SlowpulseThese two get used almost interchangeably, and they are not remotely the same thing. Heart rate counts events. HRV describes the spacing between them. You can have an unchanged heart rate and a badly shifted HRV, and the second one is often the more interesting of the two.
The mechanical difference
If your heart beats 60 times in a minute, your heart rate is 60. That figure says nothing about whether those beats were evenly spaced. In practice they never are: the interval between consecutive beats shifts continuously, and heart rate variability is a measure of how much.
A metronome-steady heart and a heart with lively beat-to-beat spacing can report the identical rate. HRV is the axis that tells them apart.
They frequently move in opposite directions
As a broad pattern, when heart rate rises HRV tends to fall, and the reverse. That is why the pairing is useful: two signals moving in opposite directions at once is a much stronger indication that something has actually changed than either drifting alone.
One number moving is ambiguous. Two numbers moving together, in the directions you would expect, is a signal.
Why HRV is the harder number to read
Heart rate is robust. Count beats, divide by minutes — the method barely affects the answer. HRV is the opposite: what you get depends on which metric was calculated, how long the recording ran, your posture, the time of day and how fast you were breathing. Two devices can report genuinely different HRV numbers for the same heart without either being wrong.
A 2024 review of the factors influencing HRV lists age, sex, time of day, alcohol, smoking, body composition, training load, heat, noise and shift work among the things that move it — and concludes that these confounders are precisely what interferes when you try to compare HRV between different people.
None of which is a reason to distrust the number. It is a reason to compare it only against itself.
How Slowpulse reads them together
Slowpulse does not act on either signal alone. It builds a baseline for both from your own Apple Health history, then looks for the pairing: heart rate climbing away from your normal while HRV drops away from yours. One without the other is not enough.
It then discounts the result using your active energy and exercise minutes, because a hard effort produces exactly that pairing for entirely ordinary reasons. That correction is the difference between an app that notices tension and an app that shouts at you for going running.
Sources
- Sammito, Thielmann & Böckelmann (2024) — Update: factors influencing heart rate variability, a narrative review, Frontiers in Physiology 15:1430458
- American Heart Association, Target Heart Rates Chart
Related
Not a medical device. Does not diagnose, treat or monitor any condition.