Chest
Volume 138, Issue 6, December 2010, Pages 1309-1315
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Original Research
COPD
Efficiency of Neural Drive During Exercise in Patients With COPD and Healthy Subjects

https://doi.org/10.1378/chest.09-2824Get rights and content

Background

It is unknown whether efficiency of neural drive as expressed by a ratio of ventilation to the diaphragm electromyogram (EMGdi) in patients with COPD differs from that of healthy subjects during exercise and whether maximal neural drive is exhibited at the point of exercise termination.

Methods

We studied 12 male patients with COPD (mean ± SD age, 62.8 ± 10.3 years; FEV1, 28.1 ± 10.2% predicted) and 12 age- and sex-matched healthy subjects (age, 61.1 ± 7.2 years, FEV1, 101.5 ± 11.9% predicted). EMGdi was recorded from a multipair esophageal electrode during a constant work (80% of maximal oxygen consumption derived from a previous incremental exercise test) treadmill exercise. Minute ventilation and oxygen consumption were also measured.

Results

Root mean square (RMS) of the EMGdi increased initially and reached a plateau at submaximal drive during constant load exercise in both patients with COPD and healthy subjects. The ratio of ventilation to EMGdi remained stable during exercise in healthy subjects from beginning to the end (100% ± 70% at the beginning and 100% ± 39% at the end, P > .05), whereas the ratio decreased gradually during exercise in patients with COPD (from 85% ± 66% to 42% ± 13%, P < .05).

Conclusions

Efficiency of neural drive decreases in patients with COPD during treadmill exercise. Neural respiratory drive reached a submaximal plateau during constant load exercise in both healthy subjects and patients with COPD, indicating that it may not be the only factor determining exercise capacity.

Section snippets

Subjects

Twelve male patients with moderate to severe COPD, aged 62.8 ± 10.3 years, with BMI 22.5 ± 3.0 kg/m2, participated in the study. All patients were clinically stable without clinically significant coexisting disease and had not had an acute exacerbation in the preceding month. No patients needed to receive supplemental oxygen at rest. Patients were allowed to take their usual medication, including inhaled bronchodilators. We also studied 12 age- (61.1 ± 7.2 years) and BMI- (22.3 ± 2.2 kg/m2)

Results

Maximal RMS of EMGdi was similar between patients with COPD and healthy subjects (245.5 ± 52.7 μV vs 210.6 ± 65.7 μV, P > .05). Neural drive as a percentage of maximal drive at rest was 18.9% ± 9.8% in patients with COPD and was significantly higher than that in healthy subjects (9.8% ± 3.2%, P < .01) (Table 1). Maximal EMG at rest was recorded from maximal inspiration from FRC to TLC in all but one patient with COPD. In contrast in normal subjects, maximal EMG was recorded during the maximal

Discussion

Our data confirm the known physiology of exercise and COPD; specifically, neural drive increases during exercise. We confirm our previous finding that, at end-exercise, EMGdi max is not reached. Additionally we show that the transmission of neural drive to ventilation is progressively impaired in COPD. Although this phenomenon has been previously explored, we believe this study is the first to quantify the problem.

Acknowledgments

Author contributions: Dr Qin: contributed to conception and design, analysis, and interpretation of data.

Dr Steier: contributed to conception and design, interpretation of data, and final approval of the version to be published.

Dr Jolley: contributed to conception and design, interpretation of data, and final approval of the version to be published.

Dr Moxham: contributed to conception and design, interpretation of data, and final approval of the version to be published.

Dr Zhong: contributed to

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      This method allows measuring directly at the diaphragm during actual exercise and thereby provide accurate assessment of the respiratory drive modulation during voluntary breathing (Qin et al., 2010). EADi has been used to measure the work of the diaphragm at different lung capacities (Beck et al., 1998), and fatigability during CO2 rebreathing efforts (Luo et al., 2001) or in whole body exercise during constant load cycling in elderly normal and COPD patients (Qin et al., 2010). So far, only Zhang et al. (2013) have measured EADi during progressive exercise to exhaustion.

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      Therefore in the present study we adopted a novel approach by measuring the ability of the participants to voluntarily activate the diaphragm was assessed during a maneuver other than exercise (specifically the IC maneuver). We chose to normalize against the largest EMGdi obtained by any maneuver prior to or during exercise rather than that obtained from the IC maneuver at rest to make sure the data was normalized by a truly maximal EMG, in line with previous studies (Qin et al., 2010; Sinderby et al., 2001). The control condition of 60% of VO2max was chosen because exercise load lower than 85% of the maximum oxygen consumption is not thought to elicit diaphragm fatigue (Johnson et al., 1993; Babcock et al., 1995).

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    Funding/Support: This study was supported by the Chinese Natural Scientific Foundation [Grant 30770937] and International Collaboration Grant supported by Chinese Ministry of Science and Technology [Grant 2005DFA30910].

    Reproduction of this article is prohibited without written permission from the American College of Chest Physicians (http://www.chestpubs.org/site/misc/reprints.xhtml).

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