Dynamic Hyperinflation and Exercise Intolerance in
Chronic Obstructive Pulmonary Disease
DENIS E. O’DONNELL, SUSAN M. REVILL, and KATHERINE A. WEBB
Respiratory Investigation Unit, Department of Medicine, Queen’s University, Kingston, Ontario, Canada
The role of dynamic hyperinflation (DH) in exercise limitation in
chronic obstructive pulmonary disease (COPD) remains to be defined. We examined DH during exercise in 105 patients with COPD
(FEV1 37 13% predicted; mean SD) and studied the relationships between resting lung volumes, DH during exercise, and
·
peak oxygen consumption ( V O2). Patients completed pulmonary
function tests and incremental cycle exercise tests. We measured
the change in inspiratory capacity (∆IC) during exercise to reflect
changes in DH. During exercise, 80% of patients showed significant DH above resting values. IC decreased 0.37 0.39 L or 14
15% predicted during exercise (p 0.0005), but with large variation in range. ∆IC correlated best with resting IC, both expressed
·
%predicted (r 0.50, p 0.0005). Peak V O2 (%predicted maximum) correlated best with the peak tidal volume attained (VT
standardized as % of predicted vital capacity) (r 0.68, p
0.0005), which, in turn, correlated strongly with IC at peak exercise (r 0.79, p 0.0005) or at rest (r 0.75, p 0.0005). The
extent of DH during exercise in COPD correlated best with resting
IC. DH curtailed the VT response to exercise. This inability to expand VT in response to increasing metabolic demand contributed
importantly to exercise intolerance in COPD.
Keywords: COPD; exercise; inspiratory capacity; dynamic lung hyperinflation; emphysema; dyspnea
Chronic obstructive pulmonary disease (COPD) is a heterogeneous disorder characterized by dysfunction of the small and
large airways, as well as destruction of the lung parenchyma
and its vasculature in highly variable combinations. The
pathophysiological hallmark of COPD is expiratory flow limitation, which, in more advanced disease, occurs even during
resting quiet breathing. As a consequence, resting lung volume (functional residual capacity [FRC]) is dynamically, and
not statically, determined. During exercise, as ventilatory demands increase in flow-limited patients, progressive air trapping and further dynamic lung hyperinflation (DH) above already increased resting values is inevitable (1, 2). Recent
studies have shown that DH during exercise contributes to
perceived respiratory discomfort (3, 4). Indirect evidence of
the importance of DH comes from studies that have demonstrated that alleviation of dyspnea following bronchodilator
therapy and lung volume reduction surgery (LVRS) was explained, in part, by reduced operating lung volumes (5, 6).
However, it is not clear from previous studies to what extent
the behavior of operating lung volumes during exercise influences peak exercise capacity in COPD. Moreover, earlier
studies have shown wide variability in the extent of DH with
(Received in original form December 26, 2000 and in revised form May 8, 2001)
Supported by the Ontario Thoracic Society and the Ontario Ministry of Health.
Correspondence and requests for reprints should be addressed to Dr. Denis
O’Donnell, Richardson House, 102 Stuart Street, c/o Kingston General Hospital,
Kingston, ON, K7L 2V7 Canada. E-mail: odonnell@post.queensu.ca
This article has an online data supplement, which is accessible from this issue’s
table of contents online at www.atsjournals.org
Am J Respir Crit Care Med Vol 164. pp 770–777, 2001
Internet address: www.atsjournals.org
exercise and the factors that determine this variability have
not been elucidated (3–7).
We hypothesized that DH and the consequent restrictive
constraints on volume expansion during exercise would contribute importantly to reduced exercise performance in COPD.
Although volume constraints are, by no means, an exclusive
source of ventilatory limitation in COPD, they are likely to be
important. They contribute to exertional dyspnea and influence breathing pattern responses during exercise. Furthermore, the operating lung volumes determine, in part, the magnitude of fractional inspiratory muscle force generation (relative
to maximum). High inflation volumes may also affect cardiac
performance and, thus, peripheral muscle function during exercise in COPD.
Therefore, the objectives of this study were (1) to determine the range and pattern of change in the various operating
lung volume components during incremental exercise in a
large COPD population; (2) to examine factors contributing
to the intersubject variability in DH during exercise; (3) to examine the relationship between resting hyperinflation,
further
·
DH during exercise, and symptom limited peak V O2; and (4)
to compare operating lung volumes and exercise performance
in a subgroup of patients with a more “emphysematous” clinical profile with patients who were matched for FEV1 but with
a better preserved diffusion capacity.
We conducted incremental cardiopulmonary cycle exercise
testing in 105 clinically stable patients with COPD and 25 healthy
age-matched control subjects. We measured and compared
ventilation, breathing pattern, operating lung volumes, metabolic factors, and exertional symptoms. We evaluated dynamic
changes in end-expiratory lung volume (EELV) from resting
FRC by collecting serial inspiratory capacity (IC) measurements throughout exercise, having established the reliability
of this measurement in a previous study (7).
