ECG Criteria for ST Elevation Myocardial Infarction (STEMI)

The diagnosis of STEMI relies on specific electrocardiographic thresholds for ST segment elevation measured at the J point. The diagnostic criteria require new ST elevation in at least two contiguous leads [8, 10]. The specific voltage thresholds vary based on patient age, sex, and lead location, as detailed below [8, 10]:

Patient Demographic	Lead Group	ST Elevation Threshold at J Point
Men under 40 years	V2 to V3	Greater than or equal to 2.5 mm
Men 40 years and older	V2 to V3	Greater than or equal to 2 mm
Women (any age)	V2 to V3	Greater than or equal to 1.5 mm
All patients	Other leads	Greater than or equal to 1 mm

These criteria for the other leads apply in the absence of left ventricular hypertrophy or left bundle branch block [8, 10].

Characteristics of ST Elevation in Acute Transmural Ischemia

When acute ischemia is transmural, the ST vector typically shifts in the direction of the outer epicardial layers, producing ST elevations over the ischemic zone [15]. In the earliest stages of ischemia, this may manifest as tall, positive hyperacute T waves [15]. Profound ST elevation across multiple leads generally indicates very severe ischemia [15].

A key distinguishing feature of ischemic ST elevation is the presence of prominent reciprocal ST depression [4]. Furthermore, ischemic ST elevations follow a predictable evolutionary pattern, typically followed within hours to days by evolving T wave inversions and often by the development of Q waves in the same lead distribution [15].

Differential Diagnosis of ST Elevation

Several nonischemic conditions can produce ST segment elevations that mimic acute myocardial infarction. Acute pericarditis is a primary differential diagnosis and presents with distinct characteristics compared to transmural ischemia [4, 5].

Acute Pericarditis vs. Acute Myocardial Infarction

The ST segment elevation in acute pericarditis is typically diffuse, involving all leads except aVR and often V1, and does not correspond to a specific coronary anatomic distribution [4, 5]. In contrast to acute myocardial infarction, pericarditis lacks prominent reciprocal ST depression and does not evolve into pathologic Q waves [4]. Additionally, PR segment depression is a common and early electrocardiographic sign of acute pericarditis, reflecting pericardial involvement overlying the atria [4, 5].

Other Differential Diagnoses

ST segment elevations simulating acute ischemia or infarction may also occur in the following conditions [15]:

• Myocarditis
• Early repolarization patterns
• Left bundle branch block
• Left ventricular hypertrophy
• Hyperkalemia
• Cerebrovascular injury