What is RMSSD?
While SDNN looks at the overall spread, RMSSD focuses on how much each beat differs from the very next beat. It is the gold standard for measuring short-term, beat-to-beat variability, which is primarily driven by the parasympathetic (vagal) nervous system.
RMSSD = √[ (1/(N−1)) · ∑(RRi+1 − RRi)² ]
SDNN vs RMSSD: The Difference
The key distinction is what gets compared:
SDNN compares every interval to the mean; RMSSD compares each interval to the next one
Worked Example
Using the same 5 RR intervals:
RR = [810, 830, 790, 850, 820]
Step 1: Calculate successive differences:
830−810 = 20
790−830 = −40
850−790 = 60
820−850 = −30
Step 2: Square them:
20² + (−40)² + 60² + (−30)² = 400 + 1600 + 3600 + 900 = 6500
Step 3: RMSSD = √(6500/4) = √1625 ≈ 40.3 ms
Why RMSSD Matters
Parasympathetic marker: RMSSD primarily reflects vagal (parasympathetic) nervous system activity. The vagus nerve can change heart rate very rapidly — from one beat to the next — which is exactly what RMSSD captures.
The sympathetic nervous system acts more slowly (over many beats), so its influence mainly shows up in longer-term metrics like SDNN, not in RMSSD.
Practical interpretation
| RMSSD (ms) | Interpretation |
|---|---|
| < 20 | Low parasympathetic activity |
| 20 – 50 | Moderate — typical range |
| > 50 | Strong vagal tone — good recovery capacity |
Key Points
- RMSSD captures rapid, beat-to-beat changes
- Primarily reflects parasympathetic (vagal) activity
- Less affected by recording duration than SDNN
- The most recommended time-domain metric for short recordings (e.g., 1–5 minutes)
- Mathematically related to SD1 in the Poincaré plot: SD1 = RMSSD / √2