Can the Apple Watch really detect or diagnose AF?

Two of the studies that are being undertaking at Barts Heart Centre use the Apple Watch to detect atrial fibrillation (the AFFU-AW study and the CSAF-AW study).

But is the Apple Watch a reliable tool to use to diagnose AF?

The Apple Watch is a wearable device that can be purchased ‘over-the-counter’. It has different functionalities for business, leisure and health. It can record the heart rhythm in two ways:

1. Using PPG (photoplethysmography). This is the scientific term for the green light shone from the watch onto the wrist to measure the blood flowing through the blood vessels under the skin and measured using a sensor in the Watch. It therefore can detect each time the blood flows as a ‘pump’ of the heart and so can plot a chart of heart beat against time. This is the same technology used in pulse oximeters used in hospitals, in Fitbits and Garmins. The Apple Watch then analyses the recorded rhythm to look for AF features such as irregularity (the specifics of the algorithm are a secret). The accuracy of this method was studied in the Apple Heart study discussed below.

1. Using a single-lead ECG. This feature is only available on Series 4 Apple Watches and above. This measures the electrical signals emitted by the heart itself (no other organ in the chest produces electrical signals in this way). When you wear the watch on one arm (let’s say right) and then tap the dial with your left, you can compare the voltage size and direction of current as it flows through the heart to cause heartbeats. This gives a more detailed view of the heart rhythm.

The Apple Heart Study is the largest reported experience using PPG to detect AF in the general population. More than 400,000 participants were recorded and the PPG identified potential AF in around 2000 patients. When these patients were followed up using formal ECG testing, AF was seen in 84% of them.

As the second method, the ECG method is relatively new it hasn’t been studied as extensively, however the reports so far suggest the agreement between the watch and traditional medical tools is >98% and the AF detection accuracy may be higher than the PPG alone.

The most recent International guidelines about AF recognised that a single-lead ECG is sufficient to make a diagnosis of AF and goes further to state that these ECGs can be recorded from a wearable device capable of this i.e. the 2nd method of the Apple Watch described above.

This feature of the Apple Watch has CE mark (European approvals) and FDA (American) approval for diagnosing AF.

Note: Apple have provided the devices for the two studies at Barts but no financial support nor involvement in the study conception, design or analysis.

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Seshadri DR, Bittel B, Browsky D, Houghtaling P, Drummond CK, Desai MY, Gillinov AM. Accuracy of Apple Watch for Detection of Atrial Fibrillation. Circulation. 2020 Feb 25;141(8):702-703. doi: 10.1161/CIRCULATIONAHA.119.044126. Epub 2020 Feb 24. PMID: 32091929.