Cardiology. The short- or longterm variation of heart rate measured by beat-to-beat analysis. Heart rate variability represents a variable marker of autonomic activity and provides useful informations on autonomic failure. Heart rate variability is influenced by factors such as age, gender, respiration and fitness. The variability of heart rate is divided into high frequency (0.15-0.40 Hz), low frequency (0.04-0.15 Hz), and very low frequency (0.0-0.04 Hz).