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(Chest. 2006;129:325-332.)
© 2006 American College of Chest Physicians

Response to One-Legged Cycling in Patients With COPD*

Thomas E. Dolmage, MSc and Roger S. Goldstein, MD

* From the Department of Respiratory Diagnostic and Evaluation Services (Mr. Dolmage), West Park Healthcare Centre; and Department of Medicine (Dr. Goldstein), University of Toronto, Toronto, ON, Canada.

Correspondence to: Roger S. Goldstein, MD, West Park Healthcare Centre, 82 Buttonwood Ave, Toronto, ON, Canada M6M 2J5; e-mail: rgoldstein{at}westpark.org

Abstract

Background: In patients with COPD, exercise intensity is often limited by the ventilatory system. We hypothesized that by exercising with a smaller muscle mass, ventilatory-limited patients would perform more high-intensity, muscle-specific work. The study objectives were as follows: (1) to determine the limitations of exercising with a smaller muscle mass, compared with conventional two-legged exercise; and (2) to determine the endurance time, using the same muscle-specific intensity, during one-legged vs two-legged exercise.

Methods: Nine patients (mean ± SD FEV1, 36 ± 13% of predicted) completed incremental exercise, and nine other patients (mean FEV1, 42 ± 16% of predicted) completed constant-power exercise. Nine healthy subjects (FEV1, 104 ± 14% of predicted) completed both tests. All subjects completed tests using two-legged and one-legged pedaling.

Results: Peak oxygen uptake (VO2peak) was similar during one-legged and two-legged incremental exercise among patients (difference, 0.03 L/min; 95% confidence interval [CI], – 0.10 to 0.16 L/min; p = 0.60), as were ventilation and dyspnea scores. VO2peak was lower during one-legged vs two-legged exercise (– 0.57 mL/min; 95% CI, – 0.81 to – 0.32 mL/min; p < 0.001) among healthy subjects with substantial ventilatory and heart rate reserve. Patients endured one-legged pedaling at a constant power longer than two-legged pedaling (16.97 min; 95% CI, 9.98 to 23.96 min; p < 0.001), resulting in greater work (12.48 kilojoules [kJ]; 95% CI, 2.58 to 22.39 kJ; p = 0.02). Healthy subjects completed similar work (– 4.02 kJ; 95% CI, – 18.59 to 10.55 kJ; p = 0.54) with one-legged vs two-legged pedaling.

Conclusion: These observations demonstrate the effectiveness of using one-legged exercise at the same muscle-specific intensity in extending the duration of exercise among patients with COPD. This has important implications for training approaches designed to enhance exercise function among ventilatory-limited patients.

Key Words: exercise tolerance • lung diseases, obstructive • muscle, skeletal • oxygen consumption

In fit, healthy, athletic individuals with a high aerobic capacity, oxygen uptake (VO2) during exercise involving large-muscle groups is limited by the cardiovascular system, such that muscle metabolic capacity exceeds the capacity of the circulation to provide oxygen.1 In contrast, VO2 by unfit individuals with a lower aerobic capacity is mainly determined by the metabolic limits of their muscles.2 In many patients with COPD, exercise is limited by the inability to increase ventilation to meet the metabolic demand. Exercise may also be limited by peripheral muscle dysfunction consequent on decreased muscle mass, decreased capillaries, decreased mitochondrial volume, and low activity of oxidative enzymes.345 Although Saey et al6 suggested that in patients with COPD, exercise limitation after cycling was associated with contractile fatigue of the quadriceps femoris, Richardson et al7 reported that a metabolic reserve existed during single-leg knee extension exercise. Rhythmic extension of one leg, which used a working muscle mass of only 12.5% of the muscle mass required for cycling, required 80% of the ventilatory capacity.

The applied mode, intensity, duration, and frequency of exercise among patients with COPD are rarely similar to those recommended for healthy individuals,8 as ventilatory constraints often prevent patients with COPD from being able to tolerate an effective training stimulus. Therefore, any approaches that might enable COPD patients to train more effectively are of interest. In one such approach, the influence of interval training was noted to be an effective way of increasing the work tolerated by COPD patients.9 However, interval training, by definition, lacks the continuous stimulus important for endurance training that may result in improved VO2 when compared with conventional training.1011

An alternate approach to increasing the work that can be tolerated is to target specific muscle groups. It is possible that partitioning exercise into a smaller muscle mass, while maintaining the same load on the muscle, will enable exercise duration to be extended. One-legged exercise, at half the load of two-legged exercise, will place the same demands on the targeted muscle, but as the total metabolic load and therefore the ventilatory load is less than the load sustained during two-legged exercise, exercise duration should be increased.

