PR INTERVAL,CAUSES OF SHORT AND LONG PR INTERVAL

CAUSES OF LONG AND SHORT PR INTERVAL IN ECG

1. PR INTERVAL Dr.G.VENKATA RAMANA MBBS DNB FAMILY MEDICINE

2. PR INTERVAL OVERVIEW The time taken for the depolarization wave to pass from its origin in the SA node, across the atria, and through the AV node into ventricular muscle is called the PR interval N ormal PR interval 0.12-0.2s ( 3 to 5 small squares) PR interval > 0.2s , first degree heart block is said to be present PR interval < 0.12 s suggests pre-excitation (the presence of an accessory pathway between the atria and ventricles) or  AV nodal ( junctional ) rhythm

3. PR INTERVAL Look for the following Is the PR interval less than 0.12 s long? Is the PR interval more than 0.2 s long? Does the PR interval vary or can it not be measured? Is the PR segment elevated or depressed?

4. SHORT PR INTERVAL I ndicates that the usual delay to conduction between the atria and the ventricles, imposed by the AV junction, has not occurred This happens if depolarization originates in the AV junction, so that it travels up to the atria and down to the ventricles simultaneously, or if it originates as normal in the sinus node but bypasses the AV junction via an additional faster conducting pathway Short PR interval causes AV nodal rhythm Wolff–Parkinson–White (WPW) syndrome Lown – Ganong –Levine (LGL) syndrome A ccelerated AV nodal conduction

5. AV nodal rhythm If depolarization is initiated from within the AV node, the wave of atrial depolarization will travel backwards through the atria at the same time as setting off forwards through the AV node towards the ventricles Thus , the time delay between atrial depolarization (the P wave) and ventricular depolarization (the QRS complex) will be reduced Any source of depolarization within the AV node can therefore cause a short PR interval , including: AV nodal escape rhythms AV ectopics AV re-entry tachycardia

6. AV nodal ( junctional ) rhythm N arrow complex, regular rhythms arising from the AV node P waves are either absent or abnormal (e.g. inverted) with a short PR interval (=retrograde P waves). ECG: Accelerated junctional rhythm demonstrating inverted P waves with a short PR interval (retrograde P waves)

7. Wolff Parkinson White syndrome In most people, conduction of electricity through the heart follows just one distinct path from atria to ventricles, namely via the AV node, bundle of His and Purkinje fibres Some people have an additional connection between the atria and the ventricles– an accessory pathway – that conducts more quickly than the AV node , so the wave of depolarization reaches the ventricles more quickly than usual and thus the PR interval is short The region of ventricle activated via the accessory pathway slowly depolarizes, giving rise to a delta wave – the first part of the QRS complex Shortly afterwards, the rest of the ventricular muscle is depolarized rapidly with the arrival of the normally conducted wave of depolarization via the AV node

8. When a short PR interval and delta wave are seen on an ECG, this is called a Wolff– Parkinson–White (WPW) pattern In most cases, this is just an incidental finding and the individual has no problems with their heart rhythm However, in some cases the presence of an accessory pathway provides a substrate for episodes of AV re-entry tachycardia (AVRT), in which case the patient is said to have WPW syndrome

9. Wolff-Parkinson-White syndrome S hort PR interval (< 0.12 s), broad QRS and a slurred upstroke to the QRS complex, the delta wave

10. Lown Ganong Levine syndrome The ‘classical’ view of LGL syndrome is that, like in WPW syndrome, patients have an accessory pathway (sometimes called the bundle of James) which bypasses the AV node However, unlike the bundle of Kent in WPW syndrome, the accessory pathway in LGL syndrome does not activate the ventricular muscle directly Instead , it simply connects the atria to the bundle of His As a result, the AV node is bypassed (so the PR interval is short ) but there is no ventricular pre-excitation (and therefore there is no delta wave)

11. Short PR interval (in the absence of a delta wave) alone is not sufficient to diagnose LGL syndrome LGL syndrome is diagnosed when patients with a short PR interval (and no delta wave) present with re-entry tachycardias Those who do not experience re-entry tachycardias are instead considered to have a normal variant of AV conduction known as accelerated AV nodal conduction

12. Lown - Ganong -Levine syndrome V ery short PR interval with normal P waves and QRS complexes and absent delta waves

13. Accelerated AV nodal conduction The presence of a short PR interval in isolation, with no history of re-entry tachycardia, is regarded as a normal variant of AV node conduction (and should not be labelled as LGL syndrome )

