Interaction of oscillatory and unidirectional flows in straight tubes and an airway cast.

Because oscillatory resistance of the respiratory system is often measured during tidal breathing, we studied the interaction between simultaneous oscillatory and unidirectional flows in three straight tubes (radius ranging from 0.3025 to 0.679 cm and length either 30.7 or 173 cm) and a central airway cast (tracheal radius 0.685 cm). Oscillatory flow was generated by a loudspeaker, airway pressure was measured with a transducer, and flow was calculated from pressure changes in an airtight enclosure mounted behind the flow source (loudspeaker plethysmograph). Oscillatory resistance, i.e., the real part of impedance, was determined from 2 to 64 Hz. In the absence of unidirectional flow, frequency dependence of resistance was observed for the two 30.7-cm-long tubes to match previously published theory. Frequency dependence of resistance for the airway cast was similar to that of the tube of comparable inlet radius. In the presence of unidirectional flow, oscillatory resistance at low frequency was independent of frequency and determined by the magnitude of the unidirectional flow. Oscillatory resistance at high frequency was frequency dependent but still influenced by the magnitude of the unidirectional flow. Our results indicate that the presence of unidirectional flow alters the oscillatory resistance of tubes and the cast at any given frequency, presumably by changing the shape of the boundary layer.