The PR interval is the time it takes for an electrical signal to travel from the top chambers of your heart to the bottom chambers, measured on an EKG (electrocardiogram). A normal PR interval lasts between 120 and 200 milliseconds, which shows up as three to five small squares on the EKG paper.
What Happens During the PR Interval
Every heartbeat starts with a small burst of electricity from a natural pacemaker (the SA node) sitting at the top of your heart. That signal spreads across both upper chambers, causing them to squeeze and push blood downward into the lower chambers. On the EKG, this electrical spread appears as the P wave, a small rounded bump.
After crossing the upper chambers, the signal reaches a relay station near the center of the heart called the AV node. This node deliberately slows the signal down by a fraction of a second. That built-in pause is critical: it gives the upper chambers time to finish emptying before the lower chambers contract. Once the delay is over, the signal shoots into the lower chambers and triggers the large spike on the EKG known as the QRS complex.
The PR interval captures this entire sequence, from the very beginning of the P wave to the very beginning of the QRS complex. It includes both the electrical spread across the upper chambers and the pause at the AV node.
PR Interval vs. PR Segment
These two terms look similar but mean different things. The PR interval measures the full duration from the start of the P wave to the start of the QRS complex. The PR segment is just the flat line between the end of the P wave and the start of the QRS, representing mostly the AV node delay. When clinicians look at segments, they care about whether the line shifts up or down from the baseline. When they look at intervals, they care about how long they last.
What a Prolonged PR Interval Means
A PR interval longer than 200 milliseconds is called first-degree AV block. Despite the alarming name, it simply means the electrical signal takes longer than usual to pass through the AV node. The signal still gets through every time, so the heart rhythm stays regular. When the interval stretches beyond 300 milliseconds, it’s considered “marked.”
In younger people, a prolonged PR interval is often caused by high vagal tone, the body’s natural tendency to keep the heart rate calm. Athletes, for example, frequently show this pattern with no symptoms at all. In older adults, the more common cause is gradual wear on the heart’s electrical wiring from fibrotic (scarring) changes in the conduction system.
Other causes include coronary heart disease, prior heart attack, low potassium or magnesium levels, certain infections (Lyme disease, rheumatic fever, endocarditis), inflammatory conditions like sarcoidosis, and some medications that slow conduction through the AV node. Many people with a mildly prolonged PR interval have no symptoms and need no treatment, but the finding sometimes prompts further evaluation depending on context.
What a Short PR Interval Means
A PR interval shorter than 120 milliseconds suggests the electrical signal is reaching the lower chambers faster than it should. The most well-known cause is Wolff-Parkinson-White (WPW) syndrome, a condition present from birth in which an extra electrical pathway connects the upper and lower chambers, bypassing the AV node’s normal delay.
On an EKG, WPW typically shows three features together: a short PR interval (under 120 ms), a widened QRS complex (over 120 ms), and a characteristic slurred upstroke at the beginning of the QRS called a delta wave. The extra pathway lets the signal arrive early but spread more slowly through the muscle, which creates that distinctive pattern. WPW can cause episodes of rapid heartbeat and, in rare cases, dangerous heart rhythms.
Normal Ranges by Age
The 120 to 200 millisecond range applies to adults. Children have naturally faster heart rates, so their PR intervals are shorter. Infants with heart rates between 100 and 150 beats per minute typically have PR intervals in the range of 80 to 110 milliseconds, occasionally stretching to 150 ms. By the teenage years, heart rates slow down and the upper limit of normal extends to around 180 ms, approaching the adult range.
Heart rate matters at every age. A faster heart rate compresses the entire cardiac cycle, including the PR interval, so the same person can have a slightly different PR interval depending on whether they’re resting or active.
How the PR Interval Is Measured
EKG paper is printed with a grid of small squares. At the standard paper speed of 25 millimeters per second, each small square represents 40 milliseconds. A normal PR interval spans three to five of these small squares. The measurement starts at the first upward deflection of the P wave and ends at the very first deflection of the QRS complex, whether that’s an upward or downward movement.
Modern EKG machines calculate the PR interval automatically and print it at the top of the report. If the machine flags it as prolonged or short, a clinician will usually confirm the measurement by hand, since artifact or unusual waveforms can sometimes throw off the automated reading.