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A Brief Overview of the SCHOLARLY PAPER

Analysis of Lung Sounds Alice Jones

Key Words Lung sounds, terminology, objective analysis.

Summery This aiticle provides a brief review of various methodologies adopted for the analysis of lung sounds. The speculated source of lung sound production and the confusion surrounding current sound description terminology are discussed. The aim of this article is to arouse physiotherapists’ interest in sound inter- pretation. as well as in research into this area.

Introduction An effective treatment programme relies on the identification of patient problems which in turn relies on a thorough and accurate assessment. Auscultation is one of the procedures commonly employed by physiotherapists and chest physicians in the investigation of a patient’s pulmonary disease. One is taught as an undergraduate that an audible sonic signal reflects the underlying pulmonary anatomy and pathophysiology. Lung sounds also provide crucial information in situations such as upper airway obstruction, misplaced tracheal tubes and spontaneous pneumothorax. In certain pathological processes such as obstructive airways disease and early interstitial pathology, lung sounds reflect more closely the disease status than radiological studies (Murphy, 1981). It has also been shown that the change in lung sound frequency may be used as an index of pulmonary congestion (Ploy-SongSang el al, 1989). However, at present, the analysis of lung sounds is still mostly qualitative and subjective, and the mechanism of lung sound production and the transmission of normal and abnormal lung sounds is still not well understood. Problems in lung sound analysis arise for the following reasons: (a) unlike heart sounds, which are uniform from beat to beat, lung sounds vary from breath to breath; (b) differing pathological processes may produce similar sounds; and (c) sophisticated instrumentation for processing the information is lacking.

Production and Transmission of Lung Sounds Studies of lung sound production were initiated as early as 1884, but the exact mechanism of lung sound production remains unknown. According to Murphy (1981), Bullar (1884) demonstrated that lung sounds were produced, not only by the glottis, but also throughout the respiratory tract, where air passes from a narrower to a wider space. Murphy also reprted that Martini and Miller (1923) showed that airways 3 mm in diameter or less took no part in the formation of bronchial sounds. In accord with Martini’s findings, Forgacs suggested that lung sound is caused by turbulence in the airstream. He supported his suggestion by a study which demonstrated that when a helium mixture was breathed by patients, the intensity of inspiratory sound was decreased (Forgacs et al, 1971). From his calculations, Forgacs suggested that flow velocity beyond the lobar or segmental bronchi is too low to generate turbulence. It has therefore been speculated that sound is produced (a) by the glottis; (b) by friction of gases against the airway walls all along the respiratory tract; and (c) as air passes from a narrower to a wider space during respiration. Gas flow is transformed into acoustic energy which is transmitted to the chest wall uiu airways or the lung parenchyma (which filters the high pitched sounds and attenuates sound of low frequency). Hannon and Lyman (1929) showed that ‘ventilation of the lung underlying the stethoscope’ was necessary to produce sound. LeBlanc and his colleagues (1970) showed that lung sound intensity varied in relation to the regional lung ventilation as demonstrated by radioactive gas distribution and speculated that breath sound intensity was a good indicator of regional pulmonary ventilation. However Ploy-Song-Sang and colleagues in 1977 showed no significant correlation between breath sound index and regional distribution of xenon’3J in ten subjects. These conflicting results may be explained by the varying lung volumes achieved during sound recording. LeBlanc’s subjects were asked to perform a series of vital capacity breaths a t different flows, while Ploy-Song-Sang’s subjects were instructed to breathe with a small tidal volume from functional residual capacity. At low lung volumes, inspired gas distributes first to the apex and then to the lung base in the upright position (Dollfuss et al, 1967), therefore ventilation seems better a t the apex. There are however insufficient data to permit further comment.

