ECG criteria
- Find the isoelectric line using the TP segment of the previous complex as the reference for assessing the PR segment
- Determine the boundaries of the segment: from the end of the P wave to the beginning of the QRS (to the Q or R wave)
- Assess the position of the segment relative to the isoelectric line in multiple leads, especially II, III, aVF, V5–V6
- When pericarditis is suspected, look for widespread PR depression, especially in lead II and the lateral leads, with reciprocal PR elevation in aVR; changes may be incomplete and are not an independent diagnostic criterion
- When atrial infarction is suspected, localized PR depression or elevation combined with P wave abnormalities and atrial rhythm disturbances may support the suspicion, but the diagnosis cannot be made from a single ECG
- Rule out baseline drift artifact and superimposition of the U wave from the previous cycle
What it looks like on ECG
The PR segment (synonym — PQ segment) is the short straight recording segment between the end of the P wave and the beginning of the QRS complex. Physiologically, it almost always lies on the isoelectric line, which is best determined using the TP segment of the preceding cycle rather than visually by eye.
Distinct from the PR segment is the PQ (PR) interval, which includes the P wave itself and the PR segment, meaning it is measured from the onset of P to the beginning of QRS. The PQ interval, rather than the PR segment, serves as a measure of AV conduction duration. The PR segment itself has no duration as a separately measured parameter in guidelines; its position relative to the isoelectric line is what matters.
There is no universal numerical threshold of normality for PR segment displacement in ECG standardization. As a reference point for pathological PR depression (especially in pericarditis), a value of about 0.5 mm (0.05 mV) or more is sometimes used, but this threshold is not an independent diagnostic criterion: assessment is always performed alongside the distribution of displacement, accompanying ST changes, and the clinical picture. Leads II, III, aVF, V5–V6 are most informative for assessing PR depression; reciprocal PR segment elevation in pericarditis is sought in lead aVR (and sometimes V1).
| Sign | Normal | Abnormal |
|---|---|---|
| Position | on the isoelectric line | widespread depression, usually in multiple leads |
| aVR | isoelectric line or minimal depression | PR segment elevation |
| Prevalence | no consistent displacement | usually widespread, but not always complete (pericarditis) or localized to the affected area (atrial infarction) |
In children and during sinus tachycardia, the PR segment may be slightly displaced downward by the atrial repolarization wave (Ta) — this is a normal variant in the absence of other signs of inflammation.
Mechanism
The P wave reflects atrial depolarization. Afterward, the atria repolarize — this repolarization wave (Ta) has a small amplitude and is usually directed opposite to the P wave. Since the P vector is positive in most standard leads, Ta displaces the PR segment downward from the isoelectric line in those leads; in aVR, where P is typically negative, the same Ta produces the opposite effect and may slightly elevate the PR segment. Normally, this wave is barely noticeable and is lost in recording noise, and the isoelectricity of the PR segment is explained by the small net atrial vector during this phase of the cycle.
In acute pericarditis, prominent PR depression is primarily associated not with enhanced physiological Ta, but with a current of injury involving the superficial (subepicardial) layers of the atria and the accompanying change in their repolarization; the changes are recorded widely across many leads, except aVR and sometimes V1, where reciprocal elevation is visible. In atrial ischemia or infarction, the mechanism is similar (injury current of the atrial wall), but PR segment changes are localized, corresponding to the affected area, and are combined with P wave morphology changes and frequently atrial arrhythmias.
Causes
- Normal variant (minimal physiological PR depression from the Ta wave, more common during tachycardia)
- Acute pericarditis (widespread PR depression + PR elevation in aVR, usually diffuse ST elevation)
- Atrial infarction (ischemia) — a potential complication of ventricular myocardial infarction, more common in inferior MI
- Pronounced sinus tachycardia with superimposition of the Ta wave
- Baseline drift artifact, incorrect calibration
Differential diagnosis
- ST depression — distinguished by localization: PR depression occurs before the QRS, ST depression occurs after the J point; it should be checked whether PR displacement is a continuation of a shifted ST segment.
- ST elevation in STEMI — pericarditis and STEMI are differentiated not by the ST segment shape (concave or convex shape is unreliable and occurs in both conditions), but by the distribution of ST elevation, the presence of reciprocal changes, pathological Q/QS waves, serial ECG evolution, clinical presentation, and biomarkers; in pericarditis, ST elevation is typically accompanied by widespread PR depression with elevation in aVR, but the absence of this combination does not exclude pericarditis, and its presence does not rule out MI.
