Flow and sound generation in human lungs: Models of wheezes and crackles

Abstract

We present a broad review of our laboratory’s contributions to the understanding of flow generated sounds which emanate from the lung and are used by clinicians to ascertain the status of health of the respiratory system. Important sounds from diseased lungs include wheezing, which occurs primarily during expiration, and crackles which occur primarily during inspiration. We have analyzed flow-induced flutter of flexible tubes as the mechanism of wheezing sounds and connected its appearance to the onset of flow limitation in a lung. Wheezes and difficulty expiring air are common to asthma and emphysema. The flutter theory is for both linear and nonlinear wall properties, while using potential flow with a friction factor as well as a coupled Orr-Sommerfeld system using the Navier– Stokes equations coupled to the wall. Both the value of the critical flow velocity that instigates flutter, as well as the flutter frequency, compares well with experiments on isolated flexible tubes as well as an excised lung. We also studied the propagation and rupture of simulated mucus plugs using carbopol 940 gels to investigate the source of crackling sounds which occur in lungs with an abnormal amount of liquid in the airways, congestive heart failure, or small airway inflammation. The non-Newtonian properties of yield stress, storage modulus, and loss modulus were matched well with normal and abnormal mucus. A collapsed airway of the 12th generation was modeled using a quasi-two-dimensional polydimethylsiloxane channel. The plug yields at a plane about one-third of the half channel width away from the lateral channel walls and then ruptures, creating a crackle sound, and reopening the model airway to gas flow. The relatively high shear stresses during rupture, and crackle sound production, may increase epithelial cell damage.