The HRV Variable Zoo

A browsable "Pokédex" of every heart-rate-variability feature HeartRateLab computes: 53 registered measures across four domains. Each entry answers the same three questions:

What is it? Definition, equation, aliases, and declared distribution family.
What does normal look like? The empirical distribution over the pooled nsrdb + nsr2db normal-sinus-rhythm cohorts (360-beat windows, n up to 61 715), overlaid with a fitted normative prior, plus a normal-range table.
What does it cost, and who introduced it? A measured wall-clock and allocation resource tier, curated use-cases, and the seminal citation.

Priors and normal-range statistics are descriptive references from a healthy cohort, not clinical thresholds. See the References page for the full bibliography. Resource tiers are measured on synthetic realistic RR at a 360-beat window (docs/zoo_gen/bench_resources.jl), from ◍◌◌◌◌ (very low) to ◍◍◍◍◍ (very high).

The C S P M column marks each feature's evidence coverage in the four fields of the HRV knowledge base: Clinical, Sports & peak-performance, and contemplative Practice (● pooled literature, ◐ individual papers, ○ sparse), and Methods & foundations (✔ seminal reference on file).

Time domain (20)

Statistics of the NN/RR intervals and their successive differences: the classic, cheapest, most-reported HRV panel.

Feature	Definition	Dist. family	Resource tier	C S P M
`mean`	Mean inter-beat interval	`Normal`	◍◌◌◌◌ very low	● ● ● ✔
`sdnn`	Standard deviation of NN intervals	`Gamma`	◍◌◌◌◌ very low	● ◐ ◐ ✔
`median`	Median inter-beat interval	`Normal`	◍◍◌◌◌ low	● ● ● ✔
`max`	Maximum inter-beat interval	`Normal`	◍◌◌◌◌ very low	● ● ● ✔
`min`	Minimum inter-beat interval	`Normal`	◍◌◌◌◌ very low	● ● ● ✔
`mean_hr`	Mean heart rate	`Normal`	◍◍◌◌◌ low	● ● ● ✔
`std_hr`	Standard deviation of heart rate	`Gamma`	◍◍◌◌◌ low	● ◐ ◐ ✔
`max_hr`	Maximum heart rate	`Normal`	◍◍◌◌◌ low	● ● ● ✔
`min_hr`	Minimum heart rate	`Normal`	◍◍◌◌◌ low	● ● ● ✔
`median_hr`	Calculate the median heart rate in BPM from the Inter-Beat-Interval…	`Normal`	◍◍◌◌◌ low	● ● ● ✔
`range_hr`	Calculate the range (max − min) of the instantaneous heart rate in BPM	`Gamma`	◍◍◌◌◌ low	● ◐ ◐ ✔
`sdsd`	Standard deviation of successive differences	`Gamma`	◍◍◌◌◌ low	● ● ◐ ✔
`range`	Range of inter-beat intervals	`Gamma`	◍◍◌◌◌ low	● ◐ ◐ ✔
`rmssd`	Root mean square of successive differences	`Gamma`	◍◍◌◌◌ low	● ● ◐ ✔
`sdann`	Standard deviation of 5-min average NN intervals	`Gamma`	◍◍◌◌◌ low	● ◐ ◐ ✔
`pnn50`	Proportion of successive differences > 50 ms	`Beta`	◍◍◌◌◌ low	● ● ◐ ✔
`pnn20`	Proportion of successive differences > 20 ms	`Beta`	◍◍◌◌◌ low	● ● ◐ ✔
`cvsd`	Coefficient of variation of successive differences	`Gamma`	◍◍◌◌◌ low	● ◐ ◐ ✔
`cvnni`	Calculate the coefficient of variation of the NN intervals (SDNN / …	`Normal`	◍◍◌◌◌ low	● ◐ ◐ ✔
`rRR`	Median relative RR interval distance	`Gamma`	◍◍◌◌◌ low	○ ○ ○ ✔

Frequency domain (12)

Power in the ULF/VLF/LF/HF bands of the RR power spectrum, plus band ratios and peak frequencies. Welch periodogram on the resampled series by default (config["freq_method"] == :welch); Lomb–Scargle on the raw unevenly-sampled series is available as an alternative.

