ECG definitions used in this guide:

• Myocardial infarction: Pathologic changes in the QRS complex reflecting ventricular activation away from the area of infarction.
• Myocardial injury: Injury always points outward from the surface that is injured. Epicardial injury: ST elevation in the distribution of the occluded artery, when viewing the epicardial surface of the ventricle. Endocardial injury: ST elevation in aVR (which views the endocardial surface of the ventricle) and produces reciprocal diffuse ST depression in the remaining leads.
• Myocardial ischemia: Various definitions are used. The term commonly refers to diffuse ST segment depression, usually withassociated T wave inversion. It usually reflects subendocardial injury, reciprocal to ST elevation in lead aVR. Ischemia may also may refer to only inverted T waves with a symmetric, sharp nadir.
• Reciprocal changes: Passive electrical reflections of a primary event viewed from either the other side of the heart, as in epicardial injury, or the other side of the ventricular wall, as in subendocardial injury.

Pathophysiology of acute myocardial infarction:
There is acute occlusion of a coronary artery by thrombus. In 7.5% of cases, there is vasospasm or abnormal thrombogenesis. The earliest signs of acute MI are manifested as myocardial injury.

Major steps in the diagnosis of myocardial infarction:
Below is a systematic method for the diagnosis of myocardial injury and/or infarction, arranged in seven steps. Following the steps will achieve the diagnosis in most cases.

• Step 1: Identify the presence of myocardial injury by ST segment deviations.
• Step 2: Identify the areas of myocardial injury by assessing lead groupings.
• Step 3: Define the primary area of involvement and identify the culprit artery producing the injury.
• Step 4: Identify the location of the lesion, in the artery in order to risk stratify the patient.
• Step 5: Identify any ECG signs of infarction found in the QRS complex.
• Step 6: Determine the age of the infarction by assessing the location of the ST segment in leads with pathologic QRS abnormalities.
• Step 7: Combine all observations into a final diagnosis.

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