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A myocardial infarction (MI), commonly known as a heart attack, typically affects the ST segment on an electrocardiogram (ECG) by presenting with elevation or depression of the ST segment, depending on the type of infarction and the stage of the event.

In the case of an ST-Elevation Myocardial Infarction (STEMI), there is significant ST segment elevation due to myocardial injury resulting from the lack of blood flow. In contrast, a Non-ST-Elevation Myocardial Infarction (NSTEMI) may present with ST segment depression, indicating myocardial ischemia rather than complete injury.

The variability in the ST segment response is crucial clinically; it helps to differentiate between these types of infarctions and aids in guiding treatment decisions. Changes in the ST segment can be seen in the acute phase of an MI and can persist as the heart heals, making this characteristic a vital aspect of ECG interpretation in the context of myocardial infarction.