PR Interval Calculator — ECG Interpretation
Enter PR interval measured from ECG. Normal range: 120–200 ms (3–5 small squares at 25mm/s).
Short
<120ms
Нормален
120–200ms
Prolonged
>200ms
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The Lown-Ganong-Levine (LGL) syndrome is a cardiac pre-excitation syndrome characterised by a short PR interval (less than 120 ms) on the 12-lead ECG combined with a normal QRS complex duration (less than 120 ms) and the presence of tachyarrhythmias — most commonly supraventricular tachycardia (SVT) or atrial fibrillation. It was first described in 1952 by Bernard Lown, William Ganong, and Samuel Levine, who identified patients presenting with syncope, palpitations, and documented SVT in the setting of a short PR interval and normal QRS morphology. The PR interval, measured from the onset of the P wave to the onset of the QRS complex, represents the time taken for electrical conduction from the sinoatrial (SA) node through the atria, across the atrioventricular (AV) node, and into the His-Purkinje system. Normal PR interval is 120 to 200 ms (equivalent to 3 to 5 small squares on a standard ECG recorded at 25 mm/s with 1 mm = 40 ms). A PR interval shorter than 120 ms implies accelerated conduction bypassing part or all of the normal delay imposed by the AV node. In LGL syndrome, the proposed mechanism involves accessory fibres called James fibres — atrionodal bypass tracts that connect the atrium directly to the lower AV node or bundle of His, bypassing the slow-conducting upper portion of the AV node. This shortens the PR interval without altering ventricular depolarisation, which proceeds normally through the His-Purkinje system, hence the QRS remains narrow. This distinguishes LGL from Wolff-Parkinson-White (WPW) syndrome, where a different accessory pathway (Bundle of Kent) directly connects atrium to ventricle, producing the characteristic delta wave and wide QRS. Clinically, the most dangerous implication of a short PR interval arises in atrial fibrillation — when the AV node's natural rate-limiting function is bypassed, rapid atrial impulses can be conducted to the ventricles at dangerously fast rates, potentially causing haemodynamic compromise or degenerating into ventricular fibrillation.
PR Interval (ms) = Time from start of P wave to start of QRS complex on ECG Normal range: 120–200 ms (3–5 small squares at standard ECG speed 25 mm/s) - Each small square = 1 mm = 40 ms at 25 mm/s - Each large square = 5 mm = 200 ms at 25 mm/s Short PR: < 120 ms (< 3 small squares) Prolonged PR: > 200 ms (> 5 small squares = first-degree AV block) Rate-corrected PR interval (Holzmann formula): PRc = PR (ms) / √(RR interval in seconds) Normal PRc: 99–140 ms (approximate) LGL Diagnostic Triad: 1. PR interval < 120 ms 2. Normal QRS duration (< 120 ms) — no delta wave 3. Symptomatic tachyarrhythmia (SVT, palpitations, syncope)
- 1Obtain a standard 12-lead ECG at 25 mm/s paper speed. Identify the P wave onset (start of the first deflection of atrial depolarisation) and the QRS onset (first deflection of ventricular depolarisation). Measure the PR interval in lead II or whichever lead shows the clearest P wave onset.
- 2Measure PR interval: count the number of small squares between P wave onset and QRS onset. Multiply by 40 ms per small square. A value under 3 small squares (< 120 ms) is defined as a short PR interval and warrants investigation for a pre-excitation syndrome.
- 3Examine the QRS complex morphology and duration. In LGL syndrome, the QRS is narrow (< 120 ms) with no delta wave — a slurred upstroke at the beginning of the QRS. The presence of a delta wave with wide QRS indicates WPW syndrome (Bundle of Kent pathway), a different and clinically distinct pre-excitation syndrome.
- 4Assess the clinical context: LGL syndrome requires the combination of short PR + normal QRS + documented or symptomatic tachyarrhythmia. An isolated short PR in an asymptomatic patient without tachyarrhythmia may represent a normal variant or enhanced AV nodal conduction rather than true LGL syndrome.
