RMSSD
RMSSD — the root mean square of successive differences between normal heartbeats — is the main time-domain measure of parasympathetic (vagal) activity in heart-rate-variability analysis. It reflects the fast, beat-to-beat changes in heart rate produced by breathing and the vagus nerve, is reliable even in short recordings, and is widely used to monitor stress, recovery and cardiovascular risk.
Physiology
Parasympathetic activity produces effects that are, in general, opposite to those of sympathetic activation: it slows the heart, reduces the force of contraction and lowers energy consumption. Unlike the sympathetic system, the vagus nerve acts on the sinus node within a single heartbeat through acetylcholine, which is broken down very quickly. Its activity rises with every exhalation and falls with every inhalation, so the heartbeat intervals lengthen and shorten with breathing — respiratory sinus arrhythmia. RMSSD captures exactly these rapid changes and is therefore regarded as the most practical index of vagal tone.
How RMSSD is calculated
The difference between each pair of neighbouring NN intervals is squared, the squares are averaged and the square root is taken. Because it uses successive differences, RMSSD is almost unaffected by slow trends in heart rate and can be measured reliably in recordings as short as 1–5 minutes. Some devices report ln RMSSD (natural logarithm), which has a more symmetrical distribution and is convenient for tracking day-to-day changes.
Reference values
| Recording | Healthy adults | Comment |
|---|---|---|
| 5-minute, at rest | 42 ± 15 ms; usual range ≈ 19–75 ms | Systematic review of 44 studies (Nunan 2010) |
| 24-hour Holter | Task Force reference ≈ 27 ± 12 ms | Lower than short-term resting values because of daytime activity |
| Wearables, 1-minute | No validated population norms | Use your personal baseline |
Interpretation
Low RMSSD
- Reduced vagal tone, sympathetic predominance
- Acute stress, poor sleep, alcohol, illness, fever, dehydration
- Insufficient recovery after training (a sustained fall below the personal baseline)
- Persistently low values: hypertension, heart failure, coronary disease, diabetes with autonomic neuropathy, depression — associated with higher risk of cardiac events
High RMSSD
- Strong vagal tone: rest, sleep, good aerobic fitness, youth
- Slow deep breathing during measurement
- Falsely high values: extrasystoles, atrial fibrillation, sinus pauses, artefacts — always check the raw ECG
How to measure and improve RMSSD
Measure every morning after waking, in the same position, for 1–5 minutes, breathing naturally, before coffee and exercise. Follow the weekly average rather than individual values.
- Regular aerobic training with adequate recovery days.
- 7–9 hours of sleep, limited alcohol, no smoking.
- Slow breathing (about 6 breaths per minute) and HRV biofeedback.
- Weight loss and good control of blood pressure and blood glucose.
Consult a doctor if RMSSD remains low for your age over weeks despite rest, especially together with palpitations, breathlessness, chest pain, dizziness, diabetes or known heart disease, or if very high values are accompanied by an irregular pulse.
References
- Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology. 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
- Nunan D, Sandercock GRH, Brodie DA. A quantitative systematic review of normal values for short-term heart rate variability in healthy adults. Pacing Clin Electrophysiol. 2010;33(11):1407–1417. doi:10.1111/j.1540-8159.2010.02841.x
- Shaffer F, Ginsberg JP. An overview of heart rate variability metrics and norms. Front Public Health. 2017;5:258. doi:10.3389/fpubh.2017.00258
- Umetani K, Singer DH, McCraty R, Atkinson M. Twenty-four hour time domain heart rate variability and heart rate: relations to age and gender over nine decades. J Am Coll Cardiol. 1998;31(3):593–601. doi:10.1016/S0735-1097(97)00554-8
This article is for general information and does not replace a consultation with a doctor. HRV values depend on the device, recording length and conditions.
Published on 11 June 2019