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Lung Sound Intensity Variability in Normal Men
Lung Sound Intensity Variability In Normal Men* A Contour Phonopneumographic Study R. Dosani, M.D., F.C.C.P.;t and S. S. Kraman, M.D., F.C.C.P.:j:
•
We describe and demonstrate a new lung sound measurement and display technique by which contour maps of lung sound intensity may be constructed. Using this technique, we have studied the lower posterolateral chest wall of ten healthy subjects. The data revealed an unexpectedly high
degree of intersubject and intrasubject variation. The reason for this variation is obscure but probably relates to the basic mechanisms of production and transmission of lung sounds.
Jt has been more than 160 years since Laennec invented the stethoscope and described lung sounds in detail. 1 Until recently, due to its subjective nature, the study oflung sounds has remained more of an art or practice than a science. In recent years, however, electronic methods of recording and displaying acoustic events have brought a degree of objectivity to the study oflung sounds. 2•3 It is generally believed that lung sound intensity is relatively uniform in normal subjects. 2 It is also customary to compare the lung sound intensity of one area over the chest wall with that of the comparable area on the opposite side,• presuming the lung sound amplitude pattern to be bilaterally symmetric. In a previous study, O'Donnell and Kraman~ demonstrated that amplitude differences of considerable magnitude may commonly exist between lung sounds recorded from opposite sides of the chest wall. They also showed that the distribution of lung sound intensity on the chest wall was subject to notable variation. That study made use of a two-dimensional (amplitude vs horizontal or vertical position) mapping arrangement, which permitted lung sound analysis over a relatively limited portion of the chest wall and so failed to reveal in detail the full extent of the size of the amplitude variations that were detected. To examine more closely and define the normal lung sound amplitude variations, we recently developed a new technique, which we call flow-corrected phonopneumography, which permits rapid and accurate determination oflung sound amplitude. In the present study, this technique was used to demonstrate graph-
ically the sound patterns at the posterolateral lung bases of ten normal subjects to determine the size and distribution of lung sound intensity variations.
*From the Department of Medicine, Section of Pulmonary Medicine, Veterans Administration Medical Center, and Division of Pulmonary Medicine, University of Kentucky Medical Center, Lexington. tCiinical Instructor. :!:Assistant Professor. Dr. Kraman is supported by NHLBI grant 5ROl-HL26234-02. Manuscript received July 6; revision accepted October 21. Reprint requests: Dr. Krarnan, VA Medical Center(lll-H ), Lexington 40511
628
MATERIALS AND METHODS
Ten male nonsmokers (aged 24 to 48 years) with no history oflung disease were studied. All subjects had normal spirometric values. 6 The right and left lower posterolateral chest walls were marked with an 18 em X 18 em grid, each consisting oflOO points (10 rows of 10 points each). The most medial vertical row was 2 em from the spine, and the lowest horizontal row at the most dependent part of the chest where lung sounds could be heard. Each row was separated from that adjacent by 2 em. The subjects were instructed to breathe deeply from functional residual capacity (FRC) through a Fleisch pneumotachograph and pressure transducer amplifier (HewlettPackard 21078 and 473304A) to provide airflow (V) signals during the recording of the lung sounds. They were asked to achieve a peak airRow of approximately 2.5 Us as displayed on an oscillosmpe screen. Two identical condenser microphones fitted with chest pieces of 14 mm diameter, 9 mm depth were used for the remrding of lung sounds. Identical free field microphone characteristics were confirmed by mmparing the output of the two microphones held side by side in front of a loudspeaker driven by a sweep generator at 50 to 1, 000 Hz. Since the chest pieces were identical, it is assumed that the characteristics would remain identical when applied to the chest wall. The lung sound recording and processing equipment has been described previously.$ The right and left microphones were used simultaneously. They were placed precisely on the chest wall with the upper margin of the chest piece touching each of the 100 points on each hemithorax in succession while the subject breathed as described previously. The horizontal rows were studied left to right, bottom to top, until one inspiratory lung sound was recorded at each point. To determine the relative lung sound intensity, the computer program measured the mean amplitude of each 25 ms segment of sound recorded while Vwas above 1.3 Us. This in effect generated a sound amplitude envelope that, if not disturbed by artifacts, would roughly follow V. The amplitude of each 25 ms sound segment was divided by the value of the simultaneously determined airflow. The result was a Vcorrected lung sound amplitude index (Ale) of each segment expressed in units of volts/Us. The threshold \• of 1.3 Us was chosen because at\' <1.3 Us, the relation between Vand lung sound intensity was not always linear. For every breath taken by the subject, the Ale of all 25 ms segments (roughly 12 per breath while V > 1.3 Us) were averaged to yield the mean Ale (Ale). This was the figure used as the amplitude of Variability of Lung Sound
Intensity (Dosani,
Kraman)
HOHCIWNLIAIII2HOH DATE: 1/7112
US: AlA
llao
AI (1)
v
AI (2)
v
"'- V·,~~
~~-~L,
0.15
0.3 TIME ISECI
I
o.•5
I
0.8
FIGURE 1. Single inspiration with microphones at the left (AI(1)) and right (AI(2)) posterior lung bases. The lung sound envelope is produced as described in the text. The airflow corrected lung sound amplitude (AI(1)lV and AI(2)lV) tends to remain linear once an inspiratory flow (V) of ± 1 Us is achieved. This demonstrates the lack of dependence of lung sound amplitude on lung volume when breathing is from FRC. The sharp peaks in sound intensity are due to random variations in lung sound intensity.
