Table 8.

Nonlinear HRV features and composite autonomic indices reported across studies.

Parameter	Studies	Percentage
SampEn, ApEn, other entropy-based	4	16.7%
SD1/SD2	4	16.7%
DFA α1 / α2	2	8.3%
REC/DET	1	4.2%
SNS/PNS index	2	8.3%

SD1 and SD2, Poincaré plot descriptors representing short-term and long-term variability of RR intervals; DFA, detrended fluctuation analysis, that quantify fractal scaling properties of HRV; REC, recurrence rate and DET, determinism, parameters derived from recurrence quantification analysis (RQA) describing dynamic patterns in the HR time series; SNS, sympathetic nervous system index; PNS, parasympathetic nervous system index, composite measures of autonomic balance derived from multiple HRV parameters; SampEn, sample entropy; ApEn, approximate entropy, used to quantify irregularity in heart rate dynamics.