Electrical Heart-Rate Measurement (ECG) Explained

Start here: symptoms need a clinician

A wearable ECG cannot replace medical care for chest pain, fainting, a racing or irregular heartbeat with breathlessness, or signs of stroke. In the US call 911 for these. The NHS lists fainting, chest pain, shortness of breath, weakness or numbness on one side and difficulty speaking among symptoms that need emergency help when they come with a fast or irregular heartbeat. [1]

What an ECG is

An electrocardiogram (ECG or EKG) records the electrical signals the heart makes each time it beats. [2] The NHS describes it as a test of the heart's electrical activity, including rate and rhythm. [3] The ECG does not send electricity into the body; it only records signals. [2]

A clinic ECG places several electrodes on the arms, legs and chest. [2] Clinicians also use portable recorders. The NHS says a portable ECG with chest electrodes is usually worn for 24 to 48 hours, sometimes up to 7 days, and the results are read by a specialist. [3] MedlinePlus notes that if symptoms come and go, a regular ECG may miss the problem, so a portable monitor may be used. [2]

Direct versus derived

Chest straps

A chest strap reads the electrical signal at the chest and sends a heart rate. A 2025 systematic review of 32 studies found the most-tested straps (the Polar H10 and Zephyr BioHarness 3.0) correlated strongly with ECG at rest and during light-to-moderate activity. Reported problems were motion artifacts, poor strap placement, sweat and loss of the skin-electrode contact. None of the reviewed devices had CE or FDA approval for clinical use, and most studies were in controlled settings. [4] A separate laboratory study found Polar H10 heart rate correlated above r = 0.99 with a gold-standard ECG during a short audiovisual task in pairs of people, which is a narrow situation. [5] Label: established for heart rate in controlled conditions; insufficient evidence for clinical-grade use. See chest-strap heart-rate monitors.

Watch and phone ECG

Consumer ECG features typically record one lead, a single view of the electrical signal. In a clinic study of 194 patients with 12-lead ECG as reference, a 6-lead handheld device and a single-lead smartwatch ECG had overall diagnostic accuracy of 98.6% and 96.9% for rhythm categories, but single-lead agreement was lower for the PR interval (ICC 0.57 versus 0.89) and QRS amplitude (0.04 versus 0.96). [6] A single lead can screen for some rhythms while giving limited information about others.

In the US, FDA regulation of "electrocardiograph software for over-the-counter use" says it can provide information for identifying arrhythmias and is not intended to provide a diagnosis. [7] Detection performance data, results and cautions for AF are in consumer ECG and atrial fibrillation features.

ECG versus the light sensor

Many wearables use optical sensors instead of electrodes; see optical heart-rate sensing (PPG). In 103 football players, light-sensor pulse rate variability read lower than ECG-based variability and was slower to show changes, so the two should not be treated as interchangeable. [8] For what variability means, see heart-rate variability explained.

Accuracy is not benefit

Good agreement with a clinic ECG shows a device measures well. It does not show that tracking your own heart rate or rhythm improves health. For adults 50 or older without symptoms, the US Preventive Services Task Force found insufficient evidence to weigh benefits and harms of screening for atrial fibrillation. [9]

Who should be careful

People with a pacemaker, other implanted device or known heart condition should ask their clinician before relying on or using a consumer ECG feature, because FDA labeling rules for this software require a list of situations where it may not operate at the expected performance level. [7] Skin irritation from electrodes is possible. [2]