DESIGN OF AN INDUCTIVE PLETHYSMOGRAPH FOR VENTILATION MEASUREMENT

We have designed an inductive plethysmograph to obtain a non-invasive measure of ventilation. Two elastic bands containing insulated wires e ncircle the chest and abdomen-the inductance of each band depends on t he enclosed cross sectional area. Each inductive band forms an element in a tank circuit, which determines the resonant frequency of a Colpi tts oscillator. By measuring the oscillator frequency, we indirectly m easure the changes in cross sectional area that occur during breathing . Independent measures of chest and abdominal cross sectional area pro vide a way to detect both normal breathing and airway obstruction. Mag netic coupling due to the mutual inductance between chest and abdomina l bands modulates the desired oscillation frequencies. When modulation is excessive, frequency locking occurs and we cannot make independent measures of chest and abdominal area. We have performed simulations t hat show that, as the chest and abdominal band oscillator frequencies are sufficiently separated, we decrease modulation and avoid frequency locking. We have compared simultaneous recordings of ventilation usin g our inductive plethysmograph and a commercial impedance pneumograph and spirometer. Recordings of normal ventilation by all methods appear similar; however, our inductive device is less prone than the impedan ce pneumograph to artifacts caused by applied pressure and body moveme nts. In addition, during simulated airway obstruction, signals from th e chest and abdominal bands are out of phase-suggesting that the induc tive technique may be useful for detecting airway obstruction.