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Terminology of Lung Sounds properties of the airway and the gas. The Perceptual differences between observers and mechanical Properties Of the airway walls are the confusion in lung sound dexriptions have also thicknew density, bending resistance, longitudinal limited the progression of lung sound analysis. tension, elasticity of parenchymal supports, and L u g so~nds were first dewrim over 160 ye- ago structural damping, as Well as channel depth by Laennec who invented the first &&howope. (GavrielY et ak 1984; Grotberg and Davis, 1980; He pointed out that lung sounds were easier to Grotberg and h i s s , 1982, 1984). In general, distinguish than describe, and the difficulties in heavier Walls, decreased bending resistance and description have led to a plethora of terms: rales, elasticity, longitudinal tension and a narrower rhonchi, crepitations, wheezes, crackles, etc. channel Will lower the critical gas velocity (flutter According to a repod published by the American vel~%‘), and with a lower flutter velocity, wheezes College of Chest Physicians (ACcp-Am, 1975), are more likely to be generated. On the basis of Laennec used the term ‘rale’ as a generic term this theory one might expect that the decreased t0 include all abnormal sounds produced by elasticity Of Supportive tissue and the increased movement of air in the bronchi or pulmonary tissue. ‘Rale’ was translated into Latin as ‘rhonchus’, and later into English as ‘wheeze’. In the same report, the American Thoracic Society (Am) recommended ‘rale’ to be used for crackling or bubbling discontinuous sounds; and ‘rhonchus’ for continuous sounds. The term ‘crackles’ was accepted as an alternative for rales and ‘wheezes’, as an alternative to rhonchi. However, these terms 2 have been used interchangeably by chest x - physicians. In 1977, the ATS Ad Hoc Committee on Pulmonary Nomenclature recommended that illstead of the previously recommended term ‘rales’, the term ‘crackles’ should be used for describing discontinuous sounds of approximately 25 msec

are termed coarse and fine crackles respectively. 0 0.8 1.6 In the same report, the term ‘wheeze’ was recommended for high pitch continuous sounds; and ‘rhonchus’ for low pitch continuous sounds. Various surveys were conducted to identify the common lung sound terminologies used by practitioners in different countries. Wilkins and colleagues (1984) found that the British journals predominantly used the term ‘crackle’ as recommended by Am, but the term ‘rhonchus’ was infrequently used, and ‘wheeze’ was further sub- classified into high and low pitch continuous 3 sounds. A study by Pasterkamp et a1 (1987) and a survey conducted at the 1988 American College 5 of Chest Physicians annual convention (Wilkins et al, 1990) also found that the lung sound termin- ology used by physicians and health care professionals was not well standardised and the recommendations of A’IS/ACCP nomenclature sub- committee were not fully accepted.

Wheezes and Crackles 1 L I 0 0.5 1 It was suggested that wheezes are generated by the

vibration (or fluttering) of the bronchial wall as

- .

b

- a

duration. Law and high pitch discontinuous sounds 0 I ’I k

(a) Frequency (kHt)

(b) Frequency (kHz)

as *Owing gas (Forgacs’ 1967; Grotberg and Fig 1: Spectral analysis of wheezes in asthmatic and now l980). According to the flutter theowl asthmatic patients. (a) Multiple peak of power at ditferent oscillation of the airway o c c u ~ when the gas frequencies - correspond to polyphonic wheeze. (b) Peaks velocity reaches a critical value which is of W e r m W r i m W at the =me frequency - correspond to monophonic wheeze. Modified from Gavriely eta/ (1984) and determined by the dimensions and physical reproduced permission

Phy.ioth.trpy, January tW5, vol81. no 1

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thickness of oedematous inflamed bronchial walls may all contribute to the lowering of flutter velocity and thus offer an explanation for the prevalence of wheezing in bronchitic and asthmatic patients (Gavriely et al, 1984). In the same paper, Gavriely et a1 reported their analysis of ten patients with audible wheezes using chest auscultation. Seven of these patients were asthmatic and three had upper airway obstruction (non-asthmatic origin). Spectral analysis (see below) of the lung sounds of asthmatic patients revealed sharp peaks of power at different frequencies, whereas in non-asthmatic patients, peaks of power were superimposed at the same frequency (fig 1). Coincidentally, these findings appear to match the clinical terms ‘polyphonic wheeze’ (a term described as having a complex musical character and regarded as a reliable sign of widespread airflow obstruction) and ‘monophonic wheeze’ (described as a single musical note of constant pitch and normally a sign of incomplete occlusion of a principal or lobar bronchus) (Forgacs, 1978). Crackles are short explosive sounds believed to be generated either by gas bubbling through fluid films, or with equalisation of pressure in the airway, when the airway abruptly opens. The timing of airway opening is believed to be associated with changes in transpulmonary pressure (Nath and Capel, 1974). Crackles (rales) were described as early as 1925, by Cabot and Dodge, as consisting of coarse and fine characteristics which were associated with low and high frequency components, respectively. In the past, studying crackles was difficult because the crackling sound is so rapid and neither an individual crackle, nor a rhythmical pattern of crackles, could be identified with the unaided ear. Nowadays, the number of crackles can be counted by computer (Murphy et al, 1989) and an individual crackle can be analysed by both waveform and spectral analysis.