METHODS
Subjects
We studied 105 clinically stable patients with COPD (FEV1 70%
predicted, FEV1/FVC 70%). Exclusion criteria included a history of
asthma, atopy, or nasal polyps; other active lung disease; significant
disease that could contribute to dyspnea or exercise limitation; and
oxygen desaturation to 75% during exercise on room air. Twentyfive age-matched ( 50 yr), healthy subjects were also studied.
Study Design
COPD subjects included patients who had performed pulmonary function tests (PFTs) and an incremental cycle exercise test during assessment before pulmonary rehabilitation or as part of screening prior to
entering various clinical research studies. Healthy normal subjects
were recruited from the local community to perform spirometry and
incremental cycle exercise tests for comparison of the behavior of operational lung volumes during exercise.
All subjects signed written informed consent at the time of their
first assessments and were aware that their test data might be used in
future analyses. Subjects were familiarized with all procedures prior
to collection of the test results evaluated in this study.
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O’Donnell, Revill, and Webb: Exercise and Hyperinflation in COPD
Procedures
Spirometry, body plethysmography, single-breath diffusing capacity
(DLCO), and maximal inspiratory mouth occlusion pressure (MIP)
were performed as previously described (8) (see online data supplement).
Chronic dyspnea was assessed using the modified Baseline Dyspnea
Index (9).
Symptom-limited incremental cycle exercise tests were conducted
as previously described (8) (see online data supplement). Subjects
breathed through a mouthpiece with noseclips in place. In the majority of subjects (COPD n 74, normal subjects n 20), flow signals
were sampled at 100 Hz using computer-based data acquisition software (CODAS; Dataq Instruments Inc., Akron, OH), from which
breath-by-breath measurements of volume, flow, and timing were calculated. In these subjects, expired air channelled through a 10-L mixing chamber was analyzed for fraction of O2 (S-3A Oyxgen Analyzer;
Applied Electrochemistry, Pasadena, CA) and CO2 (LB-2 Gas Analyzer; SensorMedics, Anaheim, CA). In all remaining subjects, breathby-breath measurements were collected using a Vmax229d Cardiopulmonary Exercise Testing Instrument (SensorMedics, Yorba Linda,
CA). Electrocardiography and pulse oximetry were monitored continuously. Blood pressure was auscultated at rest, each stage of exercise, peak exercise, and recovery. The modified Borg Scale (10) was
used to rate intensity of dyspnea (i.e., “breathing discomfort”) and leg
discomfort at rest, each stage of exercise, and peak exercise. Subjects
also specified why they stopped exercise.
Operational lung volumes: (see online data supplement). Changes
in operational lung volumes were derived from measurements of dynamic inspiratory capacity (IC), assuming that total lung capacity (TLC)
remained constant during exercise (11, 12). This has been found to be
a reliable method of tracking acute changes in lung volume (7, 11–13).
Techniques for performing and accepting IC measurements have been
previously reported (2, 3). IC was measured at the end of a steady-state
resting baseline, at 2 min intervals during exercise, and at end exercise.
Statistical Analysis
Results are means SD. COPD subgroup comparisons using unpaired Student’s t tests included (1) patients who had an emphysematous profile with DLCO 50% predicted and FRC 130% predicted
(Group A), versus patients with DLCO 50% predicted and FRC
130% predicted (Group B); (2) patients stopping primarily due to
breathing discomfort versus leg discomfort; and (3) patients with different patterns of DH with exercise.
Relationships between exercise capacity, exertional dyspnea, and
operational lung volumes in COPD were evaluated using Pearson’s
correlations. Stepwise multiple regression analysis established the best
·
predictive equations for peak V O2, Borg ratings of dyspnea, and DH
(dependent variables). Independent variables included standardized
·
exercise measurements of minute ventilation (V E), breathing pattern
·
·
(F, VT, TI, TE, TI/Ttot), gas exchange ( V CO2/ V O2, SpO2), volume constraints (IC, IRV, VT/IC, EELV, EILV), and DH (change in IC), as
well as resting pulmonary function and lung volume measurements
(expressed as % of predicted normal).
RESULTS
Subject Characteristics
Subject characteristics are summarized in Table 1. In the COPD
group as a whole, there was a wide range of airflow obstruction (FEV1 from 12 to 68% predicted), lung hyperinflation
(plethysmographic FRC from 94 to 307% predicted), diffusing
capacity (DLCO from 16 to 121% predicted), and chronic activity-related dyspnea (modified Baseline Dyspnea Index focal
scores from 2 “very severe” to 9 “mild”). The healthy control
subjects had normal spirometry and were well matched for
age, sex, and body mass index.