- 5If atrial fibrillation co-exists with a short PR interval, assess the ventricular rate carefully. Rapid ventricular rates (> 200 bpm) during AFib with a short PR suggest accessory pathway conduction and represent a medical emergency — AV nodal blocking agents (digoxin, verapamil, adenosine) are contraindicated as they may increase accessory pathway conduction and precipitate ventricular fibrillation.
- 6Apply the rate-corrected PR interval if the patient's heart rate is significantly elevated, as tachycardia shortens PR interval physiologically. The corrected PR (PRc = PR / √RR) adjusts for this effect, similar to the QTc correction principle.
- 7Refer symptomatic patients for electrophysiological study (EPS) to confirm the presence of an accessory pathway, characterise its refractory period, and assess suitability for catheter ablation — which is curative in experienced centres with success rates exceeding 95%.
Refer for electrophysiology study
The PR of 100 ms is 2.5 small squares — clearly short. The QRS is narrow at 88 ms with no delta wave, distinguishing this from WPW. The symptomatic SVT completes the LGL triad. EPS is recommended to characterise the James fibre pathway and consider radiofrequency ablation.
WPW — different accessory pathway (Bundle of Kent)
The presence of a delta wave (slurred initial QRS upstroke) and prolonged QRS (140 ms) distinguishes WPW from LGL. In WPW, the Bundle of Kent directly connects atrium to ventricle, causing pre-excitation of ventricular muscle before His-Purkinje activation — producing the delta wave. Management principles overlap but EPS mapping will differ.
AVOID digoxin, verapamil, adenosine — may precipitate VF
A ventricular rate of 240 bpm during AFib strongly suggests bypass tract conduction circumventing AV nodal rate-limiting. This is haemodynamically dangerous and can deteriorate into ventricular fibrillation. Procainamide or electrical cardioversion are appropriate acute treatments. AV nodal blocking agents are contraindicated. Urgent EPS and ablation are indicated.
No treatment required; annual review reasonable
LGL syndrome requires all three elements of the triad: short PR, normal QRS, AND symptomatic tachyarrhythmia. An isolated short PR without documented arrhythmia or symptoms may reflect enhanced AV nodal conduction, a small atrium, or a normal physiological variant. Athlete's hearts often have enhanced vagal tone that paradoxically slows conduction but athletes may also have enhanced nodal conduction. No treatment is warranted; document and monitor annually.
Emergency department ECG interpretation — identifying pre-excitation syndromes before administering rate-control medications for tachyarrhythmias
Cardiology outpatient clinics — risk stratification and shared decision-making for patients with incidentally found short PR intervals
Electrophysiology laboratories — guiding catheter ablation procedures targeting accessory pathways confirmed on EPS
Paediatric cardiology — evaluating school-age children and athletes with syncope or palpitations where pre-excitation is on the differential diagnosis
Pre-operative ECG screening — identifying patients with pre-excitation syndromes before anaesthesia where arrhythmia risk must be anticipated and anaesthetic agents selected accordingly
LGL in atrial fibrillation — medical emergency
When a patient with LGL syndrome develops atrial fibrillation, the bypass tract (James fibres) can conduct rapid atrial impulses to the ventricles at rates exceeding 200–250 bpm, bypassing AV nodal protection. This presents as a wide-complex tachycardia (if aberrant conduction occurs) or narrow-complex very rapid tachycardia. AV nodal blocking agents (digoxin, verapamil, adenosine) are absolutely contraindicated as they may accelerate accessory pathway conduction and provoke ventricular fibrillation. Treatment options include intravenous procainamide or immediate electrical cardioversion.
Distinguishing LGL from enhanced AV nodal conduction
Not all short PR intervals represent a true structural accessory pathway. Enhanced AV nodal conduction — a normal variant of fast-conducting nodal tissue — can produce PR intervals below 120 ms without any true bypass tract. During EPS, true LGL via James fibres shows a short atrial-His interval (AH < 60 ms) without a pathway-specific potential, while enhanced nodal conduction shows a short AH due to fast-pathway dominance. This distinction has therapeutic implications as ablation targets differ.
