Spodick sign on ECG

Downsloping of the TP segment (the Spodick sign) is a classic ECG feature of acute pericarditis, reflecting diffuse subepicardial inflammation.

TP Segment on ECG – Clinical Significance and Interpretation

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Introduction

The TP segment is one of the most overlooked yet clinically important portions of the ECG. Although it appears electrically silent, it serves as the true isoelectric baseline against which ST-segment deviations are measured.

Understanding the TP segment is essential for accurate interpretation of ischemia, pericarditis, atrial repolarization abnormalities, and subtle ST changes.

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What is the TP Segment?

The TP segment is the interval between the end of the T wave and the beginning of the next P wave.

Electrophysiologically, it represents:

• Completion of ventricular repolarization

• Electrical diastole

• A period with no net electrical activity

Because both atria and ventricles are electrically silent, this segment is considered the true baseline of the ECG.

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Why is TP Segment Important?

1. True Isoelectric Baseline

The TP segment is the most accurate reference for:

• Measuring ST-segment elevation

• Measuring ST-segment depression

• Identifying PR-segment deviation

Using the PR segment as baseline can be misleading in certain conditions (e.g., pericarditis), because atrial repolarization (Ta wave) may distort it.

Clinical pearl: Always compare ST deviation to the TP segment, not the PR segment.

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2. Differentiating Pericarditis from STEMI

In acute pericarditis:

• PR segment depression occurs

• ST elevation is diffuse

• TP segment may appear relatively elevated compared to depressed PR

If PR depression is present, the apparent ST elevation may partly reflect a depressed PR segment rather than true ST elevation.

Thus, the TP segment helps confirm real ST deviation.

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3. Atrial Repolarization (Ta Wave)

The Ta wave:

• Occurs after the P wave

• Is usually hidden within the PR segment

• May cause PR depression

Since the TP segment is unaffected by atrial repolarization, it remains the most reliable baseline.

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4. In Tachycardia

At high heart rates:

• TP segment shortens

• May disappear completely

• T wave merges with next P wave

In such cases, baseline identification becomes difficult, and ST assessment must be done cautiously.

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Visual Representation

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How to Measure ST Deviation Using TP Segment

Stepwise approach:

1. Identify end of T wave

2. Identify beginning of next P wave

3. Draw imaginary baseline through TP segment

4. Compare J-point and ST segment to this baseline

This prevents overcalling or undercalling ischemia.

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Special Clinical Situations

1. Atrial Flutter

Continuous atrial activity may eliminate clear TP segment.

2. Atrial Tachycardia

Baseline may be distorted.

3. Hyperkalemia

T waves may merge with P waves, obscuring TP.

4. Pericarditis

PR depression best appreciated relative to TP baseline.

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Exam Pearls (FCPS / Cardiology)

• TP segment = true isoelectric line

• ST deviation should be measured from TP, not PR

• PR depression in pericarditis is real only when compared to TP

• Loss of TP in tachycardia reduces ST accuracy

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Common Mistakes

• Using PR segment as baseline in pericarditis

• Ignoring atrial repolarization effect

• Misinterpreting PR depression as ST elevation

• Overcalling ischemia when baseline is incorrect

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Summary

The TP segment represents electrical diastole and is the most reliable isoelectric reference on ECG. Accurate ST and PR interpretation depends on recognizing and using this segment properly. In pericarditis, atrial repolarization abnormalities, and tachycardia, careful identification of the TP segment prevents diagnostic errors.

Mastering this subtle ECG detail significantly improves diagnostic precision in both exams and real-world cardiology practice.