What Does the PR Interval on an ECG Represent?

The PR interval on an ECG represents the time it takes for an electrical impulse to travel from the top of the heart (the atria) through the atrioventricular (AV) node and into the His-Purkinje system, right up to the moment the ventricles begin to contract.¹ In practical terms, it captures the brief electrical delay that keeps the upper and lower chambers of the heart firing in the right sequence. The normal range sits between about 120 and 200 milliseconds, and shifts outside that window can signal anything from a harmless athletic heart to a serious conduction disease.

Where the PR Interval Begins and Ends

On the ECG tracing, the PR interval is measured from the very start of the P wave to the very start of the QRS complex. The P wave itself reflects the electrical activation spreading across the atria. Once that wave of depolarization reaches the AV node, conduction slows dramatically on purpose. That built-in delay gives the atria time to finish squeezing blood into the ventricles before the ventricles fire. The impulse then speeds up again through the His bundle and the Purkinje fibers. The PR interval captures this entire journey: atrial activation, the AV nodal pause, and the fast sprint through the specialized conduction tissue below the node.

People sometimes confuse the PR interval with the PR segment. The segment is the flat line between the end of the P wave and the start of the QRS complex; it reflects mainly AV nodal conduction time. The interval includes the P wave plus the segment, so it covers atrial conduction as well. This distinction matters clinically because certain conditions affect the segment without necessarily changing the full interval’s duration.

What Counts as Normal

In adults, a PR interval between 120 and 200 milliseconds (0.12 to 0.20 seconds) is generally considered normal.² That range is not one-size-fits-all, though. Several factors push the number around within perfectly healthy hearts.

Heart rate is the biggest influence. As heart rate rises, the PR interval shortens; as it drops, the interval lengthens. The autonomic nervous system drives much of this: higher vagal (parasympathetic) tone slows conduction through the AV node, which stretches the PR interval, while sympathetic activation speeds it up.³ This is why a resting ECG taken after a strong cup of coffee can look different from one recorded during deep relaxation.

Age matters too. In children and adolescents, the PR interval is shorter and lengthens progressively as the heart grows.⁴ Pediatric ECG standards use age-specific reference ranges for exactly this reason, so a PR interval that looks borderline long in a teenager might be completely unremarkable in a middle-aged adult.⁵

Sex Differences and Heart Rate Adaptation

At first glance, the average PR interval looks similar in men and women. A study in young Yoruba adults found the mean PR interval was identical between sexes at 160 milliseconds.⁶ But the static average hides a more interesting dynamic difference. When heart rate changes, the PR interval adjusts along with it, and the speed of that adjustment differs substantially between men and women.

Research tracking beat-by-beat PR behavior found that the relationship between heart rate and PR duration follows a curvilinear pattern, and the curve is significantly steeper in women. Even more striking, women’s PR intervals take roughly twice as long to fully adapt after a heart rate change, reaching 95 percent of their new steady state in about 115 seconds on average compared to about 65 seconds in men.⁷ The practical upshot is that a single-snapshot ECG may catch a woman’s PR interval mid-transition, which could briefly make it look abnormal when it is simply still catching up to a new heart rate.

When the PR Interval Is Too Long

A PR interval over 200 milliseconds in an adult is called first-degree AV block. Despite the alarming name, it does not mean conduction is actually blocked; every atrial beat still reaches the ventricles, it just takes longer than expected. This is common and often discovered incidentally on a routine ECG. Still, it is not always as innocent as clinicians once assumed.

A landmark study from the Framingham Heart Study found that people with first-degree AV block faced roughly double the risk of developing atrial fibrillation and about three times the risk of eventually needing a pacemaker, compared with people whose PR intervals were normal. There was also a moderate increase in overall mortality risk.⁸ These findings shifted thinking in cardiology. A prolonged PR interval used to be brushed off in otherwise healthy people, but the data suggest it deserves at least periodic follow-up.

