Heart rate variability (HRV)

Heart rate variability is the beat-to-beat fluctuation in time between successive heartbeats, measured in milliseconds. Within a healthy sinus rhythm, higher values typically reflect stronger vagal modulation and cardiovascular adaptability, but pathologically high beat-to-beat variability (e.g. atrial fibrillation or frequent ectopic beats) does not indicate good autonomic health and must be excluded before interpretation. Declining HRV trends are associated with aging, chronic stress, and elevated all-cause mortality risk.