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ST Segment abnormalities in ECG

The ST segment represents period between ventricular depolarisation and repolarisation.
The ventricles are unable to receive any further stimulation.
The ST segment normally lies on the isoelectric line.

ST Segment Elevation
The ST segment lies above the isoelectric line.
Represents myocardial injury.
It is the hallmark of Myocardial Infarction.
The injured myocardium is slow to repolarise and remains more positively charged than the surrounding areas.
Other causes to be ruled out include pericarditis and ventricular aneurysm.

Myocardial Infarction
A medical emergency.
ST segment curves upwards in the leads looking at the threatened myocardium.
Presents within a few hours of the infarct.
Reciprocal ST depression may be present.

ST Segment Depression
Can be characterised as.

Horizontal ST Segment Depression
Myocardial Ischaemia:
Stable angina - occurs on exertion, resolves with rest and/or GTN.
Unstable angina - can develop during rest.
Non ST elevation MI - usually quite deep, can be associated with deep T wave inversion.
Reciprocal horizontal depression can occur during AMI.
Downsloping ST segment depression can be caused by digoxin.
Upward sloping ST segment depression normal during exercise.