STEMI Mimics: Spot Subtle Impostors of Myocardial Infarction

The diagnosis of ST-Elevation Myocardial Infarction (STEMI) is a time-sensitive clinical challenge with critical implications for morbidity and mortality¹. However, identifying a true STEMI based solely on ECG changes is not always straightforward, as a number of other conditions, collectively termed STEMI mimics, can produce ST-segment elevations without underlying myocardial infarction². Misinterpreting these mimics as true STEMI can lead to unnecessary cath lab activations, resulting in invasive procedures that carry inherent risks³.

While the ECG is a crucial rule-in test for STEMI diagnosis, it is not a reliable rule-out test. This means that while significant ST-elevation on an ECG strongly suggests myocardial infarction and warrants urgent action, the absence of these findings does not necessarily exclude acute coronary syndromes (ACS)⁴. It is critical to correlate ECG findings with the patient’s clinical presentation, symptoms, and additional diagnostic markers, such as troponin levels, to ensure an accurate diagnosis⁵.

In this article, we explore the most common STEMI mimics, their clinical and ECG characteristics, and the role of advanced diagnostic tools, including AI-driven solutions, in aiding accurate and timely diagnosis.

What Are STEMI Mimics?

STEMI mimics are conditions that cause ST-segment elevation on an ECG but are not due to acute coronary occlusion. These may be cardiac or non-cardiac in origin, and each presents a unique challenge to the clinician². 

Unnecessary cath lab activations due to STEMI mimics are relatively common, with studies estimating that approximately 10-36% of patients presenting with ST-segment elevation on ECGs do not have an acute coronary occlusion upon angiography^6,7. Misdiagnosing these conditions as STEMI can lead to inappropriate interventions, exposing the patient to potential complications of invasive procedures, such as contrast-induced nephropathy, radiation exposure, bleeding, and others^6,7.

The ability to distinguish these mimics is essential for patient safety and resource optimization in busy emergency settings⁷.

Common STEMI Mimics and Their Diagnostic Pitfalls

Some of the most notable conditions mimicking STEMI include subarachnoid hemorrhage, Left Bundle Branch Block (LBBB), pericarditis, spontaneous coronary artery dissection (SCAD), hyperkalemia, ventricular paced rhythm, Brugada syndrome, hypothermia, Prinzmetal’s/Variant angina, pulmonary embolism, Takotsubo cardiomyopathy, ventricular aneurysm, left ventricular hypertrophy, early repolarization, and thoracic aortic dissection. Each of these conditions can present with ECG changes that resemble STEMI, yet they require different treatment approaches, underscoring the importance of careful clinical evaluation⁸.

Below, we examine several STEMI mimics, focusing on their ECG characteristics, the mechanisms behind their resemblance to STEMI, and some of the key clinical clues that help differentiate them.

Pericarditis

Pericarditis is an inflammatory condition affecting the pericardium, which can be of infectious (often viral) or non-infectious origin, including autoimmune diseases, cancer, post-cardiac injury syndromes, post-myocardial infarction syndromes, Dressler’s syndrome and others^9,11.

ECG Features of Pericarditis

• Diffuse ST-segment elevation across multiple leads, typically concave (upward-sloping).
• PR-segment depression in leads with ST-elevation, particularly in the inferior leads.
• Absence of reciprocal changes typically seen in STEMI¹⁰.

Clinical Clues

• Sharp, pleuritic chest pain that worsens with inspiration or lying down and improves when sitting upright.
• Pericardial friction rub on auscultation.
• Absence of elevated troponin levels, or mild elevation if myocarditis is also present¹⁰.

Why It Mimics STEMI

Pericarditis can lead to localized ST elevation, but unlike STEMI, it typically lacks reciprocal ST depression, except in leads aVR and V1. Both conditions can produce concave ST elevation, but only STEMI typically results in convex or horizontal ST elevation. Additionally, if the ST elevation is greater in lead III than in lead II, this is a strong indicator of STEMI. PR-segment depression is mostly associated with viral pericarditis and tends to be a transient, early feature, lasting only a few hours¹².

Left Ventricular Hypertrophy (LVH)

LVH results from chronic pressure overload, often due to long-standing hypertension or aortic stenosis. The hypertrophied myocardium alters the electrical conduction patterns, leading to ST-segment abnormalities on the ECG¹³.

ECG Features of Left Ventricular Hypertrophy:

• High-voltage QRS complexes, especially in the precordial leads (V1-V6).
• ST-segment elevation in the anterior leads (V1-V3), often with T-wave inversions.

Strain pattern with ST-depression and T-wave inversions in the lateral leads¹⁴.

Clinical Clues:

• History of hypertension or aortic valve disease.
• Echocardiographic evidence of hypertrophy.

Why It Mimics STEMI

The elevated voltage in the anterior leads and associated strain pattern can mimic an anteroseptal infarction. LVH-induced repolarization abnormalities can lead to confusion, especially in the absence of a clear clinical picture of acute coronary syndrome¹⁵.

Early Repolarization

Early repolarization is a benign ECG variant most commonly seen in young, healthy individuals, particularly athletes. It represents a variation in the electrical activity of the heart’s repolarization phase¹⁶.

