Fig. 1.

Schematic diagram of the respiratory control system. The plant represents the gas-exchange system. The input to the plant is ventilation (V˙_E), and the output is the alveolar gas tension (P_ACO₂). The delay term represents the time it takes for the pulmonary capillary blood to reach the chemoreceptors. During sleep, the controller primarily represents the ventilatory response to CO₂, i.e. chemorelfexes. PAV works by generating pressure at the airway in proportion to a person’s inspiratory effort, thereby resulting in an increase in controller gain. The product of the plant and controller gains equals the loop gain.