- PR interval measures the time for the electrical signal to get from the top of your heart to the bottom of the heart.
- A prolonged PR interval (greater than 200 milliseconds) indicates slow conduction between the top and bottom of the heart, usually caused by a condition known as first-degree AV block.
- A short PR interval (less than 120 milliseconds) is when the electrical signal takes less time than normal to get from the top of the heart to the bottom, seen in conditions like Wolff-Parkinson-White (WPW) syndrome.
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, and why is it important to gain insight into your ECG from an experienced expert? So let’s get started with what you should know about a PR Interval.
What's a PR Interval?
When measured through an ECG, your heart’s electrical activity is identified in “waves.” To identify them more readily, physicians denote these heart waves as either P, QRS, or T waves.
P wave: 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.
PR interval is the first part of the ECG wave, indicating how fast the electrical signal travels from the atrium (upper chamber of the heart) to the ventricles (bottom chambers of the heart). That wave or electrical impulse shows whether the heartbeats are “normal,” but they can also indicate serious health concerns.
The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex. You can easily measure the PR 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 P wave and where the QRS complex begins.
What's the Normal Range for the PR Interval?
The normal PR interval range is ~120 – 200 ms (0.12-0.20s), although it can fluctuate depending on your age and health. The standard interval of the P wave can also range as low as ~90 ms (0.09s) until the onset of the QRS complex. The ECG for a child or a pregnant woman can also feature a shorter interval of the P wave.
At the same time, an older person might have a longer P wave than what is considered “normal,” even though they are still considered to be in generally good health. So a “normal” PR interval may differ based on what a doctor or medical professional determines is appropriate for them.
Is a Prolonged PR Interval Dangerous?
A prolonged PR interval is in a range that’s greater than ~200 ms (0.20s). The PR interval is usually prolonged when the electrical signal takes a long time to get through the AV node, as seen in a first-degree atrioventricular (AV) block. The AV node is the “gatekeeper” that sends an electrical signal from the top of the heart to the bottom. So, while a prolonged PR interval could mean risk (your doctor might recommend a pacemaker), it’s not necessarily always a sign of danger or a serious condition.
Even middle-aged or older adults, who are generally healthy, could have prolonged PR intervals in their ECG results. The prolonged interval means that it takes longer for the signal to move across the atria, but that could also reflect age-related changes. This is also seen in healthy individuals during sleep, in some athletes, or when someone takes certain medications like beta-blockers. You’ll need a doctor or trained medical professional to make the determination about whether the prolonged PR interval is dangerous.
Is a Short PR Dangerous?
A short PR interval is in the range of less than ~120 ms (0.12s). The PR interval can be short when the electrical signal takes less time than normal to get from the top of the heart to the bottom. This can be a normal, healthy variation called “enhanced AV node conduction.” This can also happen if there is an extra connection between the top of the heart and the bottom, as seen in Wolff-Parkinson-White (WPW) syndrome. A short PR interval can also be seen in junctional rhythms.
A short PR interval isn't necessarily dangerous since children and pregnant women sometimes have shorter PR intervals in completely normal scenarios. However, a short PR interval can be associated with symptoms like chest pain, breathing difficulty, fainting, fatigue, and anxiety. It can also be associated with life-threatening conditions like diabetic ketoacidosis, myocardial infarction (MI), acute respiratory failure, or a pulmonary embolism. To get the best sense of what your short PR interval means in your situation, consult with a medical professional.
Still Not Sure How to Measure Your PR Interval?
It’s important to understand the basics of what the PR interval is and what it may be telling you about your cardiovascular condition, but you still need an expert to interpret the results for you. 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.
At ReadMyECG, we offer the knowledge base of information you need, but we also have ECG experts standing by to answer your questions and help you understand your results. Get PR interval measurements for your smartwatch or Kardia ECGs on the ReadMyECG app from trained cardiographic technicians.