The ST segment represents a period of ventricular depolarization,it begins at J-point (junction btw QRS complex and ST segment) just before repolarization(T-wave) begins. It lasts 0.08secs and is normally Isoelectric (flat)
The ventricles are still contracting. The pattern of the ST segment reflect changes in the contractile properties of the ventricular myocardium. This can be affected by ischemia, infarction, inflammation or even hypertrophy.
ST Segment Elevation;
In Precordial leads, > 2mm above isoelectric line.
In Limb leads, > 1mm above isoelectric line.
causes: 1). Myocardial ischemia( ST elevation in affected leads)
2). Acute pericarditis(Widespread ST elevation)
3). Ventricular aneurysm
ST Segment Depression:
In precordial leads, > 1.5mm below isoelectric line
In Limb leads,> 1mm below isoelectric line
Causes: 1). Myocardial Ischemia
2). Non- ischemic causes: side effect of Digoxin, Hypokalemia, RVH or LVH, WPW, RBBB or LBBB, Hypothermia, Tachycardia, Mitral valve proplapse.
I hope this helps.