HOR·'
FIGURE 2. Lung sound amplitude contour maps of subject SKat two different times. The left and right microphones were reversed for the second trial to ensure equal microphone characteristics. While there are small differences in the two maps, the overall amplitude patterns are strikingly similar.
nied by the expiratory contour maps for comparison. In most subjects, expiration was too quiet to allow adequate recording. It can be seen that the expiratory sound patterns are not just low-intensity copies of the
the lung sound at that location. When breath sound intensity from a single point on the chest wall was analyzed repeatedly by this technique, the coefficient of variation was 10 percent. This is much improved compared to our previous technique of How-gated analysis, which required the averaging of three breath sounds at each site to yield a similar coefficient of variation. • Once the Ale at each of the 100 positions on the left and right lung bases was determined, the computer program assembled the data into a three-dimensional contour map.
'""'
RESULTS
Figure 1 shows the V corrected lung sound amplitude envelope of a single inspiration recorded simultaneously from the right and left lung bases demonstrating that at V above approximately 1 Us, the sound amplitude is determined principally by V. Figure 2 shows the bilateral contour maps of one subject perlormed twice, one hour apart. For the second trial, the left and right microphones were reversed to ensure that microphone inequality was not responsible for left-right differences in amplitude. The reproducibility of the technique is apparent. Figure 3 shows the inspiratory contour maps of one of the subjects with the loudest lung sounds, accompa-
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AMP--V·E~T HOR·' FIGURE 3. Inspiratory and expiratory contour maps of one of the subjects with the loudest lung sounds. All expiratory sounds were multiplied X 3 to allow better visualization of the contour. Differences in the general amplitude patterns apparently reflect different determinants of sound production and/or transmission in expiration. The few individual points that are much greater in amplitude than those adjacent probably represent artifacts.
CHEST I 83 I 4 I April, 1983
629
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FIGURE 4, Bilateral inspiratory contour maps for the ten subjects. For purposes of better revealing the contour, the amplitudes of some of the maps are increased by the factor marked "X."
inspiratory patterns, attesting to basic differences in their mechanism of production and/or of transmission. Figure 4 shows the inspiratory contour maps of all ten subjects and makes the following points: 1. There is marked intersubject variation in lung sound intensity. 2. There is a great degree of point-to-point variation (multiple peaks and valleys) in some subjects. 3. The amplitude patterns are not bilaterally symmetric in many areas in most of the subjects. 4. There is consistently decreased amplitude of lung sounds in the area of the scapulae. 5. There are no apparent amplitude variations due to the presence of the ribs. DISCUSSION
It is generally assumed that lung sound intensity is bilaterally symmetric, 4 and therefore the intensity of lung sounds at a given area is usually compared with that at the homologous area on the opposite side when 630
the chest is examined. Wooten et aF compared the breath sound intensity recorded over a segment of the left lung with that of a simultaneously recorded sound over an homologous segment of the right lung and found that the amplitudes were dissimilar when there was an obstruction in one of the segments, but they became similar when the obstruction was removed. The authors concluded that the comparison of simultaneously recorded amplitudes over homologous segments could be used to detect regional obstruction. Analysis of lung sounds by LeBlanc et aP showed that the amplitude of breath sounds was affected by lung volume, flow, and body position, with airflow being the major determinant (when breathing was not near residual volume). The authors compared their lung sound amplitude data with the ventilation patterns obtained by other authors using radioactive gas techniques and concluded that breath sound intensity was proportional to the regional ventilation of the variability of Lung Sound Intensity (Dosani, Kraman)