The Copenhagen ECG Study, which tracked hundreds of thousands of people over several years, confirmed the link. Individuals with a PR interval at or above the 95th percentile (roughly 196 ms in women, 204 ms in men) had a meaningfully higher risk of atrial fibrillation, with the association somewhat stronger in men.⁹ And in patients with implanted cardiac devices, a PR interval of 190 ms or longer was linked to higher rates of atrial fibrillation, heart failure hospitalization, and death.¹⁰

None of this means that a mildly long PR interval should trigger panic. Many people live their entire lives with a PR interval of 210 or 220 ms and never develop problems. But it does mean the finding warrants context. A prolonged PR interval in someone who already has risk factors for atrial fibrillation or heart failure is more clinically meaningful than one in a fit 25-year-old with no symptoms.

The Athlete’s Heart and PR Prolongation

If you are very physically active, you may have been told your ECG looks “abnormal.” Endurance athletes frequently show PR intervals at or beyond 200 milliseconds, sometimes wandering into what would technically qualify as first-degree AV block. This is driven largely by increased vagal tone, a natural adaptation to heavy training that slows resting heart rate and stretches AV conduction time.¹¹

In highly trained athletes, it is also common to see transient Wenckebach-type (Mobitz Type I) second-degree AV block, especially during sleep. In this pattern, the PR interval gradually lengthens over several beats until one P wave fails to conduct entirely, then the cycle resets. In the context of athletic conditioning, this is considered benign and typically resolves with exercise or reduced vagal tone. It becomes clinically important mainly when the person has symptoms like dizziness, fainting, or exercise intolerance, none of which are expected from training-related conduction changes.

When the PR Interval Is Too Short

A PR interval under 120 milliseconds means the electrical impulse is reaching the ventricles faster than normal. There are two broad explanations, and telling them apart matters a lot.

The more concerning cause is a physical accessory pathway, an extra strand of conducting tissue that bypasses the AV node entirely. In Wolff-Parkinson-White (WPW) syndrome, the classic ECG shows a short PR interval, a slurred initial upstroke of the QRS called a delta wave, and a widened QRS complex.¹² The danger is that this bypass tract can participate in re-entrant circuits, setting up rapid arrhythmias that occasionally become life-threatening. WPW is uncommon but well-recognized, and people with the pattern are typically referred for further evaluation.

The less concerning cause is simply enhanced AV nodal conduction, where the node itself conducts faster than average without any accessory pathway. Older literature called this Lown-Ganong-Levine (LGL) syndrome when it occurred alongside episodes of fast heart rhythm. Research into this entity found that about half of patients had evidence of enhanced conduction through both the fast and slow pathways within the AV node, which could not be explained by a simple bypass tract.¹³ In people who have a short PR interval but a normal QRS complex and no symptoms, the finding is usually benign.

Lyme Disease and the PR Interval

Lyme carditis is one of the more dramatic examples of how disease can distort the PR interval. The bacterium responsible for Lyme disease can infiltrate the heart’s conduction system, and AV block is the hallmark presentation. The disease can push PR intervals well beyond normal, and the concerning part is how rapidly things can escalate. A PR interval of 300 ms or more is associated with a higher risk of progression to complete heart block, where no impulses get through to the ventricles at all.¹⁴

The good news is that Lyme carditis almost always resolves with antibiotic treatment. In one reported case, a patient whose PR interval had stretched to 252 ms saw it normalize within two weeks of intravenous antibiotics, with no residual block on follow-up monitoring.¹⁵ Recognizing Lyme as the culprit is important because the block is temporary, and implanting a permanent pacemaker in a young, otherwise healthy person who has a treatable infection would be a significant over-treatment.¹⁶

Higher-Degree AV Blocks and What They Look Like

The PR interval also helps classify the more advanced forms of AV block. In Mobitz Type I (Wenckebach), as mentioned in the athlete context, the PR interval progressively lengthens until a beat drops. In Mobitz Type II, the PR interval stays fixed from beat to beat, but then a P wave abruptly fails to conduct with no prior warning. An unchanged PR interval before and after the dropped beat is actually the defining feature of this type.¹⁷

This distinction is not academic. Mobitz Type I is usually located within the AV node itself and tends to be relatively benign, especially in young or athletic individuals. Mobitz Type II, by contrast, typically reflects disease below the AV node in the His bundle or bundle branches, carries a higher risk of progressing to complete heart block, and much more often requires a pacemaker. Interestingly, Mobitz Type II is frequently overdiagnosed. The source of confusion is usually that the PR lengthening in Wenckebach can be subtle enough to miss, making the block look like a fixed-PR dropout when it is not.

