Hu, J; Gao, J.; and Tung, W., "Characterizing heart rate variability by scale-dependent Lyapunov exponent" (2009) Previous studies on heart rate variability ͑HRV͒ using chaos theory, fractal scaling analysis, and many other methods, while fruitful in many aspects, have produced much confusion in the literature. Especially the issue of whether normal HRV is chaotic or stochastic remains highly controversial. Here, we employ a new multiscale complexity measure, the scale-dependent Lyapunov exponent ͑SDLE͒, to characterize HRV. SDLE has been shown to readily characterize major models of complex time series including deterministic chaos, noisy chaos, stochastic oscillations, random 1 / f processes, random Levy processes, and complex time series with multiple scaling behaviors. Here we use SDLE to characterize the relative importance of nonlinear, chaotic, and stochastic dynamics in HRV of healthy, congestive heart failure, and atrial fibrillation subjects. We show that while HRV data of all these three types are mostly stochastic, the stochasticity is different among the three groups. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3152007͔Determining whether heartbeat dynamics is chaotic or stochastic is an important issue, both theoretically and clinically. The problem is difficult to solve neatly, however, since heart rate variability (HRV) may exhibit both nonlinear, and possibly chaotic, as well as stochastic behaviors. This motivates us to employ a recently developed multiscale complexity measure, the scale-dependent Lyapunov exponent (SDLE), to characterize HRV. SDLE cannot only unambiguously distinguish chaos from noise but also characterize various types of complex time series. Using SDLE, we are able to quantify the relative importance of nonlinear, chaotic, and stochastic dynamics in HRV of healthy, congestive heart failure, and atrial fibrillation subjects.