underlying lung. Ploy Song Sang and colleagues"·9 extended the work of LeBlanc et al to construct two-dimensional maps of lung sound intensity at the anterior chest wall. They compared these with the intensity of white noise introduced at the mouth to attempt to separate sound generation from transmission effects. All of these studies relied on the measurement of lung sounds at a relatively small number of locations (no more than four per hemithorax) so that the distribution of sound on the chest wall was revealed in very little detail. Significant intersubject and intrasubject variations were seen in the amplitude contour maps produced in the present study. Marked asymmetry was observed in comparable areas of opposite sides of the chest in the absence of any evidence of airways obstruction or of other lung disease. Most previous comparable lung sound studies have been performed with the assumption that the lung sound amplitude recorded at a given point on the chest wall generally represented ventilation in the underlying area. By contrast, the contour maps of the present study showed marked variation in some of the adjacent points that were separated by as little as 2 em. We do not have any definitive explanation for the lung sound amplitude variations observed in the present study. It is likely that lung sound intensity varies with some undefined factors beyond those already known, namely: airflow, lung volume, regional ventilation, and body position. These undefined factors appear to be related to the site of production of the sound and its transmission path to the chest wall. The chest wall thickness does not seem to have a predominant effect on the intensity oflung sounds, since some of our maps revealed low or equal intensity at the lateral chest wall compared to positions near the spine, where the thickness of the chest wall is much greater. One could argue that the variations observed in this study are related to variations in lung volume at the time of recording, as we did not actually measure the lung volume. This is less likely to be the case, since at and above functional residual capacity, lung volume seems to have little effect on the amplitude of lung sounds at the lung bases. 3 It is probable that much of the intrasubject variation would have been attenuated had we used larger-sized chest pieces, which would have averaged the ampli-
tudes of two to three adjacent points. This remains an open question. The finding of different intensity patterns between inspiration and expiration is not very surprising, since theoretical considerations have long predicted different sites of generation of these two sounds. 10 At least one recent study has produced evidence in support of this prediction. 11 In conclusion, we have developed a technique by which airflow corrected lung sound amplitude can be determined in great detail over a large area of the chest wall in a relatively short period of time. These data can then be displayed as a three-dimensional contour map. Analysis of these maps shows that there is a marked intersubject and intrasubject variation and absence of bilateral symmetry in lung sound amplitude patterns in normal subjects. Lung sound intensity does not appear to be closely related to chest wall thickness and the presence of the ribs, but is decreased in the areas over the scapulae. ACKNOWLEDGMENT: The authors thank Dr. Stanlev Rehm for his helpful review of this manuscript and Laurence Kojan Ong for the development of the computer programs.
REFERENCES 1 Sakula A. RTH Laennec 1781-1826. His life and work: a hit:entenary appreciation. Thorax 1981; 36:81-90 2 Murphy RLH, Holford SK. Lung sounds. Basics RD 1980; 8:1-6 3 LeBlanc P, Macklem PT. Ross WRD. Breath sounds and distribution of pulmonary ventilation. Am Rev Respir Dis 1970; 102:10-16 4 Hinshaw HC, Murray JF. Diseases of the chest. Philadelphia: WB Saunders Co, 1980:17-24 5 O'Donnell D, Kraman SS. Vesicular lung sound and amplitude mapping by Row-gated phonopneumography. J Appl Physiol 1982; 53(3):603-609 6 Morris JS, Koski A, Johnson LC. Spirometric standards for healthy, non-smoking adults. Am Rev Respir Dis 1971; 103:57-67 7 Wooten FT, Waring WW, Wegmann MJ, Anderson WF, Conley JD. Method for respiratory sound analysis. Med Instrum 1978; 12:254-257 8 Ploy Song Sang Y, Martin RR, Ross WRD, Loudon RG, Macklem PT. Breath sounds and regional ventilation. Am Rev Respir Dis 1977; 116:187-199 9 Ploy Song Sang Y, Macklem PT. Ross WRD. Distribution of regional ventilation measured by breath sounds. Am Rev Respir Dis 1978; 117:657-664 10 Farr G. The acoustics of the bronchial breath sounds. Arch Intern Med 1927; 39:286-302 11 Kraman SS. Determination of the site of production of lung sounds by subtraction phonopneumography. Am Rev Respir Dis 1980; 122:303-309
CHEST I 83 I 4 I April, 1983
631
•
We describe and demonstrate a new lung sound measurement and display technique by which contour maps of lung sound intensity may be constructed. Using this technique, we have studied the lower posterolateral chest wall of ten healthy subjects. The data revealed an unexpectedly high
degree of intersubject and intrasubject variation. The reason for this variation is obscure but probably relates to the basic mechanisms of production and transmission of lung sounds.
