In addition, we incorporated two nonlinear HRV indices: detrended fluctuation analysis (DFA) [27, 28] and multiscale entropy (MSE) [29] . DFA quantifies the presence of long-range (fractal) correlations whereas MSE measures the entropy over multiple time scales inherent in physiologic signals and is therefore a complexity measure. Both methods are available at Physionet (http://physionet.org), a research resource for complex physiologic signals [18] .