What are the ECG findings in acute myocardial infarction (AMI)?

ECG Findings in Acute Myocardial Infarction

The most important ECG findings in acute myocardial infarction (AMI) include ST-segment elevation ≥0.1 mV in two or more contiguous leads (≥0.2 mV in leads V1-V3 for men ≥40 years; ≥0.25 mV in men <40 years; ≥0.15 mV in women), new horizontal or down-sloping ST depression ≥0.05 mV in two contiguous leads, and/or T wave inversion ≥0.1 mV in two contiguous leads with prominent R wave or R/S ratio >1. 1, 2

Early ECG Changes in AMI

• Hyperacute T waves (increased T-wave amplitude with prominent symmetrical T-waves) in at least two contiguous leads are often the earliest sign of myocardial ischemia, preceding ST-segment elevation 1, 2
• Increased R-wave amplitude and width (giant R-wave with S-wave diminution) may be seen in leads exhibiting ST elevation, reflecting conduction delay in the ischemic myocardium 1
• Transient Q-waves may be observed during an episode of acute ischemia or rarely during AMI with successful reperfusion 1
• Dynamic changes in the ECG waveforms are common during acute myocardial ischemic episodes, necessitating serial ECG recordings at 15-30 minute intervals in symptomatic patients with initially non-diagnostic ECGs 1, 3

ST-Segment Elevation MI (STEMI)

• ST-segment elevation at the J-point in two or more contiguous leads is the hallmark of STEMI 1, 2
• Cut-points for ST elevation:
  • ≥0.1 mV in all leads other than V1-V3 1, 2
  • ≥0.2 mV in men ≥40 years in leads V1-V3 1, 2
  • ≥0.25 mV in men <40 years in leads V1-V3 1, 2
  • ≥0.15 mV in women in leads V1-V3 1, 2
• Prolonged new ST-segment elevation (>20 min), particularly when associated with reciprocal ST-segment depression, usually reflects acute coronary occlusion 1, 2
• More profound ST-segment shift involving multiple leads/territories correlates with greater myocardial ischemia and worse prognosis 1, 2

Non-ST Elevation MI (NSTEMI)

• New horizontal or down-sloping ST depression ≥0.05 mV in two contiguous leads 1, 2
• T wave inversion ≥0.1 mV in two contiguous leads with prominent R wave or R/S ratio >1 1, 2
• ST depression in leads V1-V3 may indicate posterior wall MI, especially when the terminal T wave is positive (ST elevation equivalent) 2

Location-Specific ECG Findings

• Inferior MI: ST-segment elevation in leads II, III, and aVF 4, 5
• Anterior MI: ST-segment elevation in precordial leads V1-V6 4, 2
• Lateral MI: ST-segment elevation in leads I, aVL, V5-V6 2
• Posterior MI: ST depression in leads V1-V3 with tall R waves and upright T waves in these leads 2, 5
• Right ventricular MI: ST elevation ≥0.05 mV in right precordial leads V3R and V4R (≥0.1 mV in men <30 years) 2, 5

Special Considerations

• In left bundle branch block (LBBB), concordant ST-segment elevation (in leads with positive QRS complexes) or depression (in leads with negative QRS complexes) suggests AMI 2, 6
• In right bundle branch block (RBBB), new ST elevation or Q waves should raise suspicion for MI despite common ST-T abnormalities in V1-V3 2
• Cardiac arrhythmias, intraventricular and atrioventricular conduction delays, and loss of precordial R wave amplitude may be associated with acute myocardial ischemia 1
• The ECG by itself is often insufficient to diagnose acute myocardial ischemia or infarction, as ST deviation may be observed in other conditions such as acute pericarditis, LV hypertrophy, LBBB, Brugada syndrome, stress cardiomyopathy, and early repolarization patterns 1, 2

Evolution of ECG Changes

• Serial ECGs are essential as AMI findings evolve over time 1, 3
• ST-segment elevation typically develops within hours of coronary occlusion 2
• T-wave inversion may persist for weeks to months after the acute event 2
• Q waves develop in many (but not all) patients with MI and may persist indefinitely 1
• Criteria for pathological Q waves indicative of prior MI: Q waves ≥0.03 sec and ≥0.1 mV deep in at least two contiguous leads 1, 4

Practical Approach to ECG Interpretation

• Always compare with previous ECGs when available to identify new changes 1, 2
• Obtain serial ECGs at 15-30 min intervals in symptomatic patients with initially non-diagnostic ECGs 1, 2, 3
• Consider continuous 12-lead ECG monitoring in high-risk patients 2, 3
• Record additional leads (posterior V7-V9, right ventricular V3R-V4R) when standard leads are non-diagnostic but clinical suspicion remains high 2
• Be aware that cyclic variations in ST segment magnitudes between normalcy and injury are common during the early phase of AMI and correspond to spontaneous intermittent coronary opening and reocclusion 3