Neural respiratory drive measured in Cystic Fibrosis - the potential use of parasternal intercostal electromyography as a clinical tool

Charles Reilly1, Katie Ward1, Caroline Jolley1, Alan Lunt2, Joerg Steier1, Caroline Elston2, Gerrard F Rafferty1, John Moxham1. 1Kings College, London, United Kingdom, 2Kings College Hospital, London, United Kingdom.

Introduction: Neural respiratory drive (NRD) quantified by electromyography (EMG) of the diaphragm is a marker of disease severity in chronic obstructive pulmonary disease (Jolley et al. Eur Respir J 2009; 33(2): 289-297). Measurement of diaphragm EMG (EMGdi) is invasive, which may limit its clinical application. The parasternal intercostal muscles are obligate respiratory muscles and recording the EMG (EMGpara) from them using surface electrodes could provide a non invasive method to assess NRD.

Methods: Fifteen patients [median (range) age 22 (20 - 26) years] with moderate to severe CF [mean (SD) FEV1 % predicted 53 (24) %] and fifteen age matched [23 (22 - 27) years] healthy subjects were studied. EMGdi was measured using a multipair oesphageal catheter and EMGpara from surface electrodes (positive electrode in the right second intercostal space, 3cm from the sternum). The EMG root mean square (RMS) was calculated and peak RMS of the resting EMG was expressed as a percentage of the peak RMS (EMG %max) obtained during maximal inspiratory capacity manoeuvres.

Results: Clear phasic inspiratory EMG activity of the diaphragm and parasternal intercostal muscles was observed. Resting median EMGdi%max was correlated to EMGpara%max (r = 0.829, p<0.0001). Median (range) EMGdi%max was 19.5 (6 - 32) % in the CF Patients and 7 (4 - 12) % in the healthy subjects (p < 0.0001). Median EMGpara%max was 11 (8 - 18) % in the CF patients and 4.8 (2.8 - 7) % in the healthy subjects (p<0.0001).In the CF patients EMGdi%max was correlated to FEV1 % predicted (r = -0.793, p<0.0001) and VC % predicted (r = -0.556, p=0.03), EMGpara%max was correlated to FEV1 % predicted (r = -0.739, p = 0.02), VC % predicted (r = -0.572, p = 0.011) and RV % predicted (r = 0.552, p = 0.04).

Conclusion: EMGpara provides a non invasive method to assess NRD and reflects disease severity in CF.