Open Journal of Respiratory Diseases, 2012, 2, 31-36 Published Online May 2012 (
Should We Measure the FEV1 or the Specific Resistance of
the Airways? An Evaluation in Patients with Either
COPD, Chronic Dyspnea or Chronic Cough*
Kinga Simon1, Viviane De Maertelaer2, André Noseda1
1The Pulmonary Division, CHU Brugmann, Brussels, Belgium
2IRIBHN and Department of Biostatistics and Medical Informatics (SBIM), Université Libre de Bruxelles, Brussels, Belgium
Received December 27, 2011; revised February 3, 2012; accepted February 13, 2012
Background: The purpose of this study was to evaluate the relative contribution of measuring the forced expiratory
volume in one second (FEV1) or the specific resistance of the airways (sRaw) in adults referred for chronic obstructive
pulmonary disease (COPD), chronic dyspnea or chronic cough. Methods: This was a prospective study of 321 subjects
referred for lung function testing, in a setting of routine clinical management, for suspicion of COPD (or follow-up of
known COPD), chronic dyspnea or chronic cough. The proportions of FEV1 values below the normal range and of
sRaw values above the normal range were compared using a Chi-square exact test of Fisher. Results: In the COPD and
chronic dyspnea groups, sRaw was as frequently abnormal as FEV1. In the chronic cough group, sRaw was increased in
56.5% of subjects, while FEV1 was decreased in solely 34.8% (p = 0.059). Conclusions: This study suggests that sRaw
may be a better tool than FEV1 to detect bronchial obstruction in patients presenting with chronic cough.
Keywords: Body Plethysmography; Chronic Cough; Specific Airway Resistance
1. Introduction
Spirometry is often considered as the gold standard for
determination of obstructive pulmonary disease. This
method has however several limitations, which has led to
resistance measurements used to clarify lung pathophysi-
ology [1]. In the recent years, there has been a progres-
sive disinterest in resistance measurements, at least in
adults. As an illustration, the 2005 American Thoracic
Society and European Respiratory Society statements did
not address resistance measurements [2]. In contrast,
resistance measurements are still considered to be par-
ticularly relevant in pediatric patients [3,4].
The present study was designed to evaluate the relative
contribution of measuring the forced expiratory volume
in one second (FEV1) or the specific resistance of the
airways (sRaw) in adults referred for lung function test-
ing (LFT) because of chronic obstructive pulmonary dis-
ease (COPD), chronic dyspnea or chronic cough. These
indications were chosen because the Belgian authorities
recently suggested that the usefulness of assessing airway
resistance is not proven in subjects with either COPD,
chronic dyspnea or chronic cough [5]. We hypothesized
that sRaw may be more sensitive than FEV1 in detecting
bronchial obstruction in these groups of patients. The
study was performed in a setting of routine clinical man-
agement and aimed to evaluate the percentage of subjects
with an abnormal test, namely a decreased FEV1 or an
increased sRaw.
2. Patients and Methods
2.1. Study Design
In the present study, we prospectively evaluated the sRaw
and the FEV1 in three groups of subjects (as defined un-
derneath) in a setting of routine clinical management.
The patients were referred by their physician for LFT
because of either COPD (group 1), chronic dyspnea (group
2) or chronic cough (group 3). The study protocol was
approved by the Ethics Committee of the CHU Brug-
mann and each subject gave written informed consent to
the analysis of his (her) lung function data.
2.2. Patients
The study was performed at the Pulmonary Division of
the CHU Brugmann between 26/10/2009 and 16/07/2010.
All males and females between 18 and 70 years old, re-
ferred for LFT because of COPD, chronic dyspnea or
chronic cough, were eligible provided they gave in-
formed consent and were able to perform technically
*Conflict of interes
: There is no conflict of interest.
opyright © 2012 SciRes. OJRD
acceptable spirometry and plethysmography. Exclusion
criteria included age less than 18 or more than 70, being
referred for LFT because of another diagnosis, being
unable to perform either spirometry or plethysmography,
as well as having any contra-indication to LFT (recent
heart attack or pneumothorax, recent ocular, abdominal
or thoracic surgery, suspicion of tuberculosis, severe
2.3. Constitution of Groups
The classification into three groups was made on basis of
the requests for LTF, written by the referring physicians.
