Technical Aspects of Effective Teaching of Auscultation

Acoustics of Heart Sounds

Origin of heart sounds and murmurs

A full discussion of this subject¹, is beyond the scope of this book but it is important that educators understand a few basic facts about the origin of heart sounds and murmurs. The first and second heart sounds originate with the closing of the atrioventricular (mitral, tricuspid) and semilunar (aortic, pulmonary) valves respectively. They can be high amplitude, short duration sounds, similar to spikes or impulse-like signals. Stenosed or malformed valves may also produce clicks and creaks, which are similar to spikes. These sounds are related in complex ways to the deceleration of blood impacting on the valves. In the case of the semilunar valves, the arterial roots also receive the impact of rebounding blood from the pulmonary and systemic arteries, and are involved in sound production. The sounds transmit to the chest surface, often circuitously, and may be expected to be loudest, or most distinct, in specific areas. Exactly where sounds are loudest in any individual may depend on chest wall anatomy, fat deposits, lung properties and internal anatomic variations. However the standard anterior listening locations are: “aortic”-right sternal border second interspace, “pulmonic”- left sternal border second interspace, “tricuspid”-left lower sternal border fourth interspace, and “mitral”-apical. In practice, one may have to move slightly from these positions to hear sounds most clearly.

Murmurs, which are sounds having significant duration compared to the more discrete first, second, third and fourth sounds, are generally produced by one of two mechanisms: turbulence (or vortex shedding) related to nonlaminar flows, or tissue vibration, as in the innocent murmur of childhood. Murmurs transmit to the chest surface in locations related to the flow of blood, in the case of turbulent murmurs, but are also influenced by anatomic features noted above. The appreciation of the different sounds created by tissue vibration and by turbulent flow is of great diagnostic importance, particularly in children, and thus deserves considerable emphasis in teaching programs.

Frequency range of heart sounds and murmurs

The audible frequency range of sound emanating from the heart and vessels is roughly 20-1000 Hz. The lower the frequency, especially below 50 Hz, the more difficult sounds are to hear. Playback systems for recordings of heart sounds and murmurs must be able to reproduce sounds accurately, especially in the lower frequency range. The third and fourth heart sounds and diastolic murmurs are notorious for being inaudible by students, and it is often necessary to increase the playback volume of recording above normal to enable these sounds to be heard. Even so, some individuals with apparently normal hearing will have great trouble hearing these low frequency sounds. Devices for reproducing heart sounds for teaching should have high and low pass filters to mimic the bell and diaphragm of the acoustic stethoscope and accentuate the low and high frequency sounds respectively.

Recordings of heart sounds and murmurs

Recording heart sounds and murmurs with fidelity requires experience, specialized equipment and much attention to detail which is not usually relevant to educators. However, since some teachers may be using electronic stethoscopes and group teaching microphones on live subjects, a few comments may be helpful. Practice with the equipment is essential before attempting to teach students. A quiet environment is essential, but seldom found in a busy clinic or inpatient unit. A steady hand is also necessary as much noise can be introduced from the skin-microphone interface, especially on hairy chests or with active children. Teachers employing electronic stethoscopes should be aware of and point out the difference in the sounds provided by acoustic and electronic stethoscopes since students will not likely be using the latter in practice. Some, but not all, electronic stethoscopes have electronic filters which model the filtering characteristics of the binaural earpiece of the acoustic stethoscope, as well as characteristics of the bell and diaphragm stethoscope head. Other electronic stethoscopes offer high volume gains providing an unrealistic listening experience compared to the acoustic models. Electronic stethoscopes can reproduce sounds from the chest that are not heard in acoustic scopes, thus using them for diagnostic purposes may be challenging.

