The evaluation of patients with chest pain is a challenging area of accident and emergency (A&E) work. In the clinical assessment of such patients, interpretation of the electrocardiogram (ECG) is an essential adjunct to the history and examination. Approximately 20% of patients presenting with chest pain will have acute myocardial infarction (AMI), 35% angina/unstable angina, and 45% non-cardiac chest pain.^{1, 2} Reaching a rapid and accurate clinical diagnosis is of great importance, particularly as urgent reperfusion treatments such as thrombolytic agents and coronary angioplasty have been shown to improve prognosis in patients with AMI—if applied appropriately and early.

The electrocardiographic criteria identifying the group of patients likely to benefit from urgent reperfusion treatments are: (1) ST segment elevation in at least two anatomically contiguous leads and (2) new left bundle branch block (LBBB); further, ST segment depression in the right precordial leads (V1–V3), indicative of posterior AMI with posterior lead ST segment elevation, may represent yet another electrocardiographic indication for urgent coronary reperfusion. The ST segment elevation associated with an evolving myocardial infarction is usually readily identifiable. A&E physicians responsible for the administration of thrombolysis, however, need to be aware of the common pseudoinfarct ST elevation patterns that are associated with non-AMI ECG syndromes in the chest pain patient as LBBB and left ventricular hypertrophy. This article will review the ECG changes associated with AMI. Others in this series review the ECG changes associated with posterior wall and right ventricular wall infarction, the diagnosis of AMI in the presence of LBBB, non-infarction ST segment elevation syndromes, and the patient with chest pain and a non-diagnostic ECG.