HRVDex — Technical Reference

Daily HRV Summaries
& Readiness

Reference for every metric HRVDex surfaces from a pre-computed daily HRV reading (Welltory-style morning check) — definitions, formulas, expected ranges, and the evidence base behind each. The core HRV measures are the established, literature-validated ones; on them HRVDex layers trend signals, readiness composites and population projections. Companion to the reading; not a substitute for clinical evaluation.

⚠️

Important: HRVDex consumes daily summaries from a short (typically one-minute) reading, so it inherits that protocol’s sensitivity to posture, breathing and timing — compare readings taken the same way each morning. Composite “readiness” scores are internal blends; projections (ANS age, VO₂max, BP) are population proxies, not measurements.

Evidence Measured Validated Emerging Experimental Heuristic fill = trust · hover a badge for source

ℹ️

No “measured” tier here. Because HRVDex ingests already-summarised HRV (it never sees the raw beats itself), its strongest rung is validated — the established HRV measures — rather than measured. For raw beat-to-beat statistics, see PulseDex or ECGDex.

Abbr:

full index →

💓

Core HRV

The established, literature-validated heart-rate-variability measures

rMSSDRoot Mean Square of Successive Differences

Core

The primary parasympathetic (vagal) marker and the most stable short-reading HRV value — HRVDex’s headline.

Formula

rMSSD = √( mean( (NN[i+1]−NN[i])² ) )

rMSSD (ms)	Read
> 42	High vagal tone / well-recovered
20–42	Typical adult range
< 20	Low — stress / fatigue

SDNNStandard Deviation of NN Intervals

Core

Overall HRV across the reading — total autonomic variability.

Formula

SDNN = √( Σ(NN−mean)² / (N−1) )

SDNN (ms)	Read
> 50	Healthy
30–50	Moderate
< 30	Reduced

LF/HFSympatho-Vagal Balance

Advanced

Ratio of low- to high-frequency power — historically read as sympatho-vagal balance; interpret directionally.

Formula

LF/HF = LF power / HF power

LF/HF	Read
1–2	Balanced
> 2.5	Sympathetic-leaning
< 0.5	Parasympathetic-leaning

HF n.u.Normalised HF

Research

High-frequency power as a share of (LF+HF) — the parasympathetic fraction.

Formula

HFnu = 100 × HF / (LF + HF)

Relative measure — compare against your own baseline; no single clinical cut-point.

SD1/SD2Poincaré Ratio

Research

Balance of short- to long-term variability from the Poincaré plot.

Formula

ratio = SD1 / SD2

Relative measure — compare against your own baseline; no single clinical cut-point.

Baevsky SIStress Index

Advanced

A geometric stress index that rises with sympathetic load / autonomic rigidity.

Formula

SI = AMo / (2 × Mode × MxDMn)

Baevsky SI	Read
< 150	Balanced
150–500	Elevated load
> 500	High sympathetic load

Toichi CVICardiac Vagal Index

Research

A log-scaled Poincaré index isolating the vagal contribution to variability.

Formula

CVI = log₁₀( L × T ) [L, T = Poincaré long/short]

Higher = stronger vagal activity; relative to baseline.

Toichi CSICardiac Sympathetic Index

Research

The sympathetic counterpart of CVI from the same Poincaré geometry.

Formula

CSI = L / T [Poincaré long ÷ short]

Higher = stronger sympathetic dominance; relative.

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♾️

Nonlinear & Trend

Complexity measures and personal-baseline trend signals

DFA α1Short-Term Scaling Proxy

Research

Detrended-fluctuation scaling exponent — the correlation structure of the rhythm; length-dependent on short readings.

Formula

α1 = slope of log F(n) vs log n, n = 4…16 beats

DFA α1	Read
0.85–1.15	Healthy correlated dynamics
< 0.75 / > 1.25	Toward random / rigid

Spectral EntSpectral Entropy

Research

Entropy of the power spectrum — how spread-out (complex) the rhythm’s frequency content is.

Formula

SE = −Σ p(f) log p(f), p = normalised PSD

Relative measure — compare against your own baseline; no single clinical cut-point.

SDNN Z-scoreSDNN vs Baseline

Advanced

Today’s SDNN as a z-score against your trailing 7-day baseline — how this reading compares to your normal.

Formula

SDNN Z = (SDNN − baseline mean) / baseline SD

SDNN Z	Read
> 0	Above your baseline
−1 to 0	Slightly below
< −1	Well below baseline

HRV Momentumln(rMSSD) Trend

Advanced

The slope of log-rMSSD over 14 days — whether your HRV trend is rising or falling.

Formula

Momentum = OLS slope of ln(rMSSD) over 14 d

Momentum	Read
> 0	Rising HRV trend
≈ 0	Flat
< 0	Declining trend

VEI (rMSSD/HR)Vagal Efficiency

Advanced

rMSSD scaled by heart rate — a vagal-coupling efficiency proxy.

