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Conceptual Overview

What is the PR Segment?

The PR segment is the flat (isoelectric) portion of the ECG tracing between the end of the P wave and the beginning of the QRS complex. It represents the delay as the electrical impulse passes through the AV node, bundle of His, and bundle branches before ventricular depolarization begins.

ECG waves, segments, and intervals diagram showing PR segment
Figure 1: ECG components - note the PR segment extends from the end of the P wave to the start of the QRS. Source: LITFL

Physiologic Significance

  • Represents: Conduction through the AV node and His-Purkinje system (silent electrical activity with no associated myocardial depolarization visible on surface ECG)
  • Normal appearance: Isoelectric (flat) - at the same baseline level as the TP segment
  • Duration: Variable, depending on heart rate and AV conduction velocity (not independently measured - it's included in the PR interval)
  • Clinical importance: Deviations from baseline (depression or elevation) indicate pathologic atrial or pericardial processes

PR Segment vs. PR Interval

Critical distinction:
  • PR Interval = P wave onset → QRS onset (includes P wave + PR segment)
  • PR Segment = P wave end → QRS onset (only the isoelectric portion)
The PR interval is measured and reported; the PR segment is evaluated for deviation from baseline.
Normal PR Segment

Identifying the Normal PR Segment

  • Location: From the end of the P wave (return to baseline after atrial depolarization) to the start of the QRS complex
  • Expected morphology: Flat, isoelectric line at the same level as the TP segment (baseline)
  • Best leads to evaluate: Limb leads (II, III, aVF, aVR) where atrial activity is most prominent
  • Normal variation: May be difficult to visualize if the PR interval is very short or if the P wave blends into the QRS

How to Measure PR Segment Deviation

  1. Establish the baseline: Use the TP segment (end of T wave to start of next P wave) as your reference isoelectric line
  2. Identify the PR segment: Locate the flat portion between P wave end and QRS start
  3. Compare levels: Determine if the PR segment is above (elevated), below (depressed), or at the same level as the TP baseline
  4. Measure deviation: Typically described qualitatively (present/absent) rather than quantitatively, but can be measured in millimeters if needed
Measurement tip: At fast heart rates, the TP segment may disappear (P waves immediately follow T waves), making baseline assessment difficult. In these cases, use the longest RR interval or a pause to identify true baseline.
PR Segment Depression

Definition & Recognition

PR segment depression occurs when the PR segment falls below the isoelectric baseline (TP segment level). This is a subtle but important ECG finding associated with atrial injury patterns.

PR segment depression in acute pericarditis
Figure 2: PR segment depression (red arrows) in acute pericarditis, most evident in leads II and V5-V6. Source: LITFL

Causes of PR Segment Depression

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Condition Mechanism Associated ECG Findings
Acute Pericarditis Atrial epicardial injury current Diffuse ST elevation (concave), PR depression in limb & precordial leads, PR elevation in aVR
Atrial Infarction Ischemia/injury to atrial myocardium May see atrial arrhythmias, changes in P wave morphology, associated ventricular MI
Atrial Injury (trauma, post-operative) Direct atrial tissue damage Variable; may see atrial arrhythmias

Acute Pericarditis - The Classic Presentation

PR segment depression is one of the most specific ECG findings for acute pericarditis, though it's only present in 60-80% of cases.

ECG Features of Acute Pericarditis

  • Stage 1 (first hours to days):
    • Widespread ST elevation (concave upward "smiley face" morphology)
    • PR segment depression in most leads (especially II, V5-V6)
    • PR segment elevation in aVR (reciprocal change)
  • Stage 2 (days): ST segments and PR segments normalize
  • Stage 3 (weeks): T wave inversion develops
  • Stage 4 (weeks to months): ECG returns to normal (or T waves remain inverted)
ECG stages of acute pericarditis showing evolution of PR and ST changes
Figure 3: Evolution of ECG changes in acute pericarditis across four stages. Source: LITFL

Differentiating Pericarditis from STEMI

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Feature Pericarditis STEMI
ST elevation distribution Diffuse, widespread (multiple territories) Regional (confined to coronary artery distribution)
ST morphology Concave upward ("smiley face") Convex upward or horizontal
Reciprocal ST depression Usually absent (except aVR, V1) Present in reciprocal leads
PR segment Depressed (except aVR - elevated) Normal (isoelectric)
Q waves Absent May develop with transmural infarction
T wave inversion timing After ST normalization (days to weeks) Early (hours to days)
Critical pearl: PR segment depression is highly specific for pericarditis. If you see diffuse ST elevation with PR depression, think pericarditis first. However, pericarditis can coexist with MI (post-MI pericarditis), so clinical correlation is essential.

