What is a Poincaré Plot?
A Poincaré plot is a scatter plot where each RR interval is plotted against the next RR interval. For every pair of consecutive intervals (RRn, RRn+1), you place one dot on the graph.
This simple technique reveals patterns in your heart rhythm that numbers alone can't show.
SD1 and SD2
An ellipse is fitted to the scatter cloud. Its two axes tell you different things:
| Axis | Direction | Measures |
|---|---|---|
| SD1 | Perpendicular to identity line | Short-term variability (beat-to-beat) |
| SD2 | Along the identity line | Long-term variability (overall trend) |
Reading the Ellipse Shape
The shape of the Poincaré ellipse is a powerful visual fingerprint of your autonomic state. Here are the key shapes to recognize:
The Shape as an Intuitive Visual Cue
Forget the numbers for a moment. Just look at the shape:
Circle → "Balanced"
Short-term and long-term variability are roughly equal. Your autonomic nervous system is well-balanced. Think of it like a ball that can roll freely in any direction — your heart has equal flexibility for quick adjustments and gradual shifts.
Elongated Ellipse (stretched along the diagonal) → "Drifting"
Your heart rate drifts up and down over time (high SD2), but doesn't jump much from one beat to the next (low SD1). This is typical during exercise, stress, or sympathetic activation. Imagine a train on tracks — it can travel far (long-term change) but can't swerve sideways (short-term change).
Fat/Wide Ellipse (spread perpendicular to diagonal) → "Jumpy"
Big beat-to-beat jumps (high SD1) but the average heart rate stays relatively stable (low SD2). This reflects strong parasympathetic (vagal) activity — the vagus nerve rapidly modulating each heartbeat. Like a person standing in place but dancing — lots of local movement, not much travel.
Tight Dot → "Rigid"
Almost no variability in any direction. The heart beats like a metronome. This can indicate extreme stress, fatigue, or autonomic dysfunction. Like a frozen gear — no flexibility at all.
The SD1/SD2 Ratio
The ratio SD1/SD2 quantifies the ellipse shape:
| SD1/SD2 | Shape | Meaning |
|---|---|---|
| ≈ 1.0 | Circle | Balanced autonomic activity |
| < 0.5 | Elongated | Sympathetic dominance, long-term drift |
| > 1.0 | Wide/fat | Parasympathetic dominance, beat-to-beat jumps |
Connecting SD1 to RMSSD
SD1 is mathematically identical to a scaled version of RMSSD:
This makes sense: SD1 measures scatter perpendicular to the identity line, which captures beat-to-beat differences — exactly what RMSSD does. The Poincaré plot gives you a visual way to see what RMSSD measures numerically.
Connecting SD2 to SDNN
SD2 relates to overall variability (SDNN) and short-term variability (RMSSD):
SD2 captures the "remaining" variability after removing the short-term component — essentially the slower, trend-like changes in heart rate.