The majority of patients with COPD (80%) stopped exercise due to severe breathing discomfort, either alone or in
combination· with leg discomfort, at a low peak oxygen consumption ( V O2) (Table 2, Figure 1). In contrast, the majority
of normal subjects (76%) stopped exercise primarily because
of leg discomfort. Compared with normal subjects during exercise, patients
with
COPD had significantly greater ventila·
·
tory slopes ( V E/ V CO2) and reduced ·ventilatory reserve at end
exercise. In this latter regard, peak V E expressed as a percentage of maximal ventilatory capacity (MVC estimated as 40
FEV1) was 92 31% versus 64 22% in patients with COPD
and normal subjects, respectively (p 0.0005). Also of note,
the exercise breathing pattern was significantly more rapid
and shallow in patients with COPD than in normal subjects.
The patients with COPD who stopped exercise primarily
due to breathing discomfort (n 64) had significantly greater
resting airflow limitation (i.e., decreased FEV1) and thoracic
hyperinflation (i.e., increased FRC with reduced IC) than
those who stopped primarily due to leg discomfort (n 19)
(Table 3). Those limited by dyspnea also had greater impairment in dynamic mechanics during exercise, that is, significantly reduced IC and IRV, increased EILV/TLC and VT/IC,
and less VT expansion during exercise (Table 3).
Measurements of Operational Lung Volumes
There are no current equations for predicting normal spirometric IC values, therefore, a predicted normal value for IC
was calculated as predicted TLC minus predicted FRC. In our
normal sample, mean resting IC was 3.11 1.13 L or 110
32% predicted; the latter value indicates that this method of
calculating a predicted normal value for IC was reasonable, or
possibly an underestimation, in this older population. In the
COPD sample, mean resting IC was significantly reduced at
1.89 0.72 L or 69 23% predicted, with measurements as
TABLE 1. SUBJECT CHARACTERISTICS*
COPD
(n 105)
Normal Subjects
(n 25)
p Value†
Male %: female %
Age, yr
Height, cm
Weight, kg
Body mass index, kg/m2
Baseline dyspnea index
64:36
66 8
167 91
72.6 18.6
25.8 5.7
5.0 1.5
60:40
63 7
70 10
76.3 14.7
26.4 4.0
11.9 0.6
NS
NS
NS
NS
NS
0.0005
Pulmonary function
FEV1, L
%predicted
FVC, L
%predicted
FEV1/FVC, %
%predicted
PEFR, L/s
%predicted
FEF50%, L/s
%predicted
MIP, cm H2O
%predicted
TLC, L
%predicted
FRC, L
%predicted
RV, L
%predicted
DLCO, ml/min/mm Hg
%predicted
0.94 0.40
37 13
2.18 0.75
60 17
43 10
62 14
2.98 1.04
43 12
0.43 0.27
11 6
54 31
65 35
7.21 1.67
122 20
5.53 1.55
174 43
4.71 1.49
219 71
11.4 5.3
57 21
2.85 0.85
106 16
3.81 1.15
100 16
75 6
106 9
6.96 2.46
94 21
3.55 1.49
85 33
79 40
94 38
ND
0.0005
0.0005
0.0005
0.0005
0.0005
0.0005
ND
ND
ND
Definition of abbreviations: COPD chronic obstructive pulmonary disease; DLCO
diffusing capacity of the lung for CO; FEF forced expiratory flow; FRC functional
residual capacity; MIP maximum inspiratory pressure; ND not done in the normal
group; NS not significantly different; PEFR peak expiratory flow rate; RV residual
volume; TLC total lung capacity.
* Values are mean SD. Normal predicted values for spirometry, maximal expiratory
flow rates, lung volumes, diffusing capacity, and inspiratory muscle strength are from
references 14 through 18, respectively.
†
Comparisons were made for demographics and %predicted values only.
