Introduction Measurement of neural respiratory drive, using parasternal intercostal muscle electromyography (EMGpara), has previously been shown to relate to pulmonary function impairment and exercise-induced breathlessness in advanced cystic fibrosis (CF). This measure reflects the load on the respiratory system and the capacity of the respiratory muscles and therefore may provide a composite measure of overall lung disease severity. In order to utilise EMGpara clinically in CF, its relationship to standard physiological outcome measures requires further investigation across a broad range of disease severities. Aim: To investigate the relationships between EMGpara and standard measures of pulmonary function and exercise performance in patients with CF.
Methods Thirty patients with clinically stable CF were recruited. EMGpara was recorded during five minutes of tidal breathing using electrodes positioned in the second intercostal space directly lateral to the sternum. Peak EMGpara per breath was averaged over the final minute of the recording and expressed as a percentage of EMGpara recorded during a maximal inspiratory manoeuvre (EMGpara%max). Spirometry, lung volumes by body plethysmography and an incremental shuttle walking test (ISWT) with breath by breath metabolic data were also performed.
Results Patient characteristics, EMGpara%max, measures of pulmonary function and exercise performance are shown in Table 1. EMGpara%max was significantly associated with residual volume/total lung capacity ratio (RV/TLC, r = 0.724, p < 0.001), forced expiratory volume in 1 second (FEV1% pred. r = -0.648, p < 0.001) and functional residual capacity/total lung capacity ratio (FRCpleth/TLC, r = 0.625, p < 0.001). EMGpara%max showed the strongest relationship with ISWT distance (r = -0.612, p < 0.001) and peak oxygen uptake (VO2peak, r = -0.665, p = 0.001). Weaker relationships were observed between ISWT distance and pulmonary function (FEV1r = 0.518 p = 0.006, RV/TLC r = -0.451 p = 0.024, FRCpleth/TLC r = -0.299 p 0.147) and VO2peak (FEV1r = 0.521 p = 0.008, RV/TLC r = -0.505 p = 0.014, FRCpleth/TLC r = -0.389 p = 0.066).
Conclusion EMGpara correlates strongly with conventional pulmonary function measures in CF and has a closer relationship with exercise capacity than standard pulmonary function parameters. EMGpara%max therefore represents a promising marker of CF lung disease severity.