PR Interval & Pre-excitation Calculator

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.

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.

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.

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.

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.

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.

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.

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.