Geometry of the Poincaré plot of RR intervals and its asymmetry in healthy adults.

The geometry of the Poincaré plot of RR intervals is considered and its basic descriptors are defined in terms of the second moment of a distribution of points in a plane. One of the standard descriptors, SD1, is redefined and used to define two new descriptors, SD1(UP) and SD1(DOWN), whose squares partition SD1(2) (the variance corresponding to short-term heart rate variability) into contributions from decelerations and accelerations of heart rate. It is shown that there is a visible and statistically highly significant asymmetry in the Poincaré plot, with the upper part, corresponding to decelerations of heart rate, larger than the lower part, which corresponds to accelerations. The effect is shown in one hundred 30 min long time series of RR intervals derived from the ECG recordings of 100 young (19-32 years old) and healthy adults. After shuffling the data to random order the asymmetry disappears, which shows that this is a genuine physiological phenomenon rather than an artefact of the method.