What are the characteristic ECG findings in myocardial infarction and how do they evolve over time for ST‑elevation versus non‑ST‑elevation infarctions and for specific locations such as posterior and right‑ventricular infarctions?

ECG Changes in Myocardial Infarction

ST-Elevation Myocardial Infarction (STEMI)

STEMI is diagnosed by new ST-segment elevation at the J-point in at least 2 contiguous leads: ≥2.5 mm in men <40 years, ≥2 mm in men ≥40 years, or ≥1.5 mm in women for leads V2-V3; and ≥1 mm in all other leads. 1

Temporal Evolution of STEMI ECG Changes

• Hyperacute T waves are often the earliest sign, appearing within minutes of coronary occlusion before ST-segment elevation develops 2, 3
• ST-segment elevation typically develops within minutes to hours, with prolonged elevation (>20 minutes) particularly when associated with reciprocal ST-depression indicating acute coronary occlusion 1
• Pathologic Q waves develop in many patients and may appear early or late in the process, persisting indefinitely as markers of prior infarction 2, 3
• T-wave inversion may occur before, during, or after the STEMI event and can persist for weeks to months 2, 3

Reciprocal Changes

• Reciprocal ST-depression ≥1 mm in leads electrically opposite to ST-elevation indicates a larger area of myocardium at risk and correlates with greater potential for myocardial salvage (salvage index 61% vs 17%, p<0.001) 2
• ST-depression in lead aVL is a highly sensitive marker for acute inferior MI, occurring in the majority of cases and sometimes representing the sole early ECG sign 4

Non-ST-Elevation Myocardial Infarction (NSTEMI)

NSTEMI encompasses all acute MI presentations without diagnostic ST-elevation, including ST-depression, T-wave inversion, or completely normal ECG findings. 1

Diagnostic Criteria

• New horizontal or down-sloping ST-depression ≥0.5 mm (0.05 mV) in two contiguous leads is diagnostic for NSTEMI 2
• T-wave inversion ≥1 mm (0.1 mV) in two contiguous leads with prominent R wave or R/S ratio >1 supports NSTEMI diagnosis 2
• The ECG may be completely normal in up to 55% of NSTEMI cases initially, necessitating serial recordings at 15-30 minute intervals 2
• Transient ST-segment changes during symptomatic episodes that resolve when asymptomatic strongly suggest severe coronary artery disease 2

Prognostic Significance

• ST-segment depression on the presenting ECG carries the highest risk of death at 6 months, with the degree of ST-depression showing a strong relationship to adverse outcomes 2
• Patients with diagnostic changes on serial ECG have 2.5 times greater risk of acute coronary syndromes, 9.6 times greater risk of life-threatening complications, and 12.3 times greater risk of death 2

Posterior Myocardial Infarction

Posterior MI is often overlooked on standard 12-lead ECG and requires specific lead placement for accurate diagnosis. 1

Standard Lead Findings

• ST-depression in leads V1-V3 with positive terminal T waves (ST-elevation equivalent) suggests posterior wall ischemia, though this finding is non-specific 1, 2
• This pattern reflects inferobasal myocardial ischemia viewed from the anterior perspective 1

Posterior Lead Confirmation

• Record posterior leads V7-V9 at the fifth intercostal space (V7 at left posterior axillary line, V8 at left mid-scapular line, V9 at left paraspinal border) when suspecting left circumflex artery occlusion 1, 2
• ST-elevation ≥0.5 mm (0.05 mV) in leads V7-V9 is diagnostic, with specificity increased at ≥1 mm (0.1 mV), particularly in men <40 years old 1, 2
• Posterior infarcts principally involve the basal and mesial levels of the left ventricle 5

Right Ventricular Infarction

Right ventricular involvement should be suspected in all patients with inferior MI and requires right precordial lead recording. 1

Diagnostic Approach

• Record right precordial leads V3R and V4R in patients with inferior MI to detect right ventricular involvement 1, 2
• ST-elevation >0.5 mm (0.05 mV) in V3R-V4R provides supportive criteria for right ventricular infarction (>1 mm in men <30 years old) 1, 2
• These additional leads increase sensitivity for AMI by 8.4% but may decrease specificity by 7.0% 6

