Physiological Reports ISSN 2051-817X
ORIGINAL RESEARCH
Airway obstruction, dynamic hyperinflation, and breathing
pattern during incremental exercise in COPD patients
Bente Frisk1,2, Birgitte Espehaug1, Jon A. Hardie2, Liv I. Strand3,4, Rolf Moe-Nilssen3, Tomas M. L.
Eagan2,5, Per S. Bakke2,5 & Einar Thorsen2,6
1
2
3
4
5
6
Centre for Evidence-Based Practice, Bergen University College, Bergen, Norway
Department of Clinical Science, University of Bergen, Bergen, Norway
Department of Global Public Health and Primary Care, University of Bergen, Bergen, Norway
Department of Physiotherapy, Haukeland University Hospital, Bergen, Norway
Department of Thoracic Medicine, Haukeland University Hospital, Bergen, Norway
Department of Occupational Medicine, Haukeland University Hospital, Bergen, Norway
Keywords
Chronic obstructive pulmonary disease,
exercise, inspiratory capacity, spirometry.
Correspondence
Bente Frisk, Centre for Evidence-Based
Practice, Bergen University College, Pb. 7030,
5020 Bergen, Norway.
Tel: +47 55 58 71 43
E-mail: bente.frisk@hib.no
Funding Information
No funding information provided.
Received: 28 November 2013; Revised: 7
January 2014; Accepted: 9 January 2014
doi: 10.1002/phy2.222
Physiol Rep, 2 (2), 2014, e00222,
doi: 10.1002/phy2.222
Abstract
Ventilatory capacity is reduced in chronic obstructive pulmonary disease
(COPD) patients. Tidal volume (VT) is lower and breathing frequency higher
at a given ventilation (VE) compared to healthy subjects. We examined
whether airflow limitation and dynamic hyperinflation in COPD patients were
related to breathing pattern. An incremental treadmill exercise test was performed in 63 COPD patients (35 men), aged 65 years (48–79 years) with a
mean forced expiratory volume in 1 sec (FEV1) of 48% of predicted
(SD = 15%). Data were averaged over 20-sec intervals. The relationship
between VE and VT was described by the quadratic equation VT =
a + bVE + cVE2 for each subject. The relationships between the curve parameters b and c, and spirometric variables and dynamic hyperinflation measured
as the difference in inspiratory capacity from start to end of exercise, were
analyzed by multivariate linear regression. The relationship between VE and
VT could be described by a quadratic model in 59 patients with median R2 of
0.90 (0.40–0.98). The linear coefficient (b) was negatively (P = 0.001) and the
quadratic coefficient (c) positively (P < 0.001) related to FEV1. Forced vital
capacity, gender, height, weight, age, inspiratory reserve volume, and dynamic
hyperinflation were not associated with the curve parameters after adjusting
for FEV1. We concluded that a quadratic model could satisfactorily describe
the relationship between VE and VT in most COPD patients. The curve
parameters were related to FEV1. With a lower FEV1, maximal VT was lower
and achieved at a lower VE. Dynamic hyperinflation was not related to breathing pattern when adjusting for FEV1.
Introduction
Exertional dyspnea is one of the main factors limiting
physical activity in patients with chronic obstructive pulmonary disease (COPD) (O’Donnell and Webb 1993;
Maltais et al. 2005; Nici et al. 2006). At a given expired
minute ventilation (VE), the tidal volume (VT) is lower
and the breathing frequency (Bf) higher in patients having
COPD compared to healthy subjects (Palange et al.
2007). The maximal ventilatory capacity is reduced
(Gallagher 1994), and is closely related to forced expired
volume in 1 sec (FEV1) (Clark et al. 1969; Potter et al.
1971).
The mechanism for the ventilatory limitation in COPD
is related to expiratory flow limitation and lung hyperinflation. The time constant for the lung, which is the
ª 2014 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of
the American Physiological Society and The Physiological Society.
This is an open access article under the terms of the Creative Commons Attribution License,
which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
2014 | Vol. 2 | Iss. 2 | e00222
Page 1
Breathing Pattern in COPD
B. Frisk et al.
product of resistance and compliance, is increased and
during progressively higher ventilatory demands, expiration may not be completed before the drive for the next
inspiration starts (Hyatt 1983). End-expiratory lung volume increases, and breathing takes place at a higher lung
volume where both resistance and compliance are lower.
