110 Interval PR

PR interval: normal 120–200 ms, first-degree and Wenckebach AV block, short PR and WPW pattern.

The PR interval (also called the PQ interval) is measured from the start of the P wave to the start of the QRS complex. It covers the whole journey of the impulse from the sinus node through the atria, the AV node and the His–Purkinje system to the ventricles. Most of the time is spent in the AV node, which deliberately slows conduction. The PR interval shortens slightly at faster heart rates and lengthens with age and high vagal tone.

ECGAV conductionHeart block

Normal adult120–200 ms

First-degree AV block> 200 ms

Short PR< 120 ms

At 25 mm/s3–5 small squares

Figure 1. The PR interval (highlighted) runs from the start of the P wave to the start of the QRS and includes the P wave and the PR segment.

Long PR interval

A PR interval longer than 200 ms with every P wave conducted is first-degree AV block. Causes include high vagal tone (athletes, sleep), medicines that slow AV conduction (beta-blockers, verapamil, diltiazem, digoxin, amiodarone), degenerative disease of the conduction system, inferior myocardial infarction, myocarditis, Lyme disease and electrolyte disorders. A PR that lengthens progressively until a beat is dropped is second-degree AV block, Mobitz type I (Wenckebach); when P waves and QRS complexes are completely unrelated, the block is complete (third-degree).

Short PR interval

A PR interval shorter than 120 ms occurs when the impulse bypasses the AV node through an accessory pathway. In Wolff–Parkinson–White (WPW) pattern, the short PR is followed by a slurred upstroke of the QRS (delta wave). People with WPW can develop fast tachycardias, and atrial fibrillation in WPW can be life-threatening. A short PR also occurs in rhythms that start low in the atria or in the AV junction.

Figure 2. Left: normal PR interval. Middle: first-degree AV block (PR over 200 ms). Right: WPW pattern, short PR with a delta wave at the start of the QRS.

When to see a doctor

Isolated mild PR prolongation without symptoms is usually benign, but fainting, dizziness, a very slow pulse or palpitations with a short PR and delta wave need prompt cardiological assessment.

References

Kusumoto FM, Schoenfeld MH, Barrett C, et al. 2018 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay. Circulation. 2019;140(8):e382–e482. doi:10.1161/CIR.0000000000000628
Page RL, Joglar JA, Caldwell MA, et al. 2015 ACC/AHA/HRS guideline for the management of adult patients with supraventricular tachycardia. Circulation. 2016;133(14):e506–e574. doi:10.1161/CIR.0000000000000311

This article is for general information and does not replace a consultation with a doctor. ECG findings must be interpreted by a physician together with symptoms and other tests.

Published on 8 May 2024