Measurements and Theory of Normal Tracheal Breath Sounds

Abstract

We studied the mechanisms by which turbulent flow induces tracheal wall vibrations detected as tracheal breath sounds (TRBSs). The effects of flow rate at transitional Reynold's numbers (1300-10,000) and gas density on spectral patterns of TRBSs in eight normal subjects were measured. TRBSs were recorded with a contact sensor during air and heliox breathing at four flow rates (1.0, 1.5, 2.0, and 2.5 l/s). We found that normalized TRBSs were proportional to flow to the 1.89 power during inspiration and to the 1.59 power during expiration irrespective of gas density. The amplitude of TRBSs with heliox was lower than with air by a factor of 0.33 ± 0.12 and 0.44 ± 0.16 during inspiration and expiration, respectively. The spectral resonance frequencies were higher during heliox than air breathing by a factor of 1.75 ± 0.2-approximately the square root of the reciprocal of the air/heliox wave propagation speed ratio. In conclusion, the flow-induced pressure fluctuations inside the trachea, which cause tracheal wall vibrations, were detected as TRBSs consist of two components:(1) a dominant local turbulent eddy component whose amplitude is proportional to the gas density and nonlinearly related to the flow; and (2) a propagating acoustic component with resonances whose frequencies correspond to the length of the upper airway and to the free-field sound speed. Therefore, TRBSs consist primarily of direct turbulent eddy pressure fluctuations that are perceived as sound during auscultation.

Respiratory Flow–Sound Relationship During Both Wakefulness and Sleep and Its Variation in Relation to Sleep Apnea

“…Normal tracheal breath sound has a broad-band spectrum below 1000 Hz; it has been shown that its shape and spectral peaks change with the geometry and pathology of the upper airway, 3,4,12,16,24,28 while its amplitude and energy change with the amount of respiratory flow. 3,10,13,38 Based on these findings, several researchers have studied various features of tracheal sound for implementing acoustical respiratory flow estimation from tracheal sound. These features include sound's average power, 11,13,14,30,40 envelope, 26 entropy, 37 and mean frequencies.…”

“…In previous studies the relationship between tracheal sound and respiratory flow was studied extensively. 3,4,10,13,38 However, these studies were all focused on data of non-OSA individuals during wakefulness. When implementing acoustical respiratory flow estimation for sleep studies, it is important to note that the relationship between tracheal sound and respiratory flow may change when one sleeps; it may also be different in non-OSA and OSA individuals.…”

“…Different mechanisms including turbulence of respiratory airflow, jet formation, and pressure fluctuations in the upper airway contribute to the production of sounds and fluid-induced vibrations. 3,4 The vibrations are transmitted to the skin through the tracheal wall and tissue beneath the skin, and can be picked up by a microphone placed over the trachea. Normal tracheal breath sound has a broad-band spectrum below 1000 Hz; it has been shown that its shape and spectral peaks change with the geometry and pathology of the upper airway, 3,4,12,16,24,28 while its amplitude and energy change with the amount of respiratory flow.…”

“…3,4 The vibrations are transmitted to the skin through the tracheal wall and tissue beneath the skin, and can be picked up by a microphone placed over the trachea. Normal tracheal breath sound has a broad-band spectrum below 1000 Hz; it has been shown that its shape and spectral peaks change with the geometry and pathology of the upper airway, 3,4,12,16,24,28 while its amplitude and energy change with the amount of respiratory flow. 3,10,13,38 Based on these findings, several researchers have studied various features of tracheal sound for implementing acoustical respiratory flow estimation from tracheal sound.…”

Using the Entropy of Tracheal Sounds to Detect Apnea during Sedation in Healthy Nonobese Volunteers

“…6,7 Tracheal sounds originate from the vibrations of the tracheal wall and surrounding soft tissues caused by gas pressure fluctuations in the trachea. 8 The signals from a microphone or a piezo-electric film transducer placed over the trachea have been processed to monitor respiratory rate Received from the Department of Biomedical Engineering, College of Basic Medical Sciences, China Medical University, Shenyang, Liaoning, P. R. China, and the Department of Anesthesiology, University of Utah, Salt Lake City, Utah. Submitted for publication April 10, 2012.…”