The effect of these changes in lung volume, resistance,
and compliance is a shorter time constant allowing complete respiratory cycles, but it is at the cost of a higher
work of breathing (Hyatt 1983; O’Donnell and Webb
1993). The VT is constrained by total lung capacity (TLC)
on the inspiratory side. The inspiratory reserve volume
(IRV) falls by increasing VT. Expiratory constraints are
more complex, influenced by the increased time constant
and inspiratory drive (Peters et al. 2006).
The relationship between VE and VT during incremental exercise can be described by three phases (Gallagher
et al. 1987). In the first phase, there is an almost linear
relationship between VE and VT. In the second phase, the
increase in VE is mainly caused by an increase in Bf and a
smaller increase in VT. In the third phase, the increase in
VE is caused by an increase in Bf only, and by the end of
this phase there can be a fall in VT (Gallagher et al.
1987). The relationship between VE and VT has previously
been described by various methods such as the maximal
VT (VTmax) or the plateau of VT and the inflection point
(O’Donnell et al. 2006), VTmax and VT at a VE of 30 L/
min (Cotes 1972), VT at given fractions of peak VE
(Neder et al. 2003), and the slope and intercept of the
first part of the response (Hey et al. 1966). However, neither of these methods account for the curvilinearity of
the response. In young healthy subjects, the individual
relationship between VE and VT has been described satisfactorily by a quadratic (Kalsas and Thorsen 2009) and a
logarithmic (Naranjo et al. 2005) relationship, but it is
not known whether these models are applicable for the
general population or patients with lung disease.
The aim of this cross-sectional study was to examine
whether a quadratic model could satisfactorily describe
the relationship between VE and VT during exercise in
COPD patients. The hypothesis was that the curve parameters of the quadratic model, which describe the breathing
pattern, were related to FEV1, IRV, and dynamic hyperinflation.
Methods
Subjects
Of the 433 patients included in the Bergen COPD Cohort
study (Eagan et al. 2010), 89 patients participated in a
pulmonary rehabilitation program during the first 2 years
of follow-up in 2006–2008. In 2011–2012, 63 of these
2014 | Vol. 2 | Iss. 2 | e00222
Page 2
patients were available for a cardiopulmonary exercise test
on a treadmill. The remaining 26 patients were deceased
or disabled.
The included patients had clinically stable COPD in
Global Initiative for Chronic Obstructive Lung Disease
(GOLD) (Rabe et al. 2007) stages II–IV and age between
48 and 79 years. Thirty-two subjects were in stage II, 23
in stage III, and eight in stage IV. All patients had a
smoking history of ≥10 pack-years, a postbronchodilation
FEV1 to forced vital capacity (FVC) ratio <0.7 and a postbronchodilator FEV1 <80% of predicted value according
to Norwegian reference values (Johannessen et al. 2006).
Patients with inflammatory disorders like rheumatoid
arthritis, systemic lupus erythematosus or other connective tissue disorders, inflammatory bowel disease, and any
active cancer in the last 5 years were not included in the
Bergen COPD Cohort study. Exclusion criteria for exercise testing were major cardiovascular disorders, a partial
pressure of oxygen in arterial blood less than 8 kPa at
rest, or exacerbations that required medical treatment
during the last 4 weeks prior to testing. The patients were
examined by a physician prior to exercise testing.
Ethics
The Western Norway Regional Research Ethics Committee approved the study. Participation in the study was
voluntary. Written and oral information was given and
written consent was obtained prior to inclusion.
Spirometry
Spirometry was conducted on a Viasys Masterscope
(Viasys, Hoechberg, Germany) before the exercise test
according to the ATS/ERS Standardization of Lung Function Testing (Miller et al. 2005). The FVC and FEV1 were
taken as the highest values from at least three satisfactory
expiratory maneuvers. The spirometer was calibrated
before each test with a 3-L calibration syringe. The body
mass index (BMI) was calculated as the body mass
divided by the square of height.
Cardiopulmonary exercise test
The patients completed an incremental exercise test to
their symptom-limited maximum on a treadmill (Woodway, model: PPS 55 med Weiss, Weil am Rhein,
Germany). The exercise protocol was a modified Bruce
protocol (Bruce 1971; Bruce et al. 1973), and started with
rest in the standing position for 2 min. The warm-up
phase lasted for 1 min with a walking speed of 1.5 km/h.
Blood pressure, electrocardiography (GE Healthcare, Cardio Soft EKG, Freiburg, Germany) and pulse oximetry
ª 2014 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of
the American Physiological Society and The Physiological Society.