ECG Changes Indicating Prior Myocardial Infarction

Pathologic Q Wave Criteria

• Any Q wave ≥0.02 seconds or QS complex in leads V2-V3 indicates prior anterior MI 1
• Q wave ≥0.03 seconds and ≥0.1 mV deep or QS complex in leads I, II, aVL, aVF, or V4-V6 in any two contiguous leads indicates prior MI 1, 2
• R wave ≥0.04 seconds in V1-V2 with R/S ≥1 and concordant positive T wave (in absence of conduction defect) indicates prior posterior MI 1

Normal Q Wave Variants to Avoid Misdiagnosis

• A QS complex in lead V1 is normal 1
• Q wave <0.03 seconds and <25% of R-wave amplitude in lead III is normal if frontal QRS axis is between 30° and 0° 1
• Q wave may be normal in aVL if frontal QRS axis is between 60° and 90° 1
• Septal Q waves <0.03 seconds and <25% of R-wave amplitude in leads I, aVL, aVF, and V4-V6 are non-pathological 1

Special Considerations and Confounding Conditions

Left Bundle Branch Block (LBBB)

• New or presumed new LBBB in a patient with ischemic symptoms warrants immediate reperfusion therapy 1, 2
• Concordant ST-elevation (in leads with positive QRS deflections) strongly suggests acute MI in the presence of LBBB 1, 2
• A previous ECG for comparison is invaluable when LBBB is present 1

Right Bundle Branch Block (RBBB)

• ST-T abnormalities in leads V1-V3 are common with RBBB, making ischemia assessment difficult in these leads 1
• New ST-elevation or Q waves in the presence of RBBB should raise suspicion for myocardial ischemia or infarction 1

Ventricular Pacing

• When ventricular pacing masks ST-T changes, consider temporary reprogramming of the pacemaker in non-dependent patients or proceed directly to coronary angiography 2

Other Mimics and Pitfalls

• ST-deviation may occur in acute pericarditis, left ventricular hypertrophy, Brugada syndrome, stress cardiomyopathy, and early repolarization patterns 1
• Pseudo-normalization of previously inverted T waves during acute chest discomfort may indicate acute myocardial ischemia 1
• Pulmonary embolism, intracranial processes, electrolyte abnormalities, hypothermia, or peri-/myocarditis may result in ST-T abnormalities 1

Serial ECG Monitoring Protocol

Initial and Repeat ECG Timing

• Obtain and interpret 12-lead ECG within 10 minutes of first medical contact for all patients with suspected acute coronary syndrome 2
• For patients with suspected MI and initially non-diagnostic ECG who remain symptomatic, obtain serial ECGs at 15-30 minute intervals to detect evolving ST-segment changes 2
• Serial 12-lead ECG monitoring detects injury in an additional 16.2% of AMI patients, representing a 34% relative increase in patients eligible for emergency reperfusion therapy 2

Continuous Monitoring Duration

• Continue ECG monitoring for 48-72 hours for all patients with acute MI 2
• Extend monitoring beyond 72 hours if hemodynamic instability, persistent ischemia, or ongoing arrhythmias are present 2
• For patients receiving fibrinolytic therapy, obtain repeat ECG at 60-180 minutes to assess reperfusion success 2

Clinical Impact of Serial Monitoring

• Patients with diagnostic changes on serial 12-lead ECG have 4.9 times greater risk of requiring revascularization procedures and 12.3 times greater risk of death 2
• Serial ECGs identify silent myocardial ischemia, which occurs frequently in patients with unstable angina and is a marker for unfavorable outcomes including death 2

Prognostic Implications of ECG Findings

• Larger ST-segment shifts involving multiple contiguous leads correlate with greater myocardial ischemia burden and worse prognosis 2
• The magnitude of ST-deviation provides independent prognostic information even after adjusting for clinical findings and biomarkers 2
• Small variations of ST-segment shift in the first 4 hours of AMI correlate with worse outcomes 2