Abstract
The aim of the present study was to use the diaphragm electromyogram (EMG(di)) to compare levels of neural respiratory drive (NRD) in a cohort of healthy subjects and chronic obstructive pulmonary disease (COPD) patients, and to investigate the relationship between NRD and pulmonary function in COPD. EMG(di) was recorded at rest and normalised to peak EMG(di) recorded during maximum inspiratory manoeuvres (EMG(di) % max) in 100 healthy subjects and 30 patients with COPD, using a multipair oesophageal electrode. EMG(di) was normalised to the amplitude of the diaphragm compound muscle action potential (CMAP(di,MS)) in 64 healthy subjects. The mean+/-sd EMG(di) % max was 9.0+/-3.4% in healthy subjects and 27.9+/-9.9% in COPD patients, and correlated with percentage predicted forced expiratory volume in one second, vital capacity and inspiratory capacity in patients. EMG(di) % max was higher in healthy subjects aged 51-80 yrs than in those aged 18-50 yrs (11.4+/-3.4 versus 8.2+/-2.9%, respectively). Observations in the healthy group were similar when peak EMG(di) or CMAP(di,MS) were used to normalise EMG(di). Levels of neural respiratory drive were higher in chronic obstructive pulmonary disease patients than healthy subjects, and related to disease severity. Diaphragm compound muscle action potential could be used to normalise diaphragm electromyogram if volitional inspiratory manoeuvres could not be performed, allowing translation of the technique to critically ill and ventilated patients.
Highlights
The mean¡SD EMG of the diaphragm (EMGdi) % max was 9.0¡3.4% in healthy subjects and 27.9¡9.9% in chronic obstructive pulmonary disease (COPD) patients, and correlated with percentage predicted forced expiratory volume in one second, vital capacity and inspiratory capacity in patients
Representative traces at rest and during maximum voluntary ventilation in a healthy subject and a COPD patient are shown in figure 1
Levels of EMGdi % max increased slightly with age, but there were no significant correlations between EMGdi % max and sex, height, weight or body mass index (BMI) in this largely non-obese population
Summary
The mean¡SD EMGdi % max was 9.0¡3.4% in healthy subjects and 27.9¡9.9% in COPD patients, and correlated with percentage predicted forced expiratory volume in one second, vital capacity and inspiratory capacity in patients. Observations in the healthy group were similar when peak EMGdi or CMAPdi,MS were used to normalise EMGdi. Levels of neural respiratory drive were higher in chronic obstructive pulmonary disease patients than healthy subjects, and related to disease severity. The translation of inspiratory muscle contraction into negative intrathoracic pressure, and of pressure changes to ventilation, is impaired as a consequence of muscle shortening, increased velocity of contraction, alteration in geometry and reduced compliance of the respiratory system. This results in high NRD in COPD, and disproportionate increases whenever airways obstruction worsens (and hyperinflation increases) or ventilatory requirements increase. The EMG of the diaphragm (EMGdi), the major inspiratory muscle during resting tidal
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