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TABLE 2. SYMPTOM-LIMITED INCREMENTAL CYCLE EXERCISE
Reason for stopping exercise (% of Group)
Breathing discomfort
Leg discomfort
Both breathing and legs
Other§
·
V O2, ml/kg/min
Heart rate, beats/min
SaO2, %
·
V E, L/min
· ·
V E/ V CO2, %
F, breaths/min
TI/Ttot
VT, L
VT, %predicted VC
VT/IC, %
IC, L
IC, %predicted
∆IC from rest, L
∆IC from rest, %predicted
IRV, L
IRV, %predicted TLC
EILV/TLC, %
EILV, %predicted TLC
COPD
Peak Exercise
(n 105)
Normal Subjects
Peak Exercise
(n 25)
Normals at a VE
Similar to Peak COPD
(n 25)
61
18
19
2
12.6 5.0
69 10
91 6
33.1 14.6
41.2 9.6
30.3 6.5
0.36 0.06
1.10 0.44
31 10
74 14
1.52 0.06
55 2
0.37 0.39
14 15
0.42 0.33
76
94 5
115 22
8*
76*
12
4
31.3 13.7*
83 11
96 2*
73.9 35.8*
32.7 3.8*
30.7 6.0
0.48 0.04*
2.41 1.04*
63 19*
74 15
3.26 1.30*
114 34*
0.17 0.46*
4 14*
0.85 0.59*
14 9*
ND
ND
15.9 3.6†
63 10
97 2*
32.4 4.9
34.3 7.4†
22.8 5.4*
0.45 0.06*
1.48 0.33*
40 9*
52 4*
3.23 1.29*
112 34*
0.13 0.51*
3 16*
1.75 1.16*
28 17*
ND
ND
Definition of abbreviations: COPD chronic obstructive pulmonary disease; EILV end-inspiratory lung volume; F respiratory frequency; IC inspiratory capacity; IRV inspiratory reserve volume; ND not done; SaO2 arterial oxygen saturation; TI/Ttot respira·
·
tory duty cycle; TLC total lung capacity; V E minute ventilation; VT tidal volume; V O2 oxygen consumption.
* p 0.0005, †p 0.01. Significant difference from COPD.
§
Other in COPD was due to general fatigue (n 1) and discomfort from noseclips (n 1); other in normal subjects was discomfort on
the bicycle seat (n 1).
Values are mean SD.
low as 0.74 L or 23% predicted. The 95% CI for resting IC
measurements was 0.14 L or 4.5% predicted within the
COPD group, indicating that a reproducibility criteria of
within 150 ml, or approximately 10%, may be appropriate for
testing IC in this population. The 95% CI for peak IC was similar at 0.12 L or 3.9% predicted.
During exercise in COPD, IC decreased significantly by
0.37 0.39 L (p 0.0005), with the change (∆) in IC ranging
between 1.42 and 0.77 L (Figures 1 and 2): this corresponds to a mean ∆IC of 18 19% or 14 15% predicted. On
average, the reduction· in IC occurred progressively throughout
exercise, with ∆IC/∆·V E during the first 2–3 min of exercise
matching the ∆IC/∆ V E in the later stages of exercise. In contrast to COPD, there was no significant change in IC from rest
to peak exercise in the normal group (∆IC 0.17 0.46 L or
4 14% predicted), although ∆IC ranged between 0.56 and
1.14 L (Figures 1 and 2). Whereas 80% of patients with
COPD significantly decreased IC during exercise (i.e., outside
the 95% confidence limits or 4.5% predicted), the majority
of our older normal subjects either increased (40% of subjects) or did not change (40% of subjects) IC during exercise.
In COPD, the VT response to exercise was limited from
both above (i.e., the TLC envelope) and below (i.e., due to a
reduced IC, which decreased even further as ventilation increased) (Figure 3). At the peak of symptom-limited exercise,
patients breathed with a tidal end-inspiratory lung volume
(EILV) that approached, but never quite reached, their TLC.
We defined this upper volume boundary as the “minimal
IRV” that could be achieved during exercise, and set its level
at the lower 95% confidence limit for peak IRV in this COPD
group (i.e., 0.35 L or 5.9% of the predicted TLC). Over half of
our patients with COPD reached a “minimal IRV” 5.9%
predicted TLC (n 56), with 15 of these patients still having
apparent
ventilatory reserve by traditional estimates (i.e.,
·
peak V E/MVC 75%).
Volume constraints on VT expansion were significantly less
in normal
subjects than in patients with COPD at a standard·
ized V E during exercise (Table 2). Even at the end of exercise
Figure 1. Inspiratory capacity (IC) measurements expressed as
liters or as a % of predicted normal in 105 patients with COPD
and a healthy age-matched normal control group (n 25) for
a given ventilation during exercise. Predicted normal IC was
calculated as predicted normal TLC minus predicted normal
FRC. Values shown are means (solid lines) 95% confidence
interval (dotted lines).
