Fig. 1
The system to develop forced expiration was a tightly sealed box with a volume of 650 ml. After intermittent application of −15 cmH2O to the box, a positive pressure of ~75 cmH2O was suddenly applied to body surface. The positive pressure was generated by blowing air against a water-filled tank. The negative pressure was generated by suctioning air through a water-filled tank. All the solenoid valves were controlled by an electronic circuit