Fractal characteristics of end-expiratory lung volume in anesthetized rats

To test the possibility that the variations in end-expiratory lung volume (EEV) might exhibit long-term correlation, we calculated the Hurst exponent H for EEV in anesthetized rats during sustained positive and negative transrespiratory pressure (i.e., (PTRP) and (NTRP), correspondingly) using the dispersional analysis method. In nine vagi-intact rats, H for EEV was 0.551+/-0.051 (mean+/-std) and 0.641 +/-0.076 (mean+/-std) with PTRP and NTRP, respectively. In both cases H was significantly different from those obtained after random shuffling of the original time series, indicating a positive long-term correlation in EEV. Also H with NTRP was significantly greater than that with PTRP (P=0.015). In six vagotomized rats, there was no significant difference in H for EEV between NTRP and PTRP. We demonstrated that long-term correlation is present in EEV. We inferred that the stimulation of vagal afferents could change the fractal property in EEV since different vagal pulmonary receptors are activated by PTRP and NTRP. Although the exact mechanisms for the fractal behavior are unknown, the results suggest that interaction of the pulmonary mechanoreceptor activity with the central respiratory pattern generator might contribute to the observed long-term correlation.