B. Frisk et al.
were monitored at rest, continuously during the test and
for 3 min into the recovery phase. A tight-fitting oronasal
mask was adjusted to each patient and checked for leaks
before starting the exercise. The integrated exercise testing
system (Care Fusion, Vmax Spectra 229, Hochberg, Germany), was calibrated every morning and immediately
before each test. The VT, Bf, oxygen uptake (VO2), carbon
dioxide production (VCO2), and heart rate (HR) were
measured on a breath by breath basis and averaged over
20-sec intervals. VE and VT were corrected to the body
temperature pressure saturated (BTPS) condition, and
VO2 and VCO2 to the standard temperature pressure
(STPD) condition.
The patients graded their level of dyspnea and leg discomfort by the Borg CR10 Scale (Borg 1998) before the test
started, every second minute during the test, and at peak
exercise. In order to measure hyperinflation during exercise, serial measurements of inspiratory capacity (IC) as
described by O’Donnell and Webb (1993) were performed.
Measurements were taken before the start of exercise, every
second minute during exercise and at peak exercise.
Patients who had a decrease in IC from rest to peak exercise (DIC) ≥0.4 L (O’Donnell and Laveneziana 2006a) were
characterized as hyperinflators, the rest as nonhyperinflators. We also calculated DIC adjusted for resting IC
(DICadj.). A reduction in DICadj. ≥20% was used as cut-off
limit for comparison of the subjects (O’Donnell and Laveneziana 2006b). The IRV was calculated as the difference
between IC at the end of the test minus the preceding VT.
Breathing Pattern in COPD
Estimated regression coefficients are presented with
95% confidence intervals (CI) and P-values. The significance level was set at 0.05. The data analyses were performed using IBM SPSS Statistics 21 (SPSS Inc. Chicago,
IL).
Results
Subject characteristics and resting pulmonary function
measurements are summarized in Table 1. The patients
were airflow limited with a mean FEV1 of 48% of the
predicted value (Fig. 1). Thirty-two patients were categorized as hyperinflators with a DIC ≥0.4 L and 31 as nonhyperinflators. The same result was demonstrated when
using a DICadj. ≥20% as cut off. The distribution of DIC
from rest to peak exercise is illustrated in Figure 2. Of
the hyperinflators 72% were men, and of the nonhyperinflators 39%. The peak responses to treadmill exercise are
presented in Table 2. There were no significant differences
in exercise time, VO2peak, VCO2peak, VEpeak, HRpeak, Borg
scores, and desaturation between the hyperinflators and
nonhyperinflators. Fifty-three (84%) of the patients
stopped exercise due to dyspnea or dyspnea in combination with leg discomfort. Ten (16%) patients stopped due
to leg discomfort only. There was approximately 10% difference in ventilation and exercise time between hyperinflators and nonhyperinflators, and the difference was
related to anthropometric characteristics and gender.
There were more men among the hyperinflators and
more women among the nonhyperinflators.
Statistical analyses
Descriptive statistics were used to characterize the study
population (mean, standard deviation [SD], and percent).
Independent samples t-tests were used to compare continuous variables and Pearson v2 tests for categorical variables. The relationship between VE and VT was described
for each individual by the quadratic model VT =
a + bVE + cVE2. The goodness of fit for the individual
patient-specific regression analysis was evaluated by the
adjusted coefficient of determination (adjusted R2) and
the F-statistic. For the latter a P-value <0.05 was required
for inclusion of the patient in further analysis. The relationship between the estimated curve parameters in the
quadratic model, the intercept (a), the slope (b), and the
curvature (c), respectively, and age, gender, height, weight,
FEV1, FVC, IRV, and DICadj. were analyzed by bivariate
and multivariate linear regression analysis. IC at rest was
also used in the multivariate analysis, but was not significant and therefore excluded from the final model.
The goodness of fit of the quadratic model was compared with the goodness of fit by a hyperbolic (inverse)
model of the form VT = a + bVE 1.
Table 1. Characteristics of the study population.
