A Preliminary Study to Assess its Potential as a
New Outcome Measure for Respiratory Therapy
Alda Marques
1, 2
, Anne Bruton
and Anna Barney
Escola Superior de Saúde da Universidade de Aveiro, Universidade de Aveiro, 3810-193 Aveiro, Portugal
School of Health Sciences, University of Southampton, Southampton, SO17 1BJ, U.K.
Institute of Sound and Vibration Research, University of Southampton, Southampton, SO17 1BJ, U.K.
Keywords: Physiotherapy, Outcome measures, Lung sounds, Crackles.
Abstract: A barrier to assess the relative effectiveness of respiratory therapies has been insufficient accurate, reliable,
and sensitive outcome measures. Lung sounds provide useful information for assessing and monitoring
respiratory patients. However, standard auscultation is too subjective to allow them to be used as an
outcome measure. In this paper, Computer Aided Lung Sound Analysis (CALSA) characterising crackles’
Initial Deflection Width (IDW) and Two Cycle Deflection (2CD) is proposed as a potential objective, non-
invasive, bedside outcome measure to assess the response to alveolar recruitment and airway clearance
interventions. A preliminary ‘repeated measures’ experimental study was conducted. Seventeen participants
with cystic fibrosis were recruited from out-patient clinics. Demographic, anthropometric and lung sound
data were collected. The intra-subject reliability of crackles’ IDW and 2CD was found to be ‘good’ to
‘excellent’, estimated by the Analysis of Variance, Intraclass Correlation Coefficient, Bland and Altman
95% limits of agreement and Smallest Real Difference. It is concluded that crackle IDW and 2CD detected
by CALSA are reliable and stable measures. In future, CALSA may be useful for assessing and monitoring
respiratory interventions in clinical settings.
The lack of reliable outcome measure has been an
obstacle to providing evidence about the relative
effectiveness of respiratory therapy interventions.
The main aims of respiratory physiotherapy include:
increasing alveolar recruitment, thereby improving
ventilation; increasing secretion removal and
therefore airway clearance; decreasing work of
breathing and consequently dyspnoea; increasing
muscle strength and endurance to increase exercise
capacity and independence in daily functioning and
increasing patients’ understanding of their lung
condition to promote self-management. Research
into airway clearance techniques has been one of the
priorities identified for research (CSP, 2002). In this
paper we explore the potential for computer aided
lung sound analysis (CALSA) to be used as an
outcome measure for respiratory therapy
Respiratory physiotherapists currently use the
following outcome measures to monitor their
interventions and evaluate their practice: sputum
quantity, respiratory function tests, tests of gas
exchange, imaging evidence and standard
auscultation techniques. Most of these are not
specifically related to the physiotherapy intervention
employed and may be affected by other factors
(Marques et al., 2006). There is no gold standard
outcome measure that is specifically related to
respiratory physiotherapy interventions. Most of the
published respiratory physiotherapy research
compares two or more active interventions rather
than an active intervention versus an inactive
control. In such studies it is never clear if differences
are not detected because the outcome measures are
not appropriate, or because the treatments being
compared are equally effective/ ineffective.
Although there are other more invasive or
laboratory-based outcome measures available, these
are generally only applicable to a research setting.
Marques A., Bruton A. and Barney A. (2009).
COMPUTER AIDED LUNG SOUND ANALYSIS - A Preliminary Study to Assess its Potential as a New Outcome Measure for Respiratory Therapy .
In Proceedings of the International Conference on Health Informatics, pages 251-256
DOI: 10.5220/0001547002510256
1.1 Lung Sounds
Normal lungs generate breath sounds as a result of
turbulent airflow in the trachea and proximal
bronchi. The airflow in the small airways and alveoli
has a very low velocity and is laminar, and therefore
silent. Turbulent flow characteristics are influenced
by airway dimensions, which are a function of body
height, body size, age, gender and airflow will all
affect breath sounds (Pasterkamp et al., 1997).
Sounds heard or recorded at the chest differ
according to the location at which they are heard or
recorded, and vary with the respiratory cycle
(Sovijarvi et al., 2000). The geometry of the bronchi
also contributes to the complexity of the thoracic
acoustics (Kompis et al., 2001) because it affects
flow, and consequently breath sounds.
Normal breath sounds can be classified as
‘abnormal’ if heard at inappropriate locations. There
are also added sounds (known as adventitious
sounds) which can be continuous (wheezes) or
discontinuous (crackles). Their presence usually
indicates a pulmonary disorder.