The PR Segment in Pericarditis

While the PR interval’s length reflects conduction speed, the PR segment’s vertical position on the tracing carries different information. When the pericardium (the sac surrounding the heart) is inflamed, the PR segment often dips below the baseline, a finding called PR depression. This is one of the more useful ECG signs for distinguishing pericarditis from a heart attack, because both conditions can cause ST-segment elevation, and the stakes of confusing them are high.

PR depression in any lead has high sensitivity for acute pericarditis (around 88 percent in one study), and when present in both limb and precordial leads, the predictive power becomes quite strong, with a positive predictive value above 96 percent for pericarditis over ST-elevation heart attack.¹⁸ PR-segment depression also turns up in people with clinically silent pericardial effusions, acting as an ECG marker of pericardial inflammation even when the person feels fine.¹⁹ This is a good example of why reading an ECG involves more than just measuring intervals. The shape and position of the waveforms relative to the baseline matter just as much as their timing.

Genetics of PR Interval Duration

Your baseline PR interval is not entirely determined by your immediate physiology. Genetic factors play a surprisingly large role. A major genome-wide association study identified 44 regions of the genome linked to PR interval variation, with 34 of those being newly discovered in people of European ancestry. The most influential region mapped to chromosome 3, where two cardiac sodium channel genes (SCN5A and SCN10A) alone harbored seven independent signals affecting PR duration.²⁰ The genes involved are enriched for ion channels, transcription factors, and cell-signaling proteins, which tracks with what we know about the biology of electrical conduction in the heart.

These findings also extend across populations. A study in African American cohorts replicated several of the known genetic associations and identified a novel locus, ITGA9, in a region already implicated in the expression of the SCN5A sodium channel gene.²¹ The practical implication is that two people of the same age, sex, heart rate, and fitness level can have meaningfully different PR intervals purely because of inherited variation in these conduction genes. This partly explains why some people have a naturally long PR interval with no clinical consequence, while others develop conduction disease. The genetic architecture suggests that PR interval is a highly polygenic trait, shaped by many small contributions from dozens of genes rather than one or two large-effect variants.

Measuring the PR Interval Accurately

In clinical practice, the PR interval is measured from a standard 12-lead ECG, and most modern machines calculate it automatically. But automated measurements are not all created equal. A comparison of seven different ECG interpretation programs found small but consistent biases between them, with the largest and smallest readings for PR interval differing by about 5 milliseconds on average. Most of the time this is clinically irrelevant, but at the extremes of measurement the programs showed substantial divergence, with some producing clinically significant errors about twice as often as others.²²

Consumer wearable devices are entering this territory too. A validation study comparing the Apple Watch single-lead ECG against the corresponding lead from a standard 12-lead ECG found that PR interval measurements showed strong agreement, with 83 percent of comparisons meeting the threshold for strong concordance. That was actually the best performance of any interval the watch measured.²³ A watch ECG is still just one lead rather than twelve, so it cannot substitute for a full clinical ECG in borderline or complex cases. But the fact that a wrist-worn device can reasonably approximate PR interval measurement is a notable development, especially for people being monitored for conditions where PR changes over time carry clinical weight.

Drugs and Other Factors That Shift the PR Interval

Plenty of medications affect AV nodal conduction and, by extension, the PR interval. Beta-blockers and calcium channel blockers (particularly the non-dihydropyridine types like verapamil and diltiazem) slow conduction through the AV node and can lengthen the PR interval. Digoxin does the same, primarily by increasing vagal tone. Antiarrhythmic drugs like amiodarone or flecainide can push the PR interval upward as well. In a study of clonidine given as oral premedication before surgery, nearly half of patients showed PR interval lengthening, though only two out of fifty crossed the 200 ms threshold.²

Electrolyte disturbances can also affect PR duration. Elevated potassium levels can slow AV conduction, as can severe magnesium imbalances. Hypothyroidism, hypothermia, and increased intracranial pressure are less common but recognized causes of PR prolongation. In each of these scenarios, the PR interval is functioning as a real-time gauge of how well the AV conduction system is performing under changing physiologic or pharmacologic conditions. When a clinician checks the PR interval on a serial ECG, they are often looking for exactly this kind of trend: is conduction getting worse, stable, or recovering?