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O’Donnell, Revill, and Webb: Exercise and Hyperinflation in COPD
TABLE 3. COPD SUBGROUPS: (1) GROUP A VERSUS GROUP B, (2) DYSPNEA VERSUS LEG LIMITED
Group B
(n 24)
Limited by
Dyspnea
(n 64)
Limited by
Leg Discomfort
(n 19)
50:50
65 7§
26.4 4.2
4.8 1.0
71:29
66 9
28.1 4.9
5.4 1.6
67:33
66 8
24.9 5.4
4.7 1.3
53:47
66 8
26.3 4.8
5.3 1.7
38 11
65 17
58 9
94
126 15*
182 38*
56 15*
38 8†
38 12
60 16
63 10
11 4
115 14
157 31
77 34
73 4
35 12*
59 17
59 14
10 6
124 21
184 43*
63 37
54 19
42 12
64 18
66 8
11 4
116 20
157 38
68 24
61 25
46
29
21
4
5.2 1.6
4.8 2.0
13.7 5.3
62 24
71 12
91 5
33.7 12.4
36.6 10.4
30.1 6.0
0.38 0.06
31 9
71 15
60 21
0.32 0.28
11 9
0.52 0.39
97
92 8
106 18
100†
0†
0
0
5.5 1.6‡
3.5 1.8†
12.4 5.3
56 23
69 9
89 6*
32.0 14.7
42.9 8.4
31.1 7.0
0.35 0.05‡
29 9
75 12
50 17‡
0.35 0.42
13 16
0.36 0.26*
6 5*
95 4‡
119 22*
0
100
0
0
4.3 1.5
6.6 1.7
12.6 3.4
57 20
70 11
93 4
32.6 8.4
38.8 7.8
28.4 4.3
0.39 0.04
33 9
70 16
64 19
0.45 0.29
17 13
0.55 0.42
98
91 9
107 24
Group A
(n 24)
Male %:Female %
Age, yr
Body mass index, kg/m2
Baseline dyspnea index
Pulmonary function
FEV1, %predicted
FVC, %predicted
FEV1/FVC, %predicted
FEF50, %predicted
TLC, %predicted
FRC, %predicted
MIP, %predicted
DLCO, %predicted
Peak exercise
Reason for stopping exercise (% of group)
Breathing discomfort
Leg discomfort
Both breathing and legs
Other
Dyspnea, Borg
Leg discomfort, Borg
·
V O2, ml/kg/min
·
V O2, %predicted max
Heart rate, %predicted max
SaO2, %
·
V E, L/min
· ·
V E/ V CO2, %
F, breaths/min
TI/Ttot
VT, %predicted VC
VT/IC, %
IC, %predicted
∆IC rest-to-peak, L
∆IC rest-to-peak, %predicted
IRV, L
IRV, %predicted TLC
EILV/TLC, %
EILV, %predicted TLC
75*
13
8
4
5.1 1.6
3.4 2.3
10.6 3.3*
51 18
66 9
90 4
32.0 13.8
42.0 6.5*
30.4 7.0
0.36 0.06
31 9
77 10
54 19
0.43 0.36
17 13 p0.07
0.34 0.24
64
95 3
119 17‡
Definition of abbreviations: COPD chronic obstructive pulmonary disease; DLCO diffusing capacity of the lung for CO; EILV end-inspiratory lung volume; F respiratory frequency; FRC functional residual capacity; IC inspiratory capacity; IRV inspiratory reserve
volume; MIP maximum inspiratory pressure; SaO2 arterial oxygen saturation; TI/Ttot respiratory duty cycle; TLC total lung capac·
·
ity; V E minute ventilation; V O2 oxygen consumption; VT tidal volume.
* p 0.05, †p 0.0005, ‡p 0.01. Significant difference between subgroups (A versus B, dyspnea versus leg discomfort).
§
Values are mean SD.
Other in Group A was discomfort from noseclips (n 1); Other in Group B was general fatigue (n 1).
in normals, IRV did not reach the same minimal level as it did
in COPD (Table 2 and Figure 3).
Increased versus No Change in DH during Exercise in COPD
Of the 84 patients who decreased IC during exercise outside the
95% CI at rest, 62 decreased IC by at least 10% predicted. This
latter subgroup (DH subgroup) was compared with the subgroup of 14 patients who did not change IC during exercise (∆IC
within 4.5% predicted). These subgroups had similar mean
baseline FEV1 %predicted, FRC %predicted, and DLCO %predicted. Although both subgroups reached a similar peak IC
%predicted (and VT/%predicted VC), patients who did not
change IC during exercise tended to have greater volume constraints at rest, that is, smaller IC (p 0.06) and IRV (p 0.08).
Reduced DLCO (Group A) versus Preserved DLCO (Group B)
As selected, Group A (n 24) had significantly greater baseline lung hyperinflation and a greater reduction in diffusing
capacity than Group B (n 24) (Table 3). These subgroups
were well matched for age, sex, height, and body mass index,
but Group A had greater exercise impairment due to exer-
tional dyspnea than Group B (Figure 4). Although the overall
extent of change in IC during exercise was similar in both subgroups, Group A had a significantly greater rate of DH, which
occurred in the early stages of exercise, than Group B. Therefore, Group A had an earlier attainment of a limiting mechanical
restriction (i.e., minimal IRV) resulting in a reduced peak
·
V O2 (Table 3 and Figure 4).
Correlates of Dynamic Hyperinflation in COPD
The total extent of change in IC (%predicted) during exercise
was determined primarily by resting volume constraints, that
is, IC expressed as %predicted (r 0.503, p 0.0005) and
IRV expressed as % of predicted TLC (r 0.497, p 0.0005).
