Subendocardial ischemia ECG

Concept of pathological change zones

According to classical Bayley concepts, when coronary blood flow is disrupted, specific zones form around the necrosis area. Each of them has distinct ECG manifestations in leads where the active electrode is positioned over the infarction area:

• Zone of ischemia manifests as the so-called "coronary" T wave—symmetrical and peaked. In subendocardial injury, it becomes tall and positive, whereas in transmural injury, it becomes negative.
• Zone of ischemic injury is characterized by RS–T segment displacement. If the injury affects only the subendocardial layers, the segment shifts below the isoelectric line (depression). In transmural injury, it elevates above the isoelectric line.
• Zone of necrosis is reflected on the ECG by the formation of a pathological Q wave (duration >30 ms) and a sharp decrease in R-wave amplitude, or the appearance of a QS complex. In isolated subendocardial ischemia, this zone is absent.

ECG changes in the hyperacute stage

The ECG picture changes dynamically depending on the stage of myocardial infarction. In the hyperacute stage, which lasts up to 2 hours from the onset of the attack, the earliest changes occur.

Within the very first minutes after the cessation of normal coronary blood flow, a zone of subendocardial ischemia forms. At this point, high coronary T waves combined with RS–T segment displacement below the isoelectric line can be recorded on the cardiogram. However, in practice, such early changes are registered extremely rarely. Subsequently, if the ischemic injury zone expands and reaches the epicardium, ST depression is replaced by ST elevation—indicating transmural ischemic injury.

ECG diagnostics of non-ST-segment elevation ACS

Acute coronary syndrome without ST-segment elevation is directly associated with damage to the distal myocardial areas close to the endocardium. This condition may present with the following signs:

1. Directly subendocardial ischemia.
2. T-wave inversion: flattening or reversion (transition from negative to positive).
3. ST-segment depression.

For ST depression to be considered a reliable diagnostic sign, it must be recorded in at least ≥2 contiguous leads. Furthermore, the depth of displacement must be ≥1 mm in standard leads and ≥2 mm in chest leads.

It is important to remember that such ECG changes may not be exclusively ischemic in origin. They occur in inflammatory, toxic, and traumatic lesions of the heart muscle. In all these cases, small-focal myocardial necrosis may develop, accompanied by the formation of negative T waves (e.g., in small-focal infarction of the anteroseptal, lateral, or apical regions of the left ventricle).

Morphological picture of non-transmural infarction

Macroscopically, myocardial infarction belongs to irregularly shaped infarctions. If necrosis is limited and localized predominantly subendocardially, a non-transmural (small-focal) infarction develops. Its key ECG feature is that changes affect only the ST segment and T wave; no pathological Q wave is formed.

At the microscopic level, profound structural disorders occur:

• Light microscopy shows myofibrillar changes at the level of Z-discs.
• Electron microscopy allows the registration of mitochondrial destruction, sarcolemma damage, pronounced sarcoplasmic edema, swelling of the sarcoplasmic reticulum, and separation of intercalated discs.

The outcome of this process triggers post-infarction remodeling. This includes wall thinning at the necrosis site, disproportionate stretching of the affected area, and subsequent dilation (enlargement) of the left ventricular cavity.

Class and exam questions

What else to study on this topic

• ECG signs of transmural myocardial infarction
• Formation of a pathological Q wave and QS complex
• Left ventricular post-infarction remodeling
• Ultrastructural changes in cardiomyocytes during necrosis
• Differential diagnostics of non-ST-segment elevation ACS

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