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; the VO₂max projection is a population proxy, not a measurement.

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.

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 composites	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
VO₂max is a population proxy, not a measurement
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 are internal blends and the VO₂max projection is a population proxy, both with no external source and 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):1043–1065. doi: 10.1161/01.CIR.93.5.1043	HRV
HR/HRV measurement & reporting methodology (2024 update to Task Force 1996)	Quigley KS, Gianaros PJ, Norman GJ, Jennings JR, Berntson GG, de Geus EJC. Publication guidelines for human heart rate and heart rate variability studies in psychophysiology — Part 1: Physiological underpinnings and foundations of measurement. Psychophysiology. 2024;61(9):e14604. doi: 10.1111/psyp.14604	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):1342–1347. 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):82–87. 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):111–115. doi: 10.1007/s00421-003-0988-y	Projection
HRV / Stress / Energy / readiness scores	HRVDex internal composites — no external source. Directional only.	Internal