ECG

Each heartbeat starts with an electrical signal that spreads through the heart muscle. An electrocardiogram (ECG, or EKG) picks up that signal through electrodes on the skin and draws it as a wave. In a clinic, electrodes go on the chest, arms and legs. A chest strap or a watch with an ECG feature uses fewer contact points but records the same kind of signal.

The tallest, sharpest part of each beat on the trace is the R wave, and the gap from one R wave to the next is the RR interval RR interval 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 . HRV research treats RR intervals from an ECG as the gold standard. Other methods, such as the optical sensors in most watches and rings, are judged by how closely they match it.

At rest, optical sensors usually come close. During movement, and under some kinds of mental stress, their agreement with ECG gets worse, sometimes by a lot. So if you want HRV during or right after exercise, an ECG chest strap is the safer choice. An ECG also shows whether the rhythm is steady or irregular, which helps when beats that start in the wrong place ( ectopic beats ectopic beats Ectopic beat A heartbeat that starts outside the heart's normal pacemaker and usually comes early. Ectopic beats are common, but they distort HRV unless removed or corrected. More about Ectopic beat ) need to be removed.

A clinical ECG is a medical test that a doctor reads for rhythm problems and other heart conditions. An HRV number from a wearable is not a substitute for one; talk to a doctor if you notice an irregular pulse or symptoms such as chest pain or fainting.

The lesson How HRV is measured: from beats to a number and the guide How to measure HRV compare ECG with other sensors.