- Enlarged atrium (P pulmonale, P mitrale) — the morphology of the P wave itself is altered, not the position of the segment following it.
- Baseline drift artifact — displacement is gradual, affects the entire cycle rather than just the PR segment, and does not fit a clear electrophysiological pattern.
Clinical significance and management
Widespread PR segment depression with elevation in aVR is one of the hallmark signs of acute pericarditis, along with diffuse concave ST elevation and the clinical picture (pain related to body position and respiration, pericardial friction rub); PR changes may be incomplete and do not serve as an independent diagnostic criterion. The physician must correlate the ECG with clinical findings, auscultation, and echocardiography, and rule out STEMI and its complications (effusion, tamponade).
Localized PR segment depression or elevation combined with a P wave morphology change in a patient with acute MI may support the suspicion of concomitant atrial involvement — such changes are nonspecific, lack a universally accepted standalone threshold, and do not allow diagnosing atrial infarction from a single ECG. The finding is evaluated together with the clinical presentation of acute MI, dynamic ECG changes, and the exclusion of pericarditis and other causes; when confirmed, the risk of atrial arrhythmias (atrial fibrillation) increases, requiring closer rhythm monitoring.
Isolated minimal PR segment deviation without other signs and without clinical symptoms is usually clinically insignificant and requires no intervention.
Common interpretation errors
- Assessing the PR segment position without referencing the TP isoelectric line — mistaking general recording drift for true depression.
- Confusing PR depression with the onset of ST depression if the exact end point of the P wave is not identified.
- Failing to check lead aVR — reciprocal PR elevation is easily missed there, despite being crucial for diagnosing pericarditis.
- Ruling out pericarditis simply because PR depression is not visible in all leads — changes may be incomplete.
- Interpreting minimal physiological PR depression during tachycardia as pathology without considering the clinical context.
- Diagnosing atrial infarction based on a single PR finding without considering the acute MI clinical context, P wave evolution, and ruling out pericarditis.
1. What is the PR segment on the ECG?
The PR segment is precisely the isoelectric segment between the end of the P wave and the beginning of the ventricular QRS complex.
2. Which ECG sign is most characteristic of acute pericarditis?
In pericarditis, PR segment depression is typically widespread across many leads, with reciprocal elevation in aVR — unlike the localized changes seen in infarction.
3. What causes the physiological wave that slightly shifts the PR segment normally?
A minor physiological PR segment shift is associated with the atrial repolarization wave (Ta); however, pronounced PR depression in pericarditis is primarily caused by an atrial current of injury rather than simply an enhancement of Ta.
Frequently asked questions
How does the PR segment differ from the PQ interval?
The PQ (PR) interval is measured from the onset of the P wave to the beginning of the QRS complex and includes the P wave itself; it specifically reflects the duration of AV conduction. The PR segment is only the section from the end of the P wave to the beginning of the QRS, excluding the P wave, and its duration is not independently standardized.
Is PR segment depression dangerous?
Minimal depression by itself can be a normal variant, especially in tachycardia. Widespread PR depression with elevation in aVR is an important sign of acute pericarditis, while local depression in the setting of an MI may support suspicion of atrial involvement; therefore, the finding must be evaluated in the clinical context rather than as a standalone diagnosis.
How to distinguish PR depression in pericarditis from changes in myocardial infarction?
In pericarditis, PR depression is typically widespread, especially in lead II and lateral leads, with reciprocal elevation in aVR, and is combined with diffuse ST elevation. In MI, ST changes usually have a territorial distribution along the ventricular wall and may be accompanied by reciprocal ST depression; PR shifts are not typical for MI, but can occur with atrial ischemia or infarction and do not rule out ACS on their own. The ST-segment morphology (concave or convex) is an unreliable sign and is not used as an independent differentiating criterion.
In which leads is PR segment depression best visualized?
Depression is most commonly sought in leads II, III, aVF, and V5–V6, and reciprocal segment elevation in aVR, occasionally in V1. Evaluation is always performed in multiple leads simultaneously rather than in a single lead.
Do we need to measure the duration of the PR segment?
No, the isolated duration of the PR segment is not a standardized parameter. The entire PQ (PR) interval and the position of the PR segment relative to the baseline are evaluated.
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Related terms
More in the section «Basics: waves, intervals, axes»
Sources
- Goldberger A. «Clinical Electrocardiography», current edition
- Hampton J. «The ECG in Practice», current edition
- Surawicz B., Knilans T. «Chou's Electrocardiography in Clinical Practice», current edition
- Orlov, V.N. «Electrocardiography Guide»
Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.