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.