RR Intervals & the Tachogram: The Raw Data Behind HRV

The one series everything else is built from

Open any article on heart rate variability and you’re quickly buried in acronyms: SDNN, RMSSD, pNN50, LF, HF. It’s easy to miss that they all describe the same underlying thing. Every one of those numbers is a different way of summarizing a single, humble series of measurements: the time between your heartbeats.

That measurement is the RR interval, the gap, in milliseconds, from one heartbeat to the next. Learn to picture that series and the rest of HRV stops being a wall of jargon and becomes a set of tools that all point at one signal. This is the foundational explainer the other HRV basics quietly assume you already understand.

What an RR interval actually is

On an ECG trace, each heartbeat produces a sharp spike called the R peak. The RR interval is simply the time from one R peak to the next. Measure it in milliseconds and you have the raw unit of HRV: a heart beating once per second has an RR interval of 1000 ms.

You’ll also see the term NN interval, “normal-to-normal.” It means the same thing, with one restriction: only the intervals between confirmed normal beats count. Once the software has thrown out the odd or misread beats (more on that below), what’s left is the NN series, and that clean series is what the metrics are actually computed from. RR is the raw recording; NN is the tidied version. If you see either term, picture the same thing: the spacing between beats.

The tachogram: your heartbeats, plotted over time

Line all your RR intervals up in order and plot them (interval length on the vertical axis, time across the horizontal) and you get a tachogram. Each point is one beat-to-beat gap; the line joining them shows how the spacing rises and falls across the reading.

Here’s the part that surprises people: a healthy resting tachogram is not a flat line. It rolls.

A healthy heart is not a metronome

Those rolling waves have a cause, and it’s a good one. As you breathe in, your heart speeds up slightly (the RR interval shortens); as you breathe out, it slows (the interval lengthens). This breathing-linked rhythm is called respiratory sinus arrhythmia, and it’s the single biggest driver of the waves you see in a resting tachogram.

As the researchers behind much of the modern HRV literature put it, a healthy heart is not a metronome: its “irregularity” is a sign that the autonomic nervous system is flexible enough to adjust the heart beat by beat. A trace with big, easy waves reflects a responsive, well-regulated system. A trace that goes flat and metronomic, every interval nearly identical, reflects the opposite: a rhythm locked in place, low in variability.

This is exactly why HRV matters in POTS, long COVID and post-viral illness. When the system is biased toward “fight or flight,” the waves shrink and the tachogram flattens. You can read more about that pattern in what HRV is and why it matters for POTS recovery and in the POTS, long COVID and MCAS overlap.

Artifacts, ectopic beats, and why your app “drops” beats

Not every point on a tachogram is a true heartbeat. Two things spoil the raw series:

• Ectopic beats: an early or extra beat that fires outside the normal rhythm. It creates one interval that’s much too short, usually followed by one that’s too long, producing a jagged spike out of step with the surrounding waves.
• Artifacts, anything the sensor got wrong: a beat it missed, a beat it counted twice, or plain motion noise. On the tachogram these also appear as sudden spikes.

The problem is that HRV metrics are sensitive to these spikes. Because a measure like RMSSD looks at the jump between neighbouring intervals, one bad beat can dramatically inflate or corrupt the number. So before computing anything, HRV software runs artifact correction: it scans for intervals that are implausibly different from their neighbours and either removes or interpolates them. That’s what’s happening when your app appears to “drop” a beat: it isn’t losing your data, it’s protecting the accuracy of the result.

From one series to every HRV number

Once you have a clean tachogram, every HRV metric is just a different question asked of the same points:

• SDNN asks: how spread out are all the intervals overall?
• RMSSD and pNN50 ask: how big are the jumps between one interval and the next?
• The frequency bands ask: how much of the waving happens at breathing speed versus slower rhythms?
• Mean RR and resting heart rate ask: what’s the average interval, and its bpm equivalent?

None of them is a separate measurement. They’re all summaries of the tachogram, which is why understanding the raw series makes every other metric click into place.

Convert heart rate to a mean RR interval

Because heart rate and RR interval are reciprocals, you can flip between them with one bit of arithmetic: RR = 60000 ÷ HR. Enter a resting heart rate and see the matching average interval, and notice that as heart rate falls, the RR interval grows.

A few reference points to anchor the relationship:

The bottom line

Strip away the acronyms and heart rate variability comes down to one series: the RR intervals between your heartbeats, plotted as a tachogram. A healthy resting trace rolls in gentle waves because breathing speeds and slows the heart. A flat, metronomic line is the warning sign, not the goal. Odd and misread beats show up as spikes and get corrected before any metric is computed, which is why your app sometimes drops a beat. And every HRV number you’ll ever track (SDNN, RMSSD, the power bands) is simply a different summary of this one underlying signal. Understand the tachogram and the rest of HRV falls into place.