Ventricular response in atrial fibrillation: random or deterministic?

The ventricular response in atrial fibrillation is often described as "chao tic," hut this has not been demonstrated in the strict mathematical sense. A defining feature of chaotic systems is sensitive dependence on initial co nditions: similar sequences evolve similarly in the short term but then div erge exponentially We developed a nonlinear predictive forecasting algorith m to search for predictability and sensitive dependence on initial conditio ns in the ventricular response during atrial fibrillation. The algorithm wa s tested for simulated R-R intervals from a Linear oscillator with and with out superimposed white noise; a chaotic signal (the logistic map) with and without superimposed white noise, and a pseudorandom signal and was then ap plied to RR intervals from 16 chronic atrial fibrillation patients. Short-t erm predictability was demonstrated for the linear oscillators, without los s of predictive ability farther into the future. The chaotic system demonst rated high shortterm predictability that declined rapidly further into the future. The pseudorandom signal was unpredictable. The ventricular response in atrial fibrillation was weakly predictable (statistically significant p redictability in 8 of 16 patients), without sensitive dependence on initial conditions. Although the R-R interval sequence is not completely unpredict able, a low-dimensional chaotic attractor does not govern the irregular ven tricular response during atrial fibrillation.