In this study, we hypothesized that patients with COPD cycling with one leg during incremental exercise would still be limited by ventilatory requirements, despite using half of their muscle mass. We also hypothesized that patients with COPD would tolerate the same muscle-specific workload during one-legged cycling for a longer duration compared with two-legged cycling. These observations are relevant to identifying innovative, disease-specific approaches to exercise training for severely impaired patients with COPD.

Materials and Methods

Subjects
Nonsmoking, clinically stable patients with COPD12 were enrolled. Patients were excluded if they were hypoxemic at rest (PaO2 < 55 mm Hg), had comorbidities that limited exercise tolerance, or were unable to provide informed consent. Healthy nonsmoking volunteers were recruited from among the health center staff. The study was approved by West Park Healthcare Centre Research Ethics Committee.

Exercise Testing
All of the 27 subjects were easily able to complete the required exercise tests, including both one-legged and two-legged cycling, with simple instruction from an experienced exercise technologist. The exercise cycle calibration13 immediately prior to and following the studies did not change. Exercise tests were carried out in a standardized manner14 with an electrically braked cycle ergometer (Collins CPX Bike model 0070; Warren E. Collins; Braintree, MA). Ventilation was measured during the incremental exercise protocol, with subjects breathing through a calibrated15 pneumotachograph (Collins/Cybermedic model 003500; Warren E. Collins). Ventilation was measured during the constant-power exercise protocol with subjects breathing through a calibrated volume sensor (Oxycon Pro 808302; Erich Jaeger GmbH; Wurzburg, Germany). For each subject, the same system was used for both one-legged and two-legged cycling. After 3 min of rest, the subjects were requested to pedal to symptom limitation. Tests were separated by at least 3 h, with no more than two tests within the same day.

One-Legged Pedaling:
Subjects cycled with their active leg while resting their inactive foot on a crossbar located midway on the head tube of the ergometer. This position was quite comfortable and was immediately accepted by the cyclist without practice or need for any position adjustments. A counterweight was not needed on the inactive pedal, and a toe clip was not used. During pilot studies, we confirmed that steady-state VO2 during one-legged and two-legged pedaling at the same power (mechanical efficiency) was similar, with agreement16 of < 80 mL/min. We confirmed that steady state17 had occurred if, after a monoexponential rise in cardiopulmonary parameters in response to a change in power, the averaged 30-s parameter remained within 1 SD (ie, VO2 remained within ± 5%) between minute 3 and minute 6 of the test, without a significant slope, as determine by linear regression analysis.

Incremental Power Exercise:
For COPD patients and healthy volunteers, the load was increased at a rate that would induce symptom limitation within 10 min. Measurements included power, heart rate (HR), and oxygen saturation by pulse oximetry (SpO2) as well as scored sensations of breathlessness and leg effort using a modified Borg scale.1819 Individual results were also expressed as a percentage of the reference values for healthy sedentary individuals.20 Pulmonary gas exchange, minute ventilation (VE), tidal volume (VT), and frequency of breathing (f) were measured for each breath. Ventilatory efficiency21 was calculated from these parameters. Using a validated method,22 dynamic hyperinflation (DH) was indicated by a positive slope of serial end-expiratory lung volume (EELV) expressed as a linear function of VE. The averaged EELV and end-inspiratory lung volume (EILV) were plotted against the averaged VE using a method described elsewhere.23

Constant-Power Exercise:
In order to test the second hypothesis, we recruited a second group of COPD patients. For the COPD patients and healthy subjects during two-legged, constant-power exercise tests, the loads were 70% and 85%, respectively, of the peak power (Ppeak) achieved on the baseline incremental test providing a symptom-limited exercise duration (t-limit) that was dependent on aerobic power and remained sensitive to change, yet avoided the likelihood of extending beyond 20 min and being limited by motivation. For COPD patients and healthy subjects, the one-legged loads were 35% and 42.5% of the Ppeak, respectively. The technologist stopped the exercise at 30 min.

Analysis
For both of the exercise protocols (incremental and constant power), there were two groups, COPD patients and healthy volunteers, and two exercise conditions, one-legged or two-legged pedaling. Primary outcomes included the peak VO2 (VO2peak) during the incremental exercise test as well as the t-limit of the submaximal constant-power exercise tests. Mean values for each variable were calculated and expressed as mean ± SD unless stated otherwise. For each test, the analysis was limited to the within-group difference of one-legged vs two-legged pedaling using a paired t test, and we calculated the 95% confidence interval (CI) around this difference.

