Heart rate variability

A healthy heart does not beat like a metronome. Even when you sit still, the gaps between beats keep changing. Heart rate variability (HRV) describes how much they change. It is calculated from a series of RR intervals RR intervals RR interval The time from one heartbeat to the next, measured in milliseconds between the R peaks of an ECG. HRV is calculated from a series of these intervals. More about RR interval recorded over anything from about a minute to 24 hours.

Most of the variation comes from the autonomic nervous system autonomic nervous system Autonomic nervous system The part of the nervous system that runs the body without conscious control, including heart rate. HRV mostly reflects how it adjusts the heart from moment to moment. More about Autonomic nervous system . The vagus nerve vagus nerve Vagus nerve The main nerve of the parasympathetic nervous system, running from the brainstem to the heart, lungs and gut. Its fast signals to the heart drive most beat-to-beat HRV. More about Vagus nerve can slow the heart in under a second, while the sympathetic sympathetic Sympathetic nervous system The "fight or flight" branch of the autonomic nervous system. It speeds up the heart and makes it pump harder, but it acts too slowly to drive beat-to-beat HRV. More about Sympathetic nervous system system takes several seconds to speed it up. Breathing, blood pressure and movement shift these signals all the time, and the gaps between beats follow.

There is no single HRV number. Apps and studies report different metrics, such as RMSSD RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next. It mainly reflects vagal activity. More about RMSSD , SDNN SDNN SDNN The standard deviation of all NN intervals in a recording, in milliseconds. It measures overall variability, and its value depends strongly on how long the recording lasts. More about SDNN or lnRMSSD lnRMSSD lnRMSSD The natural logarithm of RMSSD. Taking the log evens out the spread of RMSSD values, which makes day-to-day changes easier to compare and analyze. More about lnRMSSD , and the results depend on how long the recording was and when it was taken. A value from one app often cannot be compared with a value from another, or with a chart of norms.

The Task Force standards add a caution: HRV measures fluctuations in the nervous system’s input to the heart, not the overall level of that input. A low value can have more than one cause, so read it against your own trend rather than on its own.

Start with What is HRV? or the lesson What HRV is, in one picture.