Acute complete occlusion of a major epicardial coronary artery typically causes sufficient ischemia in the dependent myocardial cells to produce the electrocardiogram change called “epicardial injury”. There is sufficient shift of the ST baseline toward, and of the TP/PR baseline away from the involved region to provide the clinical diagnosis. Relationships between the quantities of this baseline shift and etiologic ischemia have previously been shown to provide clinical value. However, it has now become apparent that potentially reversible ischemia can be so severe, in the absence of protection from collaterals and/or ischemia preconditioning, that it alters myocardial conduction and the resultant QRS complexes. This report presents highlights from two studies in progress to determine the relative abilities of ST segment and QRS changes during coronary occlusion to predict salvage following reperfusion; and the relative effects of collaterals and ischemia preconditioning to produce each of these electrocardiogram changes.