Limitations of surface EMG estimate of parasternal intercostal to infer neural respiratory drive.

BACKGROUND: Recently, surface EMG of parasternal intercostal muscle has been incorporated in the "ERS Statement of Respiratory Muscle Testing" as a clinical technique to monitor the neural respiratory drive (NRD). However, the anatomy of the parasternal muscle risks confounding EMG "crosstalk" activity from neighboring muscles. OBJECTIVES: To determine if surface "parasternal" EMG: 1) reliably estimates parasternal intercostal EMG activity, 2) is a valid surrogate expressing neural respiratory drive (NRD). METHODS: Fine wire electrodes were implanted into parasternal intercostal muscle in 20 severe COPD patients along with a pair of surface EMG electrodes at the same intercostal level. We recorded both direct fine wire parasternal EMG (EMGPARA) and surface estimated "parasternal" EMG (SurfEMGpara) simultaneously during resting breathing, volitional inspiratory maneuvers, apnoea with extraneous movement of upper extremity, and hypercapnic ventilation. RESULTS: Surface estimated "parasternal" EMG showed spurious "pseudobreathing" activity without any airflow while real parasternal EMG was silent, during apnoea with body extremity movement. Surface estimated "parasternal" EMG did not faithfully represent real measured parasternal EMG. Surface estimated "parasternal" EMG was significantly less active than directly measured parasternal EMG during all conditions including baseline, inspiratory capacity and hypercapnic ventilation. Bland-Altman analysis showed consistent bias between direct parasternal EMG recording and surface estimated EMG during stimulated breathing. CONCLUSION: Surface "parasternal" EMG does not consistently or reliably express EMG activity of parasternal intercostal as recorded directly by implanted fine wires. A chest wall surface estimate of parasternal intercostal EMG may not faithfully express NRD and is of limited utility as a biomarker in clinical applications.

Parasternal muscle activity decreases in severe COPD with salmeterol-fluticasone propionate.

BACKGROUND: The effect of the long acting beta(2)-agonist/corticosteroid combination salmeterol-fluticasone propionate (SFC) on respiratory muscles and ventilation in severe COPD is unknown. As COPD hyperinflation worsens, diaphragm efficiency decreases, and a compensatory increase in chest wall inspiratory muscle activity occurs. If a bronchodilator successfully alleviates hyperinflation and improves diaphragm efficiency in severe COPD, then the extraordinary activation of the chest wall may be relieved. We examined directly the effect on the parasternal intercostal respiratory chest wall muscle and ventilation of four puffs of salmeterol 25 microg and fluticasone propionate 125 microg via the metered dose combination inhaler in 12 patients with severe Global Initiative on Obstructive Lung Disease stage III-IV COPD, mean FEV(1) = 0.91 L (32% predicted). METHODS: We measured parasternal intercostal electromyogram (EMG) recorded from implanted fine-wire electrodes, ventilation, and breathing pattern, during resting and CO(2)-stimulated breathing. Full pulmonary function tests were recorded at the beginning and end of the study. RESULTS: In this patient group, severe airflow obstruction and hyperinflation were poorly reversible after SFC: FEV(1) increased 4.2%, functional residual capacity decreased 1.4%, and inspiratory capacity increased 5.9%. However, with SFC there was a significant increase in minute ventilation, tidal volume, and mean inspiratory flow. There was a very large decrease in directly recorded parasternal EMG, with parasternal EMG disappearing completely in some patients after SFC. CONCLUSIONS: In severe COPD, with minimal change in hyperinflation or pulmonary mechanics, salmeterol-fluticasone induced a significant decrease in activity of the chest wall parasternal inspiratory muscle. This may be of practical benefit to reverse the extensive use of the chest wall muscles and alleviate dyspnea in severe COPD.