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How to Measure PR Interval

How to measure PR Interval

PR Interval on ECG – Measurement and Clinical Significance

How to Measure PR Interval on ECG?

Definition:

PR interval is measured from the beginning of the P wave to the beginning of the QRS complex.

Step-by-step method:

1. Use Lead II (usually best P wave visibility).

2. Identify:

Onset of P wave

Onset of QRS complex (first deflection – Q or R)

3. Count small boxes between them.


ECG Paper Speed: 25 mm/s


1 small box = 0.04 sec (40 ms)


1 large box = 0.20 sec (200 ms)



Normal PR interval:

120–200 ms

(3–5 small boxes)


Measure in multiple beats and take the average if rhythm is irregular.


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What Does PR Interval Represent?


Atrial depolarization


AV nodal conduction


His bundle conduction



It reflects AV nodal delay.



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PR Interval Abnormalities


1. Prolonged PR Interval (>200 ms)


Diagnosis: First-degree AV block


Causes:


Increased vagal tone


Drugs (β-blockers, CCBs, digoxin)


Ischemia (especially inferior MI)


Myocarditis


Electrolyte imbalance



Clinical significance:


Usually benign


May progress in structural heart disease




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2. Progressive PR Prolongation


Diagnosis: Second-degree AV block – Mobitz I (Wenckebach)


Features:


Gradually increasing PR


Dropped QRS


Cycle repeats



Common in:


Inferior MI


High vagal tone




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3. Fixed PR with Dropped Beats


Diagnosis: Mobitz II


Features:


Constant PR


Sudden dropped QRS



Significance:


Pathologic (infranodal disease)


Risk of complete heart block


Requires pacing




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4. Short PR Interval (<120 ms)


Causes:


Pre-excitation syndromes


Wolff-Parkinson-White syndrome (short PR + delta wave + wide QRS)



Junctional rhythm (P wave absent or retrograde)



Clinical importance:


Risk of SVT


Risk of AF with rapid conduction (WPW)




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5. Variable PR Interval


Seen in:


Atrial fibrillation (no true PR measurable)


Wandering atrial pacemaker


AV dissociation




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Quick Exam Points


Normal PR: 120–200 ms

> 200 ms → First-degree AV block

<120 ms → Pre-excitation

Progressive prolongation → Mobitz I

Fixed PR with dropped beat → Mobitz II





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