From the Guidelines
ECG findings in myocardial infarction (MI) include ST-segment elevation in leads facing the infarcted area, which is the hallmark sign of STEMI, as indicated by the most recent guidelines 1. The specific leads showing these changes indicate the location of the infarct:
- Anterior MI affects leads V1-V4
- Inferior MI affects leads II, III, and aVF
- Lateral MI affects leads I, aVL, V5-V6
- Posterior MI shows reciprocal changes in V1-V3 These changes are crucial for diagnosing and managing MI, as they can help identify the infarct-related artery, estimate the amount of myocardium at risk, and determine therapeutic strategy 1. Other findings may include arrhythmias such as ventricular tachycardia, ventricular fibrillation, or heart blocks, particularly in inferior MIs which can affect the AV node. The ECG changes evolve over time, progressing from hyperacute T waves to ST elevation, followed by T wave inversion and Q wave development, making serial ECGs valuable for tracking the infarct's progression 1. It is essential to acquire and interpret ECGs promptly, ideally within 10 minutes of clinical presentation, and to repeat them at 15-30 minute intervals if the initial ECG is non-diagnostic 1. The ECG criteria for diagnosing STEMI include ST-segment elevation of at least 2.5 mm in men < 40 years, 2 mm in men ≥ 40 years, or 1.5 mm in women in leads V2-V3, and/or 1 mm in other leads, in the absence of left ventricular hypertrophy or left bundle branch block 1.
From the Research
ECG Findings in Myocardial Infarction (MI)
The electrocardiogram (ECG) is a crucial tool for diagnosing and classifying myocardial infarction (MI) [ 2 ]. The 12-lead ECG can identify potential candidates for reperfusion therapy and assess the completeness and success of reperfusion [ 3 ].
Types of MI and ECG Findings
- ST-Elevation Myocardial Infarction (STEMI): Characterized by ST-segment elevation on the 12-lead ECG, accounting for approximately 30% of all MIs [ 2 ].
- Non-ST Elevation Myocardial Infarction (NSTEMI): Lacks ST-segment elevation on the 12-lead ECG, accounting for approximately 70% of all MIs [ 2 ].
- ECG Changes Mimicking AMI: Other clinical conditions, both cardiac and noncardiac, may result in ECG tracings mimicking acute myocardial infarction (AMI) [ 4 ].
ECG Variables and MI
- ST-Segment Resolution and Residual ST-Segment Elevation: Used for prognosis in acute MI [ 5 ].
- Q Waves: Related to outcome in chronic MI and independent predictor of infarct size and transmural extent of infarction [ 5 ].
- T-Wave Morphology Abnormalities and Novel Markers of Ventricular Repolarization Heterogeneity: Associated with an occluded culprit artery in NSTEMI [ 2 ].
Algorithm for Differentiating Infarct Territory
An algorithm has been constructed to facilitate discriminating MI location and culprit artery in patients with difficult-to-discern ECGs [ 6 ]. This algorithm can be used to define involved myocardial territory and culprit artery in difficult-to-discern ECGs.