Yonago Acta medica 2011;54:75–82

Detection of Pneumothorax Visualized by Computer
Analysis of Bilateral Respiratory Sounds
Nobuhiro Hayashi
Division of Emergency and Disaster Medicine, Department of Surgery, School of Medicine, Tottori
University Faculty of Medicine, Yonago 683-8504, Japan

Pneumothorax is usually diagnosed based on the attenuation of respiratory sounds of theaffected side on auscultation, but it requires a skilled technique and is limited to subjective
evaluation. Thus, we designed a device which analyzes and converts the frequency of auscultatory sounds to numerical values with a computer. With this device, the bilateral soundpressure levels were compared between groups of 25 healthy subjects and 21 patients withpneumothorax to investigate the efficacy of the diagnosing tool of pneumothorax. Whilerecording respiratory sounds of the bilateral precordial regions, the fast Fourier transform was applied with a frequency analysis software, power spectra of the auscultatory
sounds were displayed in real-time, and the sound pressure level was compared between
the bilateral sides. The difference was investigated at frequencies judged as less likely to
be influenced by cardiac sounds (200–400 Hz). No difference was observed in the control
group (n = 25, P > 0.05), but respiratory sound attenuation was detectable on the affectedside in the pneumothorax group (n = 21, P < 0.01 each for the paired Student’s t-test andWilcoxon signed-rank test). When the cutoff value was 8 dB, the sensitivity and specificity
as diagnostic tool of pneumothorax was 71.4% and 100%, respectively. This device would
facilitate the detection of occult pneumothorax at accident scenes, in emergency rooms andin intensive care units.
Key words: fast Fourier transform; frequency analysis; pneumothorax; real-time monitor

Although auscultation is an essential diagnostic
method, it has become underestimated in manycases as healthcare and medicine have advanced(Miyahara et al., 2005). However, it is a simple,
noninvasive, quick diagnostic technique and is appropriate to rapidly investigate the pathologicalcondition, and is still very useful for medical carein emergencies and disasters (Hiraide and Iwami,2001). Making a diagnosis based on auscultation
requires sufficient experience, lacking objectivity,and sharing and recording of the information are
difficult (Murata et al., 2000). I designed a record
analysis device which presents changes in thepressure of respiratory sounds as waveforms, andapplied it to healthy subjects and pneumothorax
patients. The aim of this study was to evaluate thedifference of respiratory sounds objectively by recording the sound pressure at appropriate frequencies, and to investigate the sensitivity and specificity of the device.

Materials and Methods
Two sets of a membrane-type stethoscope (Littmann
Cardiology III, 3M, St. Paul, MN) connected to a

Abbreviations: CT, computed tomography; FFT, fast Fourier transform; PC, personal computer
Present address: Clinical Department of Emergency and Critical Care Medicine, Juntendo University Urayasu Hospital,
2-1-1 Tomioka, Urayasu 279-0021, Japan75

N. Hayashi

Linear PCM recorderabStethoscope(chest piece)

Mobile PC

Monitoring headphones

Fig. 1. Record analysis device we designed.
a: Monitoring and recording device (total system).
b: A chest piece of the stethoscope with a condenser microphone.
PC, personal computer; PCM, pulse code modulation.

Condenser
microphone

Fig. 2. Block diagram.
A/D, analog-to-digital;
FFT, fast Fourier transform;PC, personal computer;
PCM, pulse code modulation.

condenser microphone (ECM-C10, SONY, Tokyo,
Japan) were prepared (Fig. 1a), and respiratory
sounds were input into a mobile personal computer(PC) via a linear pulse code modulation recorder(EDIROL R-09, Roland, Hamamatsu, Japan) andsound card (Creative Sound Blaster X-Fi Xtreme
Audio Notebook, Creative Technology, Singapore)(Figs. 1b and 2). Quantization was performed at 16bits and 48 kHz. The bilateral inputs were subjected to the fast Fourier transform (FFT) using a frequency analysis software (Realtime Analyzer version 5, Yoshimasa Electronic, Tokyo, Japan), andthe power spectrum waveform of the input information was compared between the bilateral sides(FFT size: 4096, smoothing: Hanning window at aFig. 3. Recording set-up. A pair of chest pieces is placedin the bilateral second intercostal spaces on the midclavicular line.