Feature	Definition	Dist. family	Resource tier	C S P M
`ulf`	Ultra-low frequency power (0-0.003 Hz)	`Gamma`	◍◍◍◌◌ moderate	● ◐ ◐ ✔
`vlf`	Very low frequency power (0.003-0.04 Hz)	`Gamma`	◍◍◍◌◌ moderate	● ◐ ◐ ✔
`lf`	Low frequency power (0.04-0.15 Hz)	`Gamma`	◍◍◍◌◌ moderate	● ● ● ✔
`hf`	High frequency power (0.15-0.4 Hz)	`Gamma`	◍◍◍◌◌ moderate	● ● ● ✔
`tp`	Total power (0.003-0.4 Hz)	`Gamma`	◍◍◍◌◌ moderate	● ● ● ✔
`lf_peak`	Peak frequency in LF band	`Normal`	◍◍◍◌◌ moderate	● ● ● ✔
`hf_peak`	Peak frequency in HF band	`Normal`	◍◍◍◌◌ moderate	● ● ● ✔
`lf_hf_ratio`	LF/HF power ratio	`LogNormal`	◍◍◍◌◌ moderate	● ● ● ✔
`lf_relative`	LF power as proportion of total power	`Beta`	◍◍◍◌◌ moderate	● ● ● ✔
`hf_relative`	HF power as proportion of total power	`Beta`	◍◍◍◌◌ moderate	● ● ● ✔
`lf_percentage`	LF power as percentage of total power	`Gamma`	◍◍◍◌◌ moderate	● ● ● ✔
`hf_percentage`	HF power as percentage of total power	`Gamma`	◍◍◍◌◌ moderate	● ● ● ✔

Geometric (8)

Shape descriptors of the Poincaré / Lorenz return map and the RR histogram, robust to occasional artifacts.

Feature	Definition	Dist. family	Resource tier	C S P M
`sd1`	Poincare plot short-term variability	`Gamma`	◍◍◌◌◌ low	● ◐ ◐ ✔
`sd2`	Poincare plot long-term variability	`Gamma`	◍◍◌◌◌ low	● ◐ ◐ ✔
`sd2_sd1`	Ratio of SD2 to SD1 (cardiac sympathetic index)	`LogNormal`	◍◍◌◌◌ low	● ◐ ◐ ✔
`sd1_sd2_area`	Poincare plot ellipse area	`LogNormal`	◍◍◌◌◌ low	● ◐ ◐ ✔
`cvi`	Cardiac vagal index	`Normal`	◍◍◌◌◌ low	◐ ◐ ○ ✔
`ccsi`	Corrected cardiac sympathetic index	`LogNormal`	◍◍◌◌◌ low	◐ ◐ ○ ✔
`triangular_index`	HRV triangular index	`Gamma`	◍◍◌◌◌ low	● ◐ ○ ✔
`tinn`	Triangular interpolation of NN interval histogram	`Gamma`	◍◍◍◍◍ very high	● ◐ ○ ✔

Nonlinear (13)

Entropy, complexity and fractal-scaling measures probing the nonlinear structure of cardiac control (need adequate record length).

Feature	Definition	Dist. family	Resource tier	C S P M
`apen`	Approximate entropy	`Normal`	◍◍◍◍◌ high	● ◐ ◐ ✔
`sampen`	Sample entropy	`Normal`	◍◍◍◍◌ high	● ◐ ◐ ✔
`hurst`	Hurst exponent	`Normal*`	◍◍◌◌◌ low	● ◐ ◐ ✔
`renyi0`	Renyi entropy of order 0	`Normal`	◍◍◌◌◌ low	● ◐ ◐ ✔
`renyi1`	Renyi entropy of order 1	`Normal`	◍◍◌◌◌ low	● ◐ ◐ ✔
`renyi2`	Renyi entropy of order 2	`Normal`	◍◍◌◌◌ low	● ◐ ◐ ✔
`shan_en`	Calculate the Shannon entropy of the Inter-Beat-Interval histogram	`Normal`	◍◍◌◌◌ low	● ◐ ◐ ✔
`svd_en`	Calculate the Singular Value Decomposition (SVD) entropy of the IBI…	`Normal`	◍◍◌◌◌ low	● ◐ ◐ ✔
`fuzzyen`	Calculate the fuzzy entropy of the Inter-Beat-Intervals (IBIs)	`Normal`	◍◍◍◍◍ very high	● ◐ ◐ ✔
`spec_en`	Calculate the spectral entropy of the Inter-Beat-Intervals (IBIs): …	`Normal`	◍◍◍◌◌ moderate	● ◐ ◐ ✔
`perm_en`	Calculate the permutation entropy of the Inter-Beat-Intervals (IBIs)	`Normal`	◍◍◍◌◌ moderate	● ◐ ◐ ✔
`mse`	Calculate the Multiscale Entropy (MSE) complexity index of the IBIs…	`Normal`	◍◍◍◍◌ high	● ◐ ◐ ✔
`dfa2`	DFA long-term scaling exponent alpha2	`Normal`	◍◍◍◍◌ high	◐ ○ ◐ ✔

* hurst declares Beta (theoretically bounded to (0,1)); the table shows the empirically-fitted Normal family instead, since observed values leave that interval. See hurst.

All entries

The reference dex entry is rmssd. Full per-feature detail (figure, normal-range table, resource benchmark, and citation) lives on each entry page linked above.