Objective Analysis of Lung Sounds Analysis of lung sounds has always been subjective. The clinician assumes that the pattern of lung sound intensity in normal lungs is bi-laterally symmetrical, but phonopneumographic measure- ment on opposite sides of the chest has demon- strated a considerable difference in sound intensity between the left and right lung (O’Donnell and Kraman, 1982; Dosani and Kraman, 1983). Figure 2 displays a ‘flaw-corrected phonopneumograph’ of a patient studied by Dosani and Kraman (1983). In their study, simultaneous recording of lung sounds was undertaken by two identical microphones placed on corresponding positions on the left and right chest wall. The recorded data were analysed by a computer, which was programmed to measure the mean amplitude

Fig 2: Phonopneurnogreph showing lung sound Intensity recorded at corresponding point of left and rlght chest wall of patient with normal lungs. (Y-axb: mnplihrdq X u k : poinb In horizontal l ~ w ~ l ot chest wall, taxb: polnta In wtkal rom of chest wail.) Modified from Dosani and Kraman (1983), reproduced with permission

of each 25 msec segment of the recorded sound. Their study revealed an unexpectedly high degree of variation in the pattern of lung sound intensity in normal lungs, not only between subjects but also in the left and right lungs of the same individual. Absence of pulmonary pathology was confirmed by bronchoscopy and radioactive xenon distrib- ution. Phonopneumography is basically a two- dimensional mapping of amplitude (intensity) uersus horizontal and/or vertical position on the chest wall (Kraman, 1980). With computer digitised airway phonopneumography, Tinkelman and colleagues were able to quantifl changes in airway obstruction in children and adult patients and suggested this method would be useful to detect airway obstruction in patients who were too young or too distressed to perform pulmonary function testing (Tinkelman et al, 1991). Advances in technology now permit more sophisticated computer based quantitative analysis of lung sounds. Other objective methods of lung sound analysis include waveform (timedomain) and frequency spectral analysis. %pe recordings of lung sounds stored in a computer memory can be subjected to waveform analysis and then visually displayed on a two-dimensional plot, with amplitude or intensity on the Y-axis and time on the X-axis. To isolate and analyse a single crackle, time-expanded waveform analysis is employed. The recording time is expanded or magnified by playing back slowly (normally one-eighth of the original recorded tape speed) from the computer’s memory. Lung sounds are usually analysed at a sampling frequency of 2 kHz (equivalent to 16 kHz real time)

Conventional waveform Expanded waveform

+---

Normal vesicular inspiratory sound - lnspiratory crackles

Low pitched wheezes

High pitched wheezes

Normal tracheal inspiratory sound

Fig 3: Amplltude-time plots at conventional speed and time expanded scales. Modified from Murphy eta/ (1977), reproduced with permission

for a time interval of 0.5 to 1 sec (equivalent to 62.5 or 125 ms real time) (Nath and Capel, 1974; Weiss and Carlson, 1972; Murphy et al, 1977; Mori et al, 1980). Figure 3 shows the amplitude-time plots at conventional speed and time-expanded scales. This quantitative method of investigation shows that ‘continuous’ lung sounds last for 250 msec or more, whereas ‘discontinuous’ sounds only last for 25 msec or less. Different characteristics and patterns of wheezes and crackles can be visually displayed. For example, high-pitched wheezes have a greater number of deflections per unit time compared to low-pitched wheezes. Spectral analysis involves the sound signals being amplified and recorded on an FM tape recorder (recording is performed with simultaneous measurement of gas flow using a calibrated pneumotachograph, to ensure consistent lung volume). During playback, the sound signals are filtered and then played through an analog-digital converter. The data are then divided into segments of 0.125-second duration and computer-processed. Each segment is submitted to frequency analysis by fast Fourier transformation (FFT) as described by Murphy and Sorensen (1973) (calculation as described by Cooley and Tukey (1965)). The processed data are then displayed as a three- dimensional plot; amplitude-frequency-time, to allow an overview of spectrum over time (fig 4).

10

8

6 3 E

E 4 i =

* a,

2

0 0 5 10 15 20

Frequency (kHz)

Fig 4: Spectral analysis of lung sounds displaying in amplitude-trequency-tlme axis. This diagram shows continuous sound at frequency range of 3 to 7 kHz and discontinuous sound of higher frequency at 2 ms and again at 7-10 ms. Reproduced with permission, Goncharoff et a/ (1989)

Pasterkamp and colleagues presented their analysis on a colour scale (1989) and were able to display the cardiac sounds visually by simultaneous recording of the electrocardiograph (ECG). Spectral analysis confirmed that a high- pitched wheeze had peaks of energy power at higher frequencies than a low-pitched wheeze. Using similar methods, Hidalgo et aZ(1991) showed that the frequency content of normal breath sounds in children were of a higher frequency compared to adults, explaining in part the difference in lung sound characteristics between children and adults during clinical auscultation. Spectral analysis also allowed quantitative evaluation of the filtering properties of the normal and consolidated lung (Baughman and Loudon, 1986) and confirmed the general belief that the consolidated lung allowed the transmission of higher frequency sound compared to the normal lung.