Variables
Age (years)
Pack years
Height (m)
Body
mass (kg)
BMI
FEV1 (L)
FEV1 (%
pred)
FVC (L)
FVC (%
pred)
FEV1/FVC
(%)
IC (L)
Total
(n = 63)
Women
(n = 28)
Men
(n = 35)
P-value
65.7
37.2
1.70
76.0
64.3
30.3
1.63
68.1
66.8
42.8
1.75
82.4
5.6
23.3
0.1
16.1
0.089
0.028
<0.001
0.001
6.0
22.1
0.1
17.4
6.2
18.7
0.1
15.7
26.2 5.0
1.5 0.6
48.0 14.8
25.5 5.4
1.2 0.4
48.9 13.0
26.8 4.7
1.6 0.6
47.3 16.2
0.330
0.002
0.667
3.1 0.9
82.8 15.3
2.6 0.6
83.9 16.2
3.6 0.8
81.9 14.7
<0.001
0.615
46.0 11.1
47.0 10.2
45.2 12.0
0.537
2.2 0.8
1.8 0.5
2.6 0.8
<0.001
Data are presented as mean SD. Independent t-test for
continuous variables. BMI, body mass index; FEV1, forced expiratory
volume in 1 sec; FVC, forced vital capacity; IC, inspiratory capacity.
ª 2014 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of
the American Physiological Society and The Physiological Society.
2014 | Vol. 2 | Iss. 2 | e00222
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Breathing Pattern in COPD
B. Frisk et al.
In the multivariate linear regression analyses, the linear
coefficient (b) was negatively (P = 0.001) and the quadratic coefficient (c) positively (P < 0.001) related to
FEV1. Age, gender, height, weight, FVC, IRV, and DICadj,
were not associated with the curve parameters after
adjusting for FEV1 (Table 3).
The VTmax and VE at VTmax were calculated from the
individual quadratic relationships. In adjusted linear
regression analyses, both were related to FEV1 (P < 0.001),
but not to age, gender, height, weight, FVC, and DIC.
When using the hyperbolic model, the mean constant
was 1.70 (SD = 0.52), and the curvature 14.73 (SD =
8.87). The median R2 was 0.84 (range 0.25–0.95) which was
lower than for the quadratic relationship.
12
10
Frequency
8
6
4
2
0
20
30
40
50
60
70
80
FEV1 (% pred.)
Figure 1. The distribution of FEV1 in% of predicted.
12
10
Frequency
8
6
4
2
0
–1.60 –1.40 –1.20 –1.00 –0.80 –0.60 –0.40 –0.20 0.00
0.20
0.40
Change in IC from rest to peak exercise, L
Figure 2. The distribution of change in inspiratory capacity (IC)
from rest to peak exercise.
In 59 patients, the P-value of the F-statistic for the
quadratic model was <0.05 and the R2 ranged from 0.40
to 0.98 (median of 0.90). Four patients were excluded
from further analysis, because in the individual analysis
the goodness of fit was not statistically significant. In
these patients, the exercise time was short and few data
points were available for computing the regression curve.
Two of these patients were in GOLD stage III and two in
GOLD stage IV. Figure 3 shows a random set of 14 individual responses and the mean response for the 59
patients. The mean of the estimated constant (a) was
0.18 (SD = 0.44), the mean linear coefficient (b) was
0.076 (SD = 0.035), and the mean quadratic coefficient
(c) was 0.00102 (SD = 0.00080).
2014 | Vol. 2 | Iss. 2 | e00222
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Discussion
The main findings of this study were: (1) The relationship between VT and VE during incremental exercise
could be described by a quadratic model in most COPD
patients. (2) The linear and quadratic curve parameters
were both related to FEV1. With a lower FEV1, maximal
VT was lower and achieved at a lower VE. (3) Dynamic
hyperinflation and IRV were not related to the curve
parameters.
When using a curvilinear model to describe the relationship between VE and VT, all observations throughout
the incremental exercise test are included in the analysis,
and a detailed description of the test from start to end is
provided. A limitation with other methods used to
describe the relationship between VE and VT like the Hey
et al. (1966) plot, the VT30 and VTmax (Cotes 1972), and
VT at given fractions of peak VE (Neder et al. 2003), is
that all observed data from the exercise test are not
included in the analysis. The exercise tests in these studies
were done on a cycle ergometer, and in the studies of
Cotes (1972) and Hey et al. (1966) the tests were submaximal. Breathing pattern was different with treadmill
exercise compared with cycle exercise in a study of young
and healthy subjects (Kalsas and Thorsen 2009), but no
differences in breathing pattern were observed comparing
maximal and submaximal incremental exercise test on a
cycle ergometer (Kjelkenes and Thorsen 2010). The VT30
require that a ventilation of at least 30 L/min is achieved.
In our study, 16 of the COPD patients had a peak ventilation below 30 L/min. We did not use a logarithmic
model as described by Naranjo et al. (2005), because it
does not account for VT having a maximal value.
