What are the electrocardiogram (ECG) findings of ischemia, injury, and infarction (myocardial infarction) on a 12-lead electrocardiogram (ECG)?

From the Guidelines

A 12-lead ECG is essential for identifying ischemia, injury, and infarction, with characteristic changes including T-wave inversions or flattening for ischemia, ST-segment elevation or depression for injury, and pathological Q waves for infarction, as noted in the 2014 AHA/ACC guideline for the management of patients with non-ST-elevation acute coronary syndromes 1. The ECG findings of ischemia, injury, and infarction can be summarized as follows:

• Ischemia: T-wave inversions or flattening, representing early myocardial oxygen deprivation
• Injury: ST-segment elevation or depression, indicating more severe oxygen deprivation with membrane damage
• Infarction: pathological Q waves, representing areas of dead myocardial tissue that no longer generate electrical activity The specific leads showing these changes help localize the affected heart region:
• Anterior: V1-V4
• Lateral: I, aVL, V5-V6
• Inferior: II, III, aVF
• Posterior: reciprocal changes in V1-V3 These patterns follow a typical progression in acute myocardial infarction, starting with hyperacute T waves, followed by ST elevation, then Q wave development, and finally T wave inversion, as described in the third universal definition of myocardial infarction 1. It is crucial to understand these patterns for rapid diagnosis and treatment of acute coronary syndromes, as the location and extent of ECG changes guide therapeutic decisions, including reperfusion strategies, and are supported by the AHA/ACCf/HRS recommendations for the standardization and interpretation of the electrocardiogram 1. Key points to consider when interpreting ECG findings include:
• The importance of acquiring and interpreting the ECG promptly, within 10 minutes of the patient's arrival at an emergency facility
• The need to repeat the ECG at 15- to 30-minute intervals during the first hour, especially if symptoms recur
• The potential for a normal ECG to not exclude acute coronary syndromes, and the importance of considering other diagnostic tests and clinical factors
• The impact of various conditions, such as left ventricular hypertrophy, bundle-branch blocks, and ventricular pacing, on the interpretation of ECG findings, as noted in the 2014 AHA/ACC guideline 1.

From the Research

ECG Findings of Ischemia, Injury, and Infarction

The electrocardiogram (ECG) is a crucial tool in diagnosing and managing patients with suspected acute coronary syndromes. The ECG findings of ischemia, injury, and infarction are as follows:

• Ischemia: ST segment depression in leads other than V1-V3, which usually indicates subendocardial ischemia secondary to subocclusion of the epicardial artery, distal embolization to small arteries, or supply/demand mismatch 2
• Injury: ST segment elevation in patients with active symptoms, which usually indicates acute occlusion of an epicardial artery with ongoing transmural ischemia 2
• Infarction: Q waves, which are a sign of myocardial necrosis and are often seen in patients with acute myocardial infarction 3

ECG Patterns in Acute Coronary Syndromes

The ECG patterns in acute coronary syndromes can be divided into two main categories:

• ST-elevation myocardial infarction (STEMI): characterized by ST segment elevation in two or more contiguous leads, which indicates acute occlusion of an epicardial artery with ongoing transmural ischemia 4, 2
• Non-ST-elevation myocardial infarction (NSTEMI): characterized by ST segment depression or T wave inversion in two or more contiguous leads, which indicates subendocardial ischemia secondary to subocclusion of the epicardial artery, distal embolization to small arteries, or supply/demand mismatch 4, 2

Clinical Significance of ECG Findings

The ECG findings in patients with suspected acute coronary syndromes have important clinical implications:

• Patients with STEMI should be triaged for emergent reperfusion therapy per current guidelines 2
• Patients with NSTEMI should be managed with anti-ischemic and antithrombotic therapy, and should be considered for early invasive strategy 3
• The ECG should be read along with the clinical assessment of the patient, and comparison to previous ECG and subsequent ECGs may add in the diagnosis and interpretation in difficult cases 2