Air-filled catheter systems are commonly used to measure pressures in airways and at the mouth. Airway opening pressure (Pao) is usually sampled from a side tap (lateral pressure) in a mouthpiece (Pmo), tracheal tube (Ptr), face mask (Pmask), or from a nostril plug (Pnas) (23). For nasal pressure to reflect airway pressure there must be free communication between the nostrils and mouth, with nasal flows. If Pao is measured from a side tap of a mouthpiece or a tracheal tube during a maneuver that involves gas flow, the cross-section of the device through which the subject breathes must be large enough to avoid measurement errors due to the Bernoulli effect (24). In some cases, Pao serves to estimate alveolar pressure (Pa, Palv) during dynamic respiratory efforts made against an obstructed airway (e.g., mouth pressure response to phrenic nerve stimulation). For Pao to reflect PA accurately the transmission of pressure from the alveoli to the airway has to be very fast. The time constant of transmission is the product of the flow resistance offered by the airways (Raw) and the compliance of the extrathoracic airways (Cuaw) including the mouth, cheeks, and equipment. In practice the internal volume of the measuring equipment (mouthpiece, face mask, tracheal tube) contributes negligibly to the time constant (6), but should be minimized in patients with an already increased time constant, such as patients with chronic obstructive pulmonary disease (COPD). The compliance of the cheeks can be minimized by holding them rigid with the hands.
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