Wheezes are continuous adventitious lung
sounds. The mechanisms underlying their
production appear to involve an interaction between
the airway wall and the gas moving through the
airway (Meslier et al., 1995), producing continuous
undulating sinusoidal deflections by fluttering of the
airway walls. Wheezes are clinically defined as
musical sounds and can be characterised by their
location, intensity, pitch, duration in the respiratory
cycle, and relationship to the phase of respiration
(Meslier et al., 1995).
Crackles are discontinuous adventitious sounds
and their presence may be an early sign of
respiratory disease. They are intermittent, non-
musical, brief sounds thought to be caused by the
acoustic energy generated by pressure equalization
or change in elastic stress after a sudden opening or
closing of airways (Forgacs, 1978, Nath and Capel,
1974). Their character is explosive and transient and
depends on the diameter of the airways. Their short
duration and often low intensity, makes their
discrimination and characterisation by normal
auscultation very difficult (Kiyokawa et al., 2001).
Crackles are generally characterised by the Initial
Deflection Width (IDW), i.e. the duration of the first
deflection of the crackle and the Two Cycle
Deflection (2CD), i.e. the duration of the first two
cycles of the crackle. The mean values of IDW and
2CD durations for fine and coarse crackles are
defined (ATS, 1977, Sovijarvi et al., 2000). During
respiratory disease, the involvement of different
airways is associated with the crackle frequency, i.e.,
high frequency crackles are associated with
peripheral airways and lower frequency crackles
with upper airways (Fredberg and Holford, 1983).
These measurements therefore have the potential to
be useful as an outcome measure for respiratory
1.2 Standard Auscultation
Standard auscultation via a stethoscope is an
assessment tool used by many health professionals
during chest examination in their clinical practice,
and is often used by physiotherapists to monitor
patients’ response to respiratory interventions.
However, the literature has contradictory reports
about its value in routine current practice. Some
authors argue that auscultation is an inappropriate
outcome measure because of the differences in
health professionals’ hearing acuity as well as in the
properties of stethoscopes. There can also be
different approaches to the description of
auscultatory findings, nomenclature difficulties, and
inter- and intra-observer variability (Welsby et al.,
2003) Others have argued that auscultation is an
easy, rapid, effective, non-invasive, and cost-
effective way of assessing the condition of the
airway and breathing (Chen et al., 1998). However,
agreement between observers during standard
stethoscope examination for the presence of normal
or adventitious lung sounds was found to be only
‘poor to moderate’, and clinical experience was not
found to have any clear effect on accuracy or
reliability (Brooks et al., 1993). Elphick et al. (2004)
found that using computerised acoustic analysis of
recorded lung sounds improved the reliability of
detection for all sounds when compared to listening
through a stethoscope. Therefore, although the use
of a standard stethoscope may be too subjective to
provide a useful outcome measure, the sounds
generated from the lungs provide useful information,
and relate directly to movement of air and
1.3 Computer Aided Lung Sound
There is a great deal of information derivable from
lung sounds, that is not normally readily accessible
even to experienced clinicians and exceeds the
memory capacity of most people. Lung sounds
interpretation is enhanced using CALSA through the
efficient objective data collection, generation of
permanent records of the measurements made with
HEALTHINF 2009 - International Conference on Health Informatics
easy retrievability and through graphical
representations that help with diagnosis and
management of patients’ suffering from chest
diseases (Earis and Cheetham, 2000). Digital
recordings of normal lung sounds have shown high
inter- and intra-subject repeatability with the inter-
individual variability explained by height, gender
and anatomic characteristics (Sanchez and Vizcaya,
There is some evidence in the literature to
support the hypothesis that CALSA characterising
adventitious lung sounds may be a useful outcome
measure. The number and distribution (per breath)
of adventitious sounds has been associated with
severity of disease and crackles characteristics have
been found to differ in different diseases, allowing
differentiation between conditions (Piirila, 1992).
Furthermore, the number of wheezes per respiratory
cycle has been reported as a good indicator for
airway obstruction (Baughman and Loudon, 1984).
CALSA has already been used to assess the airways’
response to bronchodilators and bronchoconstrictors
in children and in adults. Malmberg et al. (1994)
studied 11 asthmatic children (aged 10 to 14 years)
and found that spectral analysis of lung sounds can
be used to detect airways obstruction during
bronchial challenge tests. When combined with
spirometry, CALSA increased the sensitivity of
detection of pulmonary disease and was able to
provide early signs of lung disease that was not
detected by spirometry alone (Gavriely et al., 1994).
The use of CALSA has been found to be
specific, reliable, and sensitive within the limited use
to which it has been put to date but has not yet been
evaluated as an outcome measure for physiotherapy.
