C.R. Abbruscato, V.P. of Engineering, American TeleCare, Inc. Eden Prairie, MN
telecare@mn.uswest.net; www.telemed-care.com

The stethoscope was invented in the early 1800’s by Frenchman RenÈ LaÎnnec as a solution to a delicate problem. Hitherto, physicians listened to heart sounds by pressing their ears to the patient’s chest. The young Dr. LaÎnnec was examining a young woman with symptoms of heart disease. As he reported, "The patient’s age and sex did not permit direct application of the ear to the chest." Being desperate, resourceful…and a flutist… he rolled up a sheaf of paper to form a tube. Pressing this instead of his ear against her chest (see photo), he not only preserved her modesty but "was surprised and gratified at being able to hear the beating of the heart with much greater clearness and distinctness than…ever before."

The new invention was not quickly adopted. Commentary in the London Times of 1834 noted, "That it [the stethoscope] will ever come into general use…is extremely doubtful because its beneficial application requires much time and gives a good bit of trouble…There is something even ludicrous in the picture of a gray physician proudly listening through a long tube applied to the patient’s thorax." Many wooden and rubber models were tried, but none caught on until 1852, when Dr. George Cammann of New York designed a binaural stethoscope which not only transmitted more sound and blocked out extraneous sounds, but was also the first to resemble today’s instruments.

Early stethoscopes had only a bell chest piece. This excelled at picking up low frequency sounds, but missed many at higher frequencies. Several diaphragms were invented to address higher frequencies; the first efficient bell/diaphragm combination with a revolving stem was designed in Boston by Dr. Howard Sprague, in 1926. Acoustic stethoscope design culminated with the lightweight Littman model introduced in 1961, and still the mainstay of physicians and nurses.

Acoustic stethoscopes are, of course, limited in their ability to transmit and amplify sound. This problem was addressed in 1910 by S.G. Brown in London. Mr. Brown was trying to overcome a serious impediment to long distance telephony: telephone signals degraded rapidly, limiting voice transmission to about 20 miles. He invented a repeater, amplifier, and receivers that would allow transmission to 50 miles and more. To demonstrate their capabilities, he developed the first "Electrical Stethoscope and Telephone Relay" (see photo), which he found could be "…tuned so that…the passage of air through the lungs was heard as the roar of the wind through a forest of trees." He noted further that "…for the sake of experiment, the sound of the heart has been transmitted over several miles of telephone line to doctors in various parts of London and…(a)ll of them reported that the sounds received in the telephone were as good and clear as when heard locally." He concluded, "This trial proved that it is now possible for a specialist, say, in London, to examine a patient, say, in the country, stethoscopically, and to arrive at a correct diagnosis." And so was the first telestethoscope born.

The concept languished for over fifty years. There were scattered reports in the 1970’s of it use (see References), including an efficacy report by pioneers Murphy and Bird at Harvard’s original telemedicine project. More definitive academic assessment of tele-auscultation came when Mattioli and his team began publishing their series of studies in 1992. These documented that the technology then current enabled overall screening accuracy of 89.5% compared to the on-site acoustic standard.

Converting acoustic auscultation sounds to digital electric signals presents real challenges. [See sidebar: Electronic Stethoscope Technology]. To assure adequate fidelity in digital processing, with a sampling rate of 8,000 sample/second and each sample with 12-bit resolution (the ability to represent one of 4,096 ‘shades’ of sound for each sample), would require a digital stream of 96,000 bits/second (96 Kbps). Unfortunately, the fastest modems today run at about 33.6 Kbps. Sophisticated engineering and new algorithms are increasing the ability to transmit diagnostic quality sounds over regular phone lines.

References

Brown SG. A telephone relay. J of the Institution of Electrical Engineers. pp. 590-619; May 5, 1910.

Vaules DW. Auscultation by telephone (letter). NEJM 283:880-881, 1970.

Murphy RLH, Block P, Bird KT, Yurchak P. Accuracy of cardiac auscultation by microwave. Chest 63:578-581, 1973.

Mattioli LF, Goertz KK, Ardinger RH, Belmont JM, Cox R, Thomas CM. Pediatric cardiology: auscultation from 280 miles away. Kansas Medicine 93:326, 347-350.

Belmont JM, Mattioli LF, Goertz KK, Ardinger RH, Thomas CM. Evaluation of remote stethoscopy for pediatric telecardiology. Telemedicine Journal 1:133-149, 1995. [Includes a definitive bibliography of telestethoscopy – Ed.]