Results

Enrollment
A description of the patients completing the incremental and constant-power exercise tests is provided in Table l . All patients had a clear diagnosis of COPD,12 and all had stopped smoking at least 2 months before participation in the study. Hypothesis one was tested among nine COPD patients (group 1) who completed the incremental protocol. Hypothesis two was tested in nine other COPD patients (group 2) who completed the constant-power protocol. Nine healthy individuals completed both the incremental and constant-power protocols.


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  		Table 1. Baseline Pulmonary Function Among Healthy Control Subjects and COPD Patients Who Completed Incremental and Constant-Power Exercise Tests*

 
Incremental Exercise Capacity
Response of Healthy Subjects to Incremental Exercise:
During the incremental exercise test, all nine subjects stopped exercise because of their legs, with leg effort described as "somewhat heavy" (score of 4 on a scale of 10) to "maximal" (10 on a scale of 10) [Table 2 ]. None of the subjects showed evidence of ventilatory limitation.24 VO2peak was 110 ± 27% of predicted and did not exceed the predicted maximum when referenced to maximum voluntary ventilation (MVV): 86 ± 18% of predicted MVV. The increased peak VE (VEpeak) was achieved by an increase in both VT (peak VT [VTpeak], 2.05 ± 0.37 L) and f (peak f [fpeak], 43 ± 8 breaths/min). There was a normal increase in VE relative to the increase in carbon dioxide output (VCO2) [nadir of VE/VCO2 = 26 ± 3 L (air) x L (CO2)]. None of the subjects had a ventilatory-dependent increase in EELV; the mean rate of change of EELV was 0 ± 9 mL/min/L (Fig 1 ). VO2peak was 133 ± 28% of predicted (range, 28.3 to 46.2 mL/kg/min), indicating a mixed group of healthy untrained (VO2peak, 21 to 30 mL/kg/min) and physically fit subjects (VO2peak, 31 to 40 mL/kg/min). The Ppeak was 131 ± 20% of predicted. The slope of the VO2 vs power relationship (10.4 ± 1.4 mL/min/W) was normal (95% CI, 8.5 to 11 mL/min/W).24 The slope of the HR vs VO2 relationship was 53 ± 18 beats/L (O2). HR at peak exercise (HRpeak) was 96 ± 7% of predicted with a HR reserve at peak exercise of 7 ± 12 beats/min); the peak O2 pulse was 13.4 ± 3 mL of O2 per beat.


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  		Table 2. Incremental One-Legged and Two-Legged Cycle Exercise*

 

Figure 1
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  		Figure 1. The mean VT response during incremental exercise shown as the difference between EILV (triangle down) and EELV (triangle up). Two-legged (open triangles) and one-legged (solid triangles) cycling for the healthy control group (left, A) and COPD patients (right, B) are shown. The positive slope of the EELV in COPD patients indicates the presence of DH. TLC = total lung capacity.

 
Response of Healthy Subjects to One-Legged Incremental Exercise:
During the incremental exercise test, the VO2peak was less with one-legged pedaling than with two-legged pedaling (difference, – 0.57 mL/min [95% CI, – 0.81 to – 0.32 mL/min]; p < 0.001) [Table 2]. While pedaling with one leg, all subjects discontinued exercise because of leg effort. Their descriptions of breathlessness, moderate (score of 3 on a scale of 10) to very heavy (8 on a scale of 10), was the same as that described during two-legged cycling. The averaged EELV and EILV for one-legged and two-legged incremental exercise are presented in Figure 1. The VEpeak of 60.1 ± 15.7 L/min during one-legged cycling was significantly decreased compared with two-legged cycling. This difference (– 28.6 L/min [95% CI, – 41.1 to – 16.0] L/min; p < 0.001) was achieved at a lower fpeak (difference, – 6 breaths/min [95% CI, – 10 to – 2 breaths/min]; p = 0.007) and VTpeak (difference, – 0.39 L [95% CI, – 0.58 to – 0.21 L]; p < 0.001). One-legged incremental exercise was not associated with a change in ventilatory efficiency (difference in the nadir of VE/VCO2 of 2 L (air)/L (CO2) [95% CI, 0 to 5 L (air)/L (CO2)]; p = 0.06). Measured ventilatory-dependent changes in EELV of – 6 ± 7 mL/min/L were similar to those during two-legged cycling (difference, – 5 mL/min/L [95% CI, – 16 to 6 mL/min/L]; p = 0.31). The Ppeak of 97 ± 17 W was significantly less with one-legged pedaling (difference, – 94 W [95% CI, – 111 to – 77 W]; p < 0.001). The increase in VO2 with increasing power during one-legged pedaling was significantly greater than during two-legged pedaling, with a difference in slope of 2.8 mL/min/W (95% CI, 0.7 to 5.0 mL/min/W; p = 0.02). HRpeak during one-legged exercise, 148 ± 15 beats/min, was significantly less than during two-legged exercise (difference, – 24 beats/min [95% CI, – 30 to – 18 beats/min]; p < 0.001), but the slope of the HR vs VO2 relationship was similar between conditions (difference of 1 beat/L oxygen [95% CI, – 5 to 8 beats/L oxygen]; p = 0.64).

