The changes in the ECG are seen in the leads adjacent to the infarct. In the first few hours the T waves become abnormally tall (hyperacute with loss of their normal concavity) and the ST segments begin to rise.
In the first 24 hours the T wave will become inverted, as the ST elevation begins to resolve.
Pathological Q waves may appear within hours or may take greater than 24 hr.
Long term changes of ECG include persistent Q waves in 90%, persistent T waves; persistent ST elevation is rare except in the presence of a ventricular aneursym.
In non Q-wave infarcts, ST depression and T wave inversion occur without ST elevation.
There may be ST depression in the leads opposite to the site of the infarct.
Note that in insulin dependent diabetics a small infarct on ECG may 'hide' large haemodynamic changes.