The effect on sound generation of varying both gas flow rate and the viscosity of sputum-like gel in a simple tubular model

Gas flows of 2, 3, and 4 L/min were directed through a sputum-like gel with viscosities of 100, 150, and 200 P and placed in a tube similar in diamete r to a human segmental bronchus (4 mm), which was immersed in a bath of wat er. The sound produced by gas flow through the gel was recorded with a hydr ophone. Sound data were subjected to time-expanded waveforms and fast Fouri er transform (FFT) analysis. This study demonstrated that the number of cra ckles generated was directly related to the flow rate and inversely related to gel viscosity. The initial deflection width (IDW), two-cycle duration ( 2 CD), and peak-to-peak amplitude of crackles were significantly affected b y the gas flow rate but not the viscosity of the gel. A lower gas flow rate generated crackles with longer IDW and 2 CD, but higher gas flow rates gen erated crackles with higher amplitude. Peak sound intensity measured from F FT increased as flow rate increased but decreased as the viscosity of the g el increased. At low gas flows, no gel-induced crackle sound was generated within the data capture window when the most viscous gel was examined. A di gital video image of gas flow through the gel was captured, and this confir med the absence of bubbles or slug formation at low flows through 200 P gel during the 3 seconds of data acquisition. This study describes some charac teristics of crackles generated from different combinations of gas flow and gel viscosity and suggests that "coarse crackles" result from the explosio n of gas bubbles in pulmonary secretions. Health care practitioners should consider the combined effect of rate of inspiratory gas flow and sputum vis cosity during auscultation of patients' lungs.