ECG in non-ST-elevation myocardial infarction

Mechanism of ECG changes and their causes

Normally, the ST segment lies strictly on the isoelectric line (between the QRS complex and the T wave). It reflects the phase of complete ventricular depolarization—the so-called action potential plateau phase, when there is no electrical potential difference between various myocardial areas. Analysis of the QRST ventricular complex and evaluation of isoelectric line displacement is a critically important stage in the diagnosis of coronary artery disease.

When NSTE-ACS develops, primarily the distal layer of the heart muscle suffers, resulting in subendocardial ischemia. This disrupts the normal course of repolarization, manifesting as ST displacement or T-wave changes. Such changes may indicate the formation of small-focal myocardial necrosis (for example, in the anteroseptal, apical, or lateral regions of the left ventricle). It should be remembered that T-wave inversion and ST depression can be not only ischemic, but also inflammatory, toxic, or traumatic in origin.

Key ECG criteria for NSTE-ACS

For the differential diagnosis of unstable angina and myocardial infarction, pathological changes must be recorded in at least two contiguous leads. Characteristic ischemic patterns include:

• ST-segment depression ( penurunan): horizontal or downsloping. A transient or persistent depth from 0.5 mm (0.05 mV) is considered diagnostically significant. Stricter criteria require depression ≥1 mm in standard leads and ≥2 mm in chest leads. This sign is most frequently detected in leads V4–V6.
• T-wave inversion: the wave flattens, becomes negative (depth >1 mm), or reverses (was previously negative and became positive during the attack). The highest probability of acute ischemia is when pronounced symmetrical negative T waves (from 2 mm) are recorded in precordial leads.
• Transient ST elevation: a short-term elevation lasting less than 20 minutes.

Micro-displacements—ST depression of less than 0.5 mm and T inversion of less than 1 mm—are considered non-specific signs.

Exclusion criteria for NSTE-ACS

If a patient with chest pain exhibits certain changes, the diagnosis of non-ST-elevation ACS is ruled out, as the pattern corresponds to a more severe ST-elevation ACS (transmural injury). Such ECG exclusion criteria include:

1. Persistent ST-segment elevation lasting for more than 20 minutes.
2. ST elevation recorded in at least two contiguous leads.
3. Newly arisen left bundle branch block (LBBB).

Stress testing and monitoring

A normal electrocardiogram recorded outside of a pain episode (interictal period) does not rule out a diagnosis of angina or myocardial infarction. Changes are often recorded only during the pain attack itself. This precisely explains the high value of Holter monitoring and stress tests.

During a stress test (when heart rate and blood pressure normally increase in response to physical activity, and ST depression does not exceed 1 mm with preserved positive T waves), the following are considered pathological ischemic criteria:

• Development of a typical moderate-to-high-intensity angina attack.
• Horizontal or downsloping ST depression with an amplitude of ≥1 mm and a duration of 0.06–0.08 s from the J point.
• ST depression greater than 2 mm (if the resting ECG was normal).
• ST elevation greater than 1 mm (in leads without pathological Q waves, except aVR and V1).
• Increase in amplitude or appearance of negative T waves.

Example of typical dynamics: the baseline resting tracing is unremarkable. As workload increases, an angina attack occurs, and ischemic ST depression below the baseline is recorded in leads V5 and V6. After cessation of the load, the ST segment returns to normal. This indicates a positive reaction that clearly correlates with significant coronary artery stenosis (confirmed by coronary angiography).

Class and exam questions

What else to study on this topic

• Action potential phases of the cardiomyocyte and electrophysiological formation of the ST segment
• ECG criteria for left bundle branch block (LBBB)
• Topographic diagnosis: ECG signs of anterior, septal, apical, and lateral wall necrosis
• Methodology and indications for Holter monitoring in suspected coronary artery disease

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.