An interbeat interval time series (or heart rate time series) is mapped to a symbolic sequence, according to a mapping rule that accelerated heart rate in consecutive heartbeats is designated as 0 and a deceleration of heart rate is designated as 1 ( Figure 1A ). This way of mapping captures the essential dynamics of the autonomic nervous system's control of heart rate and is less sensitive to noisy fluctuations in interbeat interval time series commonly caused by ectopic heartbeats [17] . A binary, symbolic ''word'' is then defined as a n-tuple sequence derived from n+1 consecutive interbeat intervals. We determined the frequencies of each pattern of n-tuple sequences by applying a sliding window (moving one interbeat interval/step) across the entire interbeat interval time series and then ranked each n-tuple sequence according to its frequency in descending order.