(K-418) Development of Advanced Digital Stethoscope for Heart and Lung Monitoring

Modern medicine has evolved dramatically in the 20th and 21st centuries and has incorporated technology—this is almost true; stethoscopes continue the long-established technology (i.e., diaphragm and air-tube) for over a hundred years. The heart, one of the most valuable organs in the human body, is still being examined by the technology that has the same principles as a tin can telephone, while there is incredibly advanced technology, we carry with us every day. Combining a stethoscope with modern digital technology will bring more accuracy in detecting heart murmurs, arrhythmias, and other heart conditions. Being able to record bodily sounds is very important when conducting research and diagnosing disorders, however, it cannot be done without having the tools to process the sound and filter it. Understanding filtering and being able to play back sounds from specific chambers of the heart to precisely locate issues will be much more useful in gauging irregular sounds than working with a standard stethoscope. Current filter technology with multiple sound information would be able to suppress any unwanted bodily noise and to have the ability to mute specific parts of the heart and lungs for singling out a sound.

The objective of this project was to design a digital stethoscope using microphones to navigate sounds in specific chambers of the heart. In this study, as for the preliminary engineering design step, the variability of commonly available digital microphones and the microcontroller system with the Inter-IC Sound (I2C) protocol was examined.

While developing the digital stethoscope, various topics were examined: the physiology of the heart, previously reported heart sounds and patterns, and the placement of microphones to design the stethoscope to work in a wide range of clinical and research settings. The collected information was used to design this 3D printed sound capturing apparatus: a 40 mm diameter opening of parabolic shape with a microphone hole at the bottom.

This design was inspired by the parabolic microphone which has been widely used to capture small birds chirping. The omnidirectional digital microphone (INMP441, InvenSense, CA, USA) and the microcontroller (ESP32-WROOM series, Espressif Systems, China) were integrated with the I2C protocol library and programmed in Arduino IDE.   

For the preliminary recording setting, the heart sound was captured at a sampling frequency of 44.2 kHz with a 16-bit resolution. The captured sound was streamed over the USB-serial communication, recorded by the terminal software (CoolTerm) in CSV file format, and visualized in Microsoft Excel. A digital filter was considered for implementation in the system; however, for the preliminary system validation purpose, the filter was not set in the program. The measurement was conducted in a quiet room with a small sound isolation fabric.

An intensive background survey provided some information and constraints for designing the system. The survey results were able to determine how the heart works and the best microphone placement for an optimal sound range. It has been known that the frequency range of a normal heart sound, including lung breathing sounds, is about 20 to 1,200 Hz, which was adequately captured by the INMP441 microphone. Preliminary sound sampling frequency (44.2 kHz), resolution (16-bit), and the I2S protocol were more than adequate to demonstrate the system's capabilities. The 3D-printed parabolic microphone housing was able to isolate the sound between the internal body and the external ambient noise.
Although there was fundamental noise observed, the developed prototype was able to capture the heart sound, which can be seen in Figure 1(b). Unlike an ECG signal (i.e., PQRST waveform), the captured heart sound indicated the lub-dub (i.e., systolic-diastolic) sound pattern. Furthermore, the amount of lub sounds matched the number of heartbeats per min (BPM).

The prototype integrated only one INMP441 microphone for system validation purposes; however, the ESP32 microprocessor and I2S protocol would be able to extend the stereo and more microphone array possibilities in future design. More than one point of sound measurement would be able to explore further dimensions of the heart sound: appropriate noise eliminations, localization of sounds, and so on. Although the preliminary system was able to capture the heart sound and patterns, the system still needed to re-design the 3D-printed parabolic housing for optimizing microphone capabilities. The direct digital connection between the microphone and processor promised to enhance the system signal processing capabilities into the next steps.

Moreover, the digitalization of heart sounds would allow for communication; from telemedicine in isolated locations to the remote application of communicating to space. The captured sound was visualized in Excel, which was compared to the previously reported sound patterns. The preliminary system was developed with off-the-shelf components and was able to demonstrate the unique lub-dub sound patterns, which is typically expected to accomplish by high-end equipment. This initial exploration of the project provided the promise of cost-effective digital stethoscope development.