Abstract
The authors have designed an inductive plethysmograph to obtain a noninvasive measure of ventilation. Two elastic bands containing insulated wires encircle the chest and abdomen-the inductance of each band depends on the enclosed cross sectional area. Each inductive band forms an element in a tank circuit, which determines the resonant frequency of a Colpitts oscillator. By measuring the oscillator frequency, the authors indirectly measure the changes in cross sectional area that occur during breathing. Independent measures of chest and abdominal cross sectional area provide a way to detect both normal breathing and airway obstruction. Magnetic coupling due to the mutual inductance between chest and abdominal bands modulates the desired oscillation frequencies. When modulation is excessive, frequency locking occurs and one cannot make independent measures of chest and abdominal area. The authors have performed simulations that show that, as the chest and abdominal band oscillator frequencies are sufficiently separated, the modulation can be decreased to avoid frequency locking. The authors have compared simultaneous recordings of ventilation using their inductive plethysmograph and a commercial impedance pneumograph and spirometer. Recordings of normal ventilation by all methods appear similar; however, the authors' inductive device is less prone than the impedance pneumograph to artifacts caused by applied pressure and body movements. In addition, during simulated airway obstruction, signals from the chest and abdominal bands are out of phase-suggesting that the inductive technique may be useful for detecting airway obstruction.