Inferior Myocardial Infarction: Anatomical Location
An inferior myocardial infarction affects the inferior (diaphragmatic) wall of the left ventricle, and may extend to involve the right ventricle and/or posterior/posterolateral walls depending on the culprit vessel and extent of occlusion. 1, 2
Primary Anatomical Territory
- The inferior wall of the left ventricle is the primary anatomical region affected, manifesting as ST-segment elevation ≥ 0.1 mV in leads II, III, and aVF on the ECG. 2
- The blood supply to this region comes from either the right coronary artery (RCA) or the left circumflex artery (LCx), depending on which vessel is dominant and provides the posterior descending branch. 1
Right Ventricular Extension
When the RCA is occluded proximally, the right ventricle is frequently involved, which occurs in approximately 43% of inferior MI cases. 3
- Right ventricular infarction is diagnosed by ST-segment elevation ≥ 0.05 mV in right-sided chest leads V3R and V4R, with lead V4R having 93% sensitivity and 93% predictive accuracy. 2, 3
- ST elevation may also appear in lead V1 when the right ventricle is involved. 1, 2
- Right ventricular involvement carries significant prognostic implications, with in-hospital mortality of 31% versus 6% in those without RV involvement, and a 7.7-fold increased relative risk of death. 4
- The AHA/ACC/Canadian Cardiovascular Society provide a Class I, Level A recommendation to obtain right-sided leads V3R and V4R in every patient with ECG evidence of acute inferior wall ischemia/infarction. 2
Posterior/Posterolateral Extension
- ST-segment depression in precordial leads V1-V3 occurring with inferior STEMI suggests concomitant posterior or posterolateral wall ischemia, which can result from occlusion of either the RCA or LCx. 1, 2
- When posterior involvement is suspected, posterior leads V7-V9 should be recorded, using a diagnostic threshold of 0.05 mV ST elevation (or 0.1 mV in men ≥ 40 years). 2
Distinguishing the Culprit Vessel by ECG Pattern
The specific ECG pattern helps identify whether the RCA or LCx is occluded:
- RCA occlusion pattern: ST elevation in lead III > lead II, plus reciprocal ST depression in leads I and aVL. 1, 2
- Proximal RCA occlusion: Adds ST elevation in right-sided leads V3R, V4R and possibly V1, indicating right ventricular involvement. 1, 2
- LCx occlusion pattern: The ST-segment vector is directed less rightward than in RCA occlusion, with less prominent reciprocal changes in leads I and aVL. 1
Critical Timing Considerations
The ST elevation in right-sided chest leads associated with right ventricular infarction is transient and disappears within 10 hours in half of patients, much sooner than the ST elevation in the inferior leads. 1, 2, 3 Therefore, leads V3R and V4R should be recorded immediately upon presentation to avoid missing this diagnosis. 2
Clinical Pitfall
In approximately 14% of inferior MI patients, both the RCA and LCx show potentially culprit lesions on angiography, making it difficult to determine the true infarct-related artery. 5 In these cases, the Aslanger-Bozbeyoğlu ECG criterion demonstrates the best discriminative power with 81% sensitivity and 58% specificity. 5