AbstractTextbooks, clinicians, and medical teachers differ as to whether the stethoscope bell or diaphragm should be used for auscultating respiratory sounds at the chest wall. Logic and our results suggest that stethoscope diaphragms are more appropriate.HISTORICAL ASPECTSHippocrates advised immediate auscultation (the application of the ear to the patients chest) to hear transmitted sounds from within.
However, in 1816 a French doctor, Ren Thophile Hyacinth Laennec invented the stethoscope,1 which thereafter became the identity symbol of the physician.Laennec apparently had observed two children sending signals to each other by scraping one end of a long piece of solid wood with a pin, and listening with an ear pressed to the other end.2 Later, in 1816, Laennec was called to a young woman with general symptoms of a diseased heart.
Both application of his hand to the chest and percussion offered little of diagnostic assistance. Laennec was reluctant to start immediate auscultation because of the age, sex, and embonpoint (plumpness) of the patient. In his moment of embarrassment, Laennec recalled the childrens wood borne signalling.
He rolled a paper cone and applied one end over the heart and the other to his ear and discovered that heart sounds were louder than immediate auscultation. Initially Laennec built paper instruments before settling on a hollow cylinder of wood, later named stethoscope from the Greek 32 replied. Seven taught use of the bell, 15 the diaphragm, and 10 the bell and/or diaphragm.
Fifty seven doctors working on respiratory wards were sent a questionnaire; 36 replied. Nine used the bell, 19 the diaphragm, and eight bell and/or diaphragm.Stethoscope propertiesThe output from the bell divided by the output from the diaphragm at each frequency is shown in fig 2.
Outputs were equal at the 400 Hz level.Download figureOpen in new tabDownload powerpointFigure 2 Performance of the stethoscope bell and stethoscope diaphragm as measured at both earpieces. The crossover point is about 400 Hz (the differences in plots derived from the two earpieces may indicate that the microphone and preamplifier combination serving one were relatively less sensitive at higher frequencies than that serving the other).
Selective hearingForty two volunteers each heard 8000 Hz and 250 Hz played at their auditory threshold when given correct prior warning of the pitch but 26 (62%) did not hear 250 Hz when played at their previously determined auditory threshold for that frequency when falsely advised that the tone would be of high pitch, and 18 (43%) did not hear a high tone when played at their previously determined auditory threshold for that frequency when falsely advised that the tone would be of low pitch.DISCUSSIONTeaching practiceAlthough our surveys were not comprehensive, they illustrate the marked contradictory diversity of teaching and use of the stethoscope bell and diaphragm. It is obvious that both teaching and use of stethoscope bells and diaphragms is non-uniform.
Stethoscope propertiesSince stethoscope bells provide louder output than diaphragms at the low frequencies associated with the main energy of respiratory breath sounds, it has been suggested that stethoscope bells outperform diaphragms.21 Stethoscope bells would perform better in the detection of normal breath sounds since the main energy is in the low frequency range. However, provided that breath sounds are audible in the first place, it is high frequencies and harmonics that are required for characterisation and localisation.
As low frequency sounds mask high frequency sounds, it makes sense to limit the low frequency sounds as much as possible, particularly when there is cardiac and muscular interference at precisely these low frequencies. Stethoscope diaphragms will detect but limit these low frequencies so that the high pitched sounds will be less masked. Use of the bell is thus superfluous.
The performance of the human ear is astonishing. How does the ear amplify sounds and analyse their frequency? The traditional view is the place code, which suggests that different frequencies enter the cochlea and cause particular regions of the basilar membrane to vibrate, with high notes causing vibration at the base, and low notes at the apex.
When the basilar membrane vibrates it causes deflection of stereocilia on top of sensory inner hair cells and stretches tip links that link stereocilia. Hair cells then send impulses to the brain where the frequency of the stimulus can be derived from the location of the hair cells firing most rapidly.22However, the place code is a rather coarse mechanism to account for such acuity (we can distinguish two tones that differ by a fraction of a percent when played successively,23 and it is now known that hair bundles contain motile systems, which generate oscillations at a particular frequency.
When one of these non-linear dynamical systems is on the verge of vibrating it is very sensitive to disturbances at frequencies close to its characteristic frequency. In other words the cochlea is thought to contain many voices, each of which is ready to sing along with any incoming sound which falls within its own range of pitch.23 Such an ability to set our inner hair cells to await stimulation would allow the ear to focus on particular pitches of sound.
The outer hair cells of the cochlea, with their own motor intervention (about 20% of auditory nerve fibres are efferent) may be involved in sensitivity and tuning of the inner hair cells. This would perhaps allow the ear to compensate for the poorer bell qualities in practice.Selective hearingWhy do some clinicians prefer to use the bell despite its poorer performance in practice?
The false expectation results that reveal selective listening probably explain why some clinicians advocate using the bell. They can compensate for inferior delivery from the stethoscope bell by choosing what they want to heartheir hair cell stereocilia were being tuned to a brink of oscillation for an expected incoming sound of particular frequency. They would be better advised to use this skill in enhancing the appreciation of input from the stethoscope diaphragm.
CONCLUSIONSThe teaching concerning the use of stethoscope bells and diaphragms is in some disarray. A student will learn to use stethoscope bell and/or diaphragm depending on which book they read, or which tutor they are taught by rather than which is technically optimal. The stethoscope bell could be used to detect breath sounds, but the diaphragm can detect normal breath sounds without enhancing lower pitched masking sounds and can also be used to characterise and more accurately localise both normal and abnormal breath sounds.
Use of the stethoscope bell for pulmonary auscultation is both superfluous and deleterious and should not be taught as the preferred mode of pulmonary auscultation. We are aware that this evidence based message may fall on deaf ears connected to closed minds.REFERENCESLaennec RTH.
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