Bifurcations of the respiratory pattern associated with reduced lung volume in the rat.

Rats with intact vagal reflexes exhibit patterns of breathing that contain greater degrees of freedom than those seen after vagotomy. To determine how alterations in end-expiratory volume modify the respiratory pattern, continuous positive (CPAP) and negative (CNAP) airway pressure was applied to tracheal openings of nine urethan-anesthetized vagi-intact rats (+3 to -9 cmH2O). Phase portraits (e.g., volume vs. flow curves), power spectra, correlation integral curves, and inspiratory-to-expiratory duration (TI/TE) ratios are used to interpret the vagal-dependent responses to changes in mean tracheal pressure (Ptr). With CPAP, respiratory oscillation was highly periodic and one dimensional, with TI/TE near 1.0. As Ptr was reduced in a stepwise manner, transient bursts of inspiratory airflow developed local to the expiratory-inspiratory transition, with amplitude increasing proportionally with the level of CNAP. These oscillatory "expiratory interrupts" (doubling TI/TE in five of nine cases) produced highly variable and asymmetric respiratory patterns. Progressive increases in correlation dimension (maximum = 1.8-3.0) and tendencies toward broadband power spectra were seen as Ptr was lowered. The irregular phase-switching dynamics seen with CNAP (which disappeared after vagotomy) are consistent with onset of low-dimensional chaos, probably correlated with activation of feedback mechanisms responsive to lung deflation.