ECG Topic - The PR Interval

Last updated August 16, 2025

Conduction & Electrophysiology

The PR interval runs from the onset of the P wave to the onset of the QRS complex. It represents the journey of an impulse from atrial depolarization through the AV node, His–Purkinje system, and into the ventricular myocardium. What looks like a simple line segment is actually the choreography that synchronizes atrial contraction with ventricular filling.

Normal Sequence

• Atrial depolarization: Initiated in the sinus node, the impulse travels via Bachmann’s bundle and internodal pathways, activating atrial myocardium. This electrical force is large enough to be seen as the P wave.
• AV nodal delay: Within the AV node, conduction deliberately slows — an event hidden on the ECG but physiologically crucial. This pause allows the atrial kick to contribute to ventricular preload. Without it, atria and ventricles would contract nearly together, reducing stroke volume and sometimes precipitating shock in preload-dependent patients (e.g., elderly, RV infarct, severe AS).
• His–Purkinje conduction: After the pause, conduction accelerates through the His bundle, bundle branches, and Purkinje fibers, triggering coordinated ventricular depolarization — the QRS complex.

How to Assess PR Segment Elevation or Depression

Reference baseline

The true isoelectric line of the ECG is the TP segment (the flat line between the end of the T wave and the start of the next P wave).

• The PR segment (end of P wave → start of QRS) can be shifted up or down relative to the TP segment.
• Using the PR itself as the baseline is misleading, because it’s often displaced in pericarditis, atrial infarction, or atrial injury currents.

Stepwise approach:

1. Identify the TP segment in a clean lead
2. Draw a mental or caliper-based horizontal line across the TP segment — this is your baseline.
3. Compare the PR segment (the flat portion immediately after the P wave before QRS onset) against this baseline.
• PR depression = PR segment lies below the TP baseline
• PR elevation = PR segment lies above the TP baseline

PR Depression Differential Diagnosis

Normal Variant

• Baseline rule: The PR segment is usually isoelectric. However, it is sometimes found to be slightly depressed.
• Threshold: For it to be considered normal, depression must be ≤0.8 mm below the TP baseline.
• Mechanism: This benign shift comes from atrial repolarization, which pulls the PR segment downward. The atrial repolarization wave is the Tp wave — usually invisible because it’s buried in the QRS complex.

Example: normal variant in tachycardia

Acute Pericarditis

• ECG Features:
• Sinus Tachycardia – common due to pain, fever, or systemic inflammation.
• PR Depression – diffuse; best seen in limb and left precordial leads, with reciprocal PR elevation in aVR ± V1.
• Diffuse ST Elevation – usually concave up, “scooped-out” morphology across multiple leads (unlike the convex, territorial pattern of STEMI).
• Terminal QRS Notching – a subtle but supportive finding, often in the lateral precordial leads (V5–V6).
• Mechanism: Subepicardial atrial injury currents from inflamed pericardium.

Example: Pericarditis

Note the diffuse STE, PR depression, avR elevation, sinus tachycardia and terminal qrs notching (V4-V6)

Atrial Infraction

• ECG Features:
• Focal PR depression in leads reflecting the infarcted atrial wall (e.g., inferior PR depression with inferior MI).
• Reciprocal PR elevation may occur.
• Mechanism: Atrial injury currents analogous to ST shifts in ventricular MI.

Example: Atrial infraction w/ Inferior MI

Measuring the PR Interval

How to Measure:

• Time from the onset of the P wave to the start of the QRS complex
• Choose the lead with the widest, clearest P wave and widest QRS complex to avoid underestimating.
• The PR should be consistent across all leads — if not, suspect atrial ectopy, dual AV nodal physiology, or pre-excitation.

Normal Range:

• 0.12–0.20 sec (120–200 ms) is considered normal.
• ≤0.11 sec → short PR.
• Wolff Parkinson White (WPW) Pattern
• Retrograde Junctional P waves
• Lown-Ganong-Levine Syndrome
• >0.20 sec → prolonged PR
• Commonly 1st degree AV block

Example: WPW

Example: Lown-Ganong Leveine

Example: Junctional Rhythm

Example: 1st degree AV Block

References

1. Brady WJ, Mattu A, Tabas JA. Critical Decisions in Emergency and Acute Care Electrocardiography. Wiley-Blackwell; 2009.
2. Burns E, Buttner R. ECG changes in pericarditis. Life in the Fast Lane ECG Library. Accessed August 16, 2025. https://litfl.com/pr-segment-ecg-library/
3. Burns E, Buttner R. Accelerated junctional rhythm (AJR). Life in the Fast Lane ECG Library. Accessed August 16, 2025. https://litfl.com/accelerated-junctional-rhythm-ajr/
4. Chou TC, Surawicz B. Chou’s Electrocardiography in Clinical Practice. 6th ed. Saunders/Elsevier; 2008.
5. Davila C. The ECG. Self-published; updated edition 2024.
6. ECGBook. Atrial Myocardial Infarction. ECGBook. Published 2024. Accessed August 16, 2025. https://www.ecgbook.com/atrial-infarction/
7. Garcia TB. 12-Lead ECG: The Art of Interpretation. 2nd ed. Jones & Bartlett Learning; 2013.
8. Podrid PJ. Podrid’s Real-World ECGs: A Master’s Approach to the Art and Practice of Clinical ECG Interpretation. Vol 6. Cardiotext Publishing; 2016.
9. Surawicz B, Knilans TK. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric. 6th ed. Saunders Elsevier; 2008.