An updated version of Apple’s ECG feature received FDA clearance, allowing it to add abnormal heart rhythm known as Atrial Fibrillation (AFib). The news comes just over two years after the company received De Novo clearance for the ECG feature on its smartwatches.

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AFib is the most common type of irregular heartbeat aka arrhythmia. When a person has AFib, the normal beating in the upper chambers of the heart (the two atria) is irregular, and blood doesn’t flow as well as it should from the atria to the lower chambers of the heart (the two ventricles). According to the CDC, more than 454,000 hospitalizations with AFib as the primary diagnosis happen each year in the United States.4 The condition contributes to about 158,000 deaths each year.

The ECG app on the Apple Watch (Series 4, Series 5, or Series 6) can record your heartbeat and rhythm using the electrical heart sensor and then check the recording for AFib, according to Apple.

The ECG app records an electrocardiogram which represents the electrical pulses that make your heartbeat. The ECG app checks these pulses to get your heart rate and see if the upper and lower chambers of your heart are in rhythm. If they’re out of rhythm, that could be AFib.

An AFib result means the heart is beating in an irregular pattern between 50 and 120 BPM

The Apple Watch ECG app generates an ECG that is similar to a single-lead (or Lead I) ECG. In a doctor’s office, a standard 12-lead ECG is usually taken. This 12-lead ECG records electrical signals from different angles in the heart to produce twelve different waveforms. The ECG app on Apple Watch measures a waveform similar to one of those twelve waveforms. A single-lead ECG is able to provide information about heart rate and heart rhythm and enables classification of AFib. However, a single-lead ECG cannot be used to identify some other conditions, like heart attacks.

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Single-lead ECGs are often prescribed by doctors for people to wear at home or within the hospital so that the doctor can get a better look at the underlying rate and rhythm of the heart. However, the ECG app on Apple Watch allows you to generate an ECG similar to a single-lead ECG without a prescription from your doctor.

If you receive an AFib classification and you have not been diagnosed with AFib, you should talk to your doctor.