Eur Respir J, 1995, 8, 1761–1769 Copyright ERS Journals Ltd 1995 DOI: 10.1183/09031936.95.08101761 European Respiratory Journal Printed in UK - all rights reserved ISSN 0903 - 1936 SERIES 'CHEST PHYSICAL EXAMINATION' Edited by J.C. Yernault Acoustic properties of the normal chest F. Dalmay, M.T. Antonini, P. Marquet, R. Menier Acoustic properties of the normal chest. F. Dalmay, M.T. Antonini, P. Marquet, R. Laboratoire de Physiologie, Faculté de Menier. ERS Journals Ltd 1995. Médecine, Limoges, France. ABSTRACT: Laënnec invented the stethoscope in 1816 and published a treatise on auscultation in 1819. We then had to wait until the 1950s to observe develop- Correspondence: R. Menier ment of modern devices and methods of recording and signal-processing, which CHRU Dupuytren allowed objective studies of lung sounds in time and frequency fields. Physiologie et Exploration Fonctionnelle 2, rue Martin Luther-King Tracheobronchial sounds generated by ventilation originate in the upper airways, 87042 Limoges Cedex the frequency content of these sounds has led to extensive research. Consolidated France lungs act as more efficient sound conductors to the chest wall (bronchial breathing murmur). Tracheobronchial sounds contain higher frequency components com- Keywords: Normal lung sounds pared to vesicular lung sounds. sensors The origin of vesicular lung sounds has been becoming progressively clear for signal-processing about 10 yrs. It is at least partly produced locally, deep, and probably intralobu- tape-recording lar. Clearly, further studies need to be performed in order to elucidate the true tracheobronchial sounds mechanisms involved in generating vesicular lung sounds, the redistribution of intra- vesicular lung sounds pulmonary gas or vibrations caused by the stretching of lung tissue. Received: June 10 1994 The devices developed are already useful for monitoring the state of patients in Accepted after revision March 31 1995 intensive care. Sooner or later, real time analysis and automated diagnosis will become available. Eur Respir J., 1995, 8, 1761–1769. Laënnec invented the stethoscope in 1816 and pub- of the lung hoping to discover new diagnostic applica- lished his treatise on auscultation in 1819, describing tions for auscultation. acoustic events generated by ventilation of the lungs and In fact, pulmonary auscultation is widely available at systematically correlating them with anatomical and patho- the bedside; it is cheap, quick, easy to carry out and to logical findings after autopsy. "Auscultation of breath- repeat, noninvasive and totally innocuous. This is of ing sounds with a cylinder (stethoscope), produces easily particular advantage when dealing with the newborn and interpreted auditory signals capable of indicating the pres- young infants as well as the old-aged, mental patients ence and extent of most disorders of organs in the tho- and unconscious subjects, for all of whom conventional racic cage...". Auscultation of healthy individuals enables respiratory function tests can prove difficult or impossi- "ventilatory murmur" (i.e. vesicular breath sounds) to be ble. heard, and Laënnec goes on to describe a range of abnor- Since the 1950s, techniques and equipment have been mal sounds associated with different pathological states. developed that improve the detection and analysis of res- This form of semiology, however, is not without a num- piratory sounds. This has opened the door to objective ber of drawbacks; it lacks sufficient sensitivity, offers no studies of such signals [2, 3], facilitating precise topo- quantitative measurement of the sounds detected, is essen- graphical localization of lung sounds and clarification of tially a subjective clinical test, and is hindered by an their aetiology. It therefore seems likely that, sooner or imperfect system of nomenclature. However, subsequent later, real time analysis and automated diagnosis should development of X-ray radiography was so far reaching become available and that the first to benefit from this that FORGACS [1] was able to comment in 1969 that "the type of monitoring will probably be patients on inten- success of radiography in revealing structural changes in sive care wards. the lungs had a profound effect on the clinical approach to diseases of the chest. It turned the interest of physi- cians towards morphology at the expense of function and Historical summary led to the decline in the status of auscultation of the chest from its key position to a perfunctory ritual". Twenty Direct auscultation was already known to Hippocrates, five years later, the question remains of why anyone who advised application of the ear to the patient's chest should wish to further investigate the acoustic properties in order to hear sounds transmitted