PR Interval on Your Watch ECG - Short, Normal, and Prolonged

Short PR Interval, Normal PR Interval, Prolonged PR Interval
Qaly Heart
Qaly is built by Stanford engineers and cardiologists, including Dr. Marco Perez, a Stanford Associate Professor of Medicine, Stanford Cardiac Electrophysiologist, and Co-PI of the Apple Heart Study.

Key Takeaways

  • Your ECG's PR interval shows how long it takes for your heart's electrical signal to get from the top of your heart, to the bottom of your heart.
  • A prolonged PR interval is 200 milliseconds or longer, which can be a sign of First-Degree AV Block.
  • A short PR interval is 120 milliseconds or shorter, which can be a sign of Wolff-Parkinson-White Syndrome.

Got other questions on your PQRST Intervals? See the complete set of Qaly guides on PQRST Intervals:

Introduction

Hello, heart hero. In your quest to identify that irregular heart rhythm you just felt, you may have come across the term PR Interval. With your trusty watch ECG now in hand, you may be wondering, "What does PR Interval mean on my watch ECG?" Or you might be thinking, "Is a short or prolonged PR Interval dangerous?" In this guide, we'll help answer these questions. Let's dive in.

What's an Electrocardiogram?

Before diving into PR Intervals, it's important to understand exactly what an electrocardiogram, or ECG is. (If you're confident in your understanding of an ECG and how it relates to PR Intervals, though, skip on ahead to the next section for some visual examples of PR Intervals).

To start, remember how your heart beats? It produces an electrical signal, which squeezes and unsqueezes your heart, which in turn pumps your blood to your lungs for oxygen and then out to the rest of your body.

As that electrical signal flows through your heart, your ECG on your watch sees it, and shows it to you as those awesome-looking waves you know as an ECG. Every time your heart completes one heartbeat, it completes one full cycle of that electrical signal flowing through your heart. And as that electrical signal flows through your heart, it produces different waves on your ECG that capture the squeezing and relaxing of your heart with each heartbeat, which are represented on your ECG as PQRST waves.


So What's a PR Interval?

The first wave in your ECG's PQRST cycle during each of your heartbeats is the P Wave. Once your heart's electrical signal starts at the beginning of a heartbeat, it causes your heart's upper chambers, or atria, to squeeze. This squeezes the blood coming into your heart's atria down into your heart's lower chambers, or ventricles. The P Wave on your ECG captures this first squeeze in your heartbeats.

After the P Wave comes the QRS Complex. You'll recognize this one, since it's usually the tall spike in your ECG. When your heart's electrical signal moves down into your heart's lower chambers, or ventricles, it first moves through specialized wires in your heart called right and left bundle branches (also known as your His-Purkinjee system). These wires help spread your heart's electrical signal as evenly as possible. Your QRS Complex captures your electrical signal spreading down into and through your ventricles, and the beginning of the final squeeze that pushes the blood in your heart back out into the rest of your body.

Your PR Interval is measured from the beginning of your P Wave to the beginning of your QRS Complex. It measures how long it takes for your heart's electrical signal to get from the top of your heart to the bottom of your heart.

Your PR Interval, in blue.
Your PR Interval, in blue.

What's the Normal Range for a PR Interval?

Your ECG's PR Interval is measured in seconds, or milliseconds, because that's how "long" or "short" it takes for your heart to beat. The normal PR Interval range is from 120 milliseconds to 200 milliseconds.

What's a Prolonged PR Interval?

Your PR Interval is prolonged at 200 milliseconds or higher. A prolonged PR Interval happens when your heart's electrical signal takes a long time to get through your heart's atrioventricular node, or AV node. Your AV node is the “gatekeeper” that sends an electrical signal from the top of your heart to the bottom of your heart.

Here's a prolonged PR Interval caught on a Qaly member's Apple Watch ECG. Notice the PR Interval length of 240 milliseconds and the First-Degree AV Block ECG.
Here's a prolonged PR Interval caught on a Qaly member's Apple Watch ECG. Notice the PR Interval length of 240 milliseconds and the First-Degree AV Block ECG.

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Is a Prolonged PR Interval Dangerous?

While a prolonged PR Interval isn't always a cause for concern, it could be a sign of an abnormal heart rhythm. You might see a prolonged PR Interval during sleep, if you exercise frequently, or if you're taking certain medications like beta blockers. You might also see a prolonged PR Interval in your ECG if your AV node isn't functioning properly, causing your heart's electrical signal slows down. For example, here's what First-Degree AV Block looks like on your watch ECG. As always, if you're showing signs of a prolonged PR Interval, consult your healthcare provider as soon as possible to rule out any underlying heart conditions.

What's a Short PR Interval?

Your PR Interval is short at 120 milliseconds or lower. A short PR Interval happens when your heart's electrical signal takes less time than normal to get through your heart's AV node, your heart's “gatekeeper” that sends your heart's electrical signal through from the top of your heart to the bottom of your heart. You may have seen short PR Interval being called "short PR syndrome," which isn't a formal medical term.

Here's a short PR Interval caught on a Qaly member's Apple Watch ECG. Notice the PR Interval length of 109 milliseconds and the WPW ECG.
Here's a short PR Interval caught on a Qaly member's Apple Watch ECG. Notice the PR Interval length of 109 milliseconds and the WPW ECG.

Is a Short PR Interval Dangerous?

A short PR Interval can be due to a normal, healthy variation called “enhanced AV node conduction.” You might also see a short PR Interval in your ECG if there's an extra connection between the top and bottom of your heart, called Wolff-Parkinson-White Syndrome. Here's what Wolff-Parkinson-White Syndrome looks like on your watch ECG.

A short PR Interval isn't necessarily dangerous since both children and pregnant women can, at times, have shorter PR Intervals that aren't cause for concern. However, a short PR Interval can also be associated with symptoms like chest pain, difficulty breathing, fainting, fatigue, and anxiety, as well as dangerous conditions like Diabetic Ketoacidosis, Myocardial Infarction, Acute Respiratory Failure, or Pulmonary Embolism. As always, if you're showing signs of a short PR Interval, consult your healthcare provider as soon as possible to rule out any underlying heart conditions.

Conclusion

Well, that just about wraps up our guide on what PR Intervals look like on your watch ECG. We hope this could be of some help to you.

If you still need help measuring your ECGs' PR Intervals, don't worry, we understand how scary and confusing it can be to experience irregular heartbeats. That's why we created the Qaly app for you and for the hundreds of millions of people around the world who live with heart palpitations and abnormal heart rhythms. On the Qaly app, human experts will measure your ECGs' PR Intervals within minutes for clarity and peace of mind.

To get started with the Qaly app for free, grab the Qaly app from the App Store or Play Store today. If you have any more questions, or if you need our help in any other way, don't hesitate to reach out to us at support@qaly.co.

As always from the team at Qaly, stay heart healthy ❤️

Have trouble measuring your PR Intervals? On the Qaly app, human experts will measure your ECGs' PR Intervals within minutes. Get started for free today.

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Have trouble measuring your PR Intervals? On the Qaly app, human experts will measure your ECGs' PR Intervals within minutes. Get started for free today.

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