QRS Interval on Your Watch ECG (Narrow, Normal, and Wide)

QRS Interval: narrow, normal, and wide QRS interval

Key Takeaways

  • QRS interval measures the time it takes for the ventricles, or bottom part of your heart, to contract.
  • A wide QRS interval (greater than 120 milliseconds) is associated with a number of heart conditions, including right or left bundle branch block.
  • A narrow QRS interval (less than 80 milliseconds) is usually considered normal, and indicates an abnormally rapid activation of the ventricles.

The electrocardiogram (EKG or ECG) is one of the most common methods to check the electrical activity of your heartbeat. Even if a doctor or nurse has never hooked you or someone you know up to an ECG machine, you’ve still seen them being used on TV shows and movies. Nowadays, smartwatches like the Apple Watch and at-home devices like Kardia can help capture an ECG of your heart at your own convenience.

What do those squiggly lines and waves on the ECG really mean? What might the intervals mean about your cardiovascular health? Can you learn to read your own QRS intervals on your watch or Kardia ECG? Let’s get started with what you should know about the QRS Interval.

What's a QRS Interval?

When measured through an ECG, your heart’s electrical activity is identified in “waves.” Physicians denote these heart waves as either P, QRS, or T waves to identify them more readily.

P waves: The P wave is the first wave in the ECG tracing. It signifies the contraction of the upper chambers of the heart.

QRS complex: This is the easiest identifiable part of an ECG. The QRS complex looks like a spike and represents the contraction of the heart’s lower chambers.

T wave: The T wave comes after the QRS complex, which represents the relaxation of the heart’s lower chambers.

QRS Interval in blue

The QRS interval indicates how fast it takes for the ventricles, or bottom part of your heart, to contract. You can easily measure the QRS interval by looking at the ECG reading on graph paper. Each square of the graph paper represents 40 milliseconds, so you multiply that number by the number of squares between the start of your QRS complex and the end of your QRS complex.

What's a Normal Range for a QRS Interval?

The normal range for the QRS complex is 0.08 to 0.12 seconds. The QRS interval represents the depolarization of the ventricles or the time it takes for a stimulus to spread throughout the ventricles. The ventricles contract during the QRS interval. A normal QRS interval means that the conduction system of the heart is working properly, and your heart is responding to electrical signals and contracting when it should be.

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Apple Watch ECG that captured a Sinus Bradycardia with a normal QRS interval.

What Does Wide QRS Indicate? Is It Dangerous?

If your QRS complex is longer than 0.12 seconds, it is considered wide. If your ECG shows a wide QRS complex, then your ventricles (the bottom chambers of the heart) are contracting more slowly than a normal rhythm.

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Apple Watch ECG that captured a Wolff-Parkinson-White syndrome (WPW) showing a wide QRS interval of 160 milliseconds.

A wide QRS complex can be related to a ventricular issue and is often associated with the following conditions:

  • Right or left bundle branch block
  • Hyperkalaemia
  • Wolff-Parkinson-White syndrome
  • Ventricular pacing
  • Hypothermia

Whether or not a wide QRS complex is dangerous depends on several factors. First of all, the underlying condition causing the widening of the QRS complex helps determine how dangerous the condition is. Whether or not there are other cardiac symptoms and abnormalities on the ECG is another critical factor.

What Does Narrow QRS Indicate? Is It Dangerous?

A narrow QRS interval is measured at less than 0.08 seconds. According to the Mayo Clinic, cardiac arrhythmias associated with a narrow QRS are rarely life-threatening but still may present some risk. Narrow QRS complexes are usually related to one of three areas of the heart: the sinoatrial node, the atria, and/or the AV node/junction. A narrow QRS interval usually means an abnormally rapid activation of the ventricles.

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Apple Watch ECG that captured a Supraventricular Tachycardia (SVT) showing a narrow QRS interval of 77 milliseconds.

Cardiac arrhythmias associated with a narrow QRS interval are usually considered supraventricular in origin. This means that the issue lies above, or superior to the ventricles. Many different types of supraventricular arrhythmias range from mild to severe. While typically considered normal, consider having your doctor evaluate your cardiac rhythm if you think there may be abnormalities in your QRS interval.

Still Not Sure How to Measure Your QRS Interval?

It’s important to understand the basics of what the QRS interval is and what it may be telling you about your cardiovascular condition. An ECG expert can help you understand whether the ECG intervals are normal, but also whether the abnormal results might require additional medical oversight and treatment.

Hopefully, you now know how to read your own QRS intervals. But if you still want an expert opinion, get the QALY app today for QRS interval measurements for your smartwatch or Kardia ECGs from trained cardiographic technicians.

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