By stepwise multiple regression analysis, the FEF50% and
DLCO (both expressed as %predicted) added an additional 8%
to the variance in ∆IC %predicted (p 0.05 each).
The rate of· change in IC during exercise (slope of IC %predicted over V O2 %predicted maximum) correlated best with
DL/VA %predicted (r 0.412, p 0.005). Comparison of subgroups best illustrates this model: the subgroup with a reduced
DLCO had a significantly faster rate of DH, occurring early in
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2001
mined primarily by the peak IC (r 0.791, p 0.0005) (Figure 5) or the resting IC (r 0.745, p 0.0005), both expressed
as %predicted. As seen in Figure 5, the relationship between
peak VT and peak IC was strong in the 85 patients with an IC
70% predicted (r 0.866, p 0.0005), but was not significant within the 20 patients with a preserved IC (r 0.273, p
0.244). Finally, an index of the mechanical constraints on tidal
volume expansion (VT/IC) as exercise progressed was the best
correlate of · concurrent estimates of the level of ventilatory
limitation ( V E/MVC), and accounted for 43% of its variance
after accounting for repeated measurements within patients
(p 0.0005).
Correlates of Exertional Dyspnea in COPD
Figure 2. The distribution of the extent of change (∆) in IC during exercise is shown in patients with COPD (n 105) and in age-matched
normal subjects (n 25). A negative ∆IC reflects dynamic hyperinflation (DH) during exercise; each bar width corresponds to a ∆IC range
of 0.10 L. In contrast to normal subjects, the majority of patients with
COPD experienced significant DH during exercise despite reaching a
much lower peak ventilation, that is, 33 versus 64 L/min in patients
with COPD and normal subjects, respectively.
exercise, than those with a preserved DLCO (Group A versus
B, p 0.05) (Figure 4).
Correlates of Exercise Capacity in COPD
·
In COPD, the best physiological correlate of peak V O2 (expressed %predicted maximum) was the peak VT (standardized as %predicted VC) (r 0.682, p 0.0005) (Figure 5 and
·
Table 4). By stepwise multiple regression analysis, peak V O2
%predicted was best described by the combination
of peak
·
·
VT/%predicted VC, peak F, and the slope of V E/ V O2 %predicted (r2 0.816, p 0.0005). Within each of the COPD subgroups (see
above), peak VT continued to be the best correlate
·
of peak V O2 (p 0.0005 each). In turn, peak VT was deter-
The strongest correlate of exertional dyspnea intensity was an
index of the concurrent constraints on tidal volume: for all
points during exercise, after accounting for repeated measurements within patients, the VT/IC ratio accounted for 32% (p
0.0005) of the variance in concurrent Borg dyspnea· ratings.
Less important contributing variables included V E/MVC,
breathing frequency, and IRV/predicted TLC, each accounting for 25% of the variance in Borg dyspnea ratings (p 0.0005).
DISCUSSION
The novel findings of this study are as follows. (1) Although
the pattern and magnitude of DH was variable among COPD
patients during exercise, the majority (80%) demonstrated
significant dynamic increases in lung volumes above resting
values. (2) The extent of DH during exercise varied inversely
with the level of resting hyperinflation. (3) For a given level of
airway obstruction, patients with a more emphysematous clinical profile (low DLCO) had faster rates of DH, greater constraints
on tidal volume expansion
during exercise, greater dyspnea,
·
and a lower peak V O2. (4) Finally, there was a clear statistical
association between the level of resting and dynamic hyperinflation, the degree of tidal volume restriction (i.e., peak VT) during exercise, and peak exercise performance.
Operational Lung Volumes during Exercise in COPD
Serial IC measurements have been used to track dynamic
EELV during exercise for more than 30 yr (2, 3, 11, 21–23).
This approach is based on the assumption that TLC does not
change appreciably during exercise in COPD, and that reduc-
Figure 3. Changes in operational lung volumes are shown as ventilation increases with
exercise in patients with COPD and in normal subjects. “Restrictive” constraints on tidal
volume (VT, solid area) expansion during exercise are significantly greater in the COPD
group from both below (reduced IC) and
above (minimal IRV, open area).
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O’Donnell, Revill, and Webb: Exercise and Hyperinflation in COPD
Figure 4. Ventilatory responses to exercise are shown in
COPD (n 105) and its subgroups: A with a low DLCO 50%
predicted (n 24), and B with a better preserved DLCO
50% predicted (n 24). Group A had significantly (p
0.05) greater exertional dyspnea, greater levels of lung hyperinflation, and earlier attainment of a limiting mechanical
restriction (i.e., minimal IRV, shaded area) than Group B.
tions in dynamic IC must therefore reflect increases in dynamic EELV (or FRC) (11). However, regardless of any possible changes in TLC with exercise, progressive reduction of
an already diminished resting IC means that VT becomes positioned closer to the actual TLC and the upper alinear extreme
of the respiratory system’s pressure–volume relationship,
where there is increased elastic loading of the inspiratory muscles (1). The reduction of IC as exercise progresses is likely a
true reflection of shifts in EELV, rather than simply the inability to generate maximum effort because of dyspnea or
functional muscle weakness. In fact, several studies have established that dyspneic patients, even at the end of exhaustive
exercise, are capable of generating maximal inspiratory efforts
as assessed by peak inspiratory esophageal pressures (4, 13).
