Most heart attack tracings look recognizably like heart attack tracings. A small subset do not, and one of those looks like the dorsal fin of a shark cutting across the page.
A man in his forties arrived at an emergency department with chest pain that had lasted six hours. His blood pressure had fallen to 80 over 50, his heart was racing, and crackles at the base of both lungs said fluid was already backing up. His electrocardiogram showed enormous triangular waveforms across leads V2 through V4 and in leads I and aVL, with the QRS complex, ST segment and T wave fused into a single giant deflection. An echocardiogram put his ejection fraction at 25 percent. Angiography found complete thrombotic occlusion of the left main coronary artery, the vessel that supplies most of the left side of the heart.
The report was published by Vaibhav Mishra and colleagues in the Journal of Clinical and Preventive Cardiology.
One in Every Seventy Heart Attack Tracings
The shark fin pattern goes by several formal names, including triangular QRS-ST-T waveform, lambda wave and giant R wave. Whatever it is called, it is uncommon.
The best prospective measurement comes from an Italian cohort. Among 428 consecutive patients presenting with ST-elevation heart attacks, 367 met the enrollment criteria, and researchers found the triangular waveform on the admission tracing in five of them, or 1.4 percent. A later single-center review in Las Vegas found eight cases among 417 heart attack activations, or 1.9 percent.
The Italian numbers also quantify how differently those patients fared. Left main coronary artery involvement appeared in half of the triangular-waveform group compared with 0.6 percent of the others. All five experienced ventricular fibrillation, against 9.6 percent of the comparison group. Four of five developed cardiogenic shock, and two of five died during the hospital stay.
Those are small absolute numbers, and the study authors said as much. The pattern is rare enough that even a prospective cohort of several hundred yields only a handful of cases.
The Clue Hidden in Which Leads Stayed Quiet
What made this particular tracing interpretable was not only the fin shape but a combination of features around it.
The patient had developed a new right bundle branch block alongside the anterior ST elevation. The authors note that the right bundle is supplied by the first septal perforator branching off the left anterior descending artery, so new block in that conduction pathway argues for a blockage high up in the vessel.
The second clue was an absence. There was no ST elevation in leads V1 or aVR, which is atypical for a blockage confined to the left anterior descending artery. The authors argue that when the circumflex artery is also involved, as it is when the left main is occluded, the reciprocal changes it produces cancel out the elevation that would otherwise appear in those leads. Shark fin elevation in the anterolateral leads, a new right bundle branch block and that specific silence in V1 and aVR together point to the left main.
At catheterization, repeated balloon dilatation of the lesion produced poor distal flow. Intracoronary vasodilators improved it, and two overlapping drug-eluting stents were deployed from the left main into the mid left anterior descending artery. Flow was restored in both major left-sided vessels. At six months the patient was well, and his ejection fraction had recovered.
The Pattern That Gets Mistaken for Something Else
The most dangerous property of the shark fin tracing is how easily it is misread. Because the waveforms are wide and merged, they can resemble ventricular tachycardia or the changes seen with high potassium levels.
That is not theoretical. A separately published case described a 79-year-old man whose shark fin pattern was misinterpreted as ventricular tachycardia after a cardiac arrest, leading to amiodarone and repeated defibrillation attempts before the correct diagnosis was reached. He underwent successful stenting but died of multiorgan failure.
The pattern is also not exclusive to blocked coronary arteries. Cardiologists have documented it in takotsubo cardiomyopathy and in aortic dissection, which is why the tracing prompts urgent angiography rather than an assumption about the cause.
None of that changes what an ordinary person should do. The CDC lists five common heart attack symptoms: chest pain or discomfort, pain or discomfort in the jaw, neck or back, pain or discomfort in the arms or shoulder, shortness of breath, and feeling weak, lightheaded or faint. The agency's guidance is to call 911 immediately rather than arranging other transport.
What This Report Adds and What It Does Not
A single case cannot establish how reliably the shark fin pattern predicts a left main blockage. The 1.4 percent incidence figure and the outcome comparisons rest on five affected patients, which is thin ground for firm conclusions about prognosis.
What is reasonably well supported across the literature is the direction of the association. The pattern appears in the earliest hours of a heart attack, tracks with a large area of dying muscle, and identifies patients at elevated risk of ventricular fibrillation and shock.
This case adds a survivor to that record, along with a specific lead-pattern argument for localizing the blockage. It also reinforces that the finding is an emergency-department recognition problem rather than a specialist one, since the first tracing is usually read long before a cardiologist arrives.
Anyone experiencing chest discomfort with any of the associated symptoms should call emergency services rather than attempting to interpret the situation.
Key Questions Answered
What is a shark fin ECG?
It is a rare tracing in which the QRS complex, ST segment and T wave merge into one giant triangular deflection. It is also called a triangular QRS-ST-T waveform or lambda wave.
How often does it appear?
Rarely. A prospective Italian study found it in 1.4 percent of ST-elevation heart attack patients, and a US review found it in 1.9 percent of activations.
Why is it considered dangerous?
In the Italian cohort, patients with the pattern had far higher rates of left main coronary involvement, ventricular fibrillation, cardiogenic shock and in-hospital death than other heart attack patients.
What happened to the patient in this report?
Angiography showed complete clot-based blockage of his left main coronary artery. After balloon dilatation and two drug-eluting stents, flow was restored, and at six months his heart function had recovered.
Can the pattern be confused with something else?
Yes. Its wide merged waveforms can be mistaken for ventricular tachycardia or for changes caused by high potassium, which has led to treatment delays in published cases.
What symptoms should trigger a call to 911?
Chest pain or discomfort, pain in the jaw, neck, back, arms or shoulder, shortness of breath, and weakness or lightheadedness are the five symptoms the CDC highlights.