76Computer-analyzed pneumothorax detection(dB) 0

Lch/Rch–10

Sound pressure level

–20

–30

SPL [dB]–40

L –64.04–50

Frequency [Hz]

–60

L

200

R –66.79

R

200

–70

–80

–90
–100

25
40
63
100
160
250
400
630
1k
1.6 k
2.5 k
4k
6.3 k
10 k
16 k
ALL 31.5
50
80
125
200
315
500
800 1.25 k
2k
3.15 k5k
8k
12.5 k20 kFrequency(Hz)

Fig. 4. A sample of power spectra monitored on the mobile personal computer. Using this frequency analysis software,
we can measure the sound pressure level (dB) of the left and right inputs at the arbitrary time phase and the arbitrary frequency (circled). The generated value is negative, because the maximum input is processed from 0. The X axis (frequency)
is displayed on the logarithmic scale. Lch, left channel; Rch, right channel.

time constant of 125 ms, frequency weighting: flat).
The recording sites were symmetrically arrangedin the bilateral second intercostal spaces on the
midclavicular lines, and respiratory sounds were
heard with headphones (Fig. 3). The chest pieces ofthe stethoscopes were put on the chest wall by own
dignity without hands and fixation to avoid noise
artifacts. The chest piece was lightly fixed to thechest wall with paper tape for subjects with a hairychest or with well-formed pectoral muscles. Thisfrequency analysis software is capable of extracting
sound pressure levels at a specific frequency, but

the sound pressure levels are presented as negativevalues (relative values), regarding the maximum
input that the PC can process as 0 (Fig. 4). All recordings were performed after obtaining informed
consent from the subjects. The present study wasreviewed by the Ethical Review Board of Juntendo
University Urayasu Hospital.
All subjects spontaneously breathed, and therecording was performed in the emergency room
or in the intensive care unit. Although 1-cycle
breathing (a few seconds) was also possible for thisanalysis, about 10 cycles per patient were recorded.

Sound pressure level(dB) –30

–35
–40
–45
–50
–55
–60
–65
–70
–75
–80
–85
–90
–95
–100
–105
–110
–115
–120

Rch

Inhalation phase“Inhalation peak”

Exhalation phaset = 1.707 s
Time aixs

300

400

500

Frequency

(Hz)

–30
–35
–40
–45
–50
–55
–60
–65
–70
–75
–80
–85
–90
–95
–100
–105
–110
–115
–120

Fig. 5. Three-dimensional (waterfall) display of normal respiratory sounds as spectrograms of typical respiratory sounds
in the 300–900 Hz frequency band. Two peaks are shown. The 1st (posterior) peak corresponds to the inspiration phase,and the 2nd (anterior), to the expiration phase. Rch, right channel.

77

N. Hayashi

Table 1. Clinical characteristics of pneumothorax patients [n = 21]

Subjects with no chest trauma or respiratory disease were defined as normal, showing clear respiratory sounds without laterality or abnormality onchest X-ray examination. Twenty-five healthy subjects underwent recording, and served as the control group. In the frequency distribution of normal
respiratory sounds, the sound pressure levels in the
200–400 Hz range are less influenced by cardiac
sounds (Ishigaki et al., 1993). Thus, the difference was investigated at the following frequencies:
200, 211.5, 224, 237, 250, 266, 282, 298.5, 315,
335, 355, 376 and 400 Hz which were 1/12 octaveband, based on the interval of logarithmic scales.
The spectrum was observed over time, as shown in

Age (yr)
Mean ± SD
Range
GenderMale
FemaleTraumatic
Motor vehicle accident
Fall
Non-traumatic
Spontaneous
Chronic obstructive pulmonary disease

46.1 ± 19.2
18 – 83
17 (81%)4 (19%)
12
10
2
9
8
1

AL
L
59
.5
75
.0
94
.5
11
9.
14 0
9.
18 5
8.
23 5
7.
29 0
8.
37 5
6.0
47
3.0
59
5.5
75
0.
94 0
4
11 .0
90
.
15 0
00
18 .0
80
23 .0
70
.
29 0
90
37 .0
60
47 .0
30
59 .0
60
.
75 0
00
94 .0
40
.0

Sound pressure level

AL
L
59
.5
75
.0
94
.5
11
9.0
14
9.
18 5
8.
23 5
7.
29 0
8.
37 5
6.0
47
3.0
59
5.5
75
0.
94 0
4
11 .0
90
.
15 0
00
18 .0
80
23 .0
70
.
29 0
90
37 .0
60
47 .0
30
59 .0
60
.
75 0
00
94 .0
40
.0

