The irregular heart rhythm called atrial fibrillation (A-fib) is associated with increased risk of stroke. Since many individuals are unaware they have A-fib, early identification can improve outcomes. The U.S. Food & Drug Administration has designated the ECG app on the Apple Watch Series 4 as a class II medical device. This means it can be used to determine the presence of A-fib, but is not recommended for users with other known arrhythmias. In other words, it is for informational use only.