The electrocardiographic diagnosis of ischemia and infarction is more difficult when bundle branch blocks and other forms of intraventricular conduction disturbances are present. These conduction disturbances may have been present prior to the ischemic event or may have been caused by it, and by themselves, are associated with changes in the QRS complex, ST segment and T wave that may mimic and/or obscure the ECG manifestations of the ischemic event.