The evolution of a STEMI: even though ischaemia is the first thing that happens, it's not the first change that you will see on the ECG. On a normal ECG, the ST segment is on the baseline. As soon as a patient is experiencing a myocardial infarction, the ST segment will elevate within minutes. For this reason, you will not see the T wave inversion until much later. So, the first sign you will see on your ECG is the ST elevation. As the ST segment comes down within 12-24 hours, the T wave can now invert and it's possible to see on the ECG. It's usually during this time that the pathological Q wave will also develop. After one week the ST segment will be back to normal and the T wave inversion will still be present. Months later only the pathological Q wave will be present as the T wave will then convert back to positive. This is what we call the evolution of a STEMI.