In general, time-domain (waveform) analysis seems to be more widely applied to the study of discontinuous sounds, whereas frequency domain seems to be more commonly used for continuous sound analysis (Loudon and Murphy, 1984).

Mori et a1 (1980) demonstrated that most of the frequency components of a crackle were limited within a range of 0.1 to 1 kHz and each crackle was composed of two segments - an initial part, followed by a decay segment (fig 5). They suggested that the initial segment is caused by a shock wave induced by the abrupt opening of the airway. The shock wave subsequently excites a resonator in the

PhYalotherclpY. January 1995, vol81, no 1

lung causing the ‘damped sinusoid‘ decay segment. The decay segment is determined by the resonant frequency and the quality factor of the resonator. The authors hypothesised that the starting segment is determined by the pressure ratio a t the site of the airway opening, and that this pressure ratio may rise with an increase in either the stiffness of the lung, or the viscosity of mucus obstructing the airway. Wave-form analysis shows that crackles heard in patients with different pulmonary conditions have different distinct features. Crackles in patients with pulmonary fibrosis occur for shorter periods in the respiratory cycle compared to the coarse crackles heard in patients with pneumonia (Murphy et al, 1977). Patients with chronic obstructive pulmonary diseases were shown to have a shorter crackling duration than patients with alveolitis or bronchiectasis (Piirila et al, 1991). An explanation offered for this phenomenon is that in pulmonary fibrosis, the lungs are stiff and therefore the pressure ratio is high, thus leading to a short initial segment with a high amplitude (Mori et al, 1980).

Discussion Appropriate clinical judgement comes with education and experience. A good physiotherapist is able to deliver effective treatment skills by adopting a logical process of assessment, analysis, planning, intervention and evaluation. However physical examination of the pulmonary system does not always accurately reveal a patient’s pathophysiology. Monitoring devices which measure a patient’s physiological status are invaluable. Objective analysis of lung sounds was initiated over twenty years ago; however, research in this area has advanced only recently with the development of condenser microphones, minimal distortion amplification, accurate frequency filtering and availability of personal computers with software which can process large volumes of data. Crackles can now be counted by computer (Murphy et al, 1989) and patterns of wheezes and crackles with varying frequencies and duration in different disease states can be analysed quantitatively. However, the varying characteristics of the different

Fig 5 The initial (a,b,c,d) and decay (1,2,3,4) segments of a single crackle. Modified from Mori eta/ (1980), reproduced with permission

systems used for data recording and processing make interpretation of the literature difficult. There are still many unanswered basic questions, let alone clinical conundrums, such as: OHOW does the pattern of crackles in bronchitis differ from that in lung fibrosis? .Do secretions cause the crackling sound? If so, how?

0 Why are the crackles associated with asbestosis numerous whereas those associated with radio- graphically confirmed pulmonary sarcoidosis of equivalent severity are relatively few? Recent advances in computer technology even permit sound analysis on the home personal computer using the Sound BlasterTM card or signal analysis software such as Labview for WindowsTM. Theoretically, a computer neural network should be able to analyse large samples of lung sound data and define sound patterns, characterising the underlying pulmonary pathology.

Conclusion The stethoscope has not evolved since it was first developed more than 160 years ago. If the production of crackles are better understood and definite patterns of adventitious sounds associated with various pathology are clearly identified, a computerised, diagnostic stethoscope could become a reality early next century. The computer could classify the sound pattern and link this to a possible differential diagnosis for consideration by a physiotherapist. Furthermore, by storing the previous examination in the computer’s memory, the computer could compare sound pattern, quality m d intensity as a means of assessment of chest physiotherapy treatment. However, to achieve this goal, more detailed quantitative and qualitative measurements of lung sound characteristics are first required.

Author and Address for Correspondence Mrs Alice Jones MPhi/(Phty) MSc(Ed) is a university lecturer in the physiotherapy section, Depattmenl of Rehabilitation Sciences, Hong Kong Polytechnic University, Hung Hom. Kowloon, Hong Kong.

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