Sound pressure level

Fig. 5, and the time phase with the maximum wave
(consistent with the inhalation peak flow) was de(dB)termined as the “inhalation peak time0
point”. In the control group (n = 25, 19
Left channel
Right channel
males and 6 females: mean age 45.2
–20
years, range 28–70 years), the sound–40pressure levels of the bilateral inputs–60
at the inhalation peak time point weredivided into 2 subgroups of the left and–80
right sides, and the differences between–100
the mean bilateral values were ana–120
lyzed. Of patients who visited Emergency and Critical Care Medical Center of(Hz)Tottori University Hospital between AuFrequency
gust 2008 and March 2009 and ClinicalFig. 6. Mean sound pressure levels from the left and right channels in
Department of Emergency and Criticalthe control group (n = 25) at the inhalation peak time point.
Care Medicine of Juntendo University(dB)
Urayasu Hospital between June 20100and March 2011, 21 were diagnosed as
Normal side
Pneumothorax-affected side–20
pneumothorax by X-ray or computed
tomography (CT) of the chest. Twelve–40of them were traumatic and 9 were–60
non-traumatic (Table 1). No traumatic–80
patients had head injury. There was no
medical history of asthma. All traumat–100
ic patients had oxygen administration–120
according to the prehospital protocol.
One non-traumatic patient with chronic(Hz)
Frequency
obstructive pulmonary disease develFig. 7. Mean sound pressure levels of the normal and affected sides of oped hypercapnia. In the pneumothorax
the pneumothorax group (n = 21) at the inhalation peak time point. The group (n = 21), the bilateral inputs wereattenuation of the sound pressure on the affected side is shown in thedivided into 2 subgroups of the normal
119–750 Hz frequency band.
78

Computer-analyzed pneumothorax detection

Sound pressure leveland pneumothorax-affected sides, and(dB)–40
the differences between the mean valLeft channel
Right channel
NS NS NS NS NS NS NS
ues were analyzed.
–50
NS NS NSNS
The statistical significances of
NS
NS
–60
differences in sound pressure levels between the left and right-side subgroups–70
for the control group and between the–80
normal and affected-side subgroups forthe pneumothorax group were deter–90
mined by the paired t-test and Wilcoxon
signed-ranks test. The accepted level of–100
200 211.5 224 237 250 266 282 298.5 315 335 355 376 400 (Hz)significance was P < 0.05. Twenty-one
Frequency
patients (n = 20.87) as the minimum
Fig. 8. Sound pressures compared between the left and right channels insample size for a t-test was required tothe control group (n = 25). NS, not significant: P > 0.05 (paired t-test andattain a value ≥ 0.8 from the result of Wilcoxon signed-rank test).
the power analysis.
(dB)

Results

Sound pressure level

–40

–50

Normal side

Pneumothorax-affected side** ** ** ** ** ** ** ** ** ** ** ****–60
Mean sound pressure levels at the inhalation peak time point from bilateral–70
channels in the control group were–80
plotted in the 50–1000 Hz frequency
band on the logarithmic scale (Fig.
–906). Almost the identical forms wereconfirmed in the control group. In the–100
200 211.5 224 237 250 266 282 298.5 315 335 355 376 400 (Hz)pneumothorax group, mean sound presFrequency
sure levels at the inhalation peak time
Fig.
9.
Sound
pressurelevelsshow
significant differences between the
point were plotted from the normal andnormal and affected sides of patients of the pneumothorax group (n =affected sides in the 50–1000 Hz fre- 21). **P < 0.01 (paired t-test and Wilcoxon signed-rank test).
quency band on the logarithmic scale(Fig. 7). The sound pressure level was
attenuated on the affected side in the 119–750 Hz
normal and affected sides) was 8 dB, the best senrange. The difference was examined in the 200– sitivity and specificity of this device were 71.4 % (at400 Hz range, where the attenuation was especially
315 Hz) and 100 % (at 250 and 298.5 Hz), respeclarge. In the control group (n = 25), differences
tively (Table 2).
were not significant between both inputs at all frequencies in the 200–400 Hz range (Fig. 8). In thepneumothorax group (n = 21), differences were sigDiscussionnificant between the normal and affected sides at
all frequencies in the 200–400 Hz range (P < 0.01)
The presently introduced device visualizes the(Fig. 9). A non-parametric test (Wilcoxon signedsound pressure level to detect pneumothorax. By
ranks test) also showed the same results as above.
the frequency analysis of respiratory sounds ofWhen the cutoff value (the difference between thepneumothorax patients, I found that the difference79

