Subendocardial Myocardial Infarction and Its ECG Characteristics

Subendocardial myocardial infarction refers to a type of heart attack that primarily affects the innermost third of the ventricular wall, including the trabeculae carneae and papillary muscles. Unlike transmural infarctions, this type of myocardial damage typically does not result in pathological Q waves on the electrocardiogram (ECG). Instead, it presents with specific ECG patterns that can help clinicians identify the condition accurately.

ECG Features of Subendocardial Myocardial Infarction

The ECG changes in subendocardial myocardial infarction can be categorized into three main types:

1. ST-Segment Elevation or Depression Pattern

A. Predominant ST-Segment Depression: This pattern is characterized by widespread ST-segment depression in most leads, except aVR. The ST-segment depression is typically ≥ 0.1 mV and lasts for ≥ 0.08 seconds. This type may or may not be accompanied by T-wave changes. It often indicates subendocardial ischemia without full-thickness injury.

B. Predominant ST-Segment Elevation: In some cases, ST-segment elevation is observed, usually in a convex upward or "upwardly concave" morphology. The elevation is generally ≥ 0.1–0.2 mV and persists for more than 24–48 hours. This pattern shows dynamic changes over time and may eventually return to baseline, indicating reversible injury or evolving infarction.

2. T-Wave Inversion Pattern

In the T-wave type of subendocardial myocardial infarction, deep, symmetric, and inverted T waves are seen in multiple leads (excluding aVR). These T-wave inversions are typically deeper than 1 mm and tend to become more pronounced over time. This pattern often reflects significant subendocardial damage and can be an early indicator of myocardial injury.

3. Normal ECG Pattern

Surprisingly, some patients with subendocardial myocardial infarction may present with a normal or nearly normal ECG. This makes diagnosis challenging and highlights the importance of clinical correlation, serial ECGs, and biomarker testing in suspected cases.

Conclusion

Recognizing the various ECG presentations of subendocardial myocardial infarction is crucial for timely diagnosis and treatment. While the absence of Q waves can make detection difficult, attention to ST-segment changes, T-wave abnormalities, and clinical context can significantly improve diagnostic accuracy. Clinicians should remain vigilant and consider further investigation when symptoms suggest myocardial ischemia, even in the presence of a normal ECG.