Paediatric short PR — special considerations
Children have naturally shorter PR intervals than adults due to smaller atrial and AV nodal dimensions. Age-specific normal ranges apply: neonates may have PR intervals of 80–100 ms; infants 80–120 ms; young children 100–140 ms. A PR below the 2nd percentile for age with symptoms warrants paediatric cardiology evaluation. Pre-excitation syndromes in children with congenital heart disease (especially Ebstein anomaly, which has a strong association with WPW) carry additional risk.
Pregnancy and pre-excitation syndromes
Pregnancy increases susceptibility to supraventricular tachycardias due to haemodynamic changes, increased sympathetic tone, and hormonal shifts. Women with known LGL or WPW syndrome who become pregnant should have a risk assessment before conception if possible. Acute SVT in pregnancy can usually be managed with vagal manoeuvres or adenosine (safe in pregnancy). Flecainide and sotalol are generally avoided in the first trimester. If ablation is indicated, it can be performed with electrophysiology guidance and minimal fluoroscopy after the first trimester.
LGL and sudden cardiac death risk
The risk of sudden cardiac death in LGL syndrome is considered lower than in WPW syndrome because James fibres generally have longer refractory periods than Kent bundles. However, patients with very rapid accessory pathway conduction (shortest RR interval during AFib < 250 ms on EPS) are at higher risk and should be considered for ablation. Risk stratification with EPS is recommended for all symptomatic patients, particularly those with syncope, very rapid documented rates, or a family history of sudden cardiac death in the context of pre-excitation.
| PR Interval | Duration | ECG Squares | Interpretation |
|---|---|---|---|
| Short | < 120 ms | < 3 small | Pre-excitation: LGL (narrow QRS) or WPW (delta wave + wide QRS) |
| Normal | 120–200 ms | 3–5 small | Normal AV conduction — sinus rhythm |
| Borderline long | 200 ms | = 5 small | Upper limit of normal — reassess clinically |
| First-degree AV block | > 200 ms | > 5 small | Delayed AV conduction — often benign; monitor |
| Second-degree (Mobitz I) | Progressively lengthens | Variable | Wenckebach — AV node disease; cardiology review |
| Second-degree (Mobitz II) | Constant then dropped beat | Variable | Infranodal block — higher risk; consider pacing |
| Third-degree (complete) | Dissociated — no fixed PR | N/A | Complete heart block — emergency; pacemaker required |
What is the normal PR interval on an ECG?
The normal PR interval is 120 to 200 milliseconds, corresponding to 3 to 5 small squares on a standard ECG recorded at 25 mm/s (where each small square equals 40 ms). A PR shorter than 120 ms (under 3 small squares) is defined as short and may indicate pre-excitation or accelerated AV nodal conduction. A PR exceeding 200 ms (more than 5 small squares) is classified as first-degree AV block.
What causes a short PR interval?
A short PR interval can result from several mechanisms: (1) Accessory bypass tracts — James fibres in LGL syndrome (atrionodal connections) or the Bundle of Kent in WPW syndrome (atrioventricular connections); (2) Enhanced AV nodal conduction — a physiological variant where the AV node conducts faster than average, seen in some athletes and young individuals; (3) Junctional rhythms — where pacemaker activity originates in or near the AV node, with retrograde P waves appearing close to the QRS; (4) Wolff-Parkinson-White syndrome — short PR with delta wave and wide QRS.
How does LGL differ from WPW syndrome?
Both LGL and WPW involve short PR intervals, but the key distinction lies in the QRS: in LGL (James fibre pathway), the QRS is narrow and normal with no delta wave because ventricular activation proceeds normally via the His-Purkinje system. In WPW (Bundle of Kent), the accessory pathway directly connects atrium to ventricle, causing early ventricular pre-excitation — producing a delta wave (slurred QRS upstroke) and widened QRS. LGL is also less definitively characterised than WPW, and some electrophysiologists debate whether pure James fibre LGL is a distinct entity or a heterogeneous group of rapidly conducting AV nodal variants.
Why is AFib dangerous in patients with short PR or pre-excitation?
In normal physiology, the AV node acts as a gatekeeper, limiting the ventricular rate during atrial fibrillation to around 100–180 bpm by imposing physiological conduction delay and refractoriness. In pre-excitation syndromes, accessory pathways bypass this protection, allowing rapid atrial impulses (400–600 bpm in AFib) to conduct to the ventricles at very high rates — sometimes exceeding 200–300 bpm. This can cause haemodynamic collapse and potentially degenerate into ventricular fibrillation. Rates above 250 bpm during AFib in the setting of pre-excitation constitute a medical emergency.