Jt has been more than 160 years since Laennec invented the stethoscope and described lung sounds in detail. 1 Until recently, due to its subjective nature, the study oflung sounds has remained more of an art or practice than a science. In recent years, however, electronic methods of recording and displaying acoustic events have brought a degree of objectivity to the study oflung sounds. 2•3 It is generally believed that lung sound intensity is relatively uniform in normal subjects. 2 It is also customary to compare the lung sound intensity of one area over the chest wall with that of the comparable area on the opposite side,• presuming the lung sound amplitude pattern to be bilaterally symmetric. In a previous study, O'Donnell and Kraman~ demonstrated that amplitude differences of considerable magnitude may commonly exist between lung sounds recorded from opposite sides of the chest wall. They also showed that the distribution of lung sound intensity on the chest wall was subject to notable variation. That study made use of a two-dimensional (amplitude vs horizontal or vertical position) mapping arrangement, which permitted lung sound analysis over a relatively limited portion of the chest wall and so failed to reveal in detail the full extent of the size of the amplitude variations that were detected. To examine more closely and define the normal lung sound amplitude variations, we recently developed a new technique, which we call flow-corrected phonopneumography, which permits rapid and accurate determination oflung sound amplitude. In the present study, this technique was used to demonstrate graph-
ically the sound patterns at the posterolateral lung bases of ten normal subjects to determine the size and distribution of lung sound intensity variations.
*From the Department of Medicine, Section of Pulmonary Medicine, Veterans Administration Medical Center, and Division of Pulmonary Medicine, University of Kentucky Medical Center, Lexington. tCiinical Instructor. :!:Assistant Professor. Dr. Kraman is supported by NHLBI grant 5ROl-HL26234-02. Manuscript received July 6; revision accepted October 21. Reprint requests: Dr. Krarnan, VA Medical Center(lll-H ), Lexington 40511
628
MATERIALS AND METHODS
Ten male nonsmokers (aged 24 to 48 years) with no history oflung disease were studied. All subjects had normal spirometric values. 6 The right and left lower posterolateral chest walls were marked with an 18 em X 18 em grid, each consisting oflOO points (10 rows of 10 points each). The most medial vertical row was 2 em from the spine, and the lowest horizontal row at the most dependent part of the chest where lung sounds could be heard. Each row was separated from that adjacent by 2 em. The subjects were instructed to breathe deeply from functional residual capacity (FRC) through a Fleisch pneumotachograph and pressure transducer amplifier (HewlettPackard 21078 and 473304A) to provide airflow (V) signals during the recording of the lung sounds. They were asked to achieve a peak airRow of approximately 2.5 Us as displayed on an oscillosmpe screen. Two identical condenser microphones fitted with chest pieces of 14 mm diameter, 9 mm depth were used for the remrding of lung sounds. Identical free field microphone characteristics were confirmed by mmparing the output of the two microphones held side by side in front of a loudspeaker driven by a sweep generator at 50 to 1, 000 Hz. Since the chest pieces were identical, it is assumed that the characteristics would remain identical when applied to the chest wall. The lung sound recording and processing equipment has been described previously.$ The right and left microphones were used simultaneously. They were placed precisely on the chest wall with the upper margin of the chest piece touching each of the 100 points on each hemithorax in succession while the subject breathed as described previously. The horizontal rows were studied left to right, bottom to top, until one inspiratory lung sound was recorded at each point. To determine the relative lung sound intensity, the computer program measured the mean amplitude of each 25 ms segment of sound recorded while Vwas above 1.3 Us. This in effect generated a sound amplitude envelope that, if not disturbed by artifacts, would roughly follow V. The amplitude of each 25 ms sound segment was divided by the value of the simultaneously determined airflow. The result was a Vcorrected lung sound amplitude index (Ale) of each segment expressed in units of volts/Us. The threshold \• of 1.3 Us was chosen because at\' <1.3 Us, the relation between Vand lung sound intensity was not always linear. For every breath taken by the subject, the Ale of all 25 ms segments (roughly 12 per breath while V > 1.3 Us) were averaged to yield the mean Ale (Ale). This was the figure used as the amplitude of Variability of Lung Sound
Intensity (Dosani,
Kraman)
HOHCIWNLIAIII2HOH DATE: 1/7112
US: AlA
llao
AI (1)
v
AI (2)
v
"'- V·,~~
~~-~L,
0.15
0.3 TIME ISECI
I
o.•5
I
0.8
FIGURE 1. Single inspiration with microphones at the left (AI(1)) and right (AI(2)) posterior lung bases. The lung sound envelope is produced as described in the text. The airflow corrected lung sound amplitude (AI(1)lV and AI(2)lV) tends to remain linear once an inspiratory flow (V) of ± 1 Us is achieved. This demonstrates the lack of dependence of lung sound amplitude on lung volume when breathing is from FRC. The sharp peaks in sound intensity are due to random variations in lung sound intensity.