Subjects were put into group 1 (COPD) when the request
mentioned a clinical suspicion of COPD or follow-up of
a known COPD. When the request mentioned several
respiratory diseases, subjects were included provided
COPD was listed as the first diagnosis (e.g. COPD plus
bronchectasis). Subjects were put into group 2 (chronic
dyspnea) and into group 3 (chronic cough) when the re-
quest respectively mentioned chronic breathlessness or
chronic cough as the first reason to perform LTF. Pa-
tients referred for LFT for other indications (e.g. preop-
erative work-up), or for several indications with another
diagnosis listed first (e.g. known lung fibrosis and
chronic cough) were not included.
2.4. Lung Function Testing
A Zan (Waldfenster, Germany) body plethysmograph
was used. Patients were instructed in the correct tech-
nique while the plethysmography door was open. During
the maneuvers, patients were asked to firmly seal their
lips around the mouthpiece. Airflow at the mouth was
displayed against box pressure and their relationship was
computed according to an automated procedure. Signals
were digitized over a period of five breaths, with a sam-
pling rate of 50 Hz. After correction for the thermal drift
induced by the subject’s heat production, a loop was ob-
tained and midpoints at flows of +0.5 (point A) and 0.5
l·s1 (point B) were obtained. The slope of the line be-
tween points A and B defined the sRaw. At least two
technically satisfactory five breath-loops were obtained
and the reported sRaw was an arithmetical mean. Spi-
rometry was performed with the door of the box open.
The subject was instructed to slowly inspire up to the
total lung capacity and to subsequently expire a forced
vital capacity. In each subject, two to three technically
acceptable curves were obtained. Optimization of the
reading of the expiratory curves was performed as rec-
ommended by Peslin et al. [6]. The FEV1 was defined as
the volume exhaled in one second during the perform-
ance of the forced vital capacity maneuver. When a
bronchodilation or a provocation test was performed,
only the baseline values were retained for analysis.
2.5. Interpretation of Lung Function Data
To calculate the predicted values for FEV1, the regres-
sion equations of Quanjer et al. [7] were used. An indi-
vidual FEV1 value was considered as below the normal
range if it was lower than the mean predicted minus 1.64
× standard deviation (SD). The data of Pelzer and Thom-
son [8] were used as mean predicted values for sRaw,
depending on gender. An individual sRaw value was
considered as above the normal range if it was higher
than the mean predicted value plus 2 × SD.
2.6. Statistical Analyses
The variables recorded in the three groups were com-
pared using either an ANOVA procedure (means com-
parisons in case of continuous variables) or a Chi-square
exact test (proportions comparisons in case of discrete
variables). More particularly, the proportions of FEV1
values below the normal range and of sRaw values above
the normal range were compared using a Chi-square ex-
act test of Fisher. The analysis was made separately in
the three groups (COPD, chronic dyspnea, and chronic
cough). The statistical software used was SPSS version
2.7. Retrospective Review of Medical Charts
Twelve months after the study had been completed, medi-
cal charts were examined to check for the final cause
retained for chronic dyspnea and for chronic cough, in
the chronic dyspnea and chronic cough groups.
3. Results
During the study period, 321 subjects were included. The
allocation into the three groups is listed in Table 1, as
well as the mean (SD) values obtained for FEV1 and
sRaw. COPD subjects were older than subjects with
chronic dyspnea or chronic cough. The proportion of
males in the COPD group was higher than in the chronic
dyspnea group, and tended to be higher (p = 0.074) than
in the chronic cough group. As expected, FEV1 was lower,
and sRaw higher, in the COPD group than in the other
two groups (Table 2).
