Respiration Physiology (1985) 61, 137-152 137 Elsevier CHANGES IN DIAPHRAGMATIC EMG SPECTRA DURING HYPERPNEIC LOADS GARY C. SIECK, ASHER MAZAR, and MICHAEL J. BELMAN City of Hope National Medical Center, Department of Respiratory Diseases, 1500 East Duarte Road, Duarte, CA 91010. U.S.A. Abstract. 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 character- ized by the high frequency to low frequency (H/L) ratio and by the eentroid 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 (Le., 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. Centroid frequency Electromyogram Hyperpnea Diaphragm Human A characteristic shift in the power spectra of diaphragmatic EMG occurs during ventilatory muscle fatigue induced by breathing against a high inspiratory resistance (Bellemare and Grassino, 1982; Grassino et aL, 1979; Gross et al. 1979; Moxham et aL, 1982). In these studies, diaphragmatic force was estimated using trans-diaphragmatic pressure (Pdi), and ventilatory muscle fatigue was def'med as an inability to achieve a predetermined Pdi after a period of resistive breathing. BeUemare and Grassino (1982) Accepted for publication I 1 May 1985 0034-5687/85/$03.30 © 1985 Elsevier Science Publishers B.V. (Biomedical Division)