Short PR interval
Synonyms and keywords: Shortened PR interval

# Overview
A short PR interval is a term in cardiology that connotes a shortened time for the pacemaker in the atrium to conduct an impulse and activate the ventricle (the larger pumping chamber of the heart).  While it normally takes 0.12 to 0.21 seconds for the impulse to pass from the atrium to the ventricle (the normal PR interval), a short PR interval is defined as a PR interval of less than 0.12 seconds.  Although a short PR interval may be a normal variant, it is also associated with the presence of an accessory bypass tract (e.g. WPW syndrome and LGL syndrome), and close proximity of the atrial impulse to the AV node such as occurs in a premature atrial beat.  In AV dissociation, the atrial impulse does not conduct to the ventricle, but the atrium and the ventricle may separately beat at similar rates and the PR interval may vary from beat to beat.  In some beats, the PR interval may be quite short.

# Pathophysiology

## Atrial Premature Beats
Atrial premature beats, also known as atrial premature contractions or premature atrial contractions may emanate from an impulse that originates low in the atrium in close proximity to the AV node which may shorten the time for the impulse to be conducted.  Premature atrial beats which arise close to the AV node (low atrial ectopics) may activate the atria in a retrograde fashion which produces an inverted P wave with a relatively short PR interval.

## Bypass Tracts
The two subsets of preexcitation syndrome, Lown-Ganong-Levine syndrome and Wolff-Parkinson-White syndrome are associated with short PR interval due to bypass tracts that bypass the AV node, directly connecting the atria with the ventricle.
- The possible underlying pathophysiology for short PR interval in LGL syndrome can be either faster AV nodal conduction due to the rapidly conducting fibers within the AV node, or rapid conduction through Brechenmacher fibers that bypass the AV node connecting the atria with the bundle of His or the conduction through the accessory pathway James fibers that connect atria with low AV node.
- Short PR interval in WPW syndrome results from an accessory pathway, the bundle of Kent, that directly connects the atria to the ventricles bypassing the AV node.
- The difference between WPW and LGL syndrome is that LGL syndrome has a normal QRS complex following ventricular activation via the normal conduction pathway and WPW syndrome has a wide QRS complex due to the combined early ventricular activation via the abnormal accessory pathway and terminal ventricular activation via the normal conduction system.

## Junctional Rhythms
In AV junctional rhythms with retrograde atrial activation the retrograde P waves occur before the QRS complex shortening the PR interval.  Negative P waves in leads II, III and aVF point towards this diagnosis.

## Pseudo-Short PR Interval in Atrioventricular Dissociation
In Atrioventricular dissociation, the atrial impulse does not conduct to the ventricle, but the atrium and the ventricle may separately beat at similar or different rates and the PR interval may vary from beat to beat.  In some beats, the PR interval may be quite short.  Although this may appear to be a sinus rhythm with a short PR interval, there is no true impulse conduction between the atrium and ventricle, and hence the name "pseudo short PR interval".

# Causes

## Life Threatening Causes
Life-threatening causes include conditions which may result in death or permanent disability within 24 hours if left untreated.
- Acute respiratory failure
- Diabetic ketoacidosis
- Digitalis toxicity
- Myocardial infarction
- Pulmonary embolism

## Common Causes
- Premature atrial contractions

## Causes by Organ System

## Causes in Alphabetical Order