ECG Findings of Myocardial Ischemia
The ECG manifestations of acute myocardial ischemia include ST-segment elevation, ST-segment depression, and T-wave changes, which must be present in two or more contiguous leads to be diagnostic. 1
Primary ST-Segment Changes
ST-Segment Elevation
- New ST elevation at the J point in two contiguous leads with specific voltage thresholds defines acute transmural ischemia 1
- Voltage criteria vary by lead location and patient demographics: 1
- ≥0.1 mV (1 mm) in all leads except V2-V3
- In leads V2-V3: ≥0.2 mV in men ≥40 years; ≥0.25 mV in men <40 years; ≥0.15 mV in women
- Prolonged ST elevation (>20 minutes), particularly with reciprocal ST depression, typically reflects acute coronary occlusion and impending myocardial necrosis 1
ST-Segment Depression
- New horizontal or down-sloping ST depression ≥0.05 mV (0.5 mm) in two contiguous leads indicates subendocardial ischemia 1
- Widespread ST depression in multiple leads with ST elevation in aVR suggests severe multivessel disease or left main coronary artery involvement 2, 3
- ST depression in leads V1-V3 with positive terminal T waves may represent posterior (inferobasal) ischemia and functions as an "ST elevation equivalent" 1
T-Wave Abnormalities
- T-wave inversion ≥0.1 mV (1 mm) in two contiguous leads with prominent R wave or R/S ratio >1 indicates ischemia 1
- Hyperacute T waves (tall, peaked, symmetric T waves in at least two contiguous leads) represent the earliest ECG manifestation of acute ischemia, often preceding ST elevation 1
- Pseudo-normalization of previously inverted T waves during chest pain episodes indicates acute ischemia 1
Additional ECG Manifestations
QRS Complex Changes
- Loss of precordial R-wave amplitude may accompany acute ischemia 1
- Increased R-wave amplitude and width (giant R-wave with S-wave diminution) often appears in leads showing ST elevation 1
- Transient Q waves may occur during acute ischemia episodes or with successful reperfusion 1
Rhythm and Conduction Disturbances
- Cardiac arrhythmias, intraventricular conduction delays, and atrioventricular conduction delays are associated ECG signs of acute myocardial ischemia 1
Critical Diagnostic Principles
Contiguous Lead Requirement
- Changes must appear in two or more anatomically contiguous leads to establish the diagnosis 1
- Contiguous lead groups include: 1
- Anterior: V1-V6
- Inferior: II, III, aVF
- Lateral/apical: I, aVL
- Frontal plane sequence: aVL, I, -aVR, II, aVF, III
Timing and Serial ECGs
- The ECG should be obtained within 10 minutes of clinical presentation 1
- Serial recordings at 15-30 minute intervals are essential when the initial ECG is non-diagnostic but clinical suspicion remains high 1
- Dynamic changes in ST-T waveforms during acute ischemic episodes require multiple ECG acquisitions 1
Special Lead Considerations
Posterior Wall Ischemia
- Posterior leads V7-V9 at the fifth intercostal space should be recorded when circumflex artery occlusion is suspected 1
- ST elevation ≥0.05 mV in V7-V9 indicates posterior ischemia (≥0.1 mV cutpoint increases specificity) 1
Right Ventricular Involvement
- Right precordial leads V3R and V4R should be obtained in inferior MI with suspected RV involvement 1
- ST elevation >0.05 mV (>0.1 mV in men <30 years) in these leads supports RV infarction 1
Important Clinical Caveats
ECG Confounders
- The ECG alone is insufficient for diagnosis as ST deviation occurs in multiple non-ischemic conditions including acute pericarditis, LVH, LBBB, Brugada syndrome, stress cardiomyopathy, and early repolarization 1
- Always compare with prior ECG tracings when available, as baseline abnormalities significantly impact interpretation 1