ST Changes in Leads II, III, and aVF Are Indicative of Inferior Heart Ischemia

Anatomical Basis and Vessel Involvement

Inferior wall infarction with ST-segment elevation in leads II, III, and aVF results from occlusion of either the right coronary artery (RCA) or the left circumflex coronary artery (LCx), depending on which provides the posterior descending branch (i.e., which is the dominant vessel) 1, 2
When the RCA is occluded, the spatial vector of the ST segment is usually directed more to the right, resulting in greater ST-segment elevation in lead III than in lead II 1
RCA occlusion is often associated with reciprocal ST-segment depression in leads I and aVL, which have positive poles oriented to the left and superiorly 1, 3

In RCA occlusion, ST elevation is typically greater in lead III than in lead II, with associated ST depression in leads I and aVL 1, 4
In LCx occlusion, the ST segment may be elevated to a greater extent in lead II than in lead III and may be isoelectric or elevated in leads I and aVL 5, 6

When the RCA is occluded in its proximal portion, right ventricular ischemia/infarction may occur 1, 3
Right ventricular involvement results in ST-segment elevation in right-sided chest leads V3R and V4R, and often in lead V1 1, 2
Lead V4R is particularly valuable in diagnosing right ventricular involvement in the setting of an inferior wall infarction 1, 2

ST-segment depression in leads V1, V2, and V3 occurring with inferior wall ischemia/infarction may indicate posterior (now termed lateral) wall involvement 5, 7
When a dominant RCA is occluded proximally, left posterolateral and right ventricular wall involvement may be present 6

The American Heart Association and American College of Cardiology recommend recording right-sided chest leads V3R and V4R in all patients presenting with ECG evidence of acute inferior wall ischemia/infarction 5
Right-sided chest leads V3R and V4R should be recorded as rapidly as possible after the onset of chest pain in patients with inferior ST elevation, as ST elevation in these leads persists for a shorter time than the ST elevation in the inferior leads 1, 5

ECG machines should be programmed to suggest recording right-sided chest leads V3R and V4R when ST elevation greater than 0.1 mV occurs in leads II, III, and aVF 1, 5
The pattern of ST elevation in II, III, and aVF should prompt immediate evaluation for coronary occlusion, with particular attention to whether the RCA or LCx is the likely culprit vessel based on the pattern of ST changes 1, 2

Lead V4R is the single most predictive electrocardiographic finding for RV ischemia/infarction, with ST-segment elevation ≥1 mm (0.1 mV) being diagnostic, as recommended by the American College of Cardiology 8
The European Society of Cardiology Guidelines explicitly recommend recording V3R and V4R to identify concomitant RV infarction in inferior MI, but V4R carries the highest diagnostic value 9

RV infarction increases mortality from 6% to 25-30% in inferior MI patients, making recognition critical, according to the American College of Cardiology 8
The American College of Cardiology recommends that V4R changes be recorded as rapidly as possible after symptom onset, as they may be transient, resolving within 10 hours in half of patients 8

The European Society of Cardiology recommends recording V3R and V4R in all patients with inferior ST elevation (leads II, III, aVF) 9
Diagnose RV extension if V4R shows ≥1 mm ST elevation, regardless of V3R findings, as recommended by the European Society of Cardiology and the American College of Cardiology 8, 9