Airflow-Generated Sound in a Hollow Canine Airway Cast: RESULTS

Posted by James

The data were analyzed by examining the relation­ship between sound amplitude and airflow at each airway size and between amplitude and airway size at each airflow rate. Since only the aggregate airflow was measured (at the trachea), the analyses were compar­ative; absolute airflow in individual branches was not measured. We used the Lowess curve fit procedure to objectively assess trends. This method produces a smoothed curve without first presuming the shape of the function. When deemed useful, least-square re­gression curve formulas were fitted to the data.

Although frequency analysis was not a primary goal of this investigation, sound spectrograms were made of all of the data, and the sounds were audibly monitored as well. The sound nearly always had the character, and the spectrograms the appearance of, band-limited white noise with frequency components extending from 5 to 1,000 to 1,500 Hz, rarely higher. Examples are shown in Figure 3.

Table 1—Sound Amplitude in Airways Grouped by Cross-Sectional Area

Data		Airway Area, sq mm
	3.14-19.63	28.27-78.54	153.9-201	293.5
No. of samples	30	18	12	2
Log inspiratory
amplitude	2.61 (0.55)	3.57 (0.5)	3.82 (0.34)	3.23
Log expiratory
amplitude	1.33 (0.55)	2.35 (0.67)	3.03 (0.61)	4.1
Significance of
difference	p<0.0001	p<0.0001	p<0.001

The overall relationships among the examined in­dices are displayed in Figure 4. This plot displays sound amplitude against airflow and airway cross- sectional area. The data points were used to generate a mesh, in order for the pattern to be easily appreciable without undue distortion from outliers. The mesh was produced by a locally weighted, three-dimensional, multiple regression algorithm using a commercial statistical package (Systat). Only the mesh was smoothed, not the actual data points. Extrapolation beyond the data points is an artifact of the graphing program and should be ignored. Examination of the illustration reveals the following: in the expiratory direction the sound amplitude was approximately linearly related to airway cross-sectional area, with the greatest amplitude occurring in the trachea. In the inspiratory direction the greatest amplitude oc­curred in the vicinity of airways of about 200 cu mm (8-mm diameter), falling off in larger airways. A second, weaker amplitude peak occurred at airways of approximately 75 cu mm (5-mm diameter). Ampli­tude rapidly fell off within smaller airways.
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FIGURE 3. Example of sound spectra at several levels in model. Amplitude is on relative log scale.

To determine whether the differences between the amplitudes of the inspiratory and expiratory flow- generated sounds were statistically significant, we assembled the data into four groups by airway area (roughly, small, medium, or large airways and trachea). We used only those data related to the airflows of 2 and 2.5 L/s and compared the groups by paired Mest, excluding the tracheal sounds due to there being only two data points. The results showed statis­tically significant differences among all three groups, as summarized in Table 1.

FIGURE 4. Three-dimensional plot of all data. Circles are actual data points (see text for details).

Figure 5 shows separate plots of amplitude as a function of cross-sectional airway area in inspiration and expiration at various airway diameters. Figure 6A shows a representative plot of amplitude as a function of airflow in all studied airways of 14 to 16 mm in diameter. In both inspiratory and expiratory direc­tions, the plots appeared to be curvilinear, suggestive of an exponential function. We found that a linear function best fit the data when it was plotted against the square of the airflow as shown in Figure 6B. Figure 7 shows a redrawing of Figure 4 using squared airflow.
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FIGURE 5. Representative plots of amplitude as (unction of area at airflow of 1.0, 2.0, and 2.5 L/sec (Lowess smoothing). Noise level when airflow was absent averaged 5 (and never exceeded 10) units of amplitude.

Figure 6. A (top). Representative plot of amplitude as function of airflow for inspiration at all airways 14, 15, and 16 mm in diameter. В (bottom). As in Figure 6A, plot of amplitude as (unction of squared airflow. Least-squares linear regression line has been fit to data.

FIGURE 7. Three-dimensional plot of all data using squared airflow (instead of airflow, as in Figure 4).