These imposters have been fooling pre-hospital providers since…

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I don’t actually know if that’s true or not, buuuuut what I do know is that it had fooled me since 2011. This is when I got the title of Paramedic. In 2013, when I had taken a critical care paramedic course, that I found out how bad I actually was; and how bad other clinicians were.

ST-segment Elevation Myocardial Infarction (STEMI) mimics aren’t anything new, yet these rhythms and conditions “fool” many providers into thinking there might be some underlying pathology in need of emergent care when, in fact, it doesn’t. Or, even worse, the wrong care is provided from the lack of education on a topic such as this. Don’t get me wrong, there are variables with these mimicking rhythms that may be due to ischemia, injury, or infarct. Unfortunately, these misinterpreted 12-leads are often found to have ST-segment elevation that is mistaken for a STEMI causing unnecessary activation of the cath lab. This unnecessary activation is likely the better alternative to not calling the STEMI alert when there is substantial evidence suggesting PCI is needed, but we must also keep in mind that this unnecessary activation can cost hospitals $5,000 or more. Should we try to save money for the hospital by calling fewer STEMI alerts? Well, no, not necessarily, but we should educate ourselves to prevent these unnecessary losses and possible inappropriate treatment of our patients.

I have walked out of some emergency departments with my eyes to the floor, dragging my feet, and feeling incredibly stupid and embarrassed as I made my cot and returned back to service. Apparently, the ST-segment elevation in leads V1, V2, and V3 (2-3 mm in amplitude) wasn’t a STEMI. It was left ventricular hypertrophy with strain while also closely resembling previous 12-leads since this patient was a “frequent flyer.” The bradycardic rhythm I paced, that I assumed was from an MI due to the obvious ST-segment elevation, had a potassium of 7.8 mEq/L. (I spent time completing my MONA protocol instead of gathering a thorough history). The chest pain patient with ST-segment elevation in the precordial leads (V1–V4) apparently had a history of left bundle branch block and recurrent chest pain (heartburn) from consuming large amounts of a condiment known to cause “chest pain”. (Also a well-known patient to this ED.) I could go on and on with stories of misinterpreted 12-leads from myself or other providers.

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So… What is a STEMI mimic exactly? A STEMI mimic is a rhythm, more specifically a 12-lead ECG, that “mimes” or mimics ECG morphology often seen in patients with ST-segment elevation myocardial infarctions. These rhythms/interpretations are mistaken for an MI on the 12-lead ECG, and lead to inappropriate activation of cath labs and treatment of our patients. These mimicking rhythms can be benign or show some other pathology requiring attention geared toward different treatment or intervention than the standard ACS requirements.

While there are many STEMI mimics, these are the most commonly encountered by the pre-hospital provider:

➢ Benign Early Repolarization (BER)

➢ Hyperkalemia

➢ Left bundle branch block (LBBB)

➢ Left ventricular hypertrophy

➢ Pericarditis

➢ Ventricular and paced rhythms

** This list is not all-inclusive and only serves to remind clinicians of the most commonly misinterpreted, STEMI mimicking, findings obtained via 12-lead ECG. **

These rhythms all vary in morphology to some degree, but all share a common, misguiding element… ST-segment elevation! Or, at least what appears to be ST segment elevation. These rhythms are the culprit behind those embarrassing moments I, and other providers have to blame. Because of them, I feel a personal need to inform other clinicians of the dangers they pose to our pride (and patient care) through the FOAMed community and the Heavy Lies the Helmet blog.


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I would like to make a few statements and pearls before we do a quick overview of all the rhythms covered above.

1. If you encounter a “STEMI mimic” in the field, remember to correlate the signs and symptoms with the patient’s presentation. If you suspect ACS as a possibility, go with your gut instinct. Also, follow your local protocol. ☺

2. If a STEMI mimic is identified, this doesn’t mean that the patient isn’t having a coronary/cardiac event. Blood work, including cardiac profiles and further testing, is needed at the receiving facility.

3. Look for factors that favor MI:

➢ ST segment depression suggestive of ischemia or injury

➢ ST-segment elevation with reciprocal changes

➢ Convex ST-segment elevation

4. LBBB, LVH, and ventricular/paced rhythms have shared features. These features relate to an ECG pattern in which the QRS complex and the T-wave are oppositely directed, or discordant. In other words, if the QRS’ terminal direction is downward or negative, the T waves terminal direction would be upward or positive, and vice-versa. This discordance is noteworthy because the T-wave can often drag the ST-segment along with it giving it the appearance of ST-segment elevation.

5. Excellent Sources for ECG education and reference:

➢ 12-Lead ECG in Acute Coronary Syndromes

➢ 12 Lead ECG: The Art of Interpretation

➢ Life in The Fast Lane

➢ Dr. Smith’s ECG blog

➢ Critical Care Transport (AAOS)

➢ ECG’s for the Emergency Physician

➢ FOAMed podcasts

Benign Early Repolarization (BER)

This is a common ECG pattern found usually in young, healthy patients. As its name suggests, there is a repolarization issue… an early one causing change that can be mistaken for a STEMI. BER occurs in 10–15% of chest pain patients (ED patients) causing widespread ST-segment elevation that may also be mistaken as Pericarditis. These changes make BER a diagnostic challenge for clinicians.


