Fig. 8.

Theoretical intrathoracic pressure curves during the respiratory cycle under conditions of no airways resistance (curve A), normal (healthy) levels of upper airway resistance (curve B), and high levels of upper airway resistance (curve C). Ideal airflow and volume traces also shown for reference. Intrathoracic pressure is function of resistance of the upper and lower airways to airflow, as well as air volume that is associated with chest wall/lung compliance. Without airway resistance, intrathoracic pressure reflects changes in volume (dependent on chest wall/lung compliance; sinusoidal curve A), and peak tracheal displacement will occur at end of inspiration. With the addition of healthy levels of upper airway resistance, intrathoracic pressure begins to have a relatively minor dependence on airflow and resembles curve B. With a large increase in upper airway resistance, as may occur in obstructive sleep apnea (OSA), intrathoracic pressure is now largely influenced by the airflow resistive component (curve C), such that it reaches a nadir earlier, at close to maximal inspiratory airflow (× mark on curve C). Given that intrathoracic pressure is a primary load displacing the trachea, this may explain why peak tracheal displacement is reached earlier during inspiration in OSA. FRC, functional residual capacity.