Individual FEV1 values were plotted against predicted
values, separately in males and females. As shown in
Figure 1, measured values were predominantly under the
identity line in the COPD group, while a large proportion
of values were near the identity line in the chronic dysp-
nea and chronic cough groups (Figures 2 and 3). The
proportions of subjects with FEV1 within versus under
the normal range, as well as those of subjects with sRaw
within or above the normal range, are shown in Tab le 3.
In the chronic dyspnea and COPD groups, sRaw was as
frequently abnormal as FEV1 (exact Fisher test, p = 1.000
Copyright © 2012 SciRes. OJRD
Copyright © 2012 SciRes. OJRD
Table 1. Allocation of 321 subjects into three groups.
Group 1 (COPD)
n = 177
Group 2 (Chronic dyspnea)
n = 98
Group 3 (Chronic cough)
n = 46
Age (years) 58.1 (7.6) 46.5 (13.3) 50.0 (14.2)
Gender 114 M/63 F 45 M/53 F 23 M/23 F
FEV1 (% predicted) 53.0 (22.7) 81.1 (19.8) 84.9 (19.4)
sRaw (kPa.s) 3.21 (2.27) 1.44 (1.16) 1.44 (0.57)
M = males, F = females; Data in the table (continuous variables) are mean (SD).
Table 2. Comparison of age, gender, FEV1 and sRaw among groups, two by two.
COPD Chronic dyspnea COPD Chronic cough Chronic dypsnea chronic cough
Age (years) <0.001 <0.001 0.166
Gender 0.003 0.074 0.647
FEV1 (% predicted) <0.001 <0.001 0.590
sRaw (kPa.s) <0.001 <0.001 0.999
Data in the table are p values (Student t-test for age, FEV1, sRaw; Chi-square test for gender).
Table 3. Proportions of abnormal tests in 321 subjects.
Group 1 (COPD)
n = 177
Group 2 (Chronic dyspnea)
n = 98
Group 3 (Chronic cough)
n = 46
FEV1 normal 20.3% 59.2% 65.2%
Decreased 79.7% 40.8% 34.8%
sRaw normal 14.1% 59.2% 43.5%
Increased 85.9% 40.8% 56.5%
Figure 1. Measured values of forced expiratory volume in one second (FEV1) are plotted against predicted values, separately
in males and females, in the COPD group.
and p = 0.159, respectively). On the other hand, in the
chronic cough group, sRaw was increased in 56.5% of
subjects, while FEV1 was decreased in only 34.8% of
subjects; this difference reached near significance (exact
Fisher test, p = 0.059).
In the chronic dyspnea group, dyspnea remained un-
explained in 18 subjects (18%), including 10 subjects lost
for follow up. In the remainders, asthma, COPD, heart
failure and hyperventilation syndrome emerged as main
diagnoses (in, respectively, 32, 28, 7 and 6 subjects).
Less frequently, chronic dyspnea was ascribed to obesity
or beta blocker therapy (two subjects each) and to lung
Figure 2. Same legend as Figure 1, in the c hr onic dyspnea group.
Figure 3. Same legend as Figure 1, in the chronic cough group.
atelectasis, pericarditis or sarcoidosis (in a single subject
each). In the chronic cough group, cough remained un-
explained in 10 subjects (22%), including 5 subjects lost
for follow up. In the remainders, the main diagnoses
were cough-variant asthma, chronic rhinitis and sinusitis,
COPD, active smoking without COPD and post-infec-
tious cough (in respectively 12, 6, 4, 3 and 3 subjects).
Finally, angiotensin converting enzyme inhibitor-induced
cough, bronchectasis, gastroesophageal reflux and sar-
coidosis were diagnosed each in two subjects.
