The ECG in Myocardial Infarction

Dr Stephen Newell

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.

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.

• (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