Two ways to summarize variability
rMSSD summarizes the size of changes between successive beat intervals.
SDNN summarizes how widely beat intervals vary across the analyzed recording.
The values are related, but they are not interchangeable. Recording duration affects HRV estimates, especially SDNN.
Professional HRV standards commonly use five-minute stationary recordings for short-term analysis and distinguish those values from 24-hour measurements. An Apple Watch ECG is much shorter.
A short snapshot has limits
Controlled studies suggest that ultra-short recordings can approximate selected time-domain HRV measures, with rMSSD generally agreeing better than SDNN at very short durations. This research does not validate ECG Tracker+ or make a 30-second result interchangeable with a standard five-minute recording.
Standard HRV definitions use normal-to-normal beat intervals. Noise, missed beats, incorrectly marked beats, and premature beats can materially change the result.
Compare like with like
HRV varies with recording conditions and personal context. Posture, breathing, movement, time of day, recent activity, sleep, medications, alcohol, caffeine, illness, premature beats, and signal artifacts can affect the result.
For a more consistent personal comparison, capture recordings under similar conditions. A change in the app’s estimate can be worth noting without being evidence of a medical problem.
A practical comparison checklist
- Compare the same metric, recording duration, and app method.
- Record at a similar time of day and in a similar posture.
- Rest before recording and keep movement and breathing natural.
- Note recent exercise, illness, poor sleep, alcohol, caffeine, or medication changes.
- Treat a noisy recording or one containing early beats as more uncertain.
What is a normal HRV?
There is no single app threshold that is meaningful for everyone. Age, physiology, recording duration, conditions, and calculation method all matter. ECG Tracker+ does not map an HRV estimate to a diagnosis or a conventional normal range.
Is higher HRV always better?
No simple rule applies to every person or recording. A single value can reflect recording conditions, unusual beats, artifacts, or normal short-term variation.