Figure 3.

Inspiratory threshold load. (A) An idealized volume-time curve of a patient who develops increased airway resistance after the first breath, with resulting dynamic hyperinflation. (B) Airway pressure–time curve for the same patient with onset of expiratory resistance after the first breath, with resulting dynamic hyperinflation. With an increase in duty cycle, the brain signals to the inspiratory muscles to contract (*) before the return of lungs to relaxation volume and airway pressure. For air to move from the mouth to the alveoli, the patient must produce airway pressure that is negative relative to atmospheric pressure. The patient must overcome the positive pressure in the airway at the end of exhalation (intrinsic positive end-expiratory pressure) to generate the next breath. The added work associated with this condition has been termed an inspiratory threshold load.