Background During AECOPD, expiratory flow limitation results in dynamic hyperinflation (DH), respiratory neuromechanical uncoupling, and increased work of breathing causing breathlessness. We have previously demonstrated that 2nd intercostal space EMGpara, as a direct marker of neural respiratory drive (NRD), is able to detect clinical change in hospitalised AECOPD patients. We hypothesised that EMGpara has an indirect relationship with DH.
Method Patients admitted with AECOPD at a metropolitan teaching hospital were recruited. Inspiratory capacity (IC) was used as a measure of DH. EMGpara, spirometry and IC manoeuvres were measured twice daily from admission until the patient was fit for discharge. Dyspnoea scores (modified Borg score, visual analogue scale and numerical rating scale) were recorded. NRD was expressed as EMGpara%max: inspiratory EMGpara signal normalised to a maximum signal measured during a sniff manoeuvre. The changes in EMGpara%max (?EMGpara%max) and IC (ΔIC) between admission and discharge were analysed.
Results 31 patients were recruited; only 20 (65%) were able to perform spirometry and IC manoeuvres. The baseline characteristics were 69±11 years; male 55%; body mass index 26.1±7.3 kg/m2; %predicted FEV1 36.3±9.3; and 41±24 smoking pack years. The overall mean ΔEMGpara%max of the 20 patients fell by 4%, with 16 (80%) patients experiencing a fall in EMGpara%max during their admission. We observed an indirect relationship between ΔEMGpara%max and ΔIC (r=−0.52, p=0.02), and between ΔEMGpara%max and ΔFVC (r=−0.585, p=0.036). We found a correlation between ΔIC and ΔFEV1 (r=0.658, p=0.015). There were differences in ΔIC between patients whose EMGpara%max decreased during their admission and those whose EMGpara%max increased (mean difference 0.50 l; p=0.003) (Abstract S116 figure 1). There were no significant correlations between dyspnoea scores and ΔIC or ΔEMGpara%max.
Conclusion DH is a significant contributor to NRD in AECOPD. ΔEMGpara%max reflects changes in DH during hospital admission, but patient-reported dyspnoea does not indicate the degree of DH, highlighting the limitations of dyspnoea scores. Changes in DH are correlated with changes in airflow obstruction. These data provide a physiological rationale for the utility of parasternal EMG as a non-invasive and non-volitional technique to track clinical change in AECOPD patients.