Heart attack is usually detectable by ECG, but a small percentage of heart attacks, such as non-ST-segment elevation heart attacks, may not show up on ECG.
Infarction is caused by the obstruction of blood vessels supplying blood to the heart muscle, resulting in myocardial necrosis. The electrocardiogram reflects the electrical activity of the cardiomyocytes, which will change after necrosis, which is manifested in the electrocardiogram by the appearance of pathologic Q-waves, ST-segment elevation, T-wave inversion, etc., which will change dynamically with the prolongation of time.
In a few cases of infarction, the obstruction of blood vessels is incomplete, and the cardiomyocytes are not completely necrotic, which may not be obvious on the ECG. Also, if the ECG itself is a bundle branch block ECG, the occurrence of an infarction is not easily seen. Therefore, ECG is an important tool in the diagnosis of acute myocardial infarction, but not the only one.