Scientific Analysis

A structured framework for interpreting short-term HRV measurements.

Principles of interpretation

HRV must be interpreted in the context of the individual and the recording. Factors that should be considered include age, reference sex, recording method, body position, heart rate, rhythm, artefacts, breathing pattern and medication.

Age- and sex-specific reference distributions

HRV generally decreases with age and differs between male and female reference populations. The DanFunD study, a large Danish population-based cohort of adults aged 18–72 years, provides age- and sex-specific RMSSD and SDNN reference percentiles for short-term recordings. These are population reference distributions, not disease thresholds.

RMSSD reference percentiles (ms)

Age band	F P5	F P25	F P50	F P75	F P95	M P5	M P25	M P50	M P75	M P95
18-29	18.88	35.39	57.19	83.87	171.26	16.77	34.43	51.14	73.90	128.80
30-39	16.79	28.95	40.37	62.52	118.23	17.87	28.38	39.96	58.87	105.80
40-49	13.27	22.90	32.77	48.62	83.03	11.64	19.92	29.95	43.84	81.32
50-59	9.45	17.94	25.04	35.73	62.59	7.44	14.67	22.74	33.33	59.92
60-72	6.79	12.19	17.89	25.83	51.77	6.04	11.93	17.02	25.18	53.64

SDNN reference percentiles (ms)

Age band	F P5	F P25	F P50	F P75	F P95	M P5	M P25	M P50	M P75	M P95
18-29	22.50	36.76	52.54	74.06	123.49	24.45	39.33	52.02	69.71	112.10
30-39	19.64	30.70	41.50	55.67	103.11	23.50	32.86	42.71	58.28	95.73
40-49	16.13	25.48	34.16	46.03	73.90	15.86	24.80	34.04	45.83	75.14
50-59	12.27	20.75	28.07	37.38	60.60	11.18	18.69	26.09	36.85	56.43
60-72	9.00	14.55	20.20	28.36	47.01	8.34	14.73	19.92	27.72	54.97

HF and LF power have no validated age- and sex-specific reference distribution and are described without percentile placement, because spectral values are especially sensitive to breathing and analysis methodology. The LF/HF ratio is placed within the DanFunD age- and sex-specific reference percentiles using log-domain interpolation (see the Methodology page), but remains a descriptive spectral ratio. See the Evidence page for the full citation and supporting references.

Interpretation of RMSSD

RMSSD is the primary metric for short-term vagal-related assessment. Its interpretation is based on the age- and sex-specific percentile category: values below the 5th percentile are classified as “very low (below P5)”; values between the 5th and 25th percentile as “low (P5–P25)”; values between the 25th and 75th percentile as “typical (P25–P75)”; values between the 75th and 95th percentile as “high (P75–P95)”; and values above the 95th percentile as “very high (above P95)”.

Interpretation of SDNN

SDNN reflects total short-term variability. The same percentile classification is applied: very low, low, typical, high or very high, based on the DanFunD reference distribution.

Combined time-domain patterns

Reduced RMSSD

Reduced short-term beat-to-beat variability relative to the age- and sex-specific reference distribution, interpreted as reduced vagal modulation under the recording conditions.

Reduced SDNN

Reduced overall variability during the five-minute recording.

Reduced RMSSD and SDNN

A combined reduction in rapid beat-to-beat variation and overall short-term variability. This pattern may indicate reduced vagal modulation together with reduced total short-term variability.

Reduced RMSSD with preserved SDNN

Relatively reduced beat-to-beat vagal-related variability with better-preserved total five-minute variability.

Preserved RMSSD with reduced SDNN

Reduced overall short-term variability without a corresponding reduction in RMSSD. This may reflect non-vagal contributions to total variability.

High HRV values

Values above upper reference percentiles may be physiological but prompt review of bradycardia, ectopic beats, rhythm irregularity, artefacts, breathing pattern and analysis quality. Higher HRV is not automatically better.

Supporting interpretation of pNN50 and HF

pNN50 and HF power provide supporting vagal-related information. When RMSSD is preserved but pNN50 is very low (below 1%) or HF power is low (below 50 ms²), this may indicate mixed parasympathetic findings, reflecting discordance between the primary and supporting vagal-related metrics. HF values depend strongly on respiratory rate and depth.

LF and LF/HF

LF power reflects mixed autonomic and baroreflex-related influences and must not be described as a pure sympathetic marker. The LF/HF ratio describes the relative distribution of spectral power. Permitted descriptors are: relative HF predominance, broadly comparable LF and HF power, relative LF predominance, and marked relative LF predominance. LF/HF does not provide a direct quantitative measurement of sympathetic–parasympathetic balance and cannot independently establish sympathetic overactivity.

The 5HRV Autonomic Pattern Score

The 5HRV Autonomic Pattern Score combines RMSSD and the LF/HF ratio into a single numerical index ranging from −100 to +100. A negative score indicates a parasympathetic-direction shift, a score near zero indicates a central or mixed pattern, and a positive score indicates a sympathetic-direction shift. The score is a directional composite, not a diagnostic measurement.

Worked example

A 45-year-old male with a resting five-minute recording yields the following values: RMSSD 30.40 ms, SDNN 47.63 ms, HF power 125.95 ms², LF/HF ratio 3.28.

Parameter	Value
RMSSD	30.40 ms
SDNN	47.63 ms
HF power	125.95 ms²
LF/HF	3.28

The LF/HF ratio demonstrates relative LF predominance. RMSSD and SDNN must be interpreted against age- and sex-specific reference percentiles. The result does not by itself prove sympathetic overactivity or vagal failure. The 5HRV Autonomic Pattern Score for this recording is +5, classified as a central autonomic pattern.

Limitations

The 5HRV interpretation framework is based on a single five-minute resting recording under standardized conditions. It does not replace comprehensive autonomic testing, tilt-table evaluation or twenty-four-hour HRV analysis where indicated. Results describe the recording and must always be interpreted together with the context in which it was obtained.