Complexity of chaotic vasomotion is insensitive to flow and pressure but can be regulated by external control.

We have previously shown that irregular vasomotion induced by histamine in isolated rabbit ear resistance arteries is chaotic. Consistently, in the present study, such activity was found to respond in a highly unpredictable fashion to changes in flow under conditions of controlled-flow perfusion, although its fractal dimension, calculated by a standard correlation technique, was effectively independent of flow rate and remained < 4. As this statistic provides an estimate of the number of control variables that generate a chaotic time series, flow thus appears to modulate vasomotion without fundamentally contributing to its genesis. External modification of the dynamics was attempted by a negative feedback loop that regulated pump speed through an error signal derived from perfusion pressure. Irregular responses were converted to either periodic or steady-state behavior in approximately 60% of cases with an associated fall in fractal dimension. Conversely, unsuccessful control was often associated with an increase in fractal dimension, reflecting the additional complexity introduced by the feedback loop. Furthermore, control was more readily achieved in the presence of NG-nitro-L-arginine methyl ester, when time- and flow-dependent changes in endothelium-derived relaxing factor synthesis would not be expected to complicate the overall dynamics. The study suggests that vascular chaos may be economically "controlled" under both physiological and pathophysiological conditions.