Shark Fin STEMI

When we see STEMI, we are hopefully familiar with the obvious ST elevation that even an Uber driver could see.

Then we have our more subtle STEMIs or OMIs (Occlusion MI).

But what do you think about an ECG that looks like this?

This could easily be diagnosed as a wide complex sinus tachycardia but in fact it’s a STEMI. Most STEMIs you’ll encounter will be either bradycardic or less than 100 BPM. One of the most prominent reasons the patient will be tachycardic in the presence of STEMI is if they’re in cardiogenic shock. This “Giant R wave” morphology looks like a shark fin swimming right at you in the ocean. This is why it’s called a “Shark Fin STEMI” and it usually results in poor outcomes. Shark fin is the fusion of the QRS and T wave which gives it a sloping “shark fin” appearance. Now let’s look at another example of this type of ECG (the resources out there involving Shark Fin STEMIs are sort of scarce. Dr. Smith’s ECG blog has many excellent examples).

If you see this and think “wide and bizarre” you’re absolutely right. For ECGs that present wide and bizarre, it’s most often reasonable to consider hyperkalemia. It’s of high importance to scrutinize the ECG though and keep STEMI high up on your differential. Now first, you must find the J point trace it down the ECG to determine elevation and depression. Hyperkalemia rarely mimics a STEMI but it is not impossible as it is the great imitator. If you look at the morphologies of a typical patient who has hyperkalemia, and compare it to the shark fin appearance of a STEMI, they often times do look different. Hyperkalemia should be strongly considered if the QRS is nearing or over 1 big box wide.

After tracing the ECG, you can easily see that you have elevation in the anterior and lateral leads; with ST depression in II, III, and aVF. So why does this morphology occur? Well when the heart is injured due to ischemia/infarction, the cells will not work as efficiently right? So when the cells don’t work as well as they used to, you can expect issues in the phase 0-4 action potential with your electrolytes. Here is a diagram to help you remember these phases:

Some studies suggest that this appearance is due to a massive decrease in the transmural electrical conduction. Now what causes this? Electrolyte issues such as potassium loss, too much calcium, and sodium channels failing to work around the phase 2 action potential. This can cause shortening of the phase 2 action potential which gives it a wide complex look. These patients have a high probability of going into ventricular fibrillation due to “diffuse dispersion of excitability, conduction and refractoriness” (Cipriani).

Now this is not a common ECG finding (around 1.4% of STEMIs according to one study) but noticing it is extremely important. More elevation = more myocardial death. This means the heart is at great risk for pumping less efficiently. Infarcted myocardial cells lose their elasticity and become scarred so their stretch drops dramatically. And why is stretch important? Because it’s directly linked to our preload. When the preload drops in these patients, they can’t efficiently pump enough blood to perfuse the organs. So when you see a shark fin morphology, you need to keep hyperkalemia and STEMI at the forefront of your thought process.

This site is meant to be used for educational use only. We strive to push evidence based medicine with no bias to help you obtain all the important information. You should always follow your protocols that have been set in place. 

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-Scopeducation Team (Matt)


Huge shout out to Dr. Smith’s ECG blog ( check out his site for amazing ECG interpretations

Click to access Elf-cap-shape-signal-a-new-depolarization-ominous-marker-in-the-sitting-of-anterior-myocardial-infarction-complicated-with-complete-right-bundle-branch-block.pdf

Alberto Cipriani, Gianpiero D’Amico, Giulia Brunello, Martina Perazzolo Marra, Federico Migliore, Luisa Cacciavillani, Giuseppe Tarantini, Barbara Bauce, Sabino Iliceto, Domenico Corrado, Alessandro Zorzi,
The electrocardiographic “triangular QRS-ST-T waveform” pattern in patients with ST-segment elevation myocardial infarction: Incidence, pathophysiology and clinical implications, Journal of Electrocardiology (

Khalid, L., & Dhakam, S. (2008, February). A review of cardiogenic shock in acute myocardial infarction. Retrieved February 15, 2021, from

Tarantino, N., Santoro, F., Guastafierro, F., Martino, L., Scarcia, M., Ieva, R., . . . Brunetti, N. (2018, July 09). “Lambda‐Wave” st‐elevation is associated with severe prognosis in stress (takotsubo) cardiomyopathy. Retrieved February 15, 2021, from

Yan, G., & Antzelevitch, C. (1996). Cellular basis for THE Electrocardiographic J wave. Circulation, 93(2), 372-379. doi:10.1161/01.cir.93.2.372

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