How do you calculate the PR (P-R) interval on an electrocardiogram (ECG)?

How to Calculate PR Interval on ECG

The PR interval on an ECG is measured from the beginning of the P wave to the beginning of the QRS complex, using global measurements from multiple leads to detect the earliest onset of the P wave and the earliest onset of the QRS complex.

Definition and Measurement Technique

• The PR interval represents the time from the onset of atrial depolarization to the onset of ventricular depolarization 1
• To accurately measure the PR interval, use time-coherent data from multiple leads (global measurement) to detect the earliest onset of the P wave and the earliest onset of the QRS complex 1
• The normal PR interval ranges from 120-200 ms; a PR interval >200 ms indicates first-degree AV block 1

Step-by-Step Measurement Process

1. Identify the P wave:

   • Look for the first positive deflection before the QRS complex, typically most visible in leads II, III, and aVF 1
   • The P wave represents atrial depolarization 1

2. Mark the beginning of the P wave:

   • Locate the earliest point where the baseline begins to deflect upward into the P wave across all leads 1
   • This global approach ensures you don't miss the true onset of atrial depolarization 1

3. Identify the beginning of the QRS complex:

   • Mark the earliest deflection of the QRS complex (Q wave or R wave) across all leads 1
   • This represents the beginning of ventricular depolarization 1

4. Measure the interval:

   • Count the small boxes between these two points (each small box = 40 ms) 1
   • Alternatively, use the digital calipers on electronic ECG systems 1

Clinical Significance and Interpretation

• Normal PR interval: 120-200 ms 1
• First-degree AV block: PR interval >200 ms 1
• Profound first-degree AV block: PR interval ≥400 ms (requires further evaluation) 1
• Short PR interval: <120 ms (may indicate pre-excitation syndromes like Wolff-Parkinson-White) 2

Common Pitfalls to Avoid

• Single-lead measurement error: Using only one lead will systematically underestimate the PR interval; always use multiple leads for accurate global measurement 1
• Isoelectric components: When the vector orientation of a lead is perpendicular to the heart vector, an isoelectric component may be recorded, making it difficult to identify the true onset of waves in that lead 1
• Misidentifying the P wave: In some arrhythmias like atrial tachycardia or atrial flutter, P waves may be difficult to discern or may overlap with T waves 1
• Unusual P wave placement: In cases of marked first-degree AV block (PR ≥500 ms), P waves may appear immediately after the preceding QRS, complicating rhythm interpretation 3

Special Considerations

• For accurate measurements, the ECG should be recorded at standard calibration (10 mm/mV, 25 mm/s) 1
• In digitized ECGs, the recommended sampling rate is 500 samples per second to ensure adequate resolution 1
• When evaluating profound sinus bradycardia or markedly increased PR interval, assess the chronotropic response to mild aerobic activity 1
• In athletes, PR intervals up to 400 ms may be normal due to increased vagal tone 1