Which drugs are contraindicated in LGL or WPW with AFib?
AV nodal blocking agents — digoxin, verapamil, diltiazem, and adenosine — are contraindicated in pre-excitation syndromes with AFib. These drugs block the AV node preferentially, which paradoxically increases conduction through the accessory pathway by reducing refractoriness, leading to faster ventricular rates and potentially triggering ventricular fibrillation. Safe options for acute rate or rhythm control in this setting include procainamide (which slows accessory pathway conduction) and electrical cardioversion. Beta-blockers are also generally avoided in haemodynamically significant pre-excited AFib.
What is the treatment for LGL syndrome?
Asymptomatic patients with isolated short PR and no documented arrhythmia generally require no treatment. Symptomatic patients with documented SVT or AFib should undergo electrophysiological study (EPS). Catheter radiofrequency ablation of the responsible pathway is the definitive treatment with high cure rates (>95% in most centres) and low complication rates. For patients who are not candidates for or decline ablation, antiarrhythmic medications (flecainide, propafenone, or amiodarone) may be used to suppress tachyarrhythmias under specialist guidance.
What is the rate-corrected PR interval?
The PR interval shortens physiologically at higher heart rates because faster sinus rates naturally accelerate AV nodal conduction. To correct for this, a rate-corrected PR (PRc) can be calculated using the Holzmann formula: PRc = PR (ms) divided by the square root of the RR interval in seconds — analogous to the Bazett correction for QT interval. A PRc above 99 ms and below 140 ms is generally considered within normal limits. This correction is particularly useful when assessing ECGs from tachycardic patients where a borderline short PR might be rate-related rather than pathological.
Can athletes have a short PR interval normally?
Yes. Enhanced AV nodal conduction can occur as a physiological variant, sometimes seen in athletes with high sympathetic tone or individuals with naturally fast-conducting AV nodes. In the absence of symptoms (palpitations, syncope, SVT), documented arrhythmia, or QRS abnormalities, a short PR is often a benign finding requiring monitoring but no intervention. However, if an athlete has a short PR with episodes of rapid palpitations, syncope during exercise, or exertional SVT, full electrophysiological evaluation is warranted as the exercise-induced sympathetic surge can trigger life-threatening arrhythmias.
Pro Tip
When reading any ECG, always measure the PR interval systematically before reporting. A PR of less than 3 small squares (< 120 ms) should trigger an immediate check for delta waves and QRS width. If delta waves are absent and QRS is narrow, assess for LGL and document symptoms. If delta waves are present, diagnose WPW. Never give adenosine empirically to a rapid tachycardia before reviewing the ECG carefully for pre-excitation features — the stakes are high.
Did you know?
The LGL syndrome was described in 1952, but the actual existence of James fibres as a distinct anatomical entity has been debated for decades. Some electrophysiologists argue that what we call 'LGL' is actually a heterogeneous group of conditions, including enhanced AV nodal conduction and atrio-Hisian fibres, rather than a single anatomically defined pathway. Interestingly, one of the eponymous authors — Bernard Lown — was also the inventor of the DC defibrillator and the Lown classification for ventricular ectopy, making him one of the most influential cardiologists of the 20th century.
References
- ›Lown B, Ganong WF, Levine SA. The syndrome of short P-R interval, normal QRS complex and paroxysmal rapid heart action — Circulation 1952
- ›Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine, 12th Edition — Chapter on Pre-excitation Syndromes
- ›Page RL et al. 2015 ACC/AHA/HRS Guideline for the Management of Adult Patients With Supraventricular Tachycardia — JACC 2016
- ›Gollob MH et al. Identification of a gene responsible for familial Wolff-Parkinson-White syndrome — NEJM 2001
- ›Holzmann M, Scherf D. Uber Elektrokardiogramme mit verkuerzter Vorhof-Kammer-Distanz und positiven P-Zacken (original LGL mechanism description) — Ztschr Klin Med 1932