07.04.2017 change 07.04.2017

Textbooks for revisions: the stethoscope works differently than it seemed

A stethoscope - it would seem one of the simplest diagnostic tools - works differently than it was thought. "The information that has long been repeated in the textbooks for physicians has to be revised" - said Dr. Łukasz Nowak, who is working on a new generation stethoscope.

Acoustics expert Dr. Łukasz Nowak from the Institute of Fundamental Technical Problems of the Polish Academy of Sciences and his team work on a modern digital stethoscope. This device is designed to process sounds from the body so that the physician can more clearly hear all the disturbing signals coming from the patient\'s body. When working on the electronic device, Dr. Nowak\'s team investigated the properties of traditional acoustic stethoscopes, used by doctors every day. The results turned out to be surprising.

The work of the Polish team showed that a simple stethoscope works a little differently than the textbooks for future doctors teach. "To put it mildly, all medical textbooks should be revised this regard, because they mislead doctors" - believes the acoustics expert. The study on the subject appeared in the Journal of the Acoustical Society of America.


Some stethoscopes are equipped with two different tips. On one side of the chestpiece - which is pressed to the patient\'s body - there is a diaphragm, and on the other - a bell. Doctors are taught - says Dr. Nowak - that each of these stethoscope tips filters sound differently. The diaphragm of the stethoscope would suppress lower frequency sounds, which would help, inter alia, in lung examination. On the other hand, the tip called the bell would mute higher frequency sounds and thus perform better e.g. in the heart beat test.

But it turns out that this is not quite true.

"We question whether double chestpieces in stethoscopes are justified" - commented the acoustics expert and added: "Our research has shown that neither the bell tip nor the diaphragm tip has filtration properties, and the bell simply transfers quieter signals because it has less surface area".


Dr. Nowak spoke about previous studies, according to which stethoscope tips filter sound differently. "These studies were conducted in conditions not quite similar to normal auscultation" - said the scientist from the Institute of Fundamental Technical Problems PAS. The stethoscope was suspended over a loudspeaker and researchers checked the sound it transmitted. "That idea did not make much sense. After all, during the examination, the doctor presses the stethoscope against the patient\'s body and does not listen to sounds in the air" - said Nowak. He explained that the chestpiece pressed against the patient\'s body creates a new mechanical system, in which the sound propagates differently. "This effect should not be neglected" - he commented.

This is why Polish researchers have proposed a method of performing acoustic tests in which a stethoscope is placed on a patient\'s body, and the study is compared to an ECG study conducted at the same time. "Our methodology allows to objectively evaluate and compare the properties of different medical stethoscopes" - described Dr. Nowak. This method allowed the Polish researchers to overturn the myth of differences between tips.


Another myth about stethoscopes busted by Polish researchers is the myth associated with the so-called two-tone stethoscope diaphragm. If the diaphragm is pressed against the body, it is also pressed to the ring underneath. "Almost all manufacturers of stethoscopes boast such a diaphragm without referring to objective sources" - said the scientist. He explained that when the chestpiece is pressed harder against the patient\'s body, the filter effect appears - higher frequency sounds are more audible, which helps the doctor diagnose.

Polish research shows, however, that it is not the characteristic diaphragm construction that filters the sounds during the examination. The sound changes simply because when the chestpiece is pressed harger, the patient\'s tissue tends to deform and transfer the higher-frequency components better. The filter effect appears even when a microphone without any diaphragm is pressed hard against the body.


According to the scientist, during a stethoscope examination the doctor is able to recognize, for example, the murmuring in the heart or the lung cracking. These disturbing noises may be related to the turbulent flow of liquid or air in the body, which in turn can, for example, indicate structural defects in a given tissue. However, according to the researcher, we are still far from accurate description of the acoustic phenomena accompanying listening examinations.

"The stethoscope - although it seems to be a simple instrument - is, in my opinion, the least understood diagnostic device yet" - smiled Dr. Nowak.


"In 1816 French physician Rene Laennec was to listen to a patient who was a young woman. Because he was shy, he chose to roll a quire of paper into a cylinder and apply it to her body. It turned out that hea could hear the heartbeat better this way and stethoscope was invented" - said the scientist. However, he added, the possibilities offered by auscultation are much greater. And progress in this area quickly stopped. "It\'s time to move away from the solutions that imitate a rolled sheet of paper. There is tremendous possibilities for the development of auscultation, and we know how to use them" - said the researcher.

Acoustics expert from the Institute of Fundamental Technical Problems PAS wants to use new technologies in the analysis of the sounds from the body and develop the next generation of small portable electronic stethoscopes. The digital filters developed by the researchers could extract evidence of pathologies from the sound during the auscultation, provide and improve the accuracy of the doctor\'s diagnosis.

The research is carried out under the LIDER programme of the National Centre for Research and Development.

We have written about the work on a 21st century stethoscope before in PAP - Science and Scholarship in Poland.

PAP - Science and Scholarship in Poland, Ludwika Tomala

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