The infarction may involve only the subendocardium ( called a subendocardial MI ) or may spread to the epicardium or to all three layers of cardiac muscle. When all three layers are involved, the MI is termed transmural. Subendocardial MIs have less effect on wall motion and cardiac output than do transmural infarctions. When fewer grams of myocardium are affected , the characteristic “ Q ” wave on the ECG, which may also indicate, also indicate an old infarction, may not appear.