This table gives an overview of the pathology seen in myocardial infarction by time after obstruction.
For the first ~30 minutes no change at all can be seen by gross examination or by light microscopy in histopathology. However, in electron microscopy relaxed myofibrils, as well as glycogen loss and mitochondrial swelling can be seen.
Once scarring is completed, there is yet no common method of telling the actual age of the infarct, since e.g. a scar that is four months old looks identical to a scar that is ten years old.
Instead of a triphenyltetrazolium chloride dye, a LDH (lactate dehydrogenase) dye can also be used to visualize an area of necrosis.
References
Timeline of myocardial infarction pathology Wikipedia(Text) CC BY-SA