through the chest 1762 F. DALMAY ET AL. wall. However, by the time Laënnec first started prac- tubing with poor transmission properties which conduct tising medicine, at the beginning of the 19th century, the sound directly to the ears [7]. The resulting overall sen- technique had practically fallen out of favour and, indeed, sitivity is consequently low with an uneven frequency the very idea seems to have revolted him. Inspiration response, falling off by 3–6 dB per octave above 100 for his invention came one day, when he noticed two Hz. "A good quality stethoscope should have a flat res- children sending messages to one another along a woo- ponse curve with less than ±3 dB variation between 50 den stick, alternately scraping one end of the stick and and 1,200 Hz and should provide high sensitivity over then pressing it to an ear to hear the reply [4]. Laënnec this range" [7]. proceeded to experiment with various wooden cylinders and rods, finding to his satisfaction that a solid rod placed between his ear and the patient's chest significantly Modern methods of recording and signal-processing improved sound transmission, and that a rod pierced with a narrow bore was an even more effective sound con- "Lung sounds result from the vibrations within the lung ductor. and its airways that are transmitted to the chest wall. Clinical examination of pulmonary function was divid- Vibration amplitude may be less than 10 µm and is affect- ed at that time into four separate stages: questioning (i.e. ed by the method by which it is determined or trans- taking the patient's history), inspection, palpation and duced" [8]. percussion. Direct percussion had only been introduced Hannon and Lyman (see [8]) were probably the first shortly before, palpation serving to evaluate transmission to use a mechanico-electrical transducer to detect lung of vocalized vibrations from the trachea through to the sounds. Their method involved recording with a micro- chest wall. phone linked through filters to a string oscilloscope and, Laënnec initially sought to use his cylinder stethoscope thence, to a graphic recording device. to facilitate detection of whispered sounds transmitted to In 1955, MCKUSICK et al. [9] employed condenser-type the chest wall. He found that, in healthy subjects, whis- microphones and recorded the resulting electrical signals pering produces a sound of low intensity accompanied on a magnetic disc, which could be read several times by a slight distortion of the voice, being most audible successively, each time scanning at a different freq- over the axillae and scapulovertebral regions, as well as uency with a variable filter. The corresponding signal opposite sternoclavicular articulations. strength of each frequency band was then translated into He also noted that pleural effusion and pulmonary con- a proportionate level of light intensity, and the data thus solidation prevented the passage of sound waves, where- recorded on light-sensitive paper. as transmission increased when he placed his "cylinder" Most subsequent studies followed a similar general over tuberculosis cavitations or areas of underlying bronc- pattern of steps: detection, preliminary processing, record- hiectasis. Major cavitations resulted in identical voca- ing, and final processing of recorded signals. Equipment lized sounds (pectoriloquy) heard through the chest wall and technique, however, varied widely from one study and through the larynx. High-pitched sounds travelled to another and this diversity prevents any meaningful best. Laënnec associated one particular type of sound, comparison of results. egophony (a noise resembling the bleating of a goat) "Electronic recording and computer analysis of respi- with limited liquid pleural effusions. ratory sounds from the chest and trachea of normal sub- He next turned his attention to the study of "sponta- jects and patients with respiratory disease, has, over the neous" open-mouth breathing, to further develop his tech- past decade or so, become a major preoccupation of many nique for ''auscultation of breathing sounds with the aid clinical and engineering teams" [10]. of the cylinder". Healthy subjects generated vesicular The principal objective of such research is to incre- lung sounds, the "murmur of respiration", whilst patients ase understanding of the fundamental mechanisms that with pathological conditions produced adventitious lung cause breathing sounds [11–17]. The second objective sounds" ...sufficiently distinctive to permit identification is to evolve a system of classification that will enable of most organic disorders of the chest". automated processing of normal and pathological lung It was not long before several models of stethoscopes sounds. In this way, early diagnosis of pathological