Neural respiratory drive, pulmonary mechanics and breathlessness in patients with cystic fibrosis

Abstract

Background Neural respiratory drive (NRD) measured from the diaphragm electromyogram (EMGdi) reflects the load/capacity balance of the respiratory muscle pump and is a marker of lung disease severity. EMGdi measurement is invasive, but recording the EMG from the parasternal intercostal muscles using surface electrodes (sEMGpara) could provide a non-invasive method of assessing NRD and disease severity. Objectives To test the hypothesis that NRD measured by sEMGpara correlates with EMGdi, to provide an index of disease severity in cystic fibrosis (CF) and to relate to exercise-induced breathlessness. Methods 15 patients with CF (mean forced expiratory volume in 1 s (FEV 1 ) 53.5% predicted) and 15 age-matched healthy controls were studied. sEMGpara and EMGdi were recorded at rest and during exercise. sEMGpara was recorded using surface electrodes and EMGdi using a multipair oesophageal electrode catheter. Signals were normalised using the peak EMG recorded during maximum respiratory manoeuvres and expressed as EMG%max. The respiratory pattern, metabolic data, oesophageal and gastric pressures and Borg scores were also recorded. Results Mean (SD) resting sEMGpara%max and EMGdi %max were higher in patients with CF than in controls (13.1 (7)% and 18.5 (7.5)% vs 5.8 (3)% and 7.5 (2)%, respectively, p<0.001). In the patients with CF, resting sEMGpara%max and EMGdi%max were related to the degree of airways obstruction (FEV 1 ) (r¼À0.91 and r¼À0.82, both p<0.001), hyperinflation (r¼0.63 and r¼0.56, both p<0.001) and dynamic lung compliance (r¼À0.53 and r¼À0.59, both p<0.001). During exercise, sEMGpara%max and EMGdi%max were strongly correlated with breathlessness in the patients with CF before (r¼0.906, p<0.001) and after (r¼0.975, p<0.001) the onset of neuromechanical dissociation. Conclusion sEMGpara%max provides a non-invasive marker of neural drive, which reflects disease severity and exercise-induced breathlessness in CF.

Measurement of neural respiratory drive via parasternal intercostal electromyography in healthy adult subjects

“…The potential utility of parasternal intercostal muscle EMG (EMGpara) measurements as a marker of NRD has been established in both clinical and laboratory settings, in health and disease. Relationships between EMGpara and disease severity in obstructive lung diseases have been demonstrated (Maarsingh et al, 2002;Reilly et al, 2011;Steier et al, 2011;Reilly et al, 2012), along with increases in EMGpara in response to externally-imposed respiratory load in healthy subjects (Reilly et al, 2013). Differences between healthy individuals and those with lung disease have also been demonstrated Murphy et al, 2011;Reilly et al, 2011;Steier et al, 2011), via comparison to small groups of matched individuals.…”

“…Previous studies (Murphy et al, 2011;Reilly et al, 2011;Steier et al, 2011) including measurements of EMGpara in healthy individuals have recruited relatively small numbers of subjects from staff and students of respiratory physiology departments.…”

“…Subjects were instructed to remain still and not to speak throughout the recording period. EMGpara was also recorded during maximal inspiratory efforts, as described by other authors Reilly et al, 2011;Steier et al, 2011), in order to allow normalisation of the resting signal to maximal EMGpara activity. EMGpara was recorded during sniff nasal inspiratory pressure (SNIP) and maximal inspiratory pressure (PImax) manoeuvres, during inspiration to total lung capacity (TLC) and during a single fifteen-second maximum (1a), inspiration to total lung capacity (1b) and maximal sniff manoeuvre (1c) from a 32 year-old female subject, showing raw EMGpara trace ("EMGpara"), root mean square ("EMGparaRMS") and respiratory flow (1a only, inspiration positive).…”

“…NRD cannot be quantified at source in human subjects; instead, the neural input to selected important respiratory muscles in the form of the electromyogram (EMG) can be used as an index of NRD. As the primary muscle of inspiration, the diaphragm remains the best option for measurement of NRD and such measures reflect lung disease severity in chronic obstructive pulmonary disease and cystic fibrosis Reilly et al, 2011). Obtaining accurate, uncontaminated EMG signals from the diaphragm however requires the passage of an oesophageal catheter, rendering the technique unsuitable for widespread use.…”

“…Reproducibility of EMGpara%max measurements has been demonstrated (Murphy et al, 2011;Reilly et al, 2011), but these studies have mainly been undertaken in small groups of healthy individuals experienced in respiratory manoeuvres. It is not known whether naïve subjects are as able to reliably perform such maximal respiratory efforts reproducibly.…”