Moreover, we have recently shown that exercise IC measurements are both highly reproducible and responsive in patients
with severe COPD, provided due attention is taken with their
measurement (7).
The extent of DH from rest to the peak of exercise was
variable between patients (Figure 2). The change in IC correlated best with the resting IC (or IRV): those with the greatest
IC (or IRV) reduction at rest tended to have smaller changes
in both EELV and EILV with exercise (Figures 1 and 2). After accounting for the resting IC, the maximal mid expiratory
flow rate also contributed to the variance in DH: those with
higher expiratory flows available over the tidal volume operating ranges tended to have less DH. Not surprisingly, this was
a weak correlation because the measurement of forced expiratory flow rates, which is prone to measurement artifact (gas
and airway compression effects), is a very crude index of the
extent of expiratory flow limitation, which is likely the crucial
determinant of DH during exercise in COPD.
Pattern and Magnitude of DH in COPD
Independent Variables
·
Slope of dyspnea/ V O2, Borg/%predicted max
Peak exercise measurements
Peak VT, %predicted VC
Peak IC, %predicted
Peak VT/IC, %
·
Peak V E, L/min
·
Peak V E, %MVC
Peak F, breaths/min
Peak heart rate, %predicted
Peak SpO2, %
Resting measurements
IC, %predicted
IRV, %predicted TLC
FEV1, %predicted
FEV1/FVC, %predicted
VC, %predicted
FRC, %predicted
TLC, %predicted
DLCO, %predicted
DLCO/VA, %predicted
MIP, %predicted
This is the first large study to document dynamic volume components during incremental exercise in COPD. Although, in
absolute terms, a mean reduction in IC of 0.37 L seems small,
this represents a significant further reduction of an already diminished baseline value. In the COPD group, the mean
change in IC with exercise was well beyond the within-group
95% confidence interval for the resting IC measurement (i.e.,
0.14 L or 4.5% of the predicted normal value) (Figure 2).
Figure 5. In· COPD (n 105), the best correlate of peak oxygen consumption ( V O2) was the peak tidal volume attained (VT standardized
as %predicted vital capacity). In turn, the strongest correlate of peak
VT was the peak inspiratory capacity (IC).
TABLE 4. CORRELATIONS WITH SYMPTOM-LIMITED PEAK
·
V O2 (%PREDICTED MAXIMUM) IN COPD (n 105)
Pearson’s Correlation
Coefficient (r)
p Value
0.628
0.0005
0.682
0.446
0.286
0.427
0.314
0.057
0.364
0.255
0.0005
0.0005
0.004
0.0005
0.001
0.576
0.0005
0.010
0.451
0.325
0.453
0.147
0.399
0.271
0.104
0.437
0.264
0.196
0.0005
0.001
0.0005
0.145
0.0005
0.006
0.304
0.0005
0.017
0.068
Definition of abbreviations: COPD chronic obstructive pulmonary disease; DLCO
diffusing capacity of the lung for CO; F respiratory frequency; FRC functional residual capacity; IC inspiratory capacity; IRV inspiratory reserve volume; MIP
maximum inspiratory pressure; MVC estimated maximum ventilatory capacity, i.e.,
40 FEV1; SpO2 pulmonary oxygen saturation; TLC total lung capacity; VA alve·
·
olar volume; VC vital capacity; V E minute ventilation; V O2 oxygen consumption; VT tidal volume.
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AMERICAN JOURNAL OF RESPIRATORY AND CRITICAL CARE MEDICINE
Of interest, the rate of change in DH correlated inversely
with resting diffusion capacity (DLCO/VA). Patients with a
lower DLCO would be expected to have a greater propensity to
expiratory flow limitation because of reduced lung recoil and
airway tethering effects. We have previously reported that patients with COPD and a lower DLCO (an average of 32% predicted) had greater chronic activity-related dyspnea and poorer
exercise performance than patients with a similar FEV1 but
with an average DLCO of 65% predicted (24). In this study we
further explored the mechanistic link between low DLCO and
poor exercise performance. Thus, patients with a lower DLCO
(Group A) had greater resting hyperinflation, greater rates of
DH at lower exercise levels, greater exertional dyspnea, earlier attainment of critical volume constraints,
accelerated
·
·
breathing frequency, and a lower peak V E and peak V O2 than
patients with a better preserved DLCO (Group B) (Figure 3).
