Stethoscope design

SUMMARY: Some modern stethoscope designs digitize sound waves, which can be modeled and analyzed.

The stethoscope is perhaps one of the most iconic pieces of medical equipment and is used by doctors in nearly every area of clinical practice around the world. From its beginnings as a simple tube to amplify sound, in the twenty-first century the stethoscope is evolving into a highly mathematical and computerized tool. It can record, analyze, and display diagnostic information using software and algorithms developed from clinical data using a variety of concepts and techniques from statistics, signal processing, spectral analysis, and related sciences. Further, mathematical models and simulations are increasingly used to support and validate clinical results.

History and Development

French physician René Laënnec is credited with the invention of the stethoscope in 1816. The name comes from the Greek words meaning “chest” and “to examine.” Knowing that solid bodies conduct and amplify sound, Laënnec used tightly rolled and glued sheets of paper to hear patients’ heartbeats. Experimenting with cylinders of various materials, he observed that an aperture maximized the magnification of internal body sounds. His ultimate design was a straight, eight-inch wooden tube with a conical chest piece and a funnel-shaped stopper. Later physicians developed stethoscopes from materials like rosewood, papier-mâché, and even glass. The binaural form was popularized in the United States in the early 1900s by William Osler.

In the twenty-first century, the binaural acoustic stethoscope consists of a chest piece with a plastic disc (called a diaphragm) on one side and a hollow cup (called a bell) on the other. The bell transmits low-frequency sounds, and the diaphragm transmits high-frequency sounds. A majority of clinicopathological correlations and diagnostic techniques used today result from patient data acquired by physicians listening with stethoscopes or a bare ear. Refinements in design and the increasingly widespread use of stethoscopes—coupled with training—improved observations. With respect to the heart, these included better precision in timing cardiovascular sounds, focusing on segments of the cardiac cycle in turn, and devising quantitative symbols to describe sounds. On the other hand, stethoscopes have also been investigated as a vector of horizontal disease transmission in busy clinical settings like emergency rooms.

With twenty-first-century technology, pocket-sized electronic devices can record data and instantly provide doctors and patients with critical breath and heartbeat information. During the COVID-19 pandemic, some doctors preferred to use devices like chest X-rays and ultrasounds to hear chest and breath sounds instead of stethoscopes because of the close proximity traditional stethoscopes require. However, stethoscopes are unlikely to become obsolete because of their utility in individuals who are pregnant or otherwise can not undergo an X-ray. Also, they are cost-effective.

Mathematical Modeling

Electronic systems of collecting and analyzing data have begun to supplement or even supplant the use of the stethoscope. Some predict that before 2020, manual stethoscopes will become obsolete. Electronic stethoscopes convert acoustic sound waves into electrical signals, which can be amplified and enhanced, producing both visual and audio output. Software can then represent cardiopulmonary sounds graphically and interpret them using mathematical algorithms. Signals may also be recorded or transmitted, facilitating remote diagnosis and teaching. Some research suggests that mathematical methods improve accuracy in diagnosing conditions, such as heart murmurs, but some methods have not yet shown clinical usefulness. Mathematicians and physicians continue to investigate and model cardiac sounds from murmurs and prosthetic valves, as well as other types of hemodynamic data, using techniques from spectral waveform analysis and physics concepts like damped oscillations of viscoelastic systems. They have also sought to quantify pulmonary sounds, like wheezing and crackles, and address signal processing issues, such as noise reduction, amplification, and filtration.

Measuring Blood Pressure

Blood pressure is the amount of pressure exerted by the blood upon the arterial walls. A clinician uses a device known as a sphygmometer—a device that pumps air into a cuff wrapped around a patient’s arm—and listens for pulse sounds with a stethoscope, observing the height in millimeters of a column of mercury supported by the blood pressure. The sounds are known as Korotkoff sounds, named for Russian physician Nikolai Korotkoff. A contraction of the heart that causes a pulse beat that supports a column of mercury 120 millimeters high is called a “systolic reading of 120.” The reading in the period between contractions of the heart or pulses is called the “diastolic blood pressure.” If the diastolic reading is 80 millimeters, the blood pressure is recorded as 120/80 and is read as “120 over 80.” These numbers represent a ratio rather than a true fraction. The U.S. National Heart, Lung, and Blood Institute (NHLBI) defines normal blood pressure to be <120 for systolic and <80 for diastolic pressure and defines hypertension to be >140 or >90 for systolic and diastolic, respectively. These values are derived in part from statistical studies of typical human variation in blood pressure and associations with medical conditions like stroke and heart disease. Early diagnosis and appropriate treatment of hypertension is recognized as one of the most significant advances of modern medicine in reducing morbidity and mortality.

Bibliography

Bishop, P. J. “Evolution of the Stethoscope.” Journal of the Royal Society of Medicine, vol. 73, no. 6, 1980, pp. 448–56, doi: 10.1177/014107688007300611. Accessed 20 Oct. 2024.

Choudry, Misha, et al. “The History and Evolution of the Stethoscope.” Cureus, vol. 14, no. 8, 2022, p. e28171, doi.org/10.7759/cureus.28171. Accessed 20 Oct. 2024.

"How Did We Get the Stethoscope?" American Lung Association, 25 May 2022, www.lung.org/blog/stethoscopes-history. Accessed 20 Oct. 2024.

Pullan, Andrew, et al. Mathematically Modeling the Electrical Activity of the Heart: From Cell to Body Surface and Back Again. World Scientific Publishing, 2005.