Knuckle Sign on ECG: A Simple Visual Clue to Acute Pericarditis

Knuckle Sign on ECG: A Simple Visual Clue to Acute Pericarditis

The knuckle sign is a helpful electrocardiographic visual analogy used to recognize acute pericarditis, particularly by focusing attention on lead aVR. It combines characteristic ST- and PR-segment changes into an easily remembered pattern, improving bedside ECG interpretation.

What Is the Knuckle Sign?

The knuckle sign refers to the resemblance between: • PR-segment elevation with ST-segment depression in lead aVR

• The flexed shape of a finger at the knuckle joint

When viewed together, the upward deflection of the PR segment followed by a downward ST segment in aVR mimics the contour of a bent knuckle, hence the name.

ECG Basis of the Knuckle Sign

Acute pericarditis causes diffuse inflammation of the pericardium, leading to characteristic atrial and ventricular injury currents.

Key ECG features include: • Diffuse ST-segment elevation (usually concave) in most leads

• PR-segment depression in multiple leads (atrial injury)

• Reciprocal PR-segment elevation and ST-segment depression in lead aVR

Lead aVR looks at the heart from the right upper perspective and therefore shows reciprocal changes compared to the left-sided leads.

Why Lead aVR Is Central to the Knuckle Sign

In acute pericarditis: • PR depression elsewhere = PR elevation in aVR

• ST elevation elsewhere = ST depression in aVR

This combination in aVR creates the classic “knuckle” appearance: • The elevated PR segment forms the proximal knuckle

• The depressed ST segment forms the distal bend

This pattern is highly supportive of acute pericarditis and helps differentiate it from other causes of ST elevation.

Clinical Importance

The knuckle sign is useful because: • It supports the diagnosis of acute pericarditis

• It helps distinguish pericarditis from STEMI

• It reinforces careful inspection of lead aVR, which is often overlooked

Unlike myocardial infarction: • ST elevation in pericarditis is diffuse, not territorial

• PR-segment changes are common in pericarditis but rare in MI

• Reciprocal ST depression is typically absent except in aVR (and sometimes V1)

Diagnostic Value

The knuckle sign should be interpreted in conjunction with: • Typical chest pain (sharp, pleuritic, relieved by leaning forward)

• Pericardial friction rub

• Elevated inflammatory markers

• Pericardial effusion on echocardiography

It is not a standalone diagnostic criterion but a strong supportive ECG sign.

Teaching Pearl

Always inspect lead aVR when evaluating suspected pericarditis. PR elevation with ST depression in aVR—the knuckle sign—is a simple, memorable clue that reinforces the diagnosis of acute pericarditis and prevents mislabeling the ECG as acute coronary syndrome.

Summary

The knuckle sign is a visual ECG concept highlighting PR-segment elevation and ST-segment depression in lead aVR. It reflects reciprocal atrial and ventricular injury currents in acute pericarditis and serves as a practical bedside tool to improve diagnostic confidence, especially for trainees and clinicians in acute care settings.