Historically, an acute myocardial infarction (MI) was diagnosed by the presence of ST-segment elevation on the 12-lead electrocardiogram (ECG) (see discussion of the normal ECG in Chapter 36). However, all patients do not present with this finding. Instead, they are classified into one of three categories according to the presence or absence of ST- segment elevation on the ECG and positive troponin markers (see Chapter 35 for discussion of troponins):