Electrocardiogram (ECG) recording of a canine<unk>  that illustrates beat-to-beat variability in R–R interval (top) and<unk> <unk>  (bottom).<unk>  <unk>  is the physiological phenomenon of variation in the time interval between<unk> beats. 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 RR is the interval between successive Rs), and<unk>  period 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<unk>  measurement because it provides a direct reflection of cardiac electric activity.
Heart rate variability
Basal rate
Cardiac reserve
QT interval variability
Electrodermal activity
Vagal tone
Cerebral autoregulation
Coronary perfusion pressure