Although it is known that normal lung sounds are
reliable and that adventitious lung sounds have some
clinical meaning, the reliability of the specific
parameters of adventitious lung sounds (e.g.
crackles’ IDW and 2CD) has not been adequately
explored. In order for CALSA to be used as a valid
outcome measure its reliability amongst patient
populations in clinical settings must be
demonstrated. Therefore, the inter- and intra-subject
reliability of CALSA to measure crackles over short
time-periods in adult patients with cystic fibrosis
were explored.
A single group repeated measures design was
employed in which triplicate recordings were made
from the same patients over a short time period. The
study received full approval from Southampton &
South West Hampshire Research Ethics Committees
Since this was a reliability study, power
calculations were not appropriate. Potential
participants were identified via the respiratory out-
patient clinics held at two hospitals on the south
coast of the UK. Participants were included in the
study if they were 1) able to give and sign informed
consent; 2) formally diagnosed with cystic fibrosis
(CF); 3) 18 years of age or older and 4) clinically
stable for one month prior to the study (no hospital
admissions, exacerbations/ infections or change in
medication). Participants were excluded from the
study if they had co-existing lung pathologies.
Anthropometric data such as height and weight
were measured using calibrated digital scales. The
lung sound recordings were performed with a digital
stethoscope (WelchAllyn Meditron, 5079-402). The
input from the microphone was connected via an
amplifier to a laptop with customised software,
suitable for data acquisition, written in Matlab
(version 7.1).
2.1 Protocol
All participants provided informed consent prior to
data collection. Demographic and basic
anthropometric data were recorded first. The lung
sound recordings followed the guidelines defined by
Computerized Respiratory Sound Analysis for short
term acquisition (Sovijarvi et al., 2000).
Participants’ skin was marked with a pen in seven
different places to ensure consistency of stethoscope
placement: one centrally over the trachea (sternal
notch); two on the back of the chest (right and left
bases: at 5 cm from the paravertebral line and 7 cm
below the scapular angle); two on the front of the
chest (right and left: in the second intercostal space,
mid-clavicular line); two on the side of the chest
(right and left: in the fourth to fifth intercostal space,
mid axillary line). Lung sound recordings were then
performed with the digital stethoscope held by hand
over each location. Participants were asked to
breathe through the mouth during recordings. Three
sets of recordings were made for 25 seconds at each
marked location.
2.2 Analysis
Gender, date of birth, height and weight data were
entered into SPSS version 14. Body mass index
(BMI) in (kg/m
) was calculated using the formula
COMPUTER AIDED LUNG SOUND ANALYSIS - A Preliminary Study to Assess its Potential as a New Outcome
Measure for Respiratory Therapy
BMI = weight/(height)
. Descriptive statistics were
used to describe the sample.
The lung sounds were recorded with a sampling
frequency of 44.1 KHz. All files from the seven
anatomical sites were processed to detect crackles
using algorithms written in Matlab based on
Vannuccini et al.’s algorithm (1998). The data from
all files with the duration in milliseconds of the
crackles variables (IDW and 2CD) were imported to
SPSS version 14 for statistical analysis.
2.2.1 Relative Reliability
Inter-subject reliability was analysed using analysis
of variance (ANOVA). All recordings were
performed by the same researcher. The inter- and
intra-subject reliability was examined in each
recording position in each session. Therefore, the
ICC was calculated using the equation (1,k) which
uses the one-way ANOVA table: ICC (1,K) =
(BMS-WMS)/BMS, where ICC is the Intraclass
Correlation Coefficient, BMS = between subjects
mean squares; WMS = within subjects mean
squares; k = number of observers or measures. The
ICC results were analysed according to Fleiss (1986)
criteria i.e. values above 0.75 represent excellent
reliability; between 0.4 and 0.75 represent moderate
to good reliability and below 0.4 represent poor
2.2.2 Absolute Reliability
Bland-Altman plots were performed to provide
visual information about systematic bias and random
error by examining the direction and magnitude of
the scatter around the mean difference (Bland and
Altman, 1986). The standard error of measurement
(SEM) was obtained by calculating the square root
of the within subject mean square (WMS) values
obtained in the ANOVA table performed for each
recording position for each session. These values
were then used to calculate the Smallest Real
Difference (SRD), which represents the smallest
change that can be interpreted as a real difference.
Seventeen CF participants, age range of 18 to 67
years old (8 female and 9 male) and average BMI of
20.6±3.3 kg/m
, were recruited. The inter-subject
reliability analysis confirmed the expected
variability between subjects. It has been suggested
that to assess intra-rater (or test-retest) or inter-rater
reliability in reliability studies, a measure of relative
reliability, the ICC, and a measure of absolute
reliability, Bland & Altman 95% limits of
agreement, should both be reported (Rankin and
Stokes, 1998).