Response of COPD Patients to Incremental Exercise:
During the incremental exercise test, while pedaling with both legs, all patients (group 1 in Table 1) showed evidence of ventilatory limitation (Table 2). At the end of the test, breathlessness was scored as heavy (5 on a scale of 10) to maximal (10 on a scale of 10). All patients demonstrated a ventilatory-dependent increase in EELV with a mean rate of increase of 36 ± 16 mL/min/L. VEpeak was reduced to 42 ± 11% of predicted, but it was high when referenced to the MVV: 103 ± 25% of MVV predicted. The increased VEpeak was achieved largely by an increase in f; f increased from 18 ± 5 breaths/min at rest to 32 ± 7 breaths/min. The VTpeak increased from 0.71 ± 0.19 L at rest to 0.98 ± 0.26 L at end exercise. There was an excessive VE relative to the increase in VCO2, reflecting reduced ventilatory efficiency (nadir of VE/VCO2 of 48 ± 14 L (air)/L (CO2). There was arterial desaturation of 5 ± 3% from a resting SpO2 of 93 ± 1 to 88 ± 3% at end exercise. Patients demonstrated a severe cardiopulmonary impairment, with a VO2peak < 10 mL/kg/min and a reduced Ppeak (42 ± 16% of predicted). The slope (7.4 ± 1.9 mL/min/W) of the VO2 vs power relationship was less than normal (95% CI, 8.5 to 11 mL/min/W). The slope of the HR vs VO2 relationship was 53 ± 18 beats/L (O2). A low HRpeak (77 ± 9% of predicted) and considerable HR reserve at peak exercise (35 ± 14 beats/min) were observed; the peak O2 pulse (5.1 ± 1.4 mL O2/beat) was also reduced.

Response of COPD Patients to One-Legged Incremental Exercise:
VO2peak was similar between one-legged and two-legged incremental exercise conditions (difference, 0.03 L/min [95% CI, – 0.10 to 0.16 L/min]; p = 0.60) [Table 2]. During the incremental exercise test while pedaling with one leg, all patients showed evidence of ventilatory limitation and a similar description of breathlessness (heavy [5 on a scale of 10] to maximal [10 on a scale of 10]) as two-legged cycling. The ventilatory-dependent increase in EELV (39 ± 22 mL/min/L) was similar to two-legged cycling (difference, 3 mL/min/L [95% CI, – 7 to 14 mL/min/L]; p = 0.51). The averaged EELV and EILV for one-legged and two-legged cycling throughout incremental exercise are presented in Figure 1. VEpeak (28.0 ± 4.4 L/min) was similar between exercise conditions (difference, – 1.7 L/min [95% CI, – 4.8 to 1.4 L/min]; p = 0.24). Likewise, neither fpeak nor VTpeak differed between conditions (fpeak difference, – 1.0 breaths/min [95% CI, – 2.3 to 0.3 breaths/min]; p = 0.11; and VTpeak difference, – 0.02 L [95% CI, – 0.09 to 0.05 L]; p = 0.57). One-legged incremental exercise did not influence ventilatory efficiency (difference in the nadir of VE/VCO2 of – 2 L (air)/L (CO2) [95% CI, – 12 to 9 L (air)/L (CO2)]; p = 0.73). The change in SpO2 from rest to end exercise was significantly less during one-legged pedaling (difference, 3% [95% CI, 1 to 4%]; p = 0.007). While the Ppeak was significantly less with one-legged pedaling (difference, – 9 W [95% CI, – 18 to – 1 W]; p = 0.03), the increase in VO2 with increasing power was similar between conditions (difference in slope, 1.0 mL/min [95% CI, – 1.3 to 3.3 mL/min/W]; p = 0.36).