Atrial Infarction

  • Incidence: Occurs in ~10-17% of ventricular MIs but often underrecognized
  • Most common with: Inferior or posterior MI (right coronary artery occlusion affecting atrial branches)
  • ECG findings:
    • PR segment depression (atrial injury current)
    • PR segment elevation in leads V1-V3 or aVR (reciprocal)
    • Abnormal P wave morphology or duration
    • Atrial arrhythmias (AFib, atrial flutter, atrial tachycardia)
  • Clinical significance: Associated with increased risk of atrial arrhythmias, heart failure, and thromboembolic complications
Atrial infarction with PR segment depression
Figure 4: Atrial infarction showing PR segment depression with concurrent inferior STEMI. Source: LITFL
PR Segment Elevation

Definition & Recognition

PR segment elevation occurs when the PR segment rises above the isoelectric baseline. This is less common than PR depression and is typically seen as a reciprocal change.

Causes & Context

Swipe to see more
Lead(s) with PR Elevation Interpretation Associated Findings
aVR Reciprocal to diffuse PR depression (pericarditis) PR depression in other leads, diffuse ST elevation
V1-V3 Possible right atrial injury or reciprocal to left atrial injury May see atrial arrhythmias, concurrent MI
Isolated aVR elevation Normal variant or technical artifact None; clinically insignificant if isolated

Lead aVR: The Reciprocal Lead

  • In pericarditis: While most leads show PR depression, aVR typically shows PR elevation (reciprocal change)
  • Sensitivity/specificity: PR elevation in aVR combined with PR depression in other leads strongly suggests pericarditis
  • Why aVR? Lead aVR views the heart from the right shoulder, opposite the left ventricular apex; electrical changes appear inverted
Remember: PR elevation in aVR + PR depression in limb/precordial leads = classic pericarditis pattern. Think of aVR as the "reverse" or "reciprocal" lead.
Clinical Pearls
"PR depression + diffuse ST elevation = pericarditis until proven otherwise": This combination is highly specific. Don't mistake it for diffuse STEMI (which doesn't exist - STEMIs are regional).
"Check aVR for the reciprocal": In pericarditis, while most leads show PR depression, aVR will show PR elevation. This reciprocal pattern confirms the diagnosis.
"Pericarditis masquerades as MI": Young patient with chest pain and ST elevation? Before activating the cath lab, look for concave ST morphology, diffuse distribution, and PR depression. Pericarditis is common in young, otherwise healthy individuals.
Don't miss atrial infarction: PR segment changes in the context of STEMI suggest concomitant atrial injury. These patients have higher risk of atrial arrhythmias and complications. Monitor closely for AFib, flutter, and thromboembolic events.
Tachycardia obscures the PR segment: At fast heart rates, the P wave may merge with the preceding T wave, making PR segment evaluation impossible. If you suspect pericarditis but can't see the PR segment due to tachycardia, treat the tachycardia or wait for a rhythm strip with slower rate.
"Spodick's sign": Downsloping TP segment (the segment between the end of the T wave and the start of the P wave) in acute pericarditis. This is another subtle but specific finding that supports the diagnosis.
Post-MI pericarditis (Dressler syndrome): Can occur days to weeks after MI. ECG shows pericarditis pattern (PR depression, diffuse ST elevation) overlaid on MI changes (Q waves, regional ST/T abnormalities). Don't assume it's re-infarction - consider pericarditis.
Practical Tips for Evaluating the PR Segment

Step-by-Step Approach

  1. Identify the baseline: Use the TP segment as your isoelectric reference point
  2. Locate the PR segment: Find the flat portion between the end of the P wave and the start of the QRS
  3. Compare levels: Is the PR segment above, below, or at the same level as the TP baseline?
  4. Check multiple leads: PR segment deviations should be present in multiple leads (diffuse pattern suggests pericarditis; regional pattern may suggest atrial infarction)
  5. Look for associated findings: ST changes, T wave abnormalities, P wave morphology changes
  6. Correlate clinically: Chest pain? Pericardial rub? Fever? Recent MI? Context is critical

Common Pitfalls to Avoid

  • Confusing PR interval with PR segment: The interval includes the P wave; the segment does not
  • Using the wrong baseline: Don't use the ST segment or T wave as baseline; use the TP segment
  • Overlooking subtle changes: PR depression can be very subtle (0.5-1 mm); compare carefully with the TP segment
  • Ignoring tachycardia: Fast heart rates obscure the TP segment and make PR assessment difficult
  • Forgetting lead aVR: Always check aVR for reciprocal PR elevation in suspected pericarditis
Pro tip: Use calipers or a straight edge to compare the PR segment level to the TP baseline. This makes subtle deviations easier to detect.
References
  1. Farkas, Josh MD. (2015). Table of Contents - EMCrit Project. EMCrit Project. https://emcrit.org/ibcc/toc/
  2. Khan, M. G. (2007). Rapid ECG Interpretation. Humana.
  3. Sigg, D. C., Iaizzo, P. A., Xiao, Y.-F., Bin He, & Springerlink (Online Service). (2010). Cardiac Electrophysiology Methods and Models. Springer Us.
  4. Wang, K. (2012). Atlas of Electrocardiography. JP Medical Ltd.
  5. ECG Library • LITFL • ECG Library Basics. (2018). Life in the Fast Lane • LITFL • Medical Blog. https://litfl.com/ecg-library/
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