Why do you get elevation of the PR segment in some electrocardiogram (ECG) leads but depression in others in pericarditis?

PR Segment Changes in Pericarditis: The Atrial Injury Current Explanation

The PR segment shows elevation in lead aVR and depression in most other leads during pericarditis because the inflamed atrium generates an atrial injury current that vectors away from the atria (toward lead aVR's negative pole), creating a pattern opposite to the P wave polarity in each lead. 1

Mechanism of PR Segment Deviation

The key to understanding this pattern lies in recognizing that pericardial inflammation affects both the ventricles AND the atria:

• Atrial epicardial inflammation creates an atrial injury current that is directed away from the atrial epicardium (outward from the heart) 1
• This injury current vectors toward lead aVR, which has its positive electrode positioned to "look at" the heart from the right shoulder 2
• In leads where the P wave is normally upright (positive atrial depolarization), the PR segment becomes depressed because the atrial injury current opposes the normal atrial repolarization 1
• Conversely, in lead aVR where the P wave is normally inverted, the PR segment becomes elevated 1

The Reciprocal Relationship

This follows the same bioelectric principle that governs ST segment changes:

• All ECG leads are bipolar, with positive and negative poles oriented in different spatial directions 2
• When an injury current is directed toward a lead's positive pole, that segment elevates; when directed away, it depresses 2
• The PR depression in inferior and lateral leads (II, III, aVF, V3-V6) is the reciprocal of PR elevation in aVR 1
• This reciprocal pattern occurs because these leads have their positive poles oriented approximately 180° opposite to aVR 2

Typical Distribution Pattern

The characteristic ECG findings in acute pericarditis include:

• PR depression in leads I, II, aVL, aVF, and V3-V6 (where P waves are normally upright) 1
• PR elevation in lead aVR (where P waves are normally inverted) 1
• ST elevation that is widespread and concave upward in the same leads showing PR depression 1
• ST depression in aVR, frequently V1, and occasionally V2 1

Critical Diagnostic Pitfall

The combination of PR depression with multilead ST elevation can occasionally represent acute myocardial infarction (particularly left circumflex occlusion) rather than pericarditis, making clinical context essential 3:

• Look for QRS widening and QT interval shortening in leads with ST elevation, which suggests STEMI rather than pericarditis 3
• Pericarditis shows widespread ST elevation without reciprocal ST depression in opposing leads (except aVR), unlike the regional pattern of STEMI 1
• The presence of PR depression in both precordial AND limb leads has 96.7% positive predictive value for myopericarditis versus STEMI 4
• Early repolarization can also mimic this pattern but lacks the clinical symptoms and inflammatory markers of pericarditis 5

Clinical Context Matters

Only 60% of pericarditis cases show these classic ECG changes, so their absence doesn't exclude the diagnosis 1. The ECG should be interpreted alongside:

• Pleuritic chest pain characteristics 6
• Pericardial friction rub on auscultation 6
• Elevated inflammatory markers (ESR, CRP) 7
• Troponin elevation in 49% of cases, particularly with ST elevation, suggesting myopericardial involvement 1