Clinical Overview

A shortened PR interval, also written short PR interval or, when it appears without a delta wave or QRS widening, isolated short PR interval, is a PR interval — the time from the onset of the P wave to the start of the QRS complex — that measures under 120 ms (0.12 s) (LITFL, PR Interval, reviewed 2021; Healio, ECG Interpretation Tutorial, PR Interval; Kashou et al., StatPearls, P Wave, 2023). Like a prolonged PR interval, this describes a single ECG measurement, not a rhythm and not by itself an anatomic diagnosis: something is letting the impulse reach the ventricles faster than the AV node’s normal delay allows. Two broad substrates produce it. One is an accessory pathway that physically bypasses the AV node — the same substrate behind Wolff-Parkinson-White (WPW) syndrome and this dataset’s separate Ventricular Preexcitation label — but that substrate also produces a delta wave and a widened QRS complex; the moment either of those appears alongside a short PR interval, the correct read is preexcitation, not this finding. The other substrate leaves the QRS normal and narrow: either physiologically accelerated conduction through an otherwise normal AV node, or a pacemaker sited low in the atrium or in the AV junction itself, close enough to the ventricles that the resulting PR segment is inherently short (LITFL, reviewed 2021; ECG Learning Center, University of Utah, Junctional Rhythm). This page covers that second substrate — a short PR interval with a normal, narrow QRS and no delta wave.

An older eponym, Lown-Ganong-Levine (LGL) syndrome, describes a short PR interval with a normal QRS, no delta wave, and recurrent paroxysmal supraventricular tachycardia, historically attributed to an accessory “bundle of James” bypassing the AV node. Current sourcing calls this an outdated clinical diagnosis with no single confirmed anatomic correlate — the James-fiber pathway cannot be consistently demonstrated, and the pattern may instead reflect several different underlying mechanisms (StatPearls, Bhutta et al., Lown Ganong Levine Syndrome, archived 2022). [CLINICAL REVIEW NEEDED: because LGL’s underlying mechanism is unsettled in the literature, a resting ECG showing a short PR interval, normal QRS, and no delta wave cannot by itself distinguish a true accessory-pathway variant from accelerated AV-nodal conduction — the strip alone does not resolve which substrate is present.]

Mechanistically, the AV node’s conduction speed is itself rate-dependent: rising sympathetic tone or falling vagal tone speeds nodal conduction and shortens the PR interval, which is why a faster sinus rate alone can push the interval toward the lower end of normal or just under it (wikidoc, PR Interval; ScienceDirect Topics, Atrioventricular Conduction — a stable, textbook AV-node physiology point confirmed across both). A pacemaker arising low in the atrium or at the AV junction shortens the interval by a different route — the impulse simply has less distance to travel before reaching the AV node or His bundle, and a junctional pacemaker in particular can produce a retrograde P wave immediately before the QRS with an inherently short PR interval (ECG Learning Center, University of Utah, Junctional Rhythm).

For most patients, an isolated short PR interval carries no clinical significance beyond confirming the QRS and delta wave are normal. It is a common finding in children and adolescents, including competitive athletes: a 2026 cross-sectional study of 1,310 athletes aged 7-18 found a short PR interval (under 120 ms) in 26.9% using a fixed cutoff, dropping to 3.82% when an age- and sex-adjusted threshold was used instead, with no significant association to syncope, palpitations, chest pain, or exercise-induced arrhythmia — supporting its interpretation as a benign, age-dependent variant, though the authors note long-term outcome data are still needed (Komici et al., Sports Medicine - Open, 2026). A large 2025 nationwide study of over 2.2 million adult ECGs found a short PR interval in 2.9% of patients and reported it independently associated with a modestly higher risk of atrial fibrillation/flutter and a more substantial rise in all-cause mortality, alongside a lower rate of high-degree AV block or pacemaker need and no association with syncope (European Heart Journal, nationwide study, 2025). [CLINICAL REVIEW NEEDED: that 2025 cohort is the first of its scale to link an isolated short PR interval to mortality risk; the authors themselves state no clinical recommendations yet exist for this finding, so the association should be read as an open research question rather than an actionable risk marker.]

Symptoms track the underlying substrate rather than the interval itself: a purely physiologic short PR interval or a stable junctional/low-atrial rhythm is typically asymptomatic and found incidentally. When symptoms do occur — palpitations, lightheadedness, or a sensation of a racing heart — they generally signal that the underlying rhythm has changed to something faster than a simple short interval, most often a tachyarrhythmia, which by definition is no longer this finding alone.

Causes split along the same substrate lines as the mechanism above: elevated sympathetic tone from exercise, fever, anxiety, or stimulant use speeds a structurally normal AV node, and this shortening is a normal physiologic response, not disease; a low-atrial or AV-junctional pacemaker taking over as the dominant rhythm shortens the interval by anatomic proximity rather than nodal speed; and age itself plays a role, since a short PR interval is more prevalent in children and adolescents than in adults, likely reflecting a smaller heart with correspondingly less conduction distance and a still-maturing autonomic balance (Komici et al., 2026; LITFL, reviewed 2021).

