HRV - heart rate variability
Although heart rate seems to be constant at rest, large or small differences can be observed in the distance between successive heart beats with high accuracy (up to 0.001 s).
These differences are caused by the impact of the autonomic nervous system on the heart frequency. By monitoring these differences and by their description, the total activity of the autonomic nervous system can be evaluated.
SA HRV – Spectral analysis of heart frequency variability
The above-described differences in the distance between individual heartbeats are not quite regular. On the contrary, a very detailed analysis of a sufficiently long record of monitoring heart rate combined with monitoring of changes, caused by a chase of body position, enables accurate determining at what frequency the changes occur.