbations introduced by the subjects breathing jeopardize the spectral analysis of the measurement. Therefore, a combination of feedforward compensation of the excitation signal and linear feedback control are applied and discussed using measurements on a prototype device. A high-quality pressure signal is obtained, which makes it possible to obtain the respiratory impedance at low frequencies in a clinically practical way.