The ICC values range between 0.57 to 0.85 for
the crackles’ IDW variable and between 0.75 and
0.96 for the crackles’ 2CD. The ICC results were
generally found to be excellent in both groups of
participants. The crackles’ IDW variable generally
presented a lower ICC value when compared with
the crackles’ 2CD variable. Bland and Altman 95%
limits of agreement were performed and the scatter
plots were analysed in all recording positions for
both groups of participants on the three different
occasions. No systematic bias was present. The SRD
ranged from 0.33 to 0.76 ms for the crackles’ IDW
and from 1.29 to 2.66 ms for the crackles’ 2CD.
The inter- and intra-subject reliability of crackles’
IDW and 2CD measured using CALSA has been
rarely investigated. There is, however, a consensus
that the inter and intra-observer reliability of the
detection of added lung sounds among health
professionals using tape-recorders, audio-files or
standard auscultation is generally poor (Elphick et
al., 2004). The inter-subject variability for both
crackles’ variables studied (IDW and 2CD) in this
research was found to be high, as shown by the
significant ANOVA. This high inter-subject
variability for crackles’ IDW and 2CD duration was
expected due to differences in demographic and
anthropometric characteristics (Sanchez and
Vizcaya, 2003), and varying acuity of pathology.
The intra-subject reliability was found to be
‘good’ to ‘excellent’ with no systematic bias
between the repeated measures. Previous studies
have also reported high intra-subject reliability of
lung sounds in healthy subjects (Mahagnah and
Gavriely, 1994) and in healthy subjects and patients
with fibrosing alveolitis (Sovijarvi et al., 1996).
However, these studies have analysed lung sounds in
the frequency domain, in small samples of mainly
healthy subjects and have not included people with
excessive secretions making comparisons difficult.
The ICC and Bland and Altman 95% limits of
agreement have been recommended as the more
adequate methods to assess reliability (Rankin and
Stokes, 1998). The ICC for the crackles’ IDW and
2CD suggested ‘excellent’ or ‘good’ reliability in
almost all recording positions, in both groups of
HEALTHINF 2009 - International Conference on Health Informatics
participants. However, the ICC should be interpreted
with caution. Bland and Altman 95% limits of
agreement are independent of the true variability in
the observations and therefore, complement the ICC
analysis and provide detail regarding the nature of
the observed intra-subject variability (Rankin and
Stokes, 1998). The reliability assessed from Bland
and Altman techniques was found to be acceptable,
and no consistent bias was detected in any recording
position of the two crackles’ variables studied.
Therefore, crackle characterisation using CALSA
was deemed reproducible over short time periods.
The ICC has rarely been used when analysis lung
sounds, but ‘satisfactory’ reproducibility of lung
sounds based on ICC results have been reported
(Schreur et al., 1994). However, this analysis was
performed in the frequency domain where spectral
characteristics were considered and the recordings
were performed in a sound proofed room.
In this research, the SRD values, over short time
periods, for both variables studied (crackles’ IDW
and 2CD) presented a similar range of values
indicating the stability of the measure in CF
participants. Smallest real difference calculations
were not found in other published studies involving
lung sound analysis and therefore, comparisons with
the findings of this research were not possible.
As it has been demonstrated, the use of CALSA
in terms of clinical properties shows potential to be
use as an outcome measure for respiratory therapy
but this has not been previously explored. At this
point in time, the data related to lung sounds are
complex and time consuming to analyse. To be
clinically useful as an outcome measure it will be
essential to simplify, and increase the speed of, the
analytical process. Furthermore the cost-
effectiveness of implementing this outcome measure
is unknown. Validation of CALSA as a responsive
outcome measure is challenging because of the lack
of a gold standard respiratory therapy measure with
which to compare it. Studies relating to
responsiveness to change of lung sounds before and
after an intervention of known effect e.g.
bronchodilators, suction, would help in clarifying
the responsiveness of the measure, and increasing
understanding of the validity of CALSA in clinical
settings. Nevertheless, the aims proposed at the
outset for this research have been achieved.
This preliminary study suggests that the use of
CALSA to characterise crackles is reproducible over
short time periods in cystic fibrosis outpatients. This
finding gives initial confidence that CALSA has
potential as an outcome measure for respiratory
therapy interventions (such as physiotherapy for
airway clearance) but further evaluation is
We would like to thank the Fundação para a Ciência
e Tecnologia (FCT), Portugal, for funding one of the
authors (Alda Marques) during her PhD studies.
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