Spectral analysis on low frequency fluctuation in respiratory rhythm in the decerebrate cat

Spectral analysis was performed on the fluctuation in respiratory intervals in vagotomized, paralyzed, and unanesthetized decerebrate cats. The respiratory interval was measured as the time interval between the onset of phrenic neural activity. When endtidal PCO2 was set at the same level as that before immobilization, the power spectral densities of the fluctuation in respiratory intervals were found to be inversely proportional to the frequency in the low frequency range of 0.001-0.1 Hz. All the tested animals showed this type of fluctuation (1/f fluctuation). In contrast, such characteristic 1/f fluctuation disappeared when the end-tidal PCo2 was raised to 50 or 60 mmHg. The spectral profile was almost flat in the frequency range of 0.001-0.1 Hz, i.e. similar to that of "white noise". This study demonstrated that the respiratory rhythm evaluated by phrenic neural discharges exhibited 1/f fluctuation and that the spectral characteristic of the fluctuation in respiratory intervals varied depending on the end-tidal PCo2.