An ST-Elevation Myocardial Infarction (STEMI) is the most severe form of heart attack. It occurs when a coronary artery becomes completely blocked, usually by a blood clot. This blockage cuts off blood flow, causing the rapid death of heart muscle tissue (necrosis). Immediate and accurate identification of a STEMI is necessary because treatment involves restoring blood flow to the blocked artery as quickly as possible.
The Electrocardiogram (ECG) is the primary tool used for this time-sensitive diagnosis. The urgency is often summarized by the phrase “time is muscle,” emphasizing that delay results in more irreversible damage to the heart. While the ECG records the heart’s electrical activity, diagnosing a STEMI focuses on the tracing that reflects injury to the heart tissue, which necessitates immediate intervention.
Mapping the Electrical Activity: The Normal ECG Wave
The ECG tracing records the electrical events that trigger the heart’s contraction, displayed as waves and segments. The tracing begins with the P wave (atrial activation), followed by the QRS complex, which signifies the electrical activation of the ventricles.
Following contraction, the heart muscle recovers electrically, represented by the T wave. The ST segment lies between the end of the QRS complex and the start of the T wave. It is normally a flat line reflecting the plateau phase before repolarization begins.
The ST segment is compared against the isoelectric line, the flat baseline established between heartbeats. In a healthy heart, the ST segment rests directly on this baseline, indicating no injury. In a STEMI, the lack of blood flow causes an electrical current of injury that pulls the segment upward. The degree of deviation from the baseline is the primary measurement used to diagnose a STEMI.
Specific Criteria for ST Segment Elevation
Precise measurement of ST segment elevation begins at the J-point, the junction where the QRS complex ends and the ST segment begins. This point marks the baseline for measurement. For a STEMI diagnosis, the J-point elevation must meet specific minimum thresholds and be observed in a pattern.
The required magnitude of elevation varies by lead location, sex, and age. In the limb leads (I, II, III, aVL, aVF) and most chest leads (V4-V6), one millimeter of elevation is required. The criteria are stricter for the V2 and V3 leads, which are placed over the front of the heart.
In leads V2 and V3, the required elevation is higher. For men aged 40 or older, the threshold is two millimeters; for men under 40, it is 2.5 millimeters. Women must show at least 1.5 millimeters of ST elevation in these two leads to be considered significant.
This elevation must be present in at least two “contiguous leads,” meaning leads that view the same anatomical area of the heart. Observing elevation in contiguous leads confirms the abnormality is localized. The shape of the elevated ST segment also provides a clue; a convex or “tombstone” morphology is highly suggestive of a true STEMI.
Pinpointing the Location: Lead Groupings
The 12-lead ECG provides 12 distinct electrical viewpoints, allowing medical professionals to map the location of the injury within the heart muscle. These views are systematically grouped to correspond with the different anatomical walls of the heart. Identifying the specific group of leads showing ST elevation allows for the localization of the blockage and the likely identification of the affected coronary artery. Determining this specific location is important because the treatment team uses this information to plan the strategy for opening the blocked vessel.
Anatomical Localization
The 12-lead ECG groups views to localize the injury and identify the affected artery:
- Inferior Wall: Viewed by leads II, III, and aVF, suggesting a blockage in the Right Coronary Artery (RCA).
- Anterior Wall: Viewed by leads V3 and V4, typically caused by an occlusion in the Left Anterior Descending (LAD) artery.
- Septal Wall: Viewed by leads V1 and V2, often considered alongside the anterior leads.
- High Lateral Region: Seen by leads I and aVL.
- Lower Lateral Region: Seen by leads V5 and V6. Elevation in lateral leads often points toward a blockage in the Left Circumflex (LCx) artery.
Recognizing STEMI Mimics and Look-Alikes
Not every instance of ST segment elevation indicates an acute heart attack, as several other conditions can produce a similar tracing. Considering these “STEMI mimics” is necessary to avoid unnecessary urgent interventions. Two common conditions that can confuse the diagnosis are Early Repolarization and Pericarditis.
Early Repolarization
Early Repolarization is a common, harmless variant often seen in healthy young adults and athletes. This pattern causes a subtle, widespread, and concave (scooped-up) elevation of the ST segment. This differs from the convex or dome-shaped elevation seen in a true STEMI, which is typically localized to a specific region.
Pericarditis
Pericarditis, inflammation of the sac surrounding the heart, also causes ST elevation but with a distinct presentation. The elevation is usually diffuse, appearing in many leads across different anatomical regions. Pericarditis often presents with a concave ST segment and may be accompanied by a depression of the PR segment, a finding absent in an uncomplicated STEMI.