ST segment depression may be determined by measuring the vertical distance between the patient's trace and the isoelectric line at a location 2-3 millimeters from the QRS complex.
It is significant if it is more than 1 mm in V5-V6, or 1.5 mm in AVF or III.
In a cardiac stress test, an ST depression of at least 1 mm after adenosine administration indicates a reversible ischaemia, while an exercise stress test requires an ST depression of at least 2 mm to significantly indicate reversible ischaemia.
For non-transmural ischemia, the pathophysiological cause of ST depression is a slightly elevated resting potential in myocardial cells, but with the ST segment less affected, as it represents a depolarized state. Still, the resting potential is the reference line in ECG, making it display an apparent ST depression rather than an elevation of the other segments.
DEPRESSED ST/> D - Drooping valve (MV Prolapse) E - Enlargement of the left ventricle P - Potassium loss R - Reciprocal ST Depression (eg. Inferior MI) E - Encephalon Haemorrhage S - Subendocardial Infarct S - Subendocardial Ischaemia E - Embolism (Pulmonary) D - Dilated Cardiomyopathy S - Shock T - Toxicity (Digitalis/Quinidine)
^Yap, L. B.; Arshad, W.; Jain, A.; Kurbaan, A. S.; Garvie, N. W. (2005). "Significance of ST depression during exercise treadmill stress and adenosine infusion myocardial perfusion imaging". The International Journal of Cardiovascular Imaging21 (2–3): 253–258; discussion 258–60. doi:10.1007/s10554-004-2458-y. PMID16015437.edit