Quantitation of Wheezing in Acute Asthma Methods

During an acute attack of asthma, auscultation of the patients lungs is likely to be repeated many times. The severity of the attack, the improvement or deterioration, and the response to treatment are judged by several criteria, such as the patients symptoms, physical findings, pulmonary function tests, and blood gas analysis; but repeated auscultation plays an important part in assessment. Auscultation is simple, is readily available, requires only a stethoscope, and entails no risk or inconvenience to the patient. Its value is diminished by difficulty in interpreting the sounds we hear. Their complexity strains our memory. Uncertainty about which characteristics are important leads to a dependence on overall impressions. This might with advantage be replaced by concentration on a few significant elements, if these could be identified.

Methods are now available for the recording, analysis, and quantitation of pulmonary sounds. These methods allow measurement of the physical correlates of these perceived sounds and separation of the elements which combine to form them. We therefore studied pulmonary sounds in patients before and after bronchodilator treatment of acute asthma, in order to identify which characteristics changed with reduction in airflow obstruction.

Our objective was to identify those features of the wheezing sounds themselves which changed as a result of effective bronchodilator treatment. We did not, in this study, make any effort to relate these sounds to airflow rates, to pulmonary volumes, or to respiratory rate, all of which are likely to change with relief of airflow obstruction. Asthmatic subjects presenting in an acute attack were studied, which required a simple portable method of recording sounds without delaying treatment.

Treatment of Acute Asthma

Material and Methods

Patients with a known history of asthma who presented with an acute exacerbation to the emergency room at the University of Cincinnati University Hospital were invited to participate in the study. If they chose to do so after being informed of its purpose and design, their written consent was obtained. In addition to the routine history and physical examination and other tests considered to be indicated by the responsible physician, those admitted to the study had pulmonary function tests, and pulmonary sounds were recorded on magnetic tape in standard fashion.

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Pulmonary function tests were performed using a 9-L water-sealed spirometer (Collins). The forced vital capacity (FVC), forced ex piratory volume in one second (FEV,), and peak expiratory flow rate were measured and were accepted as valid only if replicate measurements varied by less than 10 percent.

Pulmonary sounds were recorded using an electronic stethoscope (3M) and a portable cassette recorder (GE model 3-5140A), as shown in Figure 1. Auscultation was performed over four areas posteriorly: the right apex, left apex, right base, and left base. Cardiac sounds could not be heard in these areas. At least five tidal breaths were recorded in each area at each testing period.

Patients were then treated with a sympathomimetic bronchodilator, either 0.3 ml of a 1/1,000 solution of epinephrine subcutaneously or 0.5 mg of terbutaline (what is it?) as an aerosol. The patients were reexamined 15 to 30 minutes after treatment, and of pulmonary function tests and recording of pulmonary sounds were repeated.

Bronchodilator Treatment

Pulmonary sounds from the same site before and after bronchodilator treatment were compared, choosing the site with the best signal-to-noise ratio. The pulmonary sounds to be analyzed were rerecorded on one channel, and an electronic signal to mark the beginning of expiration was recorded on another channel of a seven-track remote-control FM tape recorder (Ampex model PR50). The beginning of expiration could be identified easily by ear from the recorded sound signal. Segments of the sound signal, each 250 msec in duration, were analyzed in sequence, starting with the beginning of expiration and continuing with segments each starting 100 msec later than its predecessor and thus overlapping it by 60 percent. At least 25 segments were analyzed for each respiratory cycle, depending upon its duration. Each segment was digitized at 2,048 samples per second, and the frequency spectrum from 0 to 1,000 Hz, with a band-width of 6 Hz, was obtained using Fast Fourier Transformation (Hewlett-Packard spectrum analyzer 3582A). The frequency domain data were stored in the controlling computer (Hewlett-Packard 85) for subsequent graphic display.

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Figure 2 shows an example of the method of display, with the corresponding time-amplitude plot for comparison. Twenty-five segments of sound, each 100 msec later than the preceding segment, are shown from lower left to upper right. Each segment is shown as a sound spectrum, from 0 to 1,000 Hz on a linear scale horizontally. Power at each frequency is represented vertically, also on a linear scale.

The sound signal was monitored by ear as it was being analyzed. It was noted that when wheezes were audible, sharp peaks were present in the sound spectra of the corresponding segments. Examples of these peaks are seen in Figure 2 in the segments from 0.4 to 1.0 second. The height of each peak is related to the sound intensity at that frequency for that segment, and thus the intensity, frequency, and duration of wheezes can be measured. As the duration of a respiratory cycle is likely to be increased with relief of an asthmatic attack, the duration of wheezing was expressed as a proportion of the respiratory cycle duration (T1T1). In the example in Figure 2, this ratio is 0.7/2.4 seconds, or 0.29. Comparisons of observations before and after treatment were tested for statistical significance using Students paired f-test. The least-squares technique was used for correlation. Differences with a less than 5 percent probability of resulting from chance were regarded as statistically significant.

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Pulmonary SoundsFigure 1. Pulmonary sounds are auscultated with electronic stethoscope over four quadrants of chest. Pulmonary sounds are recorded on portable cassette recorder. These recordings are rerecorded on computer-controlled tape recorder. A 250-msec segment of sound is played into spectrum analyzer. Sound is analyzed using Fast Fourier Transform (FFT) technique over spectral range of 1 to 1,000 Hz. Transformed data are stored, and next segment of sound is analyzed. Sound segments are 100 msec apart, giving 60 percent overlap of analysis. After 25 to 30 segments are analyzed (2.5 to 3.0 seconds, time of respiratory cycle), plot of wave forms is produced, as shown on Figures 2, 4, and 5.

Time-amplitude waveFigure 2. Time-amplitude wave form is shown at top. Burst of activity is characteristic for wheeze. Same sound segment was analyzed as described in Figure 1. Each line represents 250-msec sound, analyzed over spectral range from 1 to 1,000 Hz. Line 1 represents beginning of expiration. On line 4, peak is identified at 250 Hz. This peak corresponds to wheeze heard on auscultation and seen on time-amplitude wave form. Peak is seen on line 4 through 10. This means wheeze was present for 700 msec. Total duration of breath was 2.4 seconds, so T*/To< is 0.292. Amplitude of peak can be measured, and this is linearly related to intensity of sound. We can measure frequency, intensity, and duration of wheezes.