4. Discussion and Conclusions
Currently accepted indications for resistance measure-
ments include the evaluation of airflow limitation beyond
spirometry, the differentiation between different types of
obstructive pulmonary diseases having similar spiromet-
ric profiles, as well as the distinction of respiratory mus-
cle weakness from obstruction as the cause of low flow
rates [9]. In asthma, sRaw has been shown to be in-
creased in patients with no or few symptoms and normal
spirometric values [10]. In these particular patients, re-
sistance measurement is crucial to correctly diagnose
asthma. Similarly, the assessment of sRaw is useful in
patients with tracheal stenosis, compressive goiter or
other obstructive lesions of the larynx and the trachea
[11]. On the other hand, it is unknown whether measur-
ing sRaw in addition to FEV1 in patients with COPD,
chronic dyspnea or chronic cough caries relevant addi-
tional information. As sRaw is measured during tidal (or
near tidal) breathing maneuvers, it may reflect the func-
tional breathing status in current everyday life better than
FEV1, which is closer to breathing associated with
coughing, singing or laughing. Where lung pathophysi-
ology is concerned, sRaw is thought to predominantly
reflect the resistance of the large proximal airways [12].
Small distal airways may however contribute to some
extent, as sRaw is sensitive to a small airway disease like
bronchiolitis obliterans syndrome [13]. On these grounds,
it may be hypothesized that assessing sRaw may be rele-
vant in subject attending for COPD, chronic dyspnea or
chronic cough.
Determination of FEV1 has several advantages. Spi-
rometry requires only a simple equipment and FEV1
shows a rather good reproducibility. In our lung function
laboratory, the mean coefficient of variation of FEV1
amounts 5.0% in a population of COPD patients [14].
Furthermore, reduced FEV1 carries additional relevant
information, as FEV1 has been shown to be an inde-
Copyright © 2012 SciRes. OJRD
pendent marker for cardiovascular mortality [15]. On the
other hand, spirometry is in a large extent dependent on
the subject cooperation. As an illustration, as many as
11.5% of subjects are unable to perform correct forced
expiratory maneuvers, even in a young population with
little or no chronic lung disease [16]. Similarly, measur-
ing sRaw via plethysmography has advantages and dis-
advantages. The equipment is more sophisticated, more
expensive and the reproducibility is somewhat weaker
than that of FEV1, with e.g. a mean coefficient of varia-
tion of 9.3% in COPD patients in our laboratory [14]. On
the other hand, sRaw is more sensitive than FEV1 in a
large variety of clinical conditions. Studying the acute
effect of physiotherapy on lung function in patients with
copious sputum production, Cochrane et al. found a
greater change in sRaw than in FEV1 [17]. In patients
with heart-lung or bilateral lung transplantation, Bassiri
et al found that serial measurements of sRaw were more
useful than those of FEV1 for early detection of bron-
chiolitis obliterans syndrome [13].
In the present study involving three groups of patients
with respectively COPD, chronic dyspnea and chronic
cough, we were able to show a larger proportion of sub-
jects with increased sRaw than that of subjects with de-
creased FEV1 only in chronic coughers. The difference
drew near statistical significance. On retrospective ex-
amination of medical charts, cough-variant asthma
emerged as the most frequent diagnosis retained to ex-
plain chronic cough. Cough-variant asthma is known to
be a frequent cause of chronic cough, particularly in non-
smokers. It may be either a precursor of typical asthma,
or persist lifelong as the sole symptom of asthma [18].
Definite diagnosis is usually based on both histamine or
metacholine inhalation testing and on resolution of cough
with anti-asthmatic therapy [19]. Studies on larger
groups of patients are needed to evaluate whether sRaw
may be a better tool than FEV1 to detect mild bronchial
obstruction in patients presenting with chronic cough.
5. Acknowledgements
The authors acknowledge Mrs F. Daimallah, Ch. Jacobs
and J. Roobaert for performing lung function testing, as
well as Mrs F. Martinez Vadillo for secretarial assis-
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List of Abbreviations
COPD: chronic obstructive pulmonary disease
FEV1: forced expiratory volume in one second
LFT: lung function testing
SD: standard deviation
sRaw: specific resistance of the airways