Premature Junctional Contraction (PJC) Vs. Premature Atrial Contraction (PAC) on Your Watch ECG

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Got other questions on PJC or PAC? See the Qaly guides on PJC and PAC:

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Introduction

Thanks to the power of sensor technology, keeping an eye on your heart health has never been easier. With the electrocardiogram (ECG) feature of your smartwatch, you can monitor heart rhythms like premature junctional contractions (PJCs) and premature atrial contractions (PACs). Understanding the difference between these two irregular heartbeats — PJC and PAC — can provide critical insights into your heart health. Let’s dive in.

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What’s a Premature Junctional Contraction (PJC)?

Premature junctional contraction (PJC) refers to an extra beat that originates in the AV junction — a part of the heart between its upper and lower chambers. On an ECG, PJCs typically appear as a narrow complex beat with an inverted P wave and/or without a preceding P wave. Sometimes, a P wave is present but occurs so close to the QRS complex that it's difficult to see. Here’s more on what a PJC looks like on your watch ECG.

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PJC read on a Apple Watch ECG through the Qaly app. — PJC read on an Apple Watch ECG through the Qaly app.

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PJCs read on a KardiaMobile ECG through the Qaly app. Notice that the Kardia ECG app did not spot the PJCs.

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What’s a Premature Atrial Contraction (PAC)?

On the other hand, premature atrial contractions are early beats that originate from the atria — the upper chambers of the heart. On an ECG, PACs are identified by an early and often different-looking P wave, followed by a QRS complex and a compensatory pause. Here’s more on what a PAC looks like on your watch ECG.

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PAC read on a Fitbit ECG through the Qaly app. Notice that the Fitbit ECG app didn't spot the PAC without the help of the Qaly ECG reader.

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PAC read on a KardiaMobile ECG through the Qaly app, this time from a heart hero in Germany 🇩🇪

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What’s the Difference Between PJC and PAC?

When comparing PAC vs PJC, it's important to remember that both refer to premature beats — extra beats that interrupt the regular rhythm of the heart. However, the location where these contractions originate differentiates them. While PJCs arise from the AV junction, PACs originate in the atria.

On an ECG, these beats can look different. PJCs usually have an inverted P wave or lack a preceding P wave. On the other hand, PACs typically exhibit an abnormal P wave before the QRS complex.

In terms of health impact, both PJCs and PACs can often be felt as a "skipped beat" or "fluttering" in the chest. They can occur in healthy hearts and single occurrences often don't require treatment. However, frequent PACs or PJCs can sometimes indicate an underlying heart condition or be a side effect of certain medications or substances such as caffeine, alcohol, or nicotine.

Remember, while your watch ECG can provide valuable data, it doesn't replace professional medical advice. Always consult a healthcare provider if you notice any irregularities in your heart rhythm. Stay informed, stay proactive, and stay heart healthy!

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Still Not Sure if It’s Premature Junctional Contraction or Premature Atrial Contraction on Your ECG?

Differentiating between PJC vs PAC on your ECG can be tricky. If you’re still looking for help interpreting your ECG, check out the Qaly app on App Store or Play Store. On Qaly, human experts will interpret your ECGs within minutes, day or night. Try out the Qaly app today!