Overview

• PR segment: flat, typically isoelectric segment representing time from end of P wave to start of QRS complex (note: does not include P wave)
• Cardiac event: impulse transmission through AV node, His bundle, bundle branches, & ventricular Purkinje system
• AV node allows for critical delay in conduction system & makes up majority of PR segment; without conduction delay, atria & ventricles would contract simultaneously & blood would not flow effectively from atria to ventricles
• Often located along baseline; normal PR depression <0.8 mm; pathological PR depression ≥0.8 mm (eg, pericarditis, atrial infarction
• PR-segment elevation/depression differential diagnosis:
• Normal variant
• Acute pericarditis
• Atrial infarction

Normal Variant

• Due to atrial repolarization (Tp wave)
• Tp wave axis: opposite of P wave axis
• If low Tp wave amplitude => PR segment practically horizontal
• If high Tp wave amplitude (eg, during exercise-induced tachycardia) => PR segment slopes downward in all leads except aVR [1]
• ECG features: in normal patients with normal HR
• PR depression <0.08 mV (0.8 mm)
• Note: taller P wave => greater PR depression
• PR elevation <0.05 mV (0.5 mm)

Acute Pericarditis

• Inflammation of pericardium
• ECG features:
• PR depression ≥0.8 mm
• Diffuse ST elevation (concave up; “saddle-shaped”)
• Reciprocal ST depression & PR elevation in aVR & V1 (not in other leads)
• Notching of terminal QRS (esp. in lateral precordial leads V4-V6)
• ± tachycardia

Atrial Infarction

• Very rare; RA infarction more common than LA infarction (may be due to higher oxygen concentration in LA blood)
• Infarction more common in atrial appendages than lateral or posterior atrial walls
• Incidence in the setting of MI:
• RA infarction in 81-98%
• LA infarction in 2-19%
• RA + LA infarction in 19-24%
• Note: variation attributed to (1) autopsy vs. clinical evidence of atrial infarction & (2) how closely it was investigated for
• Associated with
• Poor outcomes post-MI
• Increased risk of AV block
• Supraventricular arrhythmias (eg, paroxysmal atrial fibrillation, atrial tachycardia, premature atrial complexes, wandering atrial pacemaker)
• Cardiac free-wall rupture
• May contribute to low output state due to loss of atrial kick to ventricular filling
• Lewis lead ECG may better visualize atrial repolarization abnormalities than standard ECG
• ECG features:
• Major criteria: [2]
• PR elevation >0.5 mm in V5-V6 with reciprocal PR depression in V1-V2
• PR elevation >0.5 mm in I with reciprocal PR depression in II & III
• PR depression >1.5 mm in precordial leads & >1.2 mm in I, II, & III associated with any atrial arrhythmia
• Minor criteria: [2]
• Abnormal P waves
• Flattening of P wave in M- or W-shape
• Irregular or notched P wave
• Additional criteria:
• PR segment prolongation (≥0.2 s, 200 ms) [3]
• P wave axis shifts in frontal plane (normal 0° to +75°) [3]
• Abnormal atrial rhythms (eg, atrial fibrillation, atrial flutter, wandering atrial pacemaker, atrioventricular nodal rhythm) [3]
• PR depression or PR elevation in the setting of infarction without criteria for pericarditis met
• PR elevation in aVR & V1

Abbreviations: b/t, between; RA, right atrium/atrial; LA, left atrium/atrial

REFERENCES

1. Charles MA, Benziner TA, Glasser SP. Atrial injury current in pericarditis. Arch Intern Med 131:657, 1973.

2. Liu CK, Greenspan G, Piccirillo RT. Atrial infarction of the heart. Circulation 23:331-338, 1961.

3. Sivertssen E, Hoel B, Bay G, Jorgensen L. Electrocardiographic atrial complex and acute atrial myocardial infarction. Am J Cardiol 31:450-456, 1973.