Constant-Power Exercise Endurance
Response of Control Subjects to Constant-Power Exercise:
Nine control subjects completed two constant-power tests (Table 3 ). During one test, the patients pedaled with both legs (power, 163 ± 31 W). During the other test, the subjects pedaled with one leg at the same muscle-specific power (82 ± 16 W). The subjects’ t-limit of one-legged pedaling (Fig 2 ) was significantly greater than two-legged pedaling (difference, 3.56 min [95% CI, 0.25 to 6.87 min]; p = 0.04) but with similar total work (difference, – 4.02 kilojoules [kJ]; 95% CI, – 18.59 to 10.55 kJ; p = 0.54). One-legged exercise elicited a significantly lesser end-exercise VE (difference, – 18.5 L/min [95% CI, – 31.9 to – 5.1 L/min]; p = 0.01) and HRpeak (difference, – 16 beats/min [95% CI, – 22 to – 9 beats/min; p < 0.001). During one-legged pedaling, the control subjects experienced similar leg effort (difference, 0.1 [95% CI, – 1.0 to 1.1]; p = 0.91) and less intense dyspnea at end exercise (difference,– 1.8 [95% CI, – 2.8 to – 0.8]; p = 0.003).


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  		Table 3. Constant-Power One-Legged and Two-Legged Cycle Exercise*

 

Figure 2
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  		Figure 2. Individual endurance times (open symbols) during constant-power exercise are shown for two-legged and one-legged cycling for the healthy control group (left, A) and COPD patients (right, B). The mean endurance time [SD] of two-legged and one-legged cycling (closed symbols) for both groups are shown beside the respective individual values. *p < 0.05 within-group difference.

 
Response of COPD Patients to Constant-Power Exercise:
Nine patients with COPD (group 2 described in Table 1) completed two constant-power tests (Table 3). During one test, the patients pedaled with both legs (power, 38 ± 13 W). During the other test, they pedaled with one leg at the same muscle-specific power (19 ± 6 W). The patients’ t-limit of one-legged pedaling (Fig 2) was significantly greater than two-legged pedaling (difference, 16.97 min [95% CI, 9.98 to 23.96 min]; p < 0.001). They also completed significantly greater total work with one-legged cycling (difference, 12.48 kJ [95% CI, 2.58 to 22.39 kJ]; p = 0.02). These differences are underestimated, as exercise was discontinued in four patients after they had exercised for > 30 min. Increased endurance of one-legged exercise was achieved with a significantly lower end-exercise VE (difference, – 5.2 L/min [95% CI, – 9.77 to – 0.66 L/min; p = 0.03) and HR (difference, – 9 beats/min [95% CI, – 19 to 0 beats/min]; p = 0.05). During one-legged pedaling, the patients experienced less intense dyspnea at end exercise (difference, – 2.9 on a scale of 10 [– 3.9 to – 2.0 on a scale of 10]; p < 0.001).

Discussion

In this report, we have studied patients with COPD with ventilatory limitation to exercise of sufficient severity for it to limit incremental exercise even when cycling with only one leg. By applying the same muscle-specific workload, with less total ventilatory load these patients tolerated constant-power exercise longer. Therefore, their total work was markedly increased. These findings have the potential to be applied to enhance the effectiveness of exercise training in patients with COPD.7

VO2peak is a recognized indicator of cardiovascular fitness. In healthy individuals, exercise capacity is determined by the cardiovascular system and the metabolic characteristics of the working muscle.25 Differences in VO2 between one-legged and two-legged exercise reflect the differences in active muscle mass recruited to perform the task. In general, VO2peak increases in proportion to the recruited muscle mass, up to a critical level when the oxygen demand from the exercising muscle exceeds the capacity of the central pulmonary and cardiovascular systems to supply oxygen. As muscle mass is reduced, the main determinant of exercise limitation shifts from a central to a peripheral origin.