HOR·'
FIGURE 2. Lung sound amplitude contour maps of subject SKat two different times. The left and right microphones were reversed for the second trial to ensure equal microphone characteristics. While there are small differences in the two maps, the overall amplitude patterns are strikingly similar.
nied by the expiratory contour maps for comparison. In most subjects, expiration was too quiet to allow adequate recording. It can be seen that the expiratory sound patterns are not just low-intensity copies of the
the lung sound at that location. When breath sound intensity from a single point on the chest wall was analyzed repeatedly by this technique, the coefficient of variation was 10 percent. This is much improved compared to our previous technique of How-gated analysis, which required the averaging of three breath sounds at each site to yield a similar coefficient of variation. • Once the Ale at each of the 100 positions on the left and right lung bases was determined, the computer program assembled the data into a three-dimensional contour map.
'""'
RESULTS
Figure 1 shows the V corrected lung sound amplitude envelope of a single inspiration recorded simultaneously from the right and left lung bases demonstrating that at V above approximately 1 Us, the sound amplitude is determined principally by V. Figure 2 shows the bilateral contour maps of one subject perlormed twice, one hour apart. For the second trial, the left and right microphones were reversed to ensure that microphone inequality was not responsible for left-right differences in amplitude. The reproducibility of the technique is apparent. Figure 3 shows the inspiratory contour maps of one of the subjects with the loudest lung sounds, accompa-
II ao
.....
X3 II ao
AMP--V·E~T HOR·' FIGURE 3. Inspiratory and expiratory contour maps of one of the subjects with the loudest lung sounds. All expiratory sounds were multiplied X 3 to allow better visualization of the contour. Differences in the general amplitude patterns apparently reflect different determinants of sound production and/or transmission in expiration. The few individual points that are much greater in amplitude than those adjacent probably represent artifacts.
CHEST I 83 I 4 I April, 1983
629
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FIGURE 4, Bilateral inspiratory contour maps for the ten subjects. For purposes of better revealing the contour, the amplitudes of some of the maps are increased by the factor marked "X."
inspiratory patterns, attesting to basic differences in their mechanism of production and/or of transmission. Figure 4 shows the inspiratory contour maps of all ten subjects and makes the following points: 1. There is marked intersubject variation in lung sound intensity. 2. There is a great degree of point-to-point variation (multiple peaks and valleys) in some subjects. 3. The amplitude patterns are not bilaterally symmetric in many areas in most of the subjects. 4. There is consistently decreased amplitude of lung sounds in the area of the scapulae. 5. There are no apparent amplitude variations due to the presence of the ribs. DISCUSSION
It is generally assumed that lung sound intensity is bilaterally symmetric, 4 and therefore the intensity of lung sounds at a given area is usually compared with that at the homologous area on the opposite side when 630
the chest is examined. Wooten et aF compared the breath sound intensity recorded over a segment of the left lung with that of a simultaneously recorded sound over an homologous segment of the right lung and found that the amplitudes were dissimilar when there was an obstruction in one of the segments, but they became similar when the obstruction was removed. The authors concluded that the comparison of simultaneously recorded amplitudes over homologous segments could be used to detect regional obstruction. Analysis of lung sounds by LeBlanc et aP showed that the amplitude of breath sounds was affected by lung volume, flow, and body position, with airflow being the major determinant (when breathing was not near residual volume). The authors compared their lung sound amplitude data with the ventilation patterns obtained by other authors using radioactive gas techniques and concluded that breath sound intensity was proportional to the regional ventilation of the variability of Lung Sound Intensity (Dosani, Kraman)