ECG Features of Early Repolarization:

• ST-segment elevation, typically concave, in the precordial and inferior leads.
• Notching or slurring of the J-point, especially in the lateral leads.
• No reciprocal ST-segment depression or evolving ECG changes¹⁷.

Clinical Clues:

• Asymptomatic or discovered incidentally.
• No associated chest pain or cardiac risk factors^16,18.

Why It Mimics STEMI

Early repolarization can closely mimic the ST-elevations seen in anterior or inferior STEMI, especially when J-point notching or slurring is present. Differentiation relies on the absence of clinical symptoms, reciprocal changes, and the stability of the ECG over time¹⁶.

Previously Diagnosed Left Bundle Branch Block (LBBB)

LBBB is a conduction abnormality that affects the left ventricle, causing delayed depolarization and abnormal repolarization patterns. While an old, previously diagnosed LBBB is generally not concerning and often reflects underlying structural heart disease or chronic conditions, a new or presumed new LBBB may raise suspicion for an acute myocardial infarction (OMI), especially in the context of other clinical signs of ischemia^19,20.

This can be particularly challenging when interpreting the ECG, as LBBB alters the ST-segment, potentially masking or mimicking ischemic changes. In patients with suspected acute coronary syndrome, the presence of new or presumed new LBBB may raise concern for an underlying ischemic event²⁰.

ECG Features of Previously Diagnosed LBBB ECG :

• Wide QRS complexes (>120ms).
• Discordant ST-segments (ST-segment elevation in leads with a predominantly negative QRS and depression in leads with positive QRS complexes).
• Absence of normal septal Q-waves in leads I, aVL, V5, and V6²¹.

Clinical Clues:

• Often seen in patients with underlying cardiomyopathy or ischemic heart disease.
• Symptomatology may range from asymptomatic to severe heart failure¹⁹.

Why It Mimics STEMI

LBBB creates significant abnormalities in both the depolarization and repolarization of the ventricles, making it difficult to interpret ST-segments. This necessitates the use of additional diagnostic tools, such as coronary angiography, or more sophisticated AI-powered ECG interpretation models, like PMcardio, which can accurately interpret complex conduction abnormalities in LBBB and paced rhythms²².

Brugada Syndrome

Brugada syndrome is a genetic condition affecting the sodium channels of the cardiomyocytes, specifically through mutations in the SCN5A gene, predisposing individuals to ventricular arrhythmias and sudden cardiac death^23,24.

ECG Features of Brugada Syndrome

• ST-segment elevation in the right precordial leads (V1-V3) with a coved or saddleback pattern²⁵.
• T-wave inversion in the right precordial leads.
• Right bundle branch block (RBBB) pattern may also be present²⁶.

Clinical Clues:

• Typically presents in young men with a history of syncope, palpitations, or sudden cardiac arrest²³.
• Family history of sudden cardiac death or arrhythmias²⁴.
• Typically presents without chest pain and can be exacerbated by fever, which increases the risk of arrhythmias due to the temperature sensitivity of sodium channels²⁷.

Why It Mimics STEMI

Brugada syndrome can present with ST-segment elevation in the anterior leads, mimicking a STEMI involving the LAD territory. The absence of reciprocal changes and the coved morphology of the ST-elevation are key distinguishing features²⁵.

Hyperkalemia

Hyperkalemia alters cardiac depolarization and repolarization due to elevated extracellular potassium levels, affecting the resting membrane potential of cardiac myocytes²⁸.

ECG Features of Hyperkalemia:

• Peaked T-waves, particularly in the precordial leads.
• Flattening or absence of P-waves.
• Wide QRS complexes that can merge with ST-segments, creating a sine-wave appearance in severe cases²⁹.

Clinical Clues:

• History of renal failure, potassium-sparing diuretic use, or other causes of hyperkalemia.
• Symptoms of muscle weakness, fatigue, or arrhythmias^28,30.

Why It Mimics STEMI

Severe hyperkalemia can cause broad ST-elevation and wide QRS complexes that may resemble ischemic changes on the ECG. The absence of reciprocal changes, the clinical context, and rapid normalization of the ECG after correcting potassium levels are important clues³¹.

The Role of Advanced Diagnostics in Identifying STEMI Mimics

In the high-pressure environment of emergency care, where minutes matter, distinguishing between true STEMI and STEMI mimics can be a daunting task. While clinical context, patient history, and laboratory findings (such as troponin levels) are essential, the ECG remains the cornerstone of diagnosis. However, traditional ECG interpretation can be challenging in the presence of conduction abnormalities, hypertrophic changes, or benign variants.

This is where PMcardio offers a distinct advantage. By utilizing advanced deep learning algorithms trained on extensive datasets of ECGs, our AI is capable of distinguishing true myocardial infarction from common STEMI mimics with high precision²². The tool analyzes patterns that may be difficult to detect with the naked eye, reducing unnecessary cath lab activations and invasive procedures.

Conclusion

STEMI mimics pose a significant challenge to emergency care providers, with the potential for both over- and under-treatment of patients presenting with chest pain and ST-segment elevation. Careful ECG interpretation, clinical context, and advanced diagnostic tools are essential in differentiating these conditions from true STEMI. PMcardio, the AI-powered ECG platform offers clinicians an invaluable tool in making these critical decisions, ensuring that patients receive the right diagnosis and treatment^32,33.