HRV Parameters
A detailed guide to the heart rate variability metrics used in standardized five-minute assessment.
Time-domain analysis
Time-domain measures quantify the magnitude of beat-to-beat variation directly from the NN-interval time series. They are computationally straightforward, well validated and the recommended primary approach for routine short-term HRV assessment.
RMSSD
The root mean square of successive differences between consecutive NN intervals. RMSSD is the preferred primary parameter for short-term five-minute HRV assessment because it is strongly influenced by cardiac vagal modulation and is relatively robust to recording duration and respiratory effects. Values are expressed in milliseconds.
RMSSD is influenced by vagal modulation but is not a direct measurement of vagal nerve activity. It reflects the parasympathetic contribution to short-term heart rate regulation under standardized recording conditions.
SDNN
The standard deviation of all NN intervals. SDNN reflects the total variability present during the analysed recording period. In five-minute recordings, SDNN describes short-term overall variability and incorporates both vagal and sympathetic influences.
Five-minute SDNN and 24-hour SDNN are not equivalent. Twenty-four-hour SDNN incorporates circadian and slower physiological components not captured in short recordings. Reference ranges must not be mixed between short-term and long-term recording durations.
pNN50
The proportion of successive NN intervals that differ by more than 50 milliseconds, expressed as a percentage. pNN50 is a vagal-related measure of successive NN-interval variation. It is strongly influenced by age and recording conditions. No validated age- and sex-specific percentile dataset is implemented for pNN50, so it serves as a supporting rather than primary metric in the 5HRV framework.
Frequency-domain analysis
Frequency-domain analysis decomposes the NN-interval time series into its constituent frequency components. Spectral power within specific frequency bands provides information about the physiological mechanisms contributing to heart rate variability.
HF power (0.15–0.40 Hz)
High-frequency power corresponds to respiratory-frequency variability and is influenced by cardiac vagal modulation and breathing. Values depend strongly on respiratory rate, depth and spectral methodology. No universal reference range is applied to HF power in the 5HRV calculator; descriptive interpretation is recommended.
LF power (0.04–0.15 Hz)
Low-frequency power reflects mixed autonomic, vascular and baroreflex-related influences. LF power does not directly measure sympathetic activity and must not be interpreted as a pure sympathetic marker. Within the 5HRV framework, LF is described in conjunction with HF and the LF/HF ratio.
LF/HF ratio
The ratio of low-frequency to high-frequency power describes the relative distribution of spectral power between the LF and HF bands. The LF/HF ratio is not a direct quantitative measurement of sympathetic–parasympathetic balance. Permitted descriptors in the 5HRV framework are: relative HF predominance, broadly comparable LF and HF power, relative LF predominance, and marked relative LF predominance.
Relationship between time- and frequency-domain measures
Time-domain measures quantify how much beat-to-beat variation is present. Frequency-domain analysis describes where that variation occurs across frequency bands. RMSSD correlates strongly with HF power because both reflect rapid vagal-mediated variation. SDNN correlates with total spectral power. The two approaches provide complementary information.
Very-low-frequency power (VLF, 0.003–0.04 Hz) is sometimes reported by analysis software, but a single five-minute recording contains too few slow oscillatory cycles for robust interpretation. It is therefore not included in the practical 5HRV calculator. Discussion of VLF is reserved for research contexts with longer recording durations.