Effect of naloxone on spectral shifts of the diaphragm EMG during inspiratory loading

Shifts in the power spectrum of the diaphragm EMG to lower frequencies may occur in the presence of fatiguing inspiratory flow-resistive loads (IRL). However, such a shift of the centroid frequency (f(c)) could follow a reduction in central output through a differential reduction in end-inspiratory high-frequency power (HFP). In unanesthetized goats, we tested the hypothesis that activation of the endogenous opioid system by IRL would differentially reduce central respiratory output, causing a reduction in f(c). IRL was imposed for 180 min after which naloxone (0.1 mg/kg, NLX) was given. f(c) was computed from the power spectral density estimated by the Welch method. IRL reduced f(c) from 148.0 ± 9.8 (SE) Hz at base line to 141.1 ± 8.9 Hz or to 95.5 ± 1.3% of base line by 180 min (both P < 0.05). NLX increased f(c) to 148.9 ± 9.9 Hz or to 100.6 ± 1.1% of base line (both P < 0.05). The decline in f(c) during IRL was found to be the result of a reduction in HFP, predominantly toward the end of inspiration. The reversibility of this f(c) shift with NLX suggests a central mechanism conequent to elaboration of endogenous opioids and not a peripheral (muscular) event consequent to muscle fatigue.