Glossary
Poincaré plot
Also called Poincare plot, Lorenz plot, return map
A scatter plot of each heartbeat interval against the one before it. The shape of the cloud of dots shows short-term and overall variability at a glance.
To draw a Poincaré plot, take a list of RR intervals RR interval The time from one heartbeat to the next, measured in milliseconds between the R peaks of an ECG. HRV is calculated from a series of these intervals. More about RR interval and plot each one as a dot. Its position across is the previous interval; its position up is the current one. If an interval of 950 ms is followed by one of 1,000 ms, the dot goes at (950, 1000). Repeat for every beat in the recording.
If your heart beat like a clock, every dot would land on the same spot. Real recordings give a cloud stretched along the diagonal line where both intervals are equal, often roughly the shape of an ellipse. The width of the cloud, across the diagonal, shows how much each beat differs from the one before: fast, beat-to-beat variability. Its length, along the diagonal, takes in slower, longer-term changes too. These two spreads are measured as SD1 and SD2 SD1 and SD2 Two measures of the spread of dots in a Poincaré plot. SD1 is the width and tracks short-term variability (it equals RMSSD); SD2 is the length and tracks overall variability. More about SD1 and SD2 .
You can read the plot without any math. A narrow cloud means the beats change little from one to the next; a wide one means they change a lot. Unlike frequency analysis, the plot is not thrown off by slow trends in the intervals.
The Task Force standards list the same idea as a Lorenz plot, one of the geometric methods for describing HRV. The lesson What HRV is, in one picture uses this kind of picture to make HRV easy to see.