➢ Concave, ST-segment elevation; widespread but often seen in the precordial leads

➢ Notching, or slurring at the J point; fish-hook in appearance

➢ ST-segment elevation is usually minimal

  • < 2 mm in precordial leads
  • < 0.5 mm in limb leads

➢ No reciprocal changes

➢ No progression or change in ST-segment morphology over time

Example of J-point notching:


12 Lead ECG showing strong evidence of BER:



Hyperkalemia, as seen on a 12-lead ECG, is a conduction abnormality caused by increased serum potassium levels that may give a similar appearance to what we typically see in AMI/STEMI patients. The changes seen in this condition can mimic or fool clinicians, if further investigation isn’t attempted. These ECG changes seen in hyperkalemia are progressive and worsened as K+ levels increase. Without excellent history taking, this rhythm will be difficult to identify and treat appropriately.


➢ Tall, peaked T waves

  • Earliest sign
  • Usually occurs at 5.5 mEq/L
  • May be mistaken for hyperacute T-waves or ST-segment elevation

➢ QRS, T wave, and interval widening

  • Occurs at levels > 6.5 mEq/L

➢ P wave flattening

  • Occurs at levels around 6.5 – 7.0 mEq/L

➢ Sine waves; wide, bizarre appearing complexes

  • Occurs at levels > 7.0 mEq/L

Tall, peaked T waves indicative of hyperkalemia:


12 Lead ECG showing tall, peaked T waves:


Sine waves:


Left Bundle Branch Block (LBBB)

LBBB is caused by a disruption of the electrical conduction of the left bundle (both the anterior and posterior fascicle). Conduction delays and abnormalities are found in this rhythm. It shows changes that may lead the provider to think, “STEMI!” While in some cases this patient may be having a STEMI, there is a possibility that this is normal for the patient. A good history is needed when this rhythm is diagnosed. Sgarbossa Criteria can also be utilized in the presence of LBBB to assist with diagnosing MI.

Sgarbossa Criteria can be used to diagnose an MI in the presence of a LBBB.


➢ Complex duration > 120 ms

➢ Broad, monomorphic R waves in leads I and V6; no Q waves

➢ Broad, monomorphic S waves in lead V1; may have small R waves

  • R wave progression is poor

➢ Possible left axis deviation



Left Ventricular Hypertrophy (LVH)

Left ventricular hypertrophy (LVH) is often caused from hypertension and valvular heart issues. LVH gives the 12-lead an abnormal appearance often seen with ST-segment elevation in leads V1 – V3. This fools some providers into assuming ACS as a culprit since it may mimic that of a STEMI. LVH with strain is usually the reason for ST-segment changes, but all variations should be considered.

Most Commonly Used Criteria:

➢ Count the depth of the S wave in V1 or V2, whichever is deeper in mm, use that number.

➢ Count the height of the R wave in V5 or V6, whichever is taller in mm, use that number.

➢ Add the two numbers obtained together. If the number is greater than, or equal to 35 mm, LVH is present.

Other Criteria:

➢ Any precordial lead ≥ 45 mm

➢ R wave in lead aVL ≥ 11 mm

➢ R wave in lead I ≥ 12 mm

➢ R wave in lead aVF ≥ 20 mm

V2 + V5 = > 35 mm




Pericarditis, or inflammation of the pericardium, is usually caused from a viral infection. It produces chest pain and 12-lead ECG changes that may emulate or mimic a STEMI. The ST-segment elevation is diffuse due to irritation of the entire pericardium. This irritation causes a net positivity of the pericardium expressed as ST-segment elevation.


➢ PR depression

➢ Diffuse ST-segment elevation

➢ Scooping, upwardly concave ST segments

➢ Possible notching at the end of the QRS complex

Pericarditis and BER are difficult to differentiate from one another. A thorough history can assist with identification. Further research is needed to learn the differences.


About the Author


Jared Patterson, EMT-P, FP-C, CCP-C is a critical care paramedic nearing ten years in EMS; 7 as a paramedic. He works for a hospital-based 911 and transport service, as well as working in the ER to a full scope (triage, assessment, treatment, and discharge). He is also an adjunct EMT and paramedic instructor. His early career was spent as a volunteer firefighter and EMT for an interfacility transport service. Shortly after obtaining his paramedic license, he began working at his current shop; requiring all paramedics, at a minimum, to obtain endorsement by the state of Iowa as a CCP. Following a CCP course at Creighton University in Omaha, Nebraska, he began to obsess over vent management, airway management, and 12-lead interpretation. He has worked in multiple municipalities and variations of services including fire and rescue, private EMS, and county-based in both urban and rural environments. He has worked closely with other departments and ambulance companies in his local area. He has obtained both his FP-C and CCP-C within the last year through motivation of the FOAMed community. He would like to personally thank HLTH for this opportunity and, if possible, would like to continue to author blogs. He has many ideas for educational blogs, as well as “hot topics” that he frequently hears amongst his colleagues.

Burns, E. (2017, April 02). Benign Early Repolarisation • LITFL • Life in the Fast Lane Medical Blog. Retrieved from

Burns, E. (2017, April 16). ECG features of hyperkalaemia LITFL ECG Library. Retrieved from

Burns, E. (2017, April 16). Left Bundle Branch Block (LBBB). Retrieved from

Burns, E. (2017, March 18). Left Ventricular Hypertrophy. Retrieved from

Garcia, T. B., MD. (2015). 12-lead ECG: The art of interpretation. Burlington, MA: Jones & Bartlett Learning.

McEvoy, M., PhD, NRP, RN, CCRN, Rabrich, J. S., DO, FACEP, EMT-P, Murphy, M., RN, CEN, EMT-P, & Pllak, A. N., MD, FAAOS. (2018). Critical care transport. Burlington, MA: Jones & Bartlett Learning.

Nickson, C. (2017, August 29). Pericarditis ECG – LITFL Life in the Fastlane ECG Library. Retrieved from

Phalen, T., & Aehlert, B. (2011). 12-Lead ECG in Acute Coronary Syndromes. S.l.: MOSBY JEMS.


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