 During a myocardial infarction, many changes occur to the myocardium over a period of time. These are broken up into about four different timeframes that you must know and must be able to distinguish for the USMLE. First and foremost is the first 24 hours. Within the first 24 hours, we see dark modeling beginning to occur within the myocardium grossly. You can see that here in these dark spaces in the myocardium where the blood flow has been reduced and the tissue is beginning to die. This is the beginning stages of a coagulative necrosis. We also do have an increased risk at this point in time of a reperfusion injury. What this is is an oxidative stress that is caused when the blood flow returns to this portion of the heart. So when we return oxygen and blood flow, we can have risk of reperfusion injury. Complications during this first 24 hour period include arrhythmias, heart failure and cardiogenic shock. The specific arrhythmias to be concerned about are ventricular arrhythmias like V-Fib and V-Tac. The next time frame is one to three days post myocardial infarction. At this point in time is when coagulative necrosis begins to set in. That dark modeling becomes full blown necrosis and you see here a large portion of the heart becomes yellow colored. We also see acute inflammation markers at this point in time. Complications that can be seen in that one to three day period include a post infarction fibrinous pericarditis. The next time frame to be concerned about during and after a myocardial infarction is in the three to 14 day range. At this point in time we mostly see a microscopic change occurring in the heart. We see macrophages and granulation tissue specifically at the margins between the healthy and the unhealthy myocardium. Complications during this point in time include free wall rupture where we actually rupture through the heart and allow blood to flow out. That free wall rupture can lead to tamponade. Papillary muscle rupture which the papillary muscles are containing the mitral valve leaflets. So losing those papillary muscles can lead to mitral regerge. And finally we could have a issue with an IV septum rupture. An IV septum rupture can lead us to a left to right shunt. Finally two weeks to several months later we see the complete contracted scar forming where there is no more myocardium and just a fibrinous tissue that has replaced that myocardium. And you can see that here in this photo. Complications during this time include something called Dressler's syndrome. Dressler's syndrome is a persistent low grade fever along with pleuridic chest pain, pericarditis which we will see as a pericardial friction rub. And you may or may not see a pericardial effusion as well. More complications include heart failure, arrhythmias and ventricular aneurysm or a thrombus. We saw that thrombus in the previous picture but that is typically not associated with three to fourteen days. It is more often associated with two weeks and beyond where we have a thrombus forming at the apex of the heart that can cause major problems in the systemic circulation. That breaks off.