N. Hayashi

Table 2. Sensitivity and specificity of the present device at frequencies examined in the 200–400 Hz
range

Sensitivity
Specificity

200

211.5

224

237

47.6
79.2

61.9
83.3

66.7
91.7

66.7
57.1
95.8 100.0

Frequency (Hz)266
282 298.5

315

335

355

376

400

57.1
87.5

71.4
95.8

61.9
91.7

52.4
87.5

40.9
91.7

42.9
87.5

250

diagnosed by respiratory sound attenuation on theaffected side could be objectively detected in waveforms at appropriate frequencies. In addition, theabsence of the difference in healthy subjects was
confirmed.
I selected the 200–400 Hz frequency band
because the spectra of alveolar and tracheal respiratory sounds are mostly less than 500 Hz, because
the power markedly decreases at a higher frequency (Ishigaki et al., 1993; Nakano et al., 1994)
and because the main spectrum of cardiac soundnoises is 40–200 Hz (Nakano et al., 1994; Debbal
and Bereksi-Reguig, 2008). Mansy and coworkers
recorded respiratory sounds using a dog model ofpneumothorax on the affected side before and after
the onset of pneumothorax, and observed that thethe pressure level of respiratory sounds was markedly attenuated in the 200–700 Hz range (Mansy
et al., 2002a). In another study of sound transmission measurement, white noise was played via a
speaker connected to a tracheal tube inserted intoa dog, and sound transmission was recorded with
an electronic stethoscope at the chest wall. Then,the transmission was markedly attenuated in the
200–1200 Hz range (Mansy et al., 2002b). A similar tendency was noted in the present study.
I could easily use this device in the emergency room and intensive care unit in almost all
situations. Even in a noisy situation, it requires only
a few seconds to become calm. Although I did notstudy the possible limitation of this device, a question how to evaluate pneumothorax with massive
subcutaneous emphysema may be raised because
noise is marked in such cases. However, it becomesno subject of discussion because the existence of
subcutaneous emphysema is an indirect findingof pneumothorax. Furthermore, bilateral pneumo-

52.4
57.1
91.7 100.0

thoraces are undetectable because there is little
difference between both sides. In patients withpostoperative pleural adhesions or adhesive pleuraldisease such as tuberculous pleuritis and chronic
obstructive pulmonary disease, differences are also
undetectable because there are no intrathoracic
spaces between the stethoscope on the chest walland ipsilateral lung parenchyma.
I would like to investigate the association between the pneumothorax volume and level of soundpressure attenuation. Laterality was investigatedonly at a single time point (inhalation peak) in thepresent study. But our final aim is to develop an automatic laterality-detecting program by averaging
several inhalation cycles, taking advantages of thefact that the difference in sound pressure between
inhalation and exhalation in pneumothorax patientsis smaller than that in healthy subjects (Mansy etal., 2002a). Furthermore, I could investigate theapplication of the present method for hemothorax
by combining an algorithm that reduces cardiac
sounds and surrounding noise.
The presence of occult pneumothorax bytrauma is known, which could progress to tension
pneumothorax during transport or positive pressure ventilation (Rhea et al., 1989; Hill et al., 1999;
Japanese Association for the Surgery of Trauma
Development Committee, 2004), but occult pneumothorax is detectable only by CT scan. The sensitivity and specificity of routine supine anteroposterior chest X-ray as a diagnostic tool was 52%
and 100%, respectively (Soldati et al., 2008). On
the other hand, our device is more accurate as a diagnostic tool (sensitivity, 71.4%; specificity, 100%).
The representative waveforms on frequency
analysis are presented in a healthy subject and apneumothorax patient (Figs. 10a–c). Figures 10a80Computer-analyzed pneumothorax detection

(dB)

a

0

Sound pressure level–10Inhalation peak

Lch/Rch

–20
–30
–40
–50
–60
–70
–80
–90

–100

25
40
ALL 31.5(dB)

bSound pressure level

63

50

80

100

125

160

200

250

315

400

500

630

800

1k

Frequency
0
–10

1.6 k
1.25 k2k
(Hz)