Interpretation Guide

Key Features:

• Rate: not itself altered by this finding — reflects whatever the underlying rhythm happens to be; in this dataset’s matching records, sinus bradycardia and sinus tachycardia are both common contexts, more so than a plain normal sinus rate, with occasional atrial tachycardia and junctional tachycardia also appearing (search-index.json record data, this dataset)
• Rhythm: not a defining feature — the finding describes an interval, not rhythm origin or regularity; regular conduction is typical unless a coexisting arrhythmia is present
• P waves: normal upright morphology when the finding reflects accelerated sinus conduction; an inverted or retrograde P wave immediately preceding the QRS, best seen in leads II, III, and aVF, points instead to a low-atrial or AV-junctional origin
• PR interval: the defining feature — measured under 120 ms (0.12 s)
• QRS complex: normal and narrow (under 120 ms), with no slurred, slowly rising upstroke; a delta wave or QRS widening on top of a short PR interval reclassifies the finding as preexcitation, not this measurement
• ST segment: not a primary diagnostic feature; normal unless a coexisting condition is present
• T waves: not a primary diagnostic feature; normal unless a coexisting condition is present
• QT interval: not a primary diagnostic feature of this finding
• Other findings: confirm the QRS stays narrow and no delta wave is present on any lead before accepting this as an isolated short PR interval — even a subtle slurred upstroke moves the read to Wolff-Parkinson-White or ventricular preexcitation instead

Reading it correctly hinges on the same distinction the mechanism above sets up: a short PR interval with a normal, narrow QRS and no delta wave is this finding; the identical short interval with a delta wave and a widened QRS is preexcitation, a different finding with a different risk profile and treatment path.

Key Leads

• Lead II – Best single lead for the PR-interval measurement itself, and where an inverted or retrograde P wave from a low-atrial or junctional origin is most reliably seen.
• Lead V1 – Confirms the QRS stays genuinely narrow and screens specifically for a subtle slurred upstroke that would move the finding to preexcitation instead of a simple short interval.

Differential Diagnosis

• Wolff-Parkinson-White Syndrome (WPW) — the same short PR interval plus a delta wave and widened QRS from an accessory pathway, together with a documented symptomatic tachyarrhythmia; a strip with a short PR and a normal, narrow QRS belongs here instead, while a slurred QRS upstroke on top of the short PR moves it to WPW.
• Ventricular Preexcitation (VPE) — this dataset label marks the identical short-PR-plus-delta-wave ECG pattern on its own, without reference to any tachyarrhythmia; the same QRS-widening and delta-wave clue that separates this finding from WPW separates it from VPE as well.
• Atrioventricular Reentrant Tachycardia (AVRT) — the reentrant tachycardia an accessory pathway can produce; it presents as an episodic fast, regular rhythm (commonly 150-250 bpm) rather than a resting-ECG interval finding, and during orthodromic conduction the P wave follows the QRS instead of preceding it, so the PR interval is not even the relevant measurement during the episode itself.
• Junctional Escape Rhythm (AVJR) — a junctional-origin short PR interval, but specifically at an escape rate slower than the sinus node it is rescuing, typically 40-60 bpm; this finding’s own matching records more often run at a normal-to-fast rate than a slow escape rate, so bradycardic pacing with a short PR interval points toward the junctional escape rhythm rather than this label alone.
• Prolonged PR Interval (PRIE) — the mirror-image measurement: a PR interval over 200 ms rather than under 120 ms, most often from AV-nodal delay rather than accelerated conduction or an accessory pathway.

Treatment Brief

An isolated shortened PR interval itself needs no direct treatment in the large majority of cases — it is a conduction-timing finding, not a disease, and in children and athletes it is usually a benign, age-related variant.

• Confirm the QRS stays narrow and screen every lead for a delta wave before accepting the finding as an isolated short PR interval — a delta wave or QRS widening changes this to preexcitation, which carries its own risk profile and treatment approach on this product’s WPW page.
• Check P-wave morphology and axis: a normal, upright P wave supports accelerated sinus conduction, while an inverted or retrograde P wave immediately before the QRS, especially in the inferior leads, points to a low-atrial or AV-junctional origin instead.
• Re-measure on a longer strip or full 12-lead before accepting the number — calibration drift and lead misplacement both distort the PR-interval measurement.
• A stable, isolated finding in an asymptomatic patient with a normal QRS and no delta wave is managed with routine follow-up rather than active intervention; cohort data in children and athletes has not linked the isolated finding to adverse outcomes needing treatment (Komici et al., 2026).
• Escalate to the provider when a delta wave or QRS widening appears on a later strip, when the patient reports palpitations, lightheadedness, or syncope suggestive of a tachyarrhythmia, or when the underlying rhythm shifts from sinus to a sustained junctional or ectopic atrial rhythm.