Operational Lung Volumes and Exercise Intolerance
Several recent studies have confirmed that exercise intolerance in COPD is multifactorial and ultimately reflects integrated abnormalities of the ventilatory, cardiovascular, peripheral muscle, and metabolic systems in variable combinations
(18, 25–32). In our study patients in whom dyspnea was the
main symptom limiting exercise (61% of patients), ventilatory
factors played a predominant role in exercise curtailment.
Those who stopped primarily because of leg discomfort had
significantly less ventilatory constraints at peak exercise. A recent study by Diaz and coworkers (25) has shown that the
resting· IC %predicted correlated well with symptom-limited
peak V O2. The present study extends these findings to highlight the importance of ventilatory restriction during exercise
in flow-limited patients.
Compared with age-matched healthy control subjects at a
similar low level of ventilation, IC and IRV were markedly diminished in the COPD group. At this point of comparison,
VT/IC ratios in COPD and in health were 74% and 52%, respectively (Table 2). These patients with COPD, therefore,
had a very limited ability to further expand VT in the face of
the increasing metabolic demand of continued exercise. The
resting IC (not the VC) and, in particular, the dynamic IC with
exercise, represent the true operating limits for VT expansion
in any given patient. When the VT during exercise approximated the peak IC, or the dynamic EILV encroached on the
TLC envelope, further volume expansion was impossible, even
if it were possible to further increase inspiratory muscle effort.
In a· multiple regression analysis with symptom-limited
peak V O2 as the dependent variable, and several relevant
physiological measurements as independent
variables, includ·
ing FEV1, FEV1/FVC ratio, and V E/MVC, peak VT emerged
as the strongest contributory variable, explaining 47% of the
variance. Peak VT, in turn, correlated strongly with both the
resting and peak dynamic IC. It is noteworthy that this correlation was particularly strong (r 0.9) in approximately 80%
of the sample who had a diminished peak IC (i.e., 70% predicted). Similarly, the intensity of breathlessness throughout
exercise correlated better with concurrent measurements of
VT/IC (p 0.0005) than any other ventilatory variable. We
can conclude, therefore, that volume constraints contribute
importantly to both exercise intolerance and dyspnea in patients with COPD.
Previous small studies provide a basis for a possible mechanistic link between lung hyperinflation, volume restriction,
and exercise intolerance in COPD (2–7, 21–23). The greater
the increases in dynamic lung volume components during exercise, the greater the elastic and threshold loads on inspiratory muscles already burdened with increased resistive work.
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2001
With progressive DH, inspiratory muscles become functionally weakened. This combination of increased loading and reduced strength means that the inspiratory muscles are operating at a high fraction of their maximal force-generating capacity
during tidal breathing. The constrained VT response means a
greater reliance on tachypnea to increase ventilation, but this
rebounds to further aggravate DH in a vicious cycle. Progressive volume restriction in the face of increasing inspiratory effort during exercise ultimately reflects neuromechanical uncoupling of the respiratory pump, which, in turn, may contribute
to the quality and intensity of exertional dyspnea that these
patients experience (4).
The reduced peak VT, as a result of a reduced IC, has similarly been shown to correlate strongly with poor exercise performance in patients with ventilatory restriction due to interstitial lung disease (33), as well as in normal healthy subjects
where chest wall restriction was imposed (34). The contention
that restrictive mechanics, secondary to lung hyperinflation,
contribute to exercise intolerance in severe COPD is bolstered by recent interventive studies (i.e., bronchodilators and
oxygen therapy) that show that reduction of resting and/or exercise lung volumes improves exercise endurance in severe
COPD (5, 7, 35).
·
Traditionally, assessment of breathing reserve (i.e., 1 V E/
MVC ratio) has been used to assess ventilatory limitation to
exercise in COPD. This study shows that additional measurements of dynamic lung volumes during exercise provide insights into the nature of the critical ventilatory constraints on
exercise
performance. There was a strong correlation between
·
the V E/MVC and the VT/IC ratio during exercise (p 0.0005).
However, a full 14% of patients
with apparent ventilatory re·
serve at peak exercise (i.e., V E/MVC 75%) had coexisting
limiting restrictive ventilatory constraints as indicated by an
EILV of 96% of TLC (i.e., a significantly reduced peak IRV)
at the same time point.
In summary, the inability to further expand VT in response
to the increased respiratory drive of exercise contributes importantly to exercise intolerance in patients with moderate to
severe COPD. The main clinical implication of our findings is
that exercise performance and dyspnea should be improved
by therapeutic interventions that reduce operational lung volumes at rest and during exercise in severe COPD. Measurement of IC and its derived volume components during exercise
complement the traditional assessments of ventilatory constraints
and can provide additional insight into the impairment–disability interface in patients with COPD.
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