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

Short QTc Interval, Normal QTc Interval, Prolonged QTc 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 QTc interval shows how long it takes for your heart's lower chambers, or ventricles, to squeeze blood out of your heart to the rest of your body, and finally, relax, before starting the next heartbeat.
  • A prolonged QTc interval is 450 milliseconds or longer for men, and 470 milliseconds or longer for women, which can be a side effect of some medications or a sign of other heart problems like heart failure or electrolyte imbalances.
  • A short QTc interval is 350 milliseconds or shorter, which is normally a sign of a rare genetic disorder.

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 QT Interval or QTc Interval. With your trusty watch ECG now in hand, you may be wondering, "What does QTc Interval mean on my watch ECG?" Or you might be thinking, "Is a short or prolonged QTc Interval dangerous?" In this guide, we'll help answer these questions. Let's dive in.

What's an Electrocardiogram?

Before diving into QTc 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 QTc Intervals, though, skip on ahead to the next section for some visual examples of QTc Intervals or see Qaly's guide on how to read ECG strip).

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 QTc Interval?

The second wave in your ECG's PQRST cycle is your 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.

After your QRS Complex is your T Wave. On your ECG, this third and final wave shows you the unsqueezing, or relaxation of your heart's lower chambers, or ventricles. When your T Wave ends on your ECG, the cycle of blood entering and leaving your heart is complete, and you've completed one full heartbeat.

Your QT Interval​​ starts at the beginning of your QRS Complex and ends at the end of your T wave. The "c" at the end of "QTc" stands for "corrected," and the difference between QT and QTc is that QTc is your QT Interval after it's been "corrected" for heart rate. The reason your QT Interval needs to be corrected is that your QT Interval changes based on your heart rate, so to get a good sense of what's going on with your heart, it helps to correct your QT Interval measurement across your ECG with the QTc Interval. Your QTc Interval represents the time it takes for your heart's ventricles to squeeze the blood out to the rest of your body, and finally, relax.

Your QT Interval, in blue.
Your QT Interval, in blue.

What's the Normal Range for a QTc Interval?

Your QTc Interval is considered normal between 350 milliseconds and 450 milliseconds for men, and between 350 milliseconds and 470 milliseconds for women.


What's a Prolonged QTc Interval?

Your QTc Interval is considered prolonged at 450 milliseconds or higher for men, and at 470 milliseconds or higher for women.

Here's a prolonged QTc Interval caught on a Qaly member's Apple Watch ECG. Notice the QTc Interval length of 531 milliseconds and a Ventricular Bigeminy ECG.
Here's a prolonged QTc Interval caught on a Qaly member's Apple Watch ECG. Notice the QTc Interval length of 531 milliseconds and a Ventricular Bigeminy ECG.

Is a Prolonged QTc Interval Dangerous?

Your QTc Interval prolongs when your ventricle's electrical system isn't working properly. You'll typically see this as a side effect of some medications; however, you might also see this if you have other heart problems like heart failure or electrolyte imbalances. In very rare cases, a genetic abnormality can lead to prolonged QTc Interval. As always, if you're showing signs of a prolonged QTc Interval, consult your healthcare provider as soon as possible to rule out any underlying heart conditions.

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What's a Short QTc Interval?

Your QTc Interval is considered short at 350 milliseconds or lower.

Here's a short QTc Interval caught on a Qaly member's Apple Watch ECG. Notice the QTc Interval length of 317 milliseconds and a Sustained SVT ECG.
Here's a short QTc Interval caught on a Qaly member's Apple Watch ECG. Notice the QTc Interval length of 317 milliseconds and a Sustained SVT ECG.

Is a Short QTc Interval Dangerous?

Your QTc Interval can shorten when the your heart's electrical system is working abnormally quickly. The main cause of this is a very rare genetic disorder. Although it's rare, if you're showing signs of a short QTc 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 QTc 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' QTc 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' QTc 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 QTc Intervals? On the Qaly app, human experts will measure your ECGs' QTc Intervals within minutes. Get started for free today.

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App Store - Download Qaly | ECG Reader
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Have trouble measuring your QTc Intervals? On the Qaly app, human experts will measure your ECGs' QTc Intervals within minutes. Get started for free today.

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