Guideline Summary: ECG Diagnosis of Acute Myocardial Ischemia (ACC)

Diagnostic Criteria and Lead Requirements

Acute myocardial ischemia is diagnosed when ST‑segment elevation, ST‑segment depression, or T‑wave changes are present in two or more anatomically contiguous leads. 1
The requirement for contiguity applies to all ECG criteria; contiguous lead groups include anterior (V1‑V6), inferior (II, III, aVF), lateral/apical (I, aVL), and the frontal‑plane sequence (aVL, I, ‑aVR, II, aVF, III). 4, 5
Changes must appear in two or more contiguous leads to establish the diagnosis of acute ischemia. 1, 7

New ST elevation at the J point in two contiguous leads defines acute transmural ischemia. 1
Voltage thresholds: ≥0.1 mV (1 mm) in all leads except V2‑V3. 1
In leads V2‑V3, thresholds vary by sex and age: ≥0.2 mV in men ≥40 yr, ≥0.25 mV in men <40 yr, ≥0.15 mV in women. 2
Prolonged ST elevation (>20 min), especially with reciprocal ST depression, suggests acute coronary occlusion and impending myocardial necrosis. 2

New horizontal or down‑sloping ST depression ≥0.05 mV (0.5 mm) in two contiguous leads indicates subendocardial ischemia. 1, 2
ST depression in V1‑V3 with positive terminal T waves may represent posterior (inferobasal) ischemia and functions as an “ST‑elevation equivalent.” 2, 3

T‑wave inversion ≥0.1 mV (1 mm) in two contiguous leads with a prominent R wave or R/S > 1 indicates ischemia. 1, 2
Hyperacute T waves (tall, peaked, symmetric in ≥2 contiguous leads) are the earliest ECG manifestation of acute ischemia and often precede ST elevation. 4, 5
Pseudo‑normalization of previously inverted T waves during chest‑pain episodes signals acute ischemia. 6

Loss of precordial R‑wave amplitude may accompany acute ischemia. 2, 3
Increased (giant) R‑wave amplitude and width frequently appear in leads showing ST elevation. 4, 5
Transient Q waves can occur during acute ischemic episodes or after successful reperfusion. 4, 5
Cardiac arrhythmias, intraventricular conduction delays, and atrioventricular conduction delays are recognized ECG signs of acute myocardial ischemia. 2, 3

Obtain the first ECG within 10 minutes of clinical presentation for suspected acute ischemia. 1, 2
Serial ECGs at 15‑30‑minute intervals are essential when the initial tracing is nondiagnostic but clinical suspicion remains high. 1, 2
Dynamic ST‑T waveform changes during an ischemic episode require multiple ECG recordings to capture evolution. 2, 3

Record posterior leads V7‑V9 (fifth intercostal space) when circumflex artery occlusion is suspected. 2, 3
ST elevation ≥0.05 mV in V7‑V9 indicates posterior ischemia; a cut‑point of ≥0.1 mV improves specificity. 2, 3

Obtain right precordial leads V3R and V4R in inferior MI when right‑ventricular involvement is suspected. 2, 3
ST elevation >0.05 mV (or >0.1 mV in men <30 yr) in these leads supports right‑ventricular infarction. 2, 3

ECG alone is insufficient for definitive diagnosis because ST‑segment deviation occurs in numerous non‑ischemic conditions (e.g., acute pericarditis, LVH, LBBB, Brugada syndrome, stress cardiomyopathy, early repolarization). 2, 3, 6
Always compare with prior ECGs when available; baseline abnormalities markedly affect interpretation. 2, 3
Greater magnitude of ST‑segment shift or more extensive T‑wave inversion across multiple territories correlates with larger ischemic burden and poorer prognosis. 2, 3