Reproduction

For students to achieve competence and confidence with auscultation in the clinical setting, high quality reproduction of heart sounds and murmurs is essential. If the sounds they are hearing through their acoustic stethoscopes are very different from what they hear in a teaching session, insecurity will be inevitable, and they will be reluctant to rely on what they hear, apart from perhaps identifying rhythm. There is debate on the usefulness of synthetic heart sounds in teaching, with some studies indicating good transfer of skills learned from recordings of synthetic sounds, to patients. However the availability of good quality live recordings and equipment for simultaneous auscultation by several listeners to subjects enables teaching programs to easily incorporate actual heart sounds for all or part of the instruction. The equipment used for sound reproduction must be appropriate for the frequency range and loudness of the sounds, and for the teaching environment.

Current sound recordings are digital, requiring either a CD or MP3 player or computer They may also be accessed from websites using mobile devices. Minimum acceptable recording characteristics are mono (single channel) with 16 bit samples recorded at least 8 KHz. Most current laptop computers can reproduce heart sounds adequately.

Delivery of sounds to the listener requires a speaker, usually in a head set or earphones. Stethophones have the advantage of mimicking to some extent an acoustic stethoscope, having air columns in the hollow metal tubes linking the tiny speaker to the ear pieces. Both hard-shell vertical in-the-ear headsets and soft moldable ear “buds” may also give reasonable sound reproduction, although they must extend well into the ear canal and exclude external noise to some extent. We are unaware of any studies comparing the physical sound characteristics, the teaching effectiveness or clinical quality of these devices. Stereo over-the-ear headphones, both open shell and acoustic-sealed-to-the- head models, in our experience, do not faithfully reproduce some sounds, notably the innocent murmur, the most common murmur in children and arguably the most important for learners to recognize. It is important that the headsets are not used at high volume. Apart from presenting an unrealistic representation of the heart sound, the sound transducers may be driven to their mechanical extremes, producing artifacts in impulse-like sounds (S1, S2, clicks). The low frequency response for headsets is important, with a low end range extending to 20Hz desirable.

Computer and stereo room speakers are not adequate for teaching or even individual listening . Stereo room speakers, even of high quality, are extremely difficult to use for heart sound reproduction. The sound perceived is very dependent on room acoustics, size, position of speaker within the room, etc, and the low frequency sounds are often lost. Again the innocent murmur is not well reproduced by these speakers in a room setting.

Video and audio conferencing equipment often involves compression and reconstruction of the signals, which can lead to problems, especially involving the reproduction of impulse-like sounds (S1, S2, clicks). Often artifacts are produced in the vicinity of the impulse, which can be misinterpreted as splits, or multiple clicks, confusing the novice learner at remote sites.

Classroom teaching in one site

Distribution of sounds to multiple listeners in one site can be achieved in several ways. For individual students, the audio amplifier in most laptop computers is adequate to drive two headsets using an adapter to connect both headsets to the “audio out” connector. Devices using Bluetooth ® wireless transmission are more practical if synchronous listening is desired. Alternatively, mobile devices sourcing sounds from websites by groups of listeners, while not truly synchronous, do allow listening to a common source which can be the focus of discussion of the sounds².

Distributed teaching sites

Increasingly, medical students spend rotations in a variety of sites, requiring facilities for distance education. Simultaneous live and /or online teaching at several sites can be performed in several ways. For clinical clerk sessions we use a web- based archive of “unknown” heart sound recordings (usually teachingheartauscultation.com>learning programs) which students at several sites can access simultaneously. They listen with individual earbuds attached to their mobile device.

Two- way voice communication to direct the session and allow discussion takes place with a telephone audio conference using speakerphones. Thus the heart sounds are heard through a separate audio device from the discussion. In practice the teacher instructs each student to access a recording, labelled by letter only, and after all students have listened, one student is invited to describe what is heard, beginning a discussion with all students present and mentored by the teacher. This has been very well received by students over several years, and is very economical. Videoconferencing is also possible to allow visual teaching during such sessions, but the heart sounds must be delivered via a separate audio channel (from a website) to provide adequate sound quality.