ECG Basic – PR segment

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Appcardio ECG Academy · ECG Basics

PR Segment

Understand the ECG segment between atrial depolarisation and ventricular depolarisation, how to establish its baseline, and why deviations may provide clues to pericardial or atrial disease.

Foundational levelApprox. 6-minute readECG interpretation

Learning objectives

01Locate the PR segment and distinguish it from the PR interval.

02Identify the appropriate reference baseline when assessing PR deviation.

03Recognise PR-segment changes described in acute pericarditis.

04Understand the reported association between PR deviation and atrial ischaemia/infarction.

05Appreciate measurement limitations when the TP segment is unclear.

1. What is the PR segment?

The PR segment is the portion of the ECG tracing extending from the end of the P wave to the beginning of the QRS complex. It is usually flat and close to the isoelectric baseline.

Figure 1. Original Appcardio schematic identifying the PR segment. The highlighted interval begins at the end of the P wave and ends at QRS onset.

The PR segment should not be confused with the PR interval, which begins at the onset of the P wave and ends at the beginning of the QRS complex. Therefore, the PR interval includes both the P wave and the PR segment.

ECG component	Start	End
P wave	Beginning of atrial depolarisation	End of P wave
PR segment	End of P wave	Beginning of QRS complex
PR interval	Beginning of P wave	Beginning of QRS complex

2. Reference baseline and assessment

PR-segment displacement is assessed relative to the isoelectric reference, ideally the TP segment (the interval between the end of the T wave and the beginning of the next P wave). This distinction matters because the PR segment itself should not automatically be assumed to represent the baseline when it is displaced.

Practical measurement principle

Compare the PR segment with the TP segment whenever a clear TP segment is available. The supplied reference specifically emphasises that PR changes are relative to the TP baseline.

When the TP segment is difficult to identify

In some ECGs, particularly when the rate is fast or the waveform components overlap, a clear TP segment may not be present. The source illustrates PR-depression measurement in both situations: with a distinct TP segment and without a clearly defined TP segment. In the latter setting, measurement is more challenging and should be interpreted cautiously.

Clear TP segment

Use the visible TP level as the reference and assess the vertical displacement of the PR segment from that level.

Unclear TP segment

Recognise the limitation before assigning a measurement. Avoid false precision when the reference level cannot be confidently established.

3. PR-segment changes in acute pericarditis

PR-segment depression is a recognised ECG feature described in acute pericarditis. The supplied ECG reference presents it as part of a broader pattern rather than an isolated diagnostic finding.

Characteristic ECG pattern described in the source

PR-segment depression.
Widespread, concave (saddle-shaped) ST-segment elevation.
Reciprocal ST-segment depression and PR-segment elevation in leads aVR and V1.
Absence of reciprocal ST-segment depression in other leads.

Figure 2. Conceptual illustration of the direction of PR displacement described in pericarditis. It is not a reproduction of a clinical ECG.

Interpretation tip

Look for the distribution of ST- and PR-segment changes across the 12-lead ECG. The source describes PR depression in many leads with reciprocal PR elevation and ST depression in aVR and V1.

The source includes an example of PR depression in lead V5 accompanying concave ST elevation, and an example of PR elevation in aVR accompanying reciprocal ST depression. These examples illustrate why PR-segment assessment should be integrated with the overall ECG pattern.

4. PR-segment changes and atrial ischaemia

In a patient with myocardial infarction, PR-segment elevation or depression may indicate concomitant atrial ischaemia or infarction. The supplied reference reports associations with adverse outcomes after myocardial infarction, including a higher risk of atrioventricular block, supraventricular arrhythmias, and cardiac free-wall rupture.

Liu’s criteria cited for atrial ischaemia / infarction

The source lists the following ECG findings. These thresholds are reproduced as reported in the supplied educational reference; they should not be treated as a stand-alone diagnostic rule without reviewing the original study and the clinical context.

Finding	Threshold / distribution described
PR elevation in lateral leads	>0.5 mm in V5 and V6, with reciprocal PR depression in V1 and V2
PR elevation in lead I	>0.5 mm in lead I, with reciprocal PR depression in leads II and III
PR depression in precordial leads	>1.5 mm
PR depression in limb leads	>1.2 mm in leads I, II, and III
Minor criterion: P-wave morphology	M-shaped, W-shaped, irregular, or notched P waves

Clinical caution

PR deviation in myocardial infarction is a potential marker of associated atrial involvement, not proof of atrial infarction in isolation. Correlate with the full ECG, clinical presentation, haemodynamic status, and other investigations.

Inferior myocardial infarction

The supplied article illustrates profound PR-segment depression in the inferior leads in acute inferior myocardial infarction, with examples both when the TP segment is clearly visible and when it is not. The accompanying ECGs also show ST-segment elevation in the inferior leads. The cited study by Jim and colleagues examined the prognostic implications of this finding.

5. A practical approach to PR-segment assessment

Locate the segment: identify the end of the P wave and the onset of the QRS complex.
Establish the reference: use the TP segment as the baseline when it is clearly identifiable.
Assess direction: determine whether the PR segment is depressed or elevated relative to that baseline.
Review lead distribution: note whether changes are widespread, reciprocal, or concentrated in particular lead groups.
Integrate the ECG: interpret PR changes alongside ST-segment morphology, symptoms, and the clinical setting.
State limitations: if the TP baseline is indistinct, acknowledge uncertainty in measurement.

Knowledge check: Which ECG segment is the preferred reference for judging PR-segment displacement when clearly visible?

Key takeaways

The PR segment extends from the end of the P wave to the beginning of the QRS complex and is usually isoelectric.
PR-segment displacement should be judged against the TP segment when that baseline is clearly visible.
Acute pericarditis may produce PR depression, with reciprocal PR elevation in aVR and V1 as part of a broader ECG pattern.
In myocardial infarction, PR-segment deviation may suggest concomitant atrial ischaemia or infarction and has been associated with adverse outcomes.
Interpret PR changes in the context of the complete 12-lead ECG and clinical presentation.

References

Cadogan M. PR segment.Life in the Fast Lane (LITFL) ECG Library Basics. 1 August 2020. https://litfl.com/pr-segment-ecg-library/. Source material supplied for this lesson; licensed under CC BY-NC-SA 4.0 as stated on the source page.

Jim MH, Siu CW, Chan AO, Chan RH, Lee SW, Lau CP.Prognostic implications of PR-segment depression in inferior leads in acute inferior myocardial infarction. Clinical Cardiology. 2006;29(8):363–368.

Attribution and adaptation note.Educational concepts in this page are adapted and reorganised from the supplied LITFL PR-segment article. Original Appcardio schematic illustrations were independently constructed and are not reproductions of the source ECG images. Verify licensing and attribution requirements before publication or redistribution.

Medical education disclaimer: This page is intended for educational purposes and does not replace clinical assessment, professional judgement, local protocols, or specialist consultation. ECG findings must be interpreted in the context of the patient's presentation and the complete clinical record.