The quadratic model could not be used for all COPD
patients in this study. Four patients were excluded from
further analysis because the P-value of the F-statistic in
the individual analysis was not significant. The exercise
time was short and thereby few data points were available
ª 2014 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of
the American Physiological Society and The Physiological Society.
B. Frisk et al.
Breathing Pattern in COPD
Table 2. Peak responses to incremental exercise test on treadmill.
Variables
Total (n = 63)
Hyperinflators
(n = 32)
Nonhyperinflators
(n = 31)
P-value
Gender, male/female (n)
Exercise time (min)
VO2peak (L/min)
VCO2peak (L/min)
VEpeak (L/min)
HRpeak (bpm)
Dyspnea (Borg Scale)
Leg discomfort (Borg Scale)
DIC(L)
SpO2% start
SpO2% end
35/28
6.4 2.2
1.36 0.5
1.34 0.67
47.3 19.6
133 19
8.7 1.6
5.5 3.0
0.46 0.33
95.9 2.5
89.6 5.1
23/9
6.6 2.0
1.48 0.5
1.43 0.7
49.3 20.3
132 18
8.8 1.6
5.4 2.5
0.72 0.25
95.4 2.7
89.2 5.5
12/19
6.3 2.4
1.23 0.5
1.25 0.7
45.3 18.9
134 20
8.6 1.6
5.7 3.4
0.20 0.15
96.5 2.3
90.0 4.7
0.572
0.065
0.308
0.419
0.711
0.626
0.666
<0.001
0.083
0.533
Data are presented as mean SD, unless otherwise stated. VO2, oxygen uptake; VCO2, carbon dioxide production; VE, ventilation, tidal
volume; HR, heart rate; DIC, inspiratory capacity, IC at the start of the test minus IC at the end of the test; SpO2, oxygen saturation.
2
Tidal volume (Liter)
1.5
1
0.5
0
0
10
20
30
40
50
60
70
80
90
100
Minute venƟlaƟon (Liter min–1)
–0.5
Figure 3. A random set of 14 individual responses (thin lines) and
the mean response for the 59 patients (bold line).
for mathematical description of the response in these
patients. We considered other mathematical models for
all subjects including a hyperbolic model, but with respect
to R2, the parabolic was best. For the four excluded subjects, none of these models were applicable. COPD is a
progressive disease and in a general COPD population,
not all patients will have the functional capacity to complete an incremental exercise test, which is a strenuous
maneuver.
Incomplete expiration leads to accumulation of gas in
the lung, and a given ventilatory demand can only be
sustained when breathing takes place at a lung volume
having a time constant that allows complete respiratory
cycles. FEV1 is the integrated sum of maximal expiratory
flow rates during the first second of a forced exhalation.
Maximal expiratory flow rates are determined by airway
diameter, compliance of the airway wall, and gas density
(Pedersen et al. 1985). COPD is characterized by loss of
elastic properties throughout the lung, not specifically
located to the airways or the alveolar region (Hogg 2012).
In this way, FEV1 is related to both resistance and compliance, and thereby to the time constant, which is the
product of the two. A relationship between FEV1 and the
curve parameters determining the breathing pattern is
therefore not unexpected.
The TLC is expected to remain unaltered during exercise, and therefore dynamic hyperinflation can be
described as a reduction in IC from start to end of the
exercise test (∆IC), when end-expiratory lung volume
(EELV) increases (Stubbing et al. 1980; Yan et al. 1997;
Vogiatzis et al. 2005). In our study, there was no correlation between FEV1 and ∆IC, and as far as we know a
relationship between FEV1 and ∆IC has not been demonstrated in other studies. We found no relationship
between ∆IC and the curve parameters. The hyperinflators in this study were not different from the nonhyperinflators with respect to FEV1 in percent of predicted,
VO2peak, VEpeak, and Borg dyspnea score at the end of
the test. Desaturation was the same in both groups as
well. In young healthy subjects, the individual relationship between VE and VT has been described satisfactorily
by a quadratic relationship (Kalsas and Thorsen 2009),
and normal healthy subjects does not hyperinflate during
progressive exercise. This may suggest that dynamic
hyperinflation is primarily a mechanism for adjusting
the time constant of the lung to expiratory flow
limitation and is not a determinant of breathing pattern
per se.
In healthy subjects, the breathing pattern with respect
to VT and Bf has traditionally been considered a load
ª 2014 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of
the American Physiological Society and The Physiological Society.
2014 | Vol. 2 | Iss. 2 | e00222
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Breathing Pattern in COPD
B. Frisk et al.
Table 3. The relationships between the curve parameters and explanatory variables.
