High-frequency characteristics of respiratory mechanics determined by flow interruption

When flow at the airway opening is suddenly interrupted, the pressure measured just behind the point of interruption generally exhibits certain characteristic features, including some rapid and highly damped oscillations immediately after the interruption. It has previously been assumed that these oscillations reflect ringing of central airways gas. In the present study we investigated this hypothesis by performing flow interruptions during relaxed expiration in normal, tracheostomized, anesthetized, paralyzed dogs while the lungs were filled with four different gas mixtures having widely varying physical properties. We found that the power spectrum of the oscillations exhibited two peaks. The larger peak was centered about a frequency that varied approximately linearly with the inverse of the square root of the gas density. The other peak was smaller and was located at approximately 70 Hz with all gas mixtures. The area under the power spectrum of the ringing varied approximately linearly with the density of the gas mixture. These results indicate that the larger peak in the power spectrum reflects the quarter-wave resonance of the gas in the airways, whereas the smaller peak reflects a tissue resonance.