What Is a STEMI vs. NSTEMI Heart Attack?

When a heart attack occurs, cardiologists and emergency physicians classify it into types based on the pattern seen on the electrocardiogram (ECG). The two main types — STEMI and NSTEMI — reflect different degrees of coronary artery blockage and have different urgency levels for treatment. Understanding this distinction clarifies why some heart attacks require emergency catheterization within 90 minutes while others allow a somewhat longer window.

The Ecg In Heart Attack Diagnosis

The electrocardiogram records the heart's electrical activity — the waves and intervals that correspond to the electrical events of each heartbeat. Among the many components of an ECG, the ST segment — the portion of the tracing between the end of the QRS complex (ventricular depolarization) and the beginning of the T wave — is particularly important in heart attack diagnosis.

In a healthy heart with normal coronary blood flow, the ST segment lies flat along the baseline. When heart muscle is acutely ischemic (deprived of blood), the ST segment changes:

• Ischemia (insufficient but some blood flow): ST depression (the segment drops below baseline)
• Acute full-thickness injury (complete or near-complete occlusion): ST elevation (the segment rises above baseline)