Transcript ECG in MI

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