Long PR interval
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Vendhan Ramanujam M.B.B.S [2]
Synonyms and keywords: Long PR, prolonged PR, prolonged PR interval, PR prolongation, PR interval lengthened
Overview
Long PR interval is prolongation of the time required by impulse generated at the normal atrial pacemaker (SA node) to conduct via the normal conduction pathway and activate the ventricle. Normally the conduction time is 0.12 to 0.21 seconds (normal PR interval). A PR interval above 0.21 seconds is called as long PR interval. PR interval is usually longer when the conduction of atrial impulse through AV node is slowed either as a consequence of withdrawal of sympathetic tone or due to an increase in vagal inputs. It is also important to emphasize that long PR intervals can exist in subjects who are otherwise normal.[1] In atrioventricular dissociation where both atria and ventricle beat independently without conducting each others impulse either antegradely or retrogradely, PR interval may vary from beat to beat. In some beats, the PR interval may be long.
Pathophysiology
First Degree AV Block
First degree AV block may be due to conduction delay in the AV node, His-Purkinje system (made up by the bundle of His and the Purkinje fibers), or a combination of the two. The majority of cases are due to a dysfuction of the AV node; however, when first degree heart block coexists with a bundle-branch block, the cause is more likely to be a conduction delay in the His-Purkinje system. Although there are no missed beats in first degree AV block with all impulses from atria being transmitted to ventricle and thus resulting in a regular ventricular beat rhythm, the conduction delay involving AV node alone typically causes a long PR interval. If the conduction delay also involves the His-Purkinje system, the long PR interval will be associated with wide QRS complex.
Trifascicular block
Mobitz Type I Second Degree AV Block
The classic site of block in Mobitz type I second degree block is the AV node (70%-75%). In the remaining 25%-30% of the cases the site is infra-nodal (His bundle, bundle branches or fascicles). Mobitz type I is composed of two variations, classic and atypical, based on periodicity. Classic variety usually occurs within the AV node. There is a gradually increasing PR interval and eventually a dropped beat. It is also accompanied by a gradually decreasing R-R interval. The PR interval is usually shortest in the initial beat and gradually increases ending in a dropped beat and the cycle repeats. This classic Wenckebach phenomenon occurs usually with ratios of 3:2, 4:3 or 5:4. This results in grouped beating. In Atypical variant of Wenckebach pattern the conduction ratios usually exceed 6:5 or 7:6 and the last few beats of the cycle, before a dropped beat, show a relatively constant PR interval (maximum variation of 0.02 sec among them). The beats after the dropped beat again show gradually prolonging PR intervals.[2]
Atrial Premature Beats
In atrial premature beats, also known as atrial premature contractions or premature atrial contractions, with increasing prematurity, the partial penetration of the premature beat (concealed) prolongs the refractory period of the AV node and causes prolongation of the PR interval of the next sinus beat. The PR interval progressively increases until the effective refractory period of AV node is reached, when the atrial premature beats get blocked within the node and nonconducted.[3]
Pseudo-Long 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 long. Although this may appear to be a sinus rhythm with a long PR interval, there is no true impulse conduction between the atrium and ventricle, and hence the name "pseudo long PR interval".
References
- ↑ PACKARD, JM.; GRAETTINGER, JS.; GRAYBIEL, A. (1954). "Analysis of the electrocardiograms obtained from 1000 young healthy aviators; ten year follow-up". Circulation. 10 (3): 384–400. PMID 13190611. Unknown parameter
|month=ignored (help) - ↑ El-Sherif N, Aranda J, Befeler B, Lazzara R (1978). "Atypical Wenckebach periodicity simulating Mobitz II AV block". Br Heart J. 40 (12): 1376–83. PMC 483582. PMID 737095. Unknown parameter
|month=ignored (help) - ↑ Damato, AN.; Lau, SH. (1971). "Concealed and supernormal atrioventricular conduction". Circulation. 43 (6): 967–70. PMID 4102931. Unknown parameter
|month=ignored (help)