At the end of incremental exercise, patients with COPD had considerable HR reserve in the absence of any ventilatory reserve. As their metabolic requirements increased, their breathing strategy reflected their inability to significantly increase VT either during one-legged or two-legged exercise. As DH occurred, increasing breathing frequency increased ventilation. Unlike healthy individuals, EELV increased as a function of VE; the increase was similar between exercise conditions. Richardson et al7 showed that for patients with COPD, leg extension exercise can support 12.5% of the muscle mass required for conventional two-legged cycle exercise without reaching ventilatory limitation. However, leg extension requires less muscle mass than one-legged cycling. Using one-legged cycling, at 50% of the muscle mass required for two-legged exercise, a ventilatory limitation did occur. This underscores the substantial discrepancy between the reduction in peripheral muscle aerobic capacity and the reduction in ventilatory capacity among patients with COPD.

The slope of the VO2 vs power relationship ({Delta}VO2/{Delta}P), an expression of metabolic efficiency, was within the normal range24 for our control subjects and increased when exercise was performed with a smaller muscle mass, likely because the muscle was able to utilize the increased blood flow. COPD patients had a {Delta}VO2/{Delta}P below the lower limits of normal, typical of patients with a cardiopulmonary impairment24 and inefficiency26 associated with lung disease. In COPD patients, the {Delta}VO2/{Delta}P during one-legged exercise remained below the normal range.

It is accepted that during exercise there is a lag between VO2 and the instantaneous increment in power.27 The greater proportion of cardiac output diverted to the smaller working muscle mass during one-legged incremental exercise may have reduced this lag, resulting in an increased VO2 at a given submaximal workload. However, the relationship between the VO2 and VE remained the same. Therefore, during one-legged cycling, incremental exercise ended at a lower power as compared with two-legged cycling.

In healthy subjects, we observed that one-legged incremental exercise was limited by peripheral muscle symptoms. We included healthy control subjects to test our paradigm of one-legged cycling and provide a reference for the normal, nonventilatory-limited response to incremental and constant-power exercise with one-legged and two-legged pedaling. The observation that healthy subjects achieved 73% VO2peak, 68% VEpeak, and 86% HRpeak is consistent with earlier studies28293031323334 that reported a substantial ventilatory and cardiac reserve among healthy individuals during one-legged cycling. Conceivably, this training strategy may also offer advantages to some healthy individuals. Although our healthy subjects were younger than our COPD patients, healthy elderly subjects do not have a ventilatory limitation to exercise and therefore would have responded similarly to the control subjects studied.35 However, the main focus of this report is the patient with COPD, who might benefit from any approaches that help minimize their ventilatory limitations to exercise training.

We elected to enroll a subsequent group of COPD patients for measurement of one-legged constant-power exercise, after noting that these patients were able to achieve the same limitation in exercise tolerance despite their using a smaller muscle mass. If we had used the same subjects for both exercise protocols, as we did with the control group, it might have been argued that our results were limited only to this group of subjects. The enrolment of a second group of COPD patients was designed to enhance the generalizability of our observations.

Patients with severe COPD have little ventilatory reserve, such that small changes in the power demands of exercise have large effects on their endurance time.36 The maximum power that can be endured indefinitely is closely linked to their ventilation. In this study, the targeted constant power used during two-legged pedaling was set such that the respiratory system could not support exercise indefinitely. The power required for one-legged pedaling enabled patients to more than triple their exercise time.

Many COPD patients cannot tolerate conventional high-intensity exercise for sufficient time to induce changes in peripheral muscle metabolism.37 However, in this study COPD patients endured the same muscle-specific workload, during one-legged cycling, for a longer duration than two-legged cycling. The difference was of such magnitude that they performed substantially more work. The observations during the incremental and constant-power studies open the possibility of increasing the muscle-specific load as a useful approach to exercise training. This study is therefore important to those interested in approaches that might enhance exercise tolerance among patients with severe COPD.

Footnotes

Abbreviations: CI = confidence interval; DH = dynamic hyperinflation; EELV = end-expiratory lung volume; EILV = end-inspiratory lung volume; f = frequency of breathing; fpeak = frequency of breathing at peak exercise; HR = heart rate; HRpeak = heart rate at peak exercise; kJ = kilojoule; MVV = maximal voluntary ventilation; Ppeak = peak power; SpO2 = oxygen saturation by pulse oximetry; t-limit = symptom-limited exercise duration; VCO2 = carbon dioxide output; VE = minute ventilation; VEpeak = minute ventilation at peak exercise; VO2 = oxygen uptake; VO2peak = peak oxygen uptake; {Delta}VO2/{Delta}P = slope of oxygen uptake vs power relationship; VT = tidal volume; VTpeak = tidal volume at peak exercise

This study was supported in part by West Park Healthcare Centre Foundation.

Received for publication May 12, 2005. Accepted for publication July 2, 2005.

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