Formula

VEI = rMSSD / HR

Relative measure — compare against your own baseline; no single clinical cut-point.

CAICardiac Autonomic Index

Research

Geometric mean of the Poincaré axes — a combined autonomic-tone index.

Formula

CAI = √( SD1 × SD2 )

Relative measure — compare against your own baseline; no single clinical cut-point.

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🧮

Readiness Composites

Derived scores — internal blends, directional only

⚠️

All internal composites. These scores blend the validated HRV measures above into readiness/stress/recovery signals, in the spirit of consumer HRV apps. No external validation — read them as relative trends against your own history.

HRV ScoreAutonomic-Readiness Composite

Core

A 0–100 readiness headline blending vagal HRV against your baseline.

Method

blend of ln rMSSD, SDNN-Z and momentum

Internal — directional, baseline-relative.

StressHRV-Derived Stress

Core

An inverse-HRV stress estimate — rises as variability falls.

Method

inverse function of rMSSD / HF vs baseline

Internal — directional only.

EnergyHRV-Derived Energy

Advanced

A composite energy/activation estimate.

Method

blend of HRV level, balance and recovery

Internal — directional only.

ANS LoadAutonomic Burden

Advanced

A composite estimate of overall autonomic load.

Method

from SI, LF/HF and HRV deficit vs baseline

Internal — directional only.

Recov IndexRecovery vs Baseline

Advanced

How recovered the autonomic system looks relative to your baseline.

Method

ARI = today HRV / baseline HRV

Internal — directional only.

Recov DebtRecovery Debt (days)

Advanced

Accumulated days of sub-baseline recovery over the last two weeks.

Formula

Recov Debt = count(days where ARI < 0.9 in 14 d)

Internal — directional; lower is better.

CoherenceHRV Coherence

Research

A heart-rhythm coherence estimate (spectral peak sharpness around ~0.1 Hz).

Method

peak-power / total-power around the coherence band

Internal — directional only.

EFC IndexEnergy-Fatigue-Capacity

Research

A readiness blend of energy, fatigue and capacity.

Method

weighted blend of Energy, fatigue and HRV capacity

Internal — directional only.

ABSAutonomic Balance Score

Research

A combined sympathetic/parasympathetic balance index.

Method

normalised PSNS − SNS balance

Internal — directional only.

Restoration IndexRecovery Composite

Research

A composite of energy, coherence and stress as a single restoration readout.

Formula

Restoration = Energy × Coherence / Stress

Internal — directional only.

PTIParasympathetic Tone Index

Research

A parasympathetic-tone composite.

Formula

PTI = PSNS × rMSSD (normalised)

Internal — directional only.

Overtrain RiskOvertraining-Risk Proxy

Research

A directional overtraining-risk estimate from sustained autonomic load.

Method

sustained low HRV + high SNS over days

Internal — directional only.

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🔮

Projections

Population-norm projections — directional, not measurements

VO2max EstEstimated VO₂max

Research

An HRV-adjusted fitness estimate — a population proxy, not a cardiopulmonary exercise test.

Formula

VO₂max ≈ 15 × (HRmax / HRrest), HRV-adjusted

Heuristic proxy — track the trend, not the absolute.

VO2 7d AvgRolling VO₂ Baseline

Research

The smoothed 7-day average of the VO₂max estimate.

Formula

VO₂ 7d = mean( VO₂max est over 7 d )

Heuristic proxy.

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✔️

Validation Status Matrix

What is literature-validated versus experimentally derived — the key provenance table for research use

Validation refers to the underlying metric’s validation in published literature, and does not imply validation of the HRVDex implementation against a gold-standard laboratory dataset.

Metric Category	Status	Basis
rMSSD, SDNN, LF/HF, HF n.u.	● Literature-based	Task Force 1996 standards
SD1/SD2, CAI, Toichi CVI/CSI	● Literature-based	Brennan 2001; Toichi 1997
Baevsky stress index	● Literature-based	Baevsky & Chernikova
DFA α1, spectral entropy, SDNN-Z, momentum, VEI	◐ Emerging	Published / personal-baseline trend signals
HRV/Stress/Energy/readiness composites	○ Experimental composite	HRVDex internal blends; no independent validation
VO₂max (HRV-adjusted)	○ Population-derived	Population projection; not a measurement. ANS-age + HRV→BP removed 2026-06-23.

