The PR interval is the time from the onset of the P wave to the start of the QRS complex. It reflects conduction through the AV node. The normal PR interval is between 120 – 200 ms (0.12-0.20s) in duration (three to five small squares). If the PR interval is > 200 ms, first degree heart block is said to be present. PR interval < 120 ms suggests pre-excitation (the presence of an accessory pathway between the atria and ventricles) or AV nodal (junctional) rhythm.

1. Fundamentals of ECG
Approach to a patient with PR interval
abnormalities
Dr. Md.Toufiqur Rahman
MBBS, FCPS, MD, FACC, FESC, FRCP, FSCAI,
FCCP,FAPSC, FAPSIC, FAHA,FACP
Professor & head of Cardiology
CMMC, Manikganj
Ex professor of cardiology,
NICVD, Dhaka

2. Fundamentals of ECG
PR interval abnormalities in ECG
 The normal PR interval is between 120 – 200 ms (0.12-0.20s) in
duration (three to five small squares).
 If the PR interval is > 200 ms, first degree heart block is said to be present.
 PR interval < 120 ms suggests pre-excitation (the presence of an
accessory pathway between the atria and ventricles) or AV nodal
(junctional) rhythm.
 The PR interval is the
time from the onset of
the P wave to the start
of the QRS complex.
 It reflects conduction
through the AV node.

3. Fundamentals of ECG
PR interval abnormalities in ECG
Case-1: A 45 years old gentleman presented with
hypertension and taking beta blocker -atenolol for last 08
years. He had the following ECG.

4. Fundamentals of ECG
PR interval abnormalities in ECG
Case-2: A 45 years old gentleman diabetic, hypertensive and
smoker presented with severe central chest pain with
excessive sweating and vomiting for last 2 hours. On
emergency he had the following ECG.

5. Fundamentals of ECG
Case-3
A 15 years old boy presented with several episodes of sore
throat, palpitations and shortness of breath. His ASO titre
and CRP were raised. He had the following ECG.

6. Fundamentals of ECG
Causes of First Degree Heart Block
Increased vagal tone
Athletic training
Inferior MI
Mitral valve surgery
Myocarditis (e.g. Lyme disease)
Electrolyte disturbances (e.g. Hyperkalaemia)
AV nodal blocking drugs (beta-blockers, calcium
channel blockers, digoxin, amiodarone)
May be a normal variant

7. Fundamentals of ECG
Second degree AV block (Mobitz I) with prolonged PR interval
Second degree heart block, Mobitz type I
(Wenckebach phenomenon).
 The baseline PR interval is prolonged, and
then further prolongs with each successive
beat, until a QRS complex is dropped.
The PR interval before the dropped beat is
the longest (340ms), while the PR interval
after the dropped beat is the shortest
(280ms).

8. Fundamentals of ECG
Second degree AV block (Mobitz I) with prolonged PR interval
 Second degree heart block, Mobitz type I (Wenckebach
phenomenon).
 The baseline PR interval is prolonged, and then further prolongs
with each successive beat, until a QRS complex is dropped.
 The PR interval before the dropped beat is the longest (340ms),
while the PR interval after the dropped beat is the shortest
(280ms).
 The P-P interval remains relatively constant
 The greatest increase in PR interval duration is typically between
the first and second beats of the cycle.
 The RR interval progressively shortens with each beat of the cycle.
 The Wenckebach pattern tends to repeat in P:QRS groups with
ratios of 3:2, 4:3 or 5:4.

9. Fundamentals of ECG
What are the causes of Wenckebach Phenomenon?
 Drugs: beta-blockers, calcium channel
blockers, digoxin, amiodarone
 Increased vagal tone (e.g. athletes)
 Inferior MI
 Myocarditis
 Following cardiac surgery (mitral valve
repair, Tetralogy of Fallot repair)

10. Fundamentals of ECG
Short PR interval (<120ms)
A short PR interval is seen with:
Preexcitation syndromes
AV nodal (junctional)
rhythm.

11. Fundamentals of ECG
Case-4
A 21 years old man was admitted in CCU with the diagnosis of
Supraventricular tachycardia (SVT) and was being treated with
intravenous adenosine and reverted to sinus rhythm. Now he has the
following ECG at sinus rhythm.

12. Fundamentals of ECG
Case-5
A 43 years old lady presented with palpitation and
chest discomfort. She had the following ECG.

13. Fundamentals of ECG
Pre-excitation syndromes
 Wolff-Parkinson-White (WPW) and Lown-Ganong-
Levine (LGL) syndromes.
 These involve the presence of an accessory pathway
connecting the atria and ventricles.
 The accessory pathway conducts impulses faster than normal,
producing a short PR interval.
 The accessory pathway also acts as an anatomical re-entry
circuit, making patients susceptible to re-entry
tachyarrhythmias.
 Patients present with episodes of paroxsymal
supraventricular tachycardia (SVT), specifically
atrioventricular re-entry tachycardia (AVRT), and
characteristic features on the resting 12-lead ECG.

14. Fundamentals of ECG
Pre-excitation syndromes
ECG showing Pre-excitation syndrome ( WPW
syndrome- short PR interval and delta wave)

15. Fundamentals of ECG
Wolff-Parkinson-White syndrome
The characteristic features of Wolff-Parkinson-White
syndrome are a short PR interval (<120ms), broad
QRS and a slurred upstroke to the QRS complex,
the delta wave.

16. Fundamentals of ECG
Lown-Ganong-Levine syndrome
The features of Lown-Ganong-Levine syndrome LGL
syndrome are a very short PR interval with normal P
waves and QRS complexes and absent delta waves.

17. Fundamentals of ECG
Characteristics of Lown-Ganong-Levine syndrome
 Accessory pathway composed of James fibres.
 Short PR interval (<120ms);
 normal P wave axis;
 normal/narrow QRS morphology in the
presence of paroxysmal tachyarrhythmia.
 Existence of LGL is disputed and the
condition may not actually exist…the term
should not be used in the absence of
paroxysmal tachycardia

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

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

20. Fundamentals of ECG
History of LGL syndrome
 1921-1952 – association of paroxysmal tachycardia, short AV conduction time,
and normal QRS complexes reported across 11 cases, but usually attributed to
being a variant of Wolff-Parkinson-White syndrome.
 1938 – Clerc, Levy and Critesco first described ECG findings of a short PR interval,
normal QRS complex, and paroxysmal tachycardia.
 1952 – Lown, Ganong and Levine performed the first study correlating the
characteristic ECG changes with clinical findings, which distinguished patients
with paroxysmal tachycardia, short PR interval, and normal QRS complexes from
Wolff-Parkinson-White characteristics.
 1961 – James described accessory pathway connections between the atria and
distal atrioventricular node, which may have a role in the pathophysiology of LGL
syndrome
 1975 – Brechenmacher described accessory pathways between the atria and
bundle of His, which may also be involved in LGL syndrome