Innovation in Analysis of Respiratory Sounds

Background: Despite advances in technology, evaluation of respiratory physiology still depends primarily on chest auscultation. However, this method is subjective and requires sufficient training. In addition, identification of the 5 respiratory sounds specified by the International Lung Sounds Association is difficult because their frequencies overlap: The frequency of normal respiratory sound is 100 to 1000 Hz, wheeze is 100 to 5000 Hz, rhonchus is 150 Hz, coarse crackle is 350 Hz, and fine crackle is 650 Hz (1, 2). Objective: To develop a computer-assisted stethoscope to quickly and accurately identify respiratory sounds. Methods and Findings: We developed a stethoscope that uses a tablet computer to identify how much of the overall respiratory sound belongs to each of the 5 respiratory sounds. Developing the data analysis algorithm was our biggest challenge. Traditional algorithms involve a computationally intensive analysis that has difficulty quickly classifying the sounds into categories (35). In contrast, our algorithm physically and mathematically divides the overall sound into groups according to the blind-source separation method, with the approximated value based on the duration, frequency, and strength of sound. It also compares the sounds in each group with sound templates to identify the amount of overlap among each category. As a result, our algorithm is fast enough to enable real-time analysis of respiratory sounds as a patient is being examined. To create the templates, we recorded respiratory sounds in 878 persons. Three experienced respiratory physicians independently classified these sounds into the 5 categories (1, 2). At least 2 of the physicians believed that 170 persons had normal respiratory sounds and that the remainder had sounds that were mostly 1 type: 168 had rhonchi, 177 had wheezes, 178 had fine crackles, and 185 had coarse crackles. We then created templates using only respiratory sounds classified identically by all 3 physicians. To display the results, we created a pentagon chart (Figure, right). Each vertex represents 1 of the 5 respiratory sounds, and each line from the center to a vertex measures how much of that sound is present. The chart displays the measurement by plotting a point on each line, connecting each point to adjacent points with new lines, and coloring the area between the pentagon's center and the new lines in red. Videos show how our computer-assisted stethoscope displays this information (Supplements 1, 2, 3, and 4). Figure. Display of computer-assisted stethoscope. The characteristics of respiratory sounds used to create the pentagon chart are shown on the left. They include the waveform of total respiratory sound (top), the spectrogram of total respiratory sound (middle), and the 5 waveforms of the separate respiratory sounds (bottom). For the waveforms, the horizontal axis is time and the vertical axis is strength. For the spectrogram, the horizontal axis is time, the vertical axis is frequency, and the brightness of color represents the strength of the sound. The pentagon chart is shown on the right. The total area in red represents the overall volume of sound, and the proportion of red around each line from the center to each vertex represents the proportion of the overall sound that each respiratory sound contributes. Discussion: Most electronic stethoscopes focus on digitally recording respiratory sounds and less on classifying them (35). In contrast, we have developed a computer-assisted telescope that has novel ways of analyzing and displaying information and works in real time, classifies respiratory sounds into generally accepted categories, and classifies sounds when different sounds are present at the same time. We believe that the result is a new way of evaluating respiratory sounds that may help clinicians better understand what they hear. We also believe that this stethoscope could be used in medical education; the intensive care unit; the operating room; and other disciplines where invasive procedures are done, such as bronchoscopy and gastroenteroscopy, and respiratory monitoring is believed to be useful. The recordings created by the computer-assisted stethoscope can also be stored in electronic health records.