Electrocardiography of the interventricular septum

Infarction localization and septal involvement

The interventricular septum is affected in various topographic variants of myocardial infarction. The area of necrosis directly depends on which vascular bed has impaired blood flow:

• Anterior infarction: Develops due to issues in the left coronary artery basin (primarily its descending branch). In this case, ischemia affects the anterior part of the interventricular septum, as well as the apex, anterior, and lateral walls of the left ventricle.
• Posterior infarction: Occurs due to occlusion of the circumflex branch of the left coronary artery. The lesion zone includes the posterior part of the interventricular septum and the posterior wall of the left ventricle.
• Extensive infarction: Associated with occlusion of the main trunk of the left coronary artery. As a rule, it is transmural in nature.
• Right ventricular infarction: Extremely rare, but can also affect the hemodynamics of adjacent structures.

Depending on the prevalence of the lesion, necrosis of the septum and other walls can be small-focal, large-focal, or extensive.

Depth of injury and ECG classification

Damage to the interventricular septum can affect different layers of the heart muscle. The following variants are distinguished by depth:

1. Subendocardial: The focus is located beneath the endocardium.
2. Intramural: Necrosis is localized in the middle layers of the myocardial thickness.
3. Subepicardial: The lesion is located directly beneath the epicardium.
4. Transmural: The most severe form, in which necrosis penetrates all layers of the myocardium completely.

When assessing the electrical activity of the heart, myocardial infarction is divided into two major groups:

• ST-segment elevation (STEMI): Indicates the presence of transmural ischemia.
• Non-ST-segment elevation (NSTEMI): Diagnosed if ST elevation in two contiguous leads lasts less than 20 minutes and there is no acute left bundle branch block (LBBB).

It is important to remember that electrocardiography can be uninformative. A classic example is the presence of LBBB in a patient, which masks the signs of acute ischemia of the septum and other regions.

Clinical types of myocardial infarction

Damage to the interventricular septum can occur within various clinical and etiological scenarios (according to the Fourth Universal Definition):

• Type 1 (Spontaneous): Atherothrombotic variant associated with plaque erosion or rupture followed by arterial thrombosis.
• Type 2 (Ischemic imbalance): Occurs due to spasm, embolism, arrhythmia, or anemia, when myocardial oxygen demand increases while supply drops (without acute atherothrombosis).
• Type 3 (Sudden death): The patient dies before blood is drawn for biomarkers. Symptoms of ischemia, new ECG changes, or ventricular fibrillation are observed.
• Type 4a: Infarction associated with percutaneous coronary intervention (PCI), developing within 48 hours.
• Type 4b: Associated with stent thrombosis, documented by coronarography or autopsy.

Role of echocardiography and mechanical complications

If ECG data are insufficient or severe structural damage is suspected, transthoracic echocardiography comes to the forefront. It is recommended for all patients with suspected NSTEMI. An obligatory parameter of the study is the assessment of left ventricular ejection fraction (LVEF). The main goals of the method are to refine the diagnosis, choose treatment tactics, and perform differential diagnosis.

Emergency echocardiography is indicated when mechanical complications of infarction are suspected, particularly interventricular septum rupture. This life-threatening condition requires immediate imaging. The study is also performed urgently in case of:

• Acute heart failure (AHF).
• Left ventricular free wall rupture.
• Acute mitral regurgitation (chordal rupture or papillary muscle dysfunction).
• Ascending aortic dissection.
• Cardiac tamponade.
• Valvular dysfunction (e.g., aortic stenosis).

At the same time, the procedure should not delay the patient's transfer for an invasive intervention (coronarography) if there are indications for emergency revascularization.

Time dynamics

The assessment of the septum and the myocardium as a whole also depends on the time of onset of ischemia:

• Primary (acute) infarction: Lasts for up to 4 weeks from the attack until a dense scar is formed.
• Recurrent: The appearance of a new focus of necrosis against the background of an old one that has not yet scarred (within 4 weeks).
• Reinfarction: Occurs 4 weeks or more after the primary episode.

Class and exam questions

What else to study on this topic

• Diagnostic criteria for ST-elevation and non-ST-elevation myocardial infarction.
• Differential diagnosis of myocardial infarction types (Types 1–4).
• Mechanical complications of myocardial infarction: pathogenesis and hemodynamics.
• Indications for emergency coronarography in acute coronary syndrome.

Other topics in the "Additional Topics" section

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