ECG on Apple Watch Series 4

You may be aware that there is a new ECG app on the Apple Watch Series 4. An Apple Support page provides instructions on how to use the app and interpret the results. Since I'm a doctor and a geek, I'll try to provide a little more context about the Apple Watch ECG capabilities in layman's terms.

The Apple Watch ECG tracing (a portion of mine is pictured above) is a subset of what is formally known as a "12-lead ECG" that might be used in doctor's offices and hospitals. A full 12-lead ECG might look something like this:

It's called a 12-lead ECG because there are 12 combinations of the leads that are placed on a patient's body, with each lead representing the electrical signals between 2 points on the body. Those 12 leads are I, II, III, aVR, aVL, aVF, V1, V2, V3, V4, V5, and V6. The placement of those 12 leads on a patient's body and the understanding of the differences between those leads are beyond the scope of this blog, but suffice it to say that in certain scenarios, it is important to look at ECG tracings from multiple leads to tell what is going on.

A 12-lead ECG can reveal many different kinds of findings, such as:

• Arrhythmias: atrial fibrillation, atrial flutter, ventricular tachycardia, ventricular fibrillation, Wolf-Parkinson-White syndrome, sick sinus syndrome (tachy-brady syndrome), atrioventricular nodal reentrant tachycardia, multifocal atrial tachycardia, wandering atrial pacemaker, 1st/2nd/3rd-degree atrioventricular block, premature atrial contractions, premature ventricular contractions, long QT syndrome, asystole, and many others
• Ischemia (insufficient supply of oxygen to heart muscles): unstable angina and acute myocardial infarction (heart attack)
• Structural abnormalities: cardiac aneurysms, dextrocardia, congenital heart defects, hypertrophic obstructive cardiomyopathy, atrial septal defects, and others
• Miscellaneous: hyperkalemia, hypokalemia, hypercalcemia, hypocalcemia, digoxin effect, pericarditis, and many others

The Apple Watch Series 4 (and presumably beyond) can identify normal sinus rhythm (i.e., a normal rhythm that is found in healthy hearts) and atrial fibrillation (an abnormal heart rhythm in which there is no predictable periodicity to the heartbeats and the notable absence of a certain kind of electrical signal known as a P wave on the ECG), but it cannot currently identify any other problems such as the partial list above.

After demonstrating success with the detection of atrial fibrillation, I think it would be fairly straightforward for Apple to identify several other kinds of arrhythmias (from the partial list above) on a Series 4 Apple Watch simply by updating the software, and I predict that we will see this capability sometime soon. However, it makes a lot of sense to start with identifying atrial fibrillation because it is one of the most common cardiac arrhythmias, has potentially serious consequences if left undetected and untreated, and is fairly straightforward to computerize its detection on ECG.

That being said, there are some conditions such as detecting heart attacks that are unlikely to be performed on an Apple Watch because a heart attack results in ECG changes in some leads but not others, depending on its location. That is why we use all 12 leads to look for signs of ischemia in all regions of the heart. The Apple watch ECG tracing is more or less equivalent to lead I on a 12-lead ECG. It is essentially a 1-lead ECG or a "telemetry strip" reading.

I hope this sheds some light on the Apple Watch ECG app's current capabilities and limitations. Tim Cook recently stated, "if you zoom out into the future, and you look back, and you ask the question, 'What was Apple's greatest contribution to mankind,' it will be about health." With Apple's introduction of HealthKit (June 2014), the Health app (June 2014), ResearchKit (March 2015), CareKit (March 2016), and now the ECG app (December 2018), I think Apple is in an excellent position to do just that.