HRV Histogram Metrics: Mode, AMo50, MxDMn and CV

Four ways to read the shape of your heartbeat

Most HRV numbers are statistics on a list of intervals. The geometric family works differently: it treats those intervals as a histogram (a bar chart of how often each interval length shows up) and then measures the shape of that chart. Pile up a few minutes of beats and they form a hump: some interval length is most common, the rest fan out on either side. Mode, AMo50, MxDMn and CV are four different rulers held against that hump.

This lineage traces back to the Russian physiologist R.M. Baevsky, whose work on the geometry of the RR-interval distribution gave us the stress index still used today. The appeal is intuition: instead of an abstract standard deviation, you get the center, the height, and the width of a picture you can actually imagine.

Mode: the center of the histogram

Mode (Mo) is the most common interval length in the reading: the tallest bar’s position. Because it’s the interval your heart returned to most often, it tracks your underlying resting rate closely: it’s essentially a robust cousin of your mean interval, read off the histogram’s peak instead of a raw average. It’s reported in milliseconds and sits on the same scale as mean RR and resting heart rate: a Mode of 950 ms corresponds to roughly 63 bpm.

AMo50: the height of the peak

AMo50 (amplitude of the mode) is the percentage of all intervals that fall inside the single most common 50 ms bin. In histogram terms, it’s the height of the peak. When your rhythm is varied and healthy, the beats spread across many bins and no single bin holds many, AMo50 is low. When the system tightens under sympathetic strain, the rhythm concentrates onto one dominant interval, the peak spikes, and AMo50 rises. So unlike most HRV metrics, with AMo50 lower is better.

MxDMn: the width of the histogram

MxDMn is the maximum interval minus the minimum interval, reported in seconds: the histogram’s full width. A wide MxDMn means your intervals ranged freely from short to long, the mark of a flexible, healthy rhythm; a narrow one means the beats were penned into a small range, a rigid rhythm. Because it depends entirely on the two most extreme beats in the reading, a single stray or mis-detected beat can stretch it artificially, so of the four, MxDMn is the one to read as a trend and never trust on a single noisy recording.

CV: variability, normalized to your rate

CV, the coefficient of variation, solves a nagging problem: raw variability in milliseconds partly depends on how fast your heart is beating. CV divides that out. It’s SDNN divided by the mean interval, times 100, expressed as a percent:

How these feed the stress index

Three of these four (Mode, AMo50 and MxDMn) are the raw ingredients of the Baevsky stress index, the geometric measure of sympathetic strain. As the Kubios HRV analysis methods describe, the index combines them so that it climbs when the histogram is tall and narrow (high AMo50, small MxDMn, centered on a short Mode) and falls when it’s low and wide. In other words, these metrics aren’t just descriptive: they’re the geometry the stress index reads. We unpack that composite in the PNS, SNS and stress index explainer.

How Autonomic grades these metrics

Autonomic grades each geometric metric against the recovery framework’s thresholds, tuned for the few-minute readings the app captures.

Mode (ms), same scale as mean RR; too high can signal a very slow rhythm, so the bottom band brackets both extremes:

AMo50 (%), lower is better; a tall peak means a concentrated, strained rhythm:

MxDMn (seconds), wider is better:

CV (%), rate-normalized variability:

Compute your own CV from an SDNN and a mean interval:

Reading them as a recovery signal

These four rarely headline a daily check. RMSSD and pNN50 are quicker to interpret morning to morning. Where the geometric family earns its place is as a second lens on the shape of your regulation, and especially as the machinery behind the stress index. A histogram that grows taller and narrower over weeks (rising AMo50, shrinking MxDMn) tells the same story a climbing stress index does: the system is tightening. One that spreads and lowers is loosening: the direction you want in recovery.

In POTS, long COVID and post-viral dysautonomia, a concentrated, narrow histogram is common because the system is sympathetically biased; see POTS, long COVID and MCAS for that overlap, and recovery from post-viral dysautonomia for how the shape tends to open back up over a recovery arc.

The bottom line

Mode, AMo50, MxDMn and CV are the geometric HRV metrics: four rulers held against the histogram your heartbeat intervals form. Mode marks its center (your resting rate), AMo50 its peak height (concentration, which rises under strain), MxDMn its full width (range, where wider is healthier), and CV its variability normalized to heart rate. Three of them are the raw ingredients of the Baevsky stress index, which makes this family the geometry underneath one of the most useful autonomic-strain measures you can track. They’re sensitive to artifacts, so read them as trends, but followed over weeks, the changing shape of your histogram is a real and readable picture of recovery.