Unadjusted
Variable
Curve parameter a
Age
Gender
Height
Weight
FEV1
FVC
DICadj.
IRV
Adjusted
95% CI
B
P-value
B
0.003
0.088
0.273
0.001
0.192
0.005
0.006
0.240
0.761
0.448
0.666
0.732
0.059
0.938
0.187
0.224
0.009
0.039
1.209
9.9 9 10
0.560
0.184
0.001
0.066
0.632
0.133
0.774
0.927
0.016
0.874
0.026
0.218
0.001
0.014
0.041
12.3 9 10
0.053
0.016
20.1 9 10
0.007
P-value
1
Curve parameter b1
Age
Gender
Height
Weight
FEV1
FVC
DICadj.
IRV
36.6 9 10
0.014
0.015
2.4 9 10
0.020
0.001
0.001
0.020
5
Curve parameter c1
Age
Gender
Height
Weight
FEV1
FVC
DICadj.
IRV
1.4 9
9.1 9
0.002
0.6 9
0.001
27.0 9
1.8 9
0.001
10
10
5
10
5
10
10
5
5
5
5
0.422
0.668
0.193
0.334
<0.001
0.026
0.026
0.013
1.4 9
38.1 9
0.001
0.2 9
0.001
12.6 9
0.6 9
6.6 9
5
5
5
10
10
5
10
5
10
10
10
5
5
5
5
0.013
0.375
3.497
0.008
0.151
0.477
0.010
0.448
to
to
to
to
to
to
to
to
0.030
0.296
1.079
0.009
0.968
0.110
0.012
0.580
0.423
0.814
0.294
0.981
0.008
0.215
0.847
0.798
0.002
0.011
0.131
0.001
0.083
0.006
0.001
0.032
to
to
to
to
to
to
to
to
0.001
0.040
0.213
0.001
0.022
0.038
0.001
0.045
0.439
0.254
0.633
0.698
0.001
0.157
0.622
0.726
1.82 to 4.72
0.001 to 13.52
0.004 to 0.003
1.52 to 1.12
0.001 to 0.002
0.001 to 32.52
2.32 to 1.12
0.001 to 0.001
0.389
0.144
0.663
0.810
<0.001
0.577
0.477
0.868
95% confidence interval (CI) examined by linear regression in multivariate analyses (P < 0.05). FEV1, forced expired volume in 1 sec; FVC,
forced vital capacity; DIC, inspiratory capacity, IC at the start of the test minus IC at the end of the test; DICadj., DIC adjusted for resting IC;
IRV, inspiratory reserve volume.
1
The relationship between VE and VT was described by a quadratic model (VT = a + bVE + cVE2).
2
Values are given multiplied by 10 5.
compensating mechanism to minimize the work of
breathing (Otis et al. 1950; Widdicombe and Nadel 1963;
Poon 1987). However, direct evidence for such a mechanism being operative is lacking. Dynamic hyperinflation
and a lower IRV are not load compensating mechanisms
and could therefore be independent phenomena. The
importance of hyperinflation can, however, not be
ignored as it is by itself related to dyspnea, respiratory
effort, and work of breathing. The constraint for the
expansion of VT on the inspiratory side set by TLC, and
how close the patients breathe in relationship to TLC, will
also be associated with a higher work of breathing.
The participants in this study had participated in a pulmonary rehabilitation program. The patients recruited
could therefore be biased to have higher functional capacity than the common COPD population. The distribution
among GOLD stages were 32 patients in stage II, 23 in
2014 | Vol. 2 | Iss. 2 | e00222
Page 6
stage III and eight in stage IV, respectively. There were
fewer patients with more serious disease as represented by
GOLD stage IV and the most severely ill patients were
not able to participate in the study. However, 49% of the
patients were in GOLD stages III and IV. We therefore
assume that our study population is representative for the
common COPD patients met in outpatient clinics or in
hospitals.
Conclusion
The curvilinear model provides a method to describe the
breathing pattern during exercise in most COPD patients.
The curve parameters were related to FEV1. With a lower
FEV1, maximal VT was lower and achieved at a lower VE.
Dynamic hyperinflation and IRV were not related to
breathing pattern when adjusting for FEV1.
ª 2014 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of
the American Physiological Society and The Physiological Society.
B. Frisk et al.
Acknowledgments
The authors thank Eli Nordeide, Lene Svendsen, and
Michael Storebø for participation in data collection and
for help in organizing the study. We also wish to
acknowledge the patients who participated in the study.
Conflict of Interest
None declared.
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