underlying lung. Ploy Song Sang and colleagues"·9 extended the work of LeBlanc et al to construct two-dimensional maps of lung sound intensity at the anterior chest wall. They compared these with the intensity of white noise introduced at the mouth to attempt to separate sound generation from transmission effects. All of these studies relied on the measurement of lung sounds at a relatively small number of locations (no more than four per hemithorax) so that the distribution of sound on the chest wall was revealed in very little detail. Significant intersubject and intrasubject variations were seen in the amplitude contour maps produced in the present study. Marked asymmetry was observed in comparable areas of opposite sides of the chest in the absence of any evidence of airways obstruction or of other lung disease. Most previous comparable lung sound studies have been performed with the assumption that the lung sound amplitude recorded at a given point on the chest wall generally represented ventilation in the underlying area. By contrast, the contour maps of the present study showed marked variation in some of the adjacent points that were separated by as little as 2 em. We do not have any definitive explanation for the lung sound amplitude variations observed in the present study. It is likely that lung sound intensity varies with some undefined factors beyond those already known, namely: airflow, lung volume, regional ventilation, and body position. These undefined factors appear to be related to the site of production of the sound and its transmission path to the chest wall. The chest wall thickness does not seem to have a predominant effect on the intensity oflung sounds, since some of our maps revealed low or equal intensity at the lateral chest wall compared to positions near the spine, where the thickness of the chest wall is much greater. One could argue that the variations observed in this study are related to variations in lung volume at the time of recording, as we did not actually measure the lung volume. This is less likely to be the case, since at and above functional residual capacity, lung volume seems to have little effect on the amplitude of lung sounds at the lung bases. 3 It is probable that much of the intrasubject variation would have been attenuated had we used larger-sized chest pieces, which would have averaged the ampli-
tudes of two to three adjacent points. This remains an open question. The finding of different intensity patterns between inspiration and expiration is not very surprising, since theoretical considerations have long predicted different sites of generation of these two sounds. 10 At least one recent study has produced evidence in support of this prediction. 11 In conclusion, we have developed a technique by which airflow corrected lung sound amplitude can be determined in great detail over a large area of the chest wall in a relatively short period of time. These data can then be displayed as a three-dimensional contour map. Analysis of these maps shows that there is a marked intersubject and intrasubject variation and absence of bilateral symmetry in lung sound amplitude patterns in normal subjects. Lung sound intensity does not appear to be closely related to chest wall thickness and the presence of the ribs, but is decreased in the areas over the scapulae. ACKNOWLEDGMENT: The authors thank Dr. Stanlev Rehm for his helpful review of this manuscript and Laurence Kojan Ong for the development of the computer programs.
REFERENCES 1 Sakula A. RTH Laennec 1781-1826. His life and work: a hit:entenary appreciation. Thorax 1981; 36:81-90 2 Murphy RLH, Holford SK. Lung sounds. Basics RD 1980; 8:1-6 3 LeBlanc P, Macklem PT. Ross WRD. Breath sounds and distribution of pulmonary ventilation. Am Rev Respir Dis 1970; 102:10-16 4 Hinshaw HC, Murray JF. Diseases of the chest. Philadelphia: WB Saunders Co, 1980:17-24 5 O'Donnell D, Kraman SS. Vesicular lung sound and amplitude mapping by Row-gated phonopneumography. J Appl Physiol 1982; 53(3):603-609 6 Morris JS, Koski A, Johnson LC. Spirometric standards for healthy, non-smoking adults. Am Rev Respir Dis 1971; 103:57-67 7 Wooten FT, Waring WW, Wegmann MJ, Anderson WF, Conley JD. Method for respiratory sound analysis. Med Instrum 1978; 12:254-257 8 Ploy Song Sang Y, Martin RR, Ross WRD, Loudon RG, Macklem PT. Breath sounds and regional ventilation. Am Rev Respir Dis 1977; 116:187-199 9 Ploy Song Sang Y, Macklem PT. Ross WRD. Distribution of regional ventilation measured by breath sounds. Am Rev Respir Dis 1978; 117:657-664 10 Farr G. The acoustics of the bronchial breath sounds. Arch Intern Med 1927; 39:286-302 11 Kraman SS. Determination of the site of production of lung sounds by subtraction phonopneumography. Am Rev Respir Dis 1980; 122:303-309
CHEST I 83 I 4 I April, 1983
631