Exhalation peak

Lch/Rch

–20
–30
–40
–50
–60
–70
–80
–90

–100

25
40
ALL 31.5

50

63

80

100

125

160

200

250

315

400

500

630

800

Frequency(dB) 0c–10Sound pressure leveland b show stationary waveforms at the inhalation and exhalation phases in the healthy
subject. No difference was noted between thebilateral inputs (sound pressure levels) at theinhalation peak point in the 200–400 Hz frequency band. In the waveforms of the 65-yearold female patient with chest contusion by a
motor vehicle accident (Fig. 10c), pneumothorax
was not observed on the initial antero-posterior
X-ray examination but diagnosed by CT scanshowing left occult pneumothorax (Fig. 10d).
Only an instructor physician with abundant
experience could identify the difference of respiratory sounds in the emergency room, andthe distinction was so subtle that residents andnurses recorded it as no difference, but the device could detect the waveform attenuation ofleft respiratory sounds around 200 to 400 Hz in
waveforms.
Reportedly, thoracic drainage is necessary
for about 20% of patients with occult pneumothorax (Livingston and Hauser, 2008). For suchpatients, continuous monitoring with the present
device may detect changes in the pathologicalcondition in an early stage. Recently, a digital
signal processing equipment which subtracts
the sound wave of surrounding noise from thetotal input sound has spread, and it is used forheadphones and digital hearing aids. If thesenoise reduction technologies are combined in
the future, the present device may be useful as a
low-cost, light and compact mobile respiratory

1k

1.6 k
1.25 k2k(Hz)

Inhalation peak

Lch/Rch

–20
–30
–40
–50
–60
–70
–80
–90

–100
ALL

25

40
31.5

63
50

100
80

160
125

250
200

400
315

Frequency

630
500

1k
800

1.6 k
1.25 k2k(Hz)

dFig. 10. Representative waveforms and images recorded
from a healthy control and a pneumothorax patient.
a: Waveforms of the control at the inhalation peak time
point. No difference is recognized between the bilateral channels in the 200–400 Hz range.
b: Waveforms of the control at the exhalation phase.
c: Waveforms at the inhalation peak of the 65-year-old
female patient with left occult pneumothorax due to
Chest X-ray
Chest CTa motor vehicle accident. The difference between thebilateral channels is visually recognized in the 200–400 Hz range.
d: Images by supine antero-posterior chest X-ray (left) and chest CT (right) of the patient. Only the front part of the left
lung is marginally collapsed (arrows).
CT, computed tomography; Lch, left channel; Rch, right channel.

81

N. Hayashi

monitor equipped in ambulances, for remote medical care, and as an educational material.
With a newly designed record analysis device
which visualizes changes in the respiratory soundpressure as waveforms, difference in respiratory
sound attenuation on the pneumothorax-affectedside could be objectively detected in the waveformsof pneumothorax patients. The device is easily applicable without any skill in the emergency room.

Trauma. 6th ed. New York: McGraw-Hill; 2008. p.
525–552.
7 Mansy HA, Royston TJ, Balk RA, Sandler RH. Pneumothorax detection using computerized analysis ofbreath sounds. Med Biol Eng Comput 2002a;40:526–
532.
8 Mansy HA, Royston TJ, Balk RA, Sandler RH.
Pneumothorax detection using pulmonary acoustic
transmission measurements. Med Biol Eng Comput
2002b;40:520–525.
9 Miyahara S, Tanaka K, Funayama M, Kiyasu S, Soda
H, Oka M, et al. Lung sounds analysis using electronicstethoscope. Yakuri To Rinsho 2005;15:511–518 (inJapanese with English abstract).
10 Murata A, Kudoh S, Takasaki Y, Shibuya A, Muratake T,
Nakajima M. The newly developed phononeumograph.
Yakuri To Rinsho 2000;10:355–361 (in Japanese withEnglish abstract).
11 Nakano H, Sano K, Maekawa J, Narita N. Relationshipbetween power spectra of breath sounds and inspiratory
flows at different frequencies. Nippon Kokyuki Gakkai
Zasshi 1994;32:1142–1148 (in Japanese).
12 Rhea JT, Novelline RA, Lawrason J, Sacknoff R, Oser A,
et al. The frequency and significance of thoracic injuries
detected on abdominal CT scans of multiple trauma patients. J Trauma 1989;29:502–505.
13 Soldati G, Testa A, Sher S, Pignataro G, Sala ML, Silveri NG. Occult traumatic pneumothorax: diagnostic
accuracy of lung ultrasonography in the emergency department. Chest 2008;133:204–211.

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Received September 20, 2011; accepted October 17, 2011
Corresponding author: Nobuhiro Hayashi

82