14. PROLONGED PR INTERVAL Prolongation of the PR interval is a common finding and indicates that conduction through the AV node has been delayed When this delay is constant for each cardiac cycle , and each P wave is followed by a QRS complex, it is referred to as first-degree AV block

15. In first-degree AV block, conduction through the AV node is slower than usual and the PR interval is therefore prolonged

16. First-degree AV block is a common feature of vagally induced bradycardia , as an increase in vagal tone decreases AV nodal conduction It may also be a feature of: Ischaemic heart disease Hyperkalemia or hypokalemia A cute rheumatic myocarditis Lyme disease D rugs B eta blockers R ate-modifying calcium channel blockers Digoxin

17. DOES THE PR INTERVAL VARY OR CAN IT NOT BE MEASURED? Normally , the PR interval is constant In some conditions, however, the interval between P waves and QRS complexes changes, giving rise to a variable PR interval Sometimes a P wave is not followed by a QRS complex at all and so the PR interval cannot be measured If the PR interval gradually lengthens with each beat, until one P wave fails to produce a QRS complex, the patient has Mobitz type I AV block If the PR interval is fixed and normal, but occasionally a P wave fails to produce a QRS complex, the patient has Mobitz type II AV block If alternate P waves are not followed by QRS complexes, the patient has 2:1 AV block If there is no relationship between P waves and QRS complexes, the patient has third-degree (complete) AV block

18. When Mobitz type I AV block occurs at the level of the AV node it is generally regarded as ‘ benign ’,and a permanent pacemaker is not required unless the frequency of ‘dropped’ ventricular beats causes a symptomatic bradycardia When the block is infranodal ( as identified by electrophysiological testing) there is a stronger indication for pacing,even if patients are asymptomatic Mobitz type I AV block may require pacing prior to surgery

19. Mobitz type II AV block Characteristic features are: M ost P waves are followed by a QRS complex T he PR interval is normal and constant O ccasionally , a P wave is not followed by a QRS complex. Mobitz type II AV block is thought to result from abnormal conduction below the AV node ( infranodal ) and is considered more serious than Mobitz type I as it can progress without warning to third-degree (complete) heart block Referral to a cardiologist is therefore recommended, as a pacemaker may be required

20. 2:1 AV block S pecial form of second-degree heart block in which alternate P waves are not followed by QRS complexes U sually requires pacing

21. Third-degree AV block In third-degree AV block (‘complete heart block’), there is complete interruption of conduction between atria and ventricles, so that the two are working independently QRS complexes usually arise as the result of a ventricular escape rhythm in which the QRS complexes are usually broad However , if the level of AV block is located in or just below the AV node, a junctional escape rhythm may arise with narrow QRS complexes Third-degree AV block usually requires pacing

22. C haracteristic features of complete heart block are: P wave rate is faster than ventricular QRS complexes P waves bear no relationship to the ventricular QRS complexes I f block occurs in the AV node, QRS complexes are usually narrow due to a subsidiary pacemaker arising in the bundle of His I f block occurs below the AV node, QRS complexes are usually broad due to a subsidiary pacemaker arising in the left or right bundle branches

23. Causes of third-degree AV block Congenital Acquired D rug toxicity (e.g. anti- arrhythmics ) F ibrosis/calcification of the conduction system M yocardial ischaemia /infarction I nfection (e.g. Lyme disease) M yocardial infiltration (e.g. amyloid, sarcoid ) Metabolic disorders (e.g. hypothyroidism ) Neuromuscular diseases (e.g. myotonic muscular dystrophy ) C ardiac procedures (e.g. ablation procedures, aortic valve surgery )

24. ATRIOVENTRICULAR DISSOCIATION Atrioventricular dissociation occurs when the ventricular (QRS) rate is higher than the atrial (P wave) rate The opposite is found in third degree AV block Atrioventricular dissociation usually occurs in the context of an escape rhythm(from the AV junction or ventricles) during sinus bradycardia , or an acceleration in a subsidiary focus in the AV junction or ventricles which then overtakes the sinoatrial node, which continues firing independently

25. IS THE PR SEGMENT ELEVATED OR DEPRESSED? The PR segment, between the end of the P wave and the start of the QRS complex, is usually flat and isoelectric PR segment depression can occur in pericarditis PR segment depression is a specific ECG feature of pericarditis and may be seen in any leads except aVR and V1 (where there may be PR segment elevation) PR segment depression (or, rarely, PR segment elevation) can also be seen if there is atrial involvement with an acute coronary syndrome In the setting of acute inferior myocardial infarction, the presence of ≥1.2 mm PR segment depression is associated with worse outcomes

26. PR depression and ST elevation in V5 Reciprocal PR elevation and ST depression in aVR