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What does the Apple Watch Series 4's EKG actually do?

The latest Apple Watch feature could save your life. Here's why.

James Martin/CNET

The Apple Watch Series 4 promises to look after your health better than its predecessors by taking extra care of your heart. New features introduced this week go beyond older Apple Watch ($399 at Walmart) models, which could simply measure intermittent heart rate during workouts and rest.

The newest Apple Watch is the first consumer smartwatch with the ability to generate an electrocardiogram test (ECG or EKG -- Apple uses ECG), which can tell you about your heart health.

You know those machines you see in a hospital that display a patient's heartbeat as a squiggly line? That's a live EKG.

So what exactly can an EKG tell you about your heart? And how can the Apple Watch mimic something that's traditionally used in hospitals? Let's find out.

Now playing: Watch this: Apple Watch Series 4: Bigger screen with EKG sensor

What is an electrocardiogram?

According to the American Heart Association, an EKG is merely "a test that measures the electrical activity of the heartbeat." An electrical signal causes first the top and then the bottom heart valves to squeeze in order to pump blood.

Monitoring the patient's wellbeing

That green line at the top is an electrocardiogram.

bymuratdeniz / Getty Images

With an EKG, doctors can examine the rhythm and speed of how this electrical signal moves through your heart to detect all sorts of problems, from harmless irregular rhythms (also called arrhythmias) to cardiac arrest.

EKGs can also tell doctors if parts of your heart are overworked or enlarged. Those two conditions can mean heart disease or heart valve problems.

By the way, an EKG is not the same as an echocardiogram. Echocardiograms are essentially an ultrasound of your heart that can uncover tumors, heart valve problems, blockages and more.

How does the Apple Watch conduct an EKG?

The Apple Watch Series 4 is the first smartwatch to get FDA clearance for EKG monitoring. Previously, you could only use the Apple Watch to conduct an EKG if you purchased the KardiaMobile watch band.

Electrodes on the back of the watch and in the digital crown measure your heart's electrical pulses. By holding your finger on the crown while the watch is on your wrist, these sensors work together to perform an EKG.


The process works like this: You open the ECG App (available later this year) and get a prompt to hold your finger to the crown. Once you do, the watch starts measuring the electrical signal and shows your heart's real-time waveform -- those classic squiggly lines that visualize a heartbeat -- in real time. 

The app records the full waveform and saves it so that you can show it to your doctor if needed.

The ECG App and the watch's S4 processor then parse the data it collected and gives you a heart rhythm classification -- more about that in a second.

Traditional EKG machines have 12 leads with electrodes that are attached all over your body to measure the electrical signals. Apple compares what the Apple Watch Series 4 does to a single-lead EKG, which research shows is just as effective at measuring the heart's electrical signals as a 12-lead machine.


The ECG App can tell you if you have a sinus rhythm or atrial fibrillation.


What can the Apple Watch tell me about my heart?

From what we know from Apple, the ECG App can give you two heart rhythm classifications; a normal "sinus" rhythm or atrial fibrillation (AFib), a serious condition that needs your doctor's attention. If the watch gives you the AFib classification, it will prompt you to go to the doctor.

Once the ECG App is available, we'll be able to explore all of its features.

With WatchOS 5, the Apple Watch's built-in heart rate sensor also automatically keep tabs on your heart rate and alerts you if it detects signs of AFib.

This is a big deal because if you go into your doctor for a EKG and your heart is beating normally at that time, the test won't show anything. But if you have an irregular heartbeat that shows up intermittently, such as paroxysmal AFib, your watch might be able to catch it.

What it can't tell you

Despite our advances in medical care, the Apple Watch Series 4 and devices like it still can't diagnose you with a medical condition. Nor are they at all a replacement for regular medical tests or health screenings.

The benefit of having an EKG built into the Apple Watch is that it can alert you to previously unknown issues and urge you to get checked out by a doctor.

Does everyone need an EKG on their wrist? Of course not. But with more medical technology at our fingertips, there's a greater chance of catching medical concerns that may have gone undiscovered or detecting an abnormality before it becomes a life-threatening condition.

The Apple Watch Series 4 is the just the beginning -- expect to see more and more consumer medical devices in the coming years.

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