Changes in diaphragmatic EMG spectra during hyperpneic loads

Changes in electromyographic (EMG) activity of the diaphragm were examined in human subjects during different levels of voluntary hyperpnea. Diaphragmatic EMG was recorded using both surface and esophageal electrodes. The power spectra density (PSD) of the EMG signal was calculated and characterized by the high frequency to low frequency (H/L) ratio and by the centroid frequency (Fc). During high levels of voluntary hyperpnea, a significant decrease in both the H/L ratio and Fc occurred, which was similar in diaphragmatic EMG recorded by either surface or esophageal electrodes. This similarity in EMG spectral changes suggested that when diaphragmatic EMG was recorded using surface electrodes, there was only minimal contamination from the activity of other chest wall muscles. Changes in EMG Fc were detected during levels of hyperpnea which could be readily sustained. Thus, diaphragmatic EMG spectral changes were not characteristic of imminent ventilatory failure (i.e., an inability to sustain a target level of ventilation). In contrast, significant changes in EMG H/L ratio were observed only during hyperpneic loads which could not be sustained. This difference in sensitivity between the Fc and H/L ratio was due to the increased variability of H/L ratio and suggests that the H/L ratio may fail to detect small but significant shifts in EMG spectra. The relationship between the rate of decrease in EMG Fc and the level of hyperpnea was not statistically significant. We conclude that diaphragmatic EMG spectral changes do occur during hyperpneic loads, but we question the specificity of using diaphragmatic EMG spectral changes in predicting ventilatory failure.