Metric Tier Definitions

Tier	Meaning	Examples
Core	Headline, universally interpretable	rMSSD, SDNN, HRV Score, Stress
Advanced	Established support, fuller analysis	LF/HF, Baevsky SI, SDNN-Z, VEI
Research	Exploratory / emerging / composites	DFA, Toichi, ANS ratios, projections

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🗺️

Formula → Citation Map

Every computed metric mapped to its primary source

Metric / Formula	Primary Citation	Category
rMSSD, SDNN, LF/HF, HF n.u.	Task Force 1996	HRV
SD1/SD2; CAI √(SD1×SD2)	Brennan 2001	HRV
Toichi CVI / CSI	Toichi 1997	HRV
Baevsky stress index	Baevsky & Chernikova 2017	HRV
DFA α1	Peng 1995	Nonlinear
VO₂max (HR-ratio)	Uth–Sørensen 2004	Projection
HRV/Stress/readiness; ANS-age & BP proxies	HRVDex internal — no external source	Internal

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𝑓

Formula Provenance Index

Compact audit index — every external formula mapped to its source

Formula	Source / Author	Year	Reference
Time-domain HRV standards	Task Force ESC/NASPE	1996	Circulation. 93(5):1043–65
SD1/SD2 Poincaré geometry	Brennan, Palaniswami & Kamen	2001	IEEE TBME. 48(11):1342–7
CVI = log₁₀(L·T) ; CSI = L/T	Toichi et al.	1997	J Auton Nerv Syst. 62(1–2):79–84
Baevsky SI = AMo/(2·Mo·MxDMn)	Baevsky & Chernikova	2017	Cardiometry. 10:66–76
DFA α1	Peng et al.	1995	Chaos. 5(1):82–7
VO₂max = 15·(HRmax/HRrest)	Uth, Sørensen et al.	2004	Eur J Appl Physiol. 91(1):111–5

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⚠️

Known Limitations

The interpretation context for daily-summary HRV

ℹ️

These limitations are inherent to short, pre-summarised HRV readings. They do not invalidate HRVDex outputs but define the appropriate interpretation context.

📱 Source & Protocol

Consumes pre-summarised daily HRV — raw beats are not seen here
Short (~1-min) readings are sensitive to posture and breathing
Take readings the same way each morning for comparability
Source-app summarisation choices affect the numbers

🧠 Interpretation

Absolute HRV varies with age/fitness — track your own trend
LF/HF as “balance” is contested; read directionally
Frequency metrics need adequate reading length
No EEG / no raw morphology — no staging or beat-shape claims

📊 Algorithmic

Readiness/stress/energy/recovery are internal composites
ANS age, VO₂max, BP are population proxies, not measurements
Baselines need ~7–14 days of readings to stabilise
Normative bands are general, not personally calibrated

⚖️ Regulatory

Not FDA cleared or CE marked as a medical device
Not for clinical diagnosis or treatment decisions
Personal, research, and wellness use only
Discuss medical concerns with a qualified clinician

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📚

Academic References

Primary sources for the HRV measures HRVDex surfaces

⚠️

Provenance note. Composite readiness scores and BP/ANS-age projections are internal blends or population proxies with no external source, labelled as such. The core and nonlinear HRV measures use the verified canon below.

Method / Metric	Primary Citation	Category
rMSSD, SDNN, LF/HF, HF n.u.	Task Force of the ESC and NASPE. Heart rate variability: standards of measurement, physiological interpretation, and clinical use. Circulation. 1996;93(5):1043–65. doi: 10.1161/01.CIR.93.5.1043	HRV
Poincaré SD1/SD2; CAI √(SD1×SD2)	Brennan M, Palaniswami M, Kamen P. Do existing measures of Poincaré plot geometry reflect nonlinear features of HRV? IEEE Trans Biomed Eng. 2001;48(11):1342–7. doi: 10.1109/10.959330	HRV
Toichi CVI / CSI	Toichi M, Sugiura T, Murai T, Sengoku A. A new method of assessing cardiac autonomic function and its comparison with spectral analysis and coefficient of variation of R-R interval. J Auton Nerv Syst. 1997;62(1–2):79–84. doi: 10.1016/S0165-1838(96)00112-9	HRV
Baevsky stress index	Baevsky RM, Chernikova AG. Heart rate variability analysis: physiological foundations and main methods. Cardiometry. 2017;10:66–76. doi: 10.12710/cardiometry.2017.10.6676	HRV
DFA α1	Peng C-K, Havlin S, Stanley HE, Goldberger AL. Quantification of scaling exponents… in nonstationary heartbeat time series. Chaos. 1995;5(1):82–7. doi: 10.1063/1.166141	Nonlinear
VO₂max (HR-ratio method)	Uth N, Sørensen H, Overgaard K, Pedersen PK. Estimation of VO₂max from the ratio between HRmax and HRrest. Eur J Appl Physiol. 2004;91(1):111–5. doi: 10.1007/s00421-003-0988-y	Projection
HRV / Stress / Energy / readiness scores; BP & ANS-age proxies	HRVDex internal composites and population projections — no external source. Directional only.	Internal