1. The ECG in Myocardial Infarction Dr Stephen Newell

4. The ECG
An upward deflection on the ECG represents depolarisation moving towards the viewing electrode, and a downward deflection represents depolarisation moving away from the viewing electrode.
The P wave represents atrial depolarisation - there is little muscle in the atrium so the deflection is small.
The Q wave represents depolarisation at the bundle of His; again, this is small as there is little muscle there.
The R wave represents the main spread of depolarisation, from the inside out, through the base of the ventricles. This involves large amounts of muscle so the deflection is large.
The S wave shows the subsequent depolarisation of the rest of the ventricles upwards from the base of the ventricles.
The T wave represents repolarisation of the myocardium. This is a relatively slow process - hence the smooth curved deflection.

5. ECG changes in myocardial infarction
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.
In Type 1 DM a small infarct on ECG may hide large haemodynamic changes.

11. (hyperacute) the mirror image of acute injury in leads V1-3
(fully evolved) tall R wave, tall upright T wave in leads V1-3
usually associated with inferior and/or lateral wall MI