Inferolateral Myocardial Infarction on ECG

What it looks like on ECG

Inferolateral myocardial infarction combines two anatomically contiguous regions — the inferior wall, which is projected in II, III, aVF, and the lateral wall, reflected in I, aVL (high lateral) and V5–V6 (low lateral). In the acute stage of STEMI, ST-segment elevation appears in these leads, often merging with the T wave (hyperacute T waves at the onset of the process). The elevation shape is convex upward, straight, or concave; a concave shape alone does not exclude STEMI and does not automatically classify changes as early repolarization — the diagnosis is made based on thresholds in contiguous leads, reciprocal changes, dynamics, and clinical presentation.

In isolated inferior wall involvement without high lateral wall involvement, one of the earliest and most sensitive signs is reciprocal ST depression in aVL and/or I — it sometimes appears before ST elevation becomes apparent in II, III, aVF. If the high lateral wall is involved in the process, ST elevation, rather than depression, is expected in I and aVL.

The ratio of ST-segment elevation between leads helps identify the culprit artery: ST elevation in III greater than in II, plus ST elevation in V1 or depression in I, favors right coronary artery (RCA) occlusion; the reverse pattern (II ≥ III, unchanged or elevated in I) points to the circumflex artery (LCx). When RCA occlusion is proximal to the takeoff of right ventricular branches, one should look for ST elevation in V3R–V4R (RV infarction) and ST depression in V1–V3 with elevation in V7–V9 (concomitant posterior infarction).

As the infarction evolves, ST elevation decreases, the T wave becomes inverted in the same leads, and a pathological Q wave may form — these are signs of the subacute and scar stages. A scar-stage ECG may show only a pathological Q wave in II, III, aVF and/or I, aVL, V5–V6 without ST-T dynamics; in some patients, inferolateral MI presents as non-ST-segment elevation ACS, in which case localization cannot be reliably determined from a single ECG alone — it is refined using coronary angiography and imaging data.

Real ECGs: 8 examples

ECG from the open PTB-XL dataset (Wagner P. et al., Scientific Data, 2020; PhysioNet), CC BY 4.0 license. Rendering by WhiteCoat Pro.

Mechanism

The underlying mechanism is transmural myocardial ischemia of the inferior and lateral walls of the LV due to acute thrombotic occlusion of a coronary artery supplying both zones. Most often, this is a dominant right coronary artery, which in most individuals gives off the posterior descending artery to the inferior wall and may participate in supplying the lateral divisions, or the circumflex branch of the left coronary artery, which supplies the lateral wall and frequently the posterior part of the inferior wall in left-dominant circulation. The damaged epicardial layer loses its negative charge relative to the endocardium, and the injury vector is directed toward the electrode overlying the injury zone — hence ST elevation in the inferior and lateral leads; in leads electrically opposite to the injury zone, reciprocal ST depression may be recorded.

Causes

• Atherothrombotic occlusion of the right coronary artery (dominant)
• Atherothrombotic occlusion of the circumflex branch of the left coronary artery
• Embolic occlusion (cardioembolism, thrombosis in atrial fibrillation or endocarditis)
• Spontaneous coronary artery dissection (SCAD), more frequent in young and middle-aged women
• Coronary spasm (Prinzmetal variant angina) with a transient pattern of inferolateral ST elevation
• Multivessel disease involving the RCA/LCx basin

Differential diagnosis

• Isolated inferior MI — no ST elevation in I, aVL, V5–V6, only inferior leads; reciprocal ST depression in I/aVL is possible.
• Isolated lateral/high lateral MI — ST elevation in I, aVL (and/or V5–V6) without elevation indicating an inferior component; reciprocal ST depression is possible in inferior leads.
• Early repolarization — concave ST elevation, J-point notch, no dynamics on serial ECGs; the shape of ST alone is not a reliable criterion for differentiation.
• Pericarditis — diffuse concordant ST elevation in almost all leads, PR depression; localized reciprocal ST depression outside of aVR and sometimes V1 is not typical for a classical presentation; no pathological Q waves.
• Brugada syndrome — saddleback or coved ST elevation limited to the right chest leads V1–V3, not involving inferior and lateral leads; accompanied by characteristic QRS morphology rather than typical MI waves.
• Hyperkalemia — can cause pseudoinfarction ST elevation, including locally in inferior or other leads, but a tall peaked T wave, shortening or disappearance of the P wave, PR prolongation, and QRS widening are more typical; when suspected, ST elevation localization does not rule out electrolyte imbalance — potassium levels must be checked.
• Inferoposterior MI without lateral component — ST depression in V1–V3 without elevation in I, aVL, V5–V6; posterior leads V7–V9 are needed for confirmation.

Clinical significance and management

Inferolateral ST elevation meeting STEMI criteria requires immediate reperfusion. In patients with symptom onset within 12 hours, primary PCI is preferred if the expected time from STEMI diagnosis to recanalization (guidewire crossing) is ≤120 minutes. If this is unachievable, and in the absence of contraindications, fibrinolysis should be performed as early as possible followed by transfer to a PCI center; in late presentation (significant delay from symptom onset), the strategy may differ and requires individual evaluation.

In inferior STEMI, right-sided chest leads V3R–V4R (primarily V4R) should be recorded as early as possible, without delaying reperfusion, to detect signs of right ventricular involvement — ST elevation here alters fluid management tactics and increases the risk of hypotension with nitrates. It is important to remember that these changes can be transient and disappear upon late recording, so the absence of ST elevation in V3R–V4R does not reliably rule out RV infarction: if clinical suspicion persists (hypotension, jugular venous distension with clear lungs), the time of recording from symptom onset is considered and supplemented by imaging. Posterior leads V7–V9 should also be acquired if extension to the posterior wall is suspected.

With RCA occlusion, the risk of bradyarrhythmias is high: sinus bradycardia, AV blocks of various degrees (up to complete) due to ischemia of the AV node, which in most cases is supplied by a branch of the RCA. Rhythm monitoring and preparedness for temporary pacing are required. Extensive involvement with lateral wall recruitment (LCx basin) may be accompanied by a larger mass of ischemic myocardium and LV dysfunction; individual risk of mechanical complications (e.g., papillary muscle rupture with mitral regurgitation) and ventricular arrhythmias is assessed based on clinical status, imaging, and coronary anatomy, rather than solely on the number of leads with ST elevation.

Common interpretation errors

• Evaluating only II, III, aVF and missing concomitant ST elevation in I, aVL, V5–V6, leading to underestimation of the extent of damage.
• Failing to compare ST-elevation amplitude in II and III — missing an indication of the culprit artery.
• Failing to record V3R–V4R leads in inferior/inferolateral STEMI, missing signs of right ventricular involvement.
• Automatically interpreting ST depression in aVL as an independent finding without considering that it may be a reciprocal sign of an inferior MI without high lateral elevation.
• Confusing an isolated lateral MI with an inferolateral MI — diagnosing an inferolateral MI requires changes in both lead groups.
• Diagnosing a prior MI based on any Q wave with a duration of 0.02–0.03 s without regard to lead, amplitude, and clinical context.

Frequently asked questions

Related terms

More in the section "Ischemia and Myocardial Infarction"

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Sources

• AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram, 2009
• ESC Guidelines for the management of acute coronary syndromes, 2023
• Hampton J.R. The ECG in Practice
• Orlov V.N. Guide to Electrocardiography

Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.