Shark-Fin Sign on ECG: A Life-Threatening STEMI Pattern Often Misinterpreted as Wide QRS

Faraz Afzal
Jan 21
4 min read

Updated: 2 days ago

The shark-fin sign on ECG is a rare but extremely high-risk electrocardiographic pattern seen in acute myocardial infarction. It is most commonly associated with left main coronary artery occlusion or proximal LAD occlusion and carries a very high risk of cardiogenic shock, malignant arrhythmias, and cardiac arrest.

Because the ECG appears to show an extremely wide QRS complex, the pattern is frequently misinterpreted by automated ECG algorithms and clinicians, leading to potentially fatal delays in reperfusion.

What Is the Shark-Fin Sign on ECG?

The shark-fin sign describes a triangular waveform formed by the fusion of the QRS complex, ST segment, and T wave into a single large deflection. Rather than a true conduction abnormality, the apparent QRS widening is caused by massive, early-onset ST-segment elevation.

Key electrocardiographic features include:

Loss of clear separation between QRS, ST segment, and T wave
Massive ST elevation beginning immediately after QRS onset
A triangular or “shark-fin” morphology
Often absent or indiscernible P waves

The ECG appearance resembles the dorsal fin of a shark, which gives the sign its name.

Why the Shark-Fin Sign Is So Dangerous

The shark-fin pattern is strongly associated with:

Left main coronary artery occlusion
Proximal LAD occlusion
Rapid development of:
- cardiogenic shock
- ventricular fibrillation
- pulseless electrical activity
- sudden cardiac arrest

Although the pattern occurs in only about 1–2% of anterior STEMI cases, reported series show that nearly all patients develop severe hemodynamic compromise, often before or during reperfusion.

Mortality remains extremely high despite prompt intervention.

Why the ECG Is Commonly Misinterpreted as Wide QRS

Automated ECG systems frequently label this pattern as:

extreme QRS widening
ventricular rhythm
nonspecific intraventricular conduction delay

This misinterpretation occurs because:

ST elevation is so pronounced that it visually merges with the QRS complex
The algorithm incorrectly measures the combined QRS–ST waveform

A critical diagnostic step is comparison with a prior ECG:

Similar QRS morphology in some leads argues against new bundle branch block
Sudden “QRS widening” in a patient with chest pain should be assumed ischemic until proven otherwise

Clinical Case: Collapse During Cycling

Background

A man in his early 60s with known atrial fibrillation and a mechanical aortic valve developed sudden chest pain while cycling and collapsed.

Prehospital ECG

The ECG was automatically interpreted as showing an extremely wide QRS complex (~255 ms).

On Hospital Arrival

The patient was hypotensive, confused, pale, and diaphoretic. Transthoracic echocardiography revealed severely reduced left ventricular systolic function. He was accepted for emergent coronary angiography.

Course in the Cath Lab

The patient suffered cardiac arrest. Mechanical chest compressions were initiated. During ongoing CPR, angiography revealed acute thrombotic occlusion of the left main coronary artery, which was stented.

Return of spontaneous circulation was achieved, and the patient was intubated and started on vasopressors. Despite reperfusion, he developed refractory respiratory and circulatory failure and ultimately died in pulseless electrical activity.

Additional ECG Observations

P waves may be absent
The rhythm may appear nodal or idioventricular
Ventricular fibrillation is a common initial arrest rhythm
Similar rhythms can also be observed during reperfusion

Key Learning Points

The shark-fin sign on ECG is one of the highest-risk STEMI patterns
Apparent extreme QRS widening may represent massive ST-segment elevation
Automated ECG interpretation should never be trusted blindly
In patients with chest pain and a shark-fin pattern, assume left main or proximal LAD occlusion until proven otherwise
Immediate recognition and emergent reperfusion are critical, though prognosis remains poor

Final Comment

This ECG pattern is uncommon, but when present it represents one of the most malignant electrocardiographic presentations of acute myocardial infarction. Recognizing the shark-fin sign — and understanding that it is not true QRS widening - can be lifesaving.

FAQ

1) What is the shark-fin sign on ECG?

The shark-fin sign is a rare, high-risk ECG pattern in which the QRS complex, ST segment, and T wave merge into a single large triangular waveform. It is usually caused by massive ST-segment elevation due to acute left main or proximal LAD coronary artery occlusion.

2) Why is the shark-fin sign often misinterpreted as a wide QRS?

Because the ST-segment elevation begins very early and is extremely pronounced, it obscures the boundary between the QRS complex and the ST segment. Automated ECG algorithms may therefore measure the entire waveform as QRS duration.

3) Which coronary artery occlusion is most commonly associated with the shark-fin sign?

The shark-fin pattern is most commonly associated with acute occlusion of the left main coronary artery or the proximal left anterior descending (LAD) artery.

4) How dangerous is the shark-fin sign?

It represents one of the most malignant ECG patterns in acute myocardial infarction. Most patients rapidly develop cardiogenic shock or cardiac arrest, and the risk of ventricular fibrillation is very high.

5) How common is the shark-fin sign in STEMI?

The pattern is rare, occurring in approximately 1–2% of anterior STEMI cases, but it is strongly associated with severe hemodynamic collapse and high mortality.

6) Can P waves be absent in the shark-fin sign?

Yes. P waves may be absent or difficult to identify, and the rhythm may appear nodal or idioventricular. Similar rhythms can also be observed during reperfusion.

7) What is the key clinical takeaway?

Do not rely blindly on automated ECG interpretation. In patients with chest pain and apparent extreme QRS widening, always consider massive ST-segment elevation (shark-fin sign) and treat as a very high-risk STEMI.