taken from https://www.reddit.com/r/apple/comments/9fkb3t/im_an_emergency_physician_and_wanted_to_talk/

I'm an Emergency Physician and read/interpret ECGs on a daily basis. I wanted to talk about the Apple Watch's new ECG feature as there are a lot of misconceptions about this new function. Apple Watch's new ECG feature is pretty neat and a lot of people here are understandably excited about it, but it does have limitations.

I also wanted to do a brief tutorial on the basics of ECGs.

 

Electrocardiography aka ECG or EKG

At it's basic core, ECGs measure voltage activity in the heart. As the heart beats, electrical activity moves along the heart and shows up as the bumps and spikes in the ECG.

Voltages are measured across gradients meaning you need at least two electrodes with vectors go across the heart. An ECG computer measures these voltages and calculates certain vectors or "leads" based on the orientation of the electrodes and displays them as a rhythm strip, that looks something like this.

Some of these vectors are direct measurements of certain electrodes, while others are calculated by the computer across multiple electrodes.

 

The most comprehensive ECG is known as a 12-lead ECG.

This ECG uses 10 electrodes, 4 on each limb, and 6 in a specific orientation around the heart in order to represent 12 separate vectors across the heart. In some ways you can think about this type of ECG as almost a 3D measurement of electrical activity in the heart.

 

The heart monitors typically used in the hospital settings usually use 3-5 electrodes, limiting the number of leads that are measured, but still useful for other diagnostic tests as they are typically continuous.

 

What is Apple Watch's ECG?

The Apple Watch ECG is a single lead ECG, measuring Lead I. This is great for measuring the rate and rhythm of the heart which can be very useful for the screening of atrial fibrillation.

Atrial Fibrillation Rhythm Strips

The normal bottom strip is nice and regular and also includes something called a P-wave that represents the beat of the atrium and is supposed to be before every big spike. The top strip represents afib, the spikes are disorderly and not regular, it is also notably missing that P-wave.

In it's current state, the Apple Watch will continuously monitor your heart beat with it's optical sensor, and if it detects an arrhythmia, prompt you with a notification at which point you can conduct an ECG.

 

Atrial Fibrillation

Atrial Fibrillation (aka a-fib) is the most common cardiac arrhythmia, and up to 25% of people above the age of 40 will experience at least one episode of afib. It occurs when the normal path of electrical activity is interrupted and disorganized.

Atrial fibrillation can occur in episodes that may only last a few minutes and go back to normal rhythm by itself. Some of these episodes can be precipitated by certain events like drinking alcohol, being sick, or other medical conditions. Some people will have recurrent episodes of self resolving afib, also known as paroxymal atrial fibrillation with no known cause. The extreme of this are people who have persistent afib and who's rhythm can be very difficult to flip back back to normal.

Having atrial fibrillation in it of itself is not necessarily an emergency. Many people with atrial fibrillation are completely asymptomatic and others may only have the feeling of palpitations or an irregular heart beat.

Where atrial fibrillation can get dangerous is if it causes vital sign abnormalities, where your blood pressure can get so low you pass out, you feel short or breath, or your heart rate is dangerously fast. If your heart rate is greater than 100-110 or so, you are in a state called atrial fibrillation with rapid ventricular response. At this point we would control your heart rate with IV medications in an emergent setting.

While brief periods of asymptomatic afib can be benign, sustained atrial fibrillation increases your risk for stroke, clots in the lung, and heart failure. Depending on the patient's risk factors, some people will require blood thinners to reduce these risks. The longer your heart is in uncontrolled afib, the more difficult it is to reverse as there can be an element of cardiac remodeling.

 

What the Apple Watch is not

  • At this point in time, the Apple Watch ECG feature is not indicated for the detection of any heart conditions except Atrial Fibrillation.
  • It is also not indicated for people who already have a diagnosis of atrial fibrillation, who should be seeing a physician regularly.
  • It is NOT capable of ruling in or out a heart attack. Even a full 12-lead ECG can miss certain heart attacks.
  • It is also NOT considered an FDA approved medical device as some people have claimed it to be. The FDA simply released clearance letters, also known as a 510k Pre Market notification clearance, that also explicitly state that it is not intended for people under the age of 22. It is considered as an over-the-counter (OTC) device and classified as Class II, which is the same class as things like condoms and home-pregnancy kits.
  • It is also not a continuous monitor of your heart's electrical activity. It is only capable of measuring an ECG while your other hand is on the crown.
    • A single electrode ECG is also physically impossible. In order to measure electrical activity, there needs to be a complete circuit that passes through the heart. Not even a wireless device on the other hand can get around this as it wouldn't be part of the same electrical circuit.