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].
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].
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].
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].
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