Calculate HRV metrics RMSSD, SDNN and pNN50 from RR intervals in ms or seconds, or from beat-to-beat heart rate, plus mean RR and average heart rate.

Default filter: 300–2000 ms; threshold: 50 ms; sample SDNN. Exclusions break original adjacency.

At least 3 values in original order. Separate with commas, semicolons or whitespace; use decimal points and no thousands separators.

Changing this unit reinterprets the list; its numbers stay unchanged.

Filtering and metric settings

SDNN/RMSSD Ratio Formula (HRV-derived)

The ratio mode uses the following arithmetic formula with SDNN and RMSSD from the same recording and preprocessing. It is undefined when RMSSD is zero and is not a diagnostic or autonomic-balance score.

SDNN / RMSSD Ratio = (SDNN) / (RMSSD)

Variables:

  • SDNN/RMSSD Ratio is unitless (dimensionless)
  • SDNN is the standard deviation of normal-to-normal (NN) intervals (typically reported in ms)
  • RMSSD is the root mean square of successive differences of NN intervals (typically reported in ms)

To calculate the SDNN/RMSSD ratio, divide SDNN by RMSSD. If both are in the same time unit (for example, ms), the units cancel and the result has no units.

What is Heart Rate Variability (HRV)?

Heart Rate Variability (HRV) refers to the variation in time between heartbeats (beat-to-beat variation), influenced by the autonomic nervous system. HRV is commonly summarized using different metrics (for example, SDNN and RMSSD). In general, HRV can be influenced by many factors (for example, sleep, illness, training load, stress, medications, alcohol, time of day, and measurement conditions), so interpretation is often most meaningful when comparing to your own baseline over time.

For interval modes, RMSSD uses only originally adjacent accepted pairs; excluded intervals break adjacency. SDNN offers sample N − 1 or population N denominators. The displayed pNNx divides the count strictly above the chosen threshold by accepted interval count, following the total-NN-count convention when those beats are verified normal-to-normal; the alternate usable-pair percentage appears in details. Default filtering is 300–2000 ms and does not detect ectopic beats or replace clinical preprocessing. Heart rate from mean RR is not the arithmetic mean of individual BPM values. Recording duration and conditions affect comparison. See the original PhysioNet pNNx documentation.

How to Calculate the SDNN/RMSSD Ratio?

The following steps outline how to calculate the SDNN/RMSSD ratio using the formula SDNN/RMSSD = SDNN / RMSSD.

  1. First, determine the value of SDNN (standard deviation of NN intervals) in a time unit (commonly milliseconds, ms).
  2. Next, determine the value of RMSSD (root mean square of successive differences of NN intervals) in the same time unit (commonly ms).
  3. Next, use the formula SDNN/RMSSD = SDNN / RMSSD to calculate the ratio (unitless).
  4. Finally, calculate the ratio value.
  5. Select SDNN / RMSSD ratio mode above to check this arithmetic example. The interval modes instead compute metrics from an ordered series.

Example Problem:

Use the following variables as an example problem to test your knowledge:

SDNN (standard deviation of NN intervals) = 50 ms

RMSSD (root mean square of successive differences of NN intervals) = 20 ms, so the SDNN/RMSSD ratio = 50 ÷ 20 = 2.5 (unitless).