Heart Rate Variability Overview on Wikipedia
Wikipedia has a good overall description of Heart Rate Variability. The first three paragraphs are shown below. Click on the link at the bottom of the page to read more.
Heart rate variability (HRV) is the physiological phenomenon of variation in the time interval between heartbeats. It is measured by the variation in the beat-to-beat interval.
Other terms used include “cycle length variability”, “R–R variability” (where R is a point corresponding to the peak of the QRS complex of the ECG wave; and R–R is the interval between successive Rs), and “heart period variability”. Measurement of the RR interval is used to derive heart rate variability.
Methods used to detect beats include ECG, blood pressure, ballistocardiograms, and the pulse wave signal derived from a photoplethysmograph (PPG). ECG is considered the gold standard for HRV measurement because it provides a direct reflection of cardiac electric activity.
