The Apple Watch has two distinct mechanisms for recording heart rate.
One is PPG (photoplethysmography), which uses the light on the back of the watch to measure changes in light absorption with each heartbeat. This allows it to measure beat-to-beat timing intervals, but it does not record the cardiac rhythm. In other words, an extra beat from the top chamber (premature atrial complex, PAC) or from the bottom chamber (premature ventricular complex, PVC) could both alter timing and would not be differentiated by the light on the back of the watch. Additionally, PPG can be subject to artifact—for example, with changes in body position. As a result, a very brief increase in heart rate to 150 or 160 beats per minute, lasting only a few seconds and associated with no symptoms, is generally not concerning and does not warrant additional evaluation.
On the other hand, the ECG function, which requires placing your opposite hand on the crown of the watch, records the electrical signals of the heart. Although the watch’s algorithm is generally accurate, PACs, PVCs, or other sources of variability may cause the algorithm to render a diagnosis of “inconclusive,” or at times indicate atrial fibrillation when atrial fibrillation is not actually present. However, because this is a true recording of the heart’s electrical activity, a physician can review the actual tracing and make a clinical determination.
Apple Watch ECG tracings can therefore be quite useful clinically, although they are not as robust as a Kardia 6-lead ECG, which has more leads and less smoothing/filtering, making it easier for human interpretation (p-waves are easier to see, generally). Watch tracings are also less comprehensive than wearable patches or implantable loop recorders used for long-term monitoring that provide longer data recordings, but may at times also have artifact.
Very brief episodes of high heart rates without symptoms are not concerning. Rhythms that are associated with symptoms should be recorded using an ECG at the time of the symptoms, and those recordings can then be reviewed by a clinician.
At Mayo Clinic, there is currently an ongoing study called the Heart and Voice Study, which allows participants to record voice segments on their phones and, if they have an Apple Watch or an Android-compatible watch, to contribute data from those devices as well, including ECG signals from an apple watch. The watch ECG data are then automatically transmitted to a dashboard in the Mayo Clinic electronic medical record, and made available to clinicians. While this is a research study, the recorded ECGs can be reviewed and used to help address clinical questions.