The Apple Watch used a sensor to intermittently measure changes in blood flow as long as the user carried the device. The signals then generated pulse intervals over one minute which ran through an algorithm to infer the presence of an irregular ventricular rhythm. Further evaluation was conducted for the group with abnormal results. On the other hand, a 12-lead electrocardiogram measures electrical activity over a very short period of time. It’s just one quick snapshot of the heart, therefore, episodic atrial fibrillation can occur outside that quick peek.