PR Interval Variation as a Predictor

Beyond the absolute value of the PR interval, researchers have found that how much the PR interval bounces around from beat to beat can carry its own diagnostic information. Because the AV node is heavily influenced by the autonomic nervous system, and because autonomic fluctuations tend to be more pronounced in people prone to atrial fibrillation, variation in the PR interval has been explored as an early marker. In patients with frequent premature atrial contractions, greater PR interval variability was associated with a higher likelihood of developing new-onset atrial fibrillation.³ This is still an area of active research, and PR variability has not entered routine clinical screening. But it highlights how the PR interval encodes more than just a snapshot of conduction time. Treated as a dynamic signal that changes breath to breath and beat to beat, it can reveal information about the autonomic nervous system’s grip on the heart.

1
PubMed Central. PR interval behavior during exercise stress test
2
Anaesthesia, Pain & Intensive Care. Effect of oral clonidine premedication on PR interval in patients undergoing rhinoplasty and functional endoscopic sinus surgery (FESS)
3
PubMed Central. Electrical PR Interval Variation Predicts New Occurrence of Atrial Fibrillation in Patients With Frequent Premature Atrial Contractions
4
PubMed Central. Pediatric Electrocardiogram in Preparticipation Screening: Narrative Review of Normal Values in Key Features
5
PubMed. Electrocardiogram Standards for Children and Young Adults Using Z-Scores
6
Nigerian Journal of Physiological Sciences. Mean Values, Normal Limits and Sex Differences of Heart Rate, QT Interval, QTc, PR Interval and QRS Duration among Young Adults of Yoruba Ethnicity
7
Scientific Reports. Physiologic heart rate dependency of the PQ interval and its sex differences
8
JAMA. Long-term Outcomes in Individuals With Prolonged PR Interval or First-Degree Atrioventricular Block
9
PubMed. Risk of atrial fibrillation as a function of the electrocardiographic PR interval: results from the Copenhagen ECG Study
10
PubMed Central. Prolonged PR interval and incidence of atrial fibrillation, heart failure admissions, and mortality in patients with implanted cardiac devices: A real-world survey
11
PubMed. The electrocardiogram and the athlete
12
PubMed Central. Hide and Seek: Intermittent Preexcitation Wolff-Parkinson-White Syndrome Case Report and Management Overview
13
PubMed. Characteristics of atrio-His conduction in the short PR interval, normal QRS complex syndrome. Evidence for enhanced slow-pathway conduction
14
HeartRhythm Case Reports. Lyme carditis presenting with an incessant atrioventricular nodal reentrant tachycardia masking a variable atrioventricular block
15
Heart Rhythm Case Reports. Accelerated Junctional Rhythm in Lyme Carditis: A Case Report and Literature Review
16
PubMed. Diagnosis and Treatment of Lyme Carditis: JACC Review Topic of the Week
17
PubMed Central. Mobitz type II second-degree atrioventricular block: a commonly overdiagnosed and misinterpreted arrhythmia
18
PubMed Central. PR depression is useful in the differential diagnosis of myopericarditis and ST elevation myocardial infarction
19
PubMed. Clinical correlates of PR-segment depression in asymptomatic patients with pericardial effusion
20
Nature Communications. PR interval genome-wide association meta-analysis identifies 50 loci associated with atrial and atrioventricular electrical activity
21
PubMed Central. Novel loci associated with PR interval in a genome-wide association study of 10 African American cohorts
22
PubMed. Comparison of PR, QRS, and QT interval measurements by seven ECG interpretation programs
23
PubMed Central. A comparison of manual electrocardiographic interval and waveform analysis in lead 1 of 12-lead ECG and Apple Watch ECG: A validation study