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 (subendocardial ischemia) injured cells are closer to the inside of heart wall, resulting in a systolic injury current. A systolic injury current results from a greater depolarization in healthier cells. Because the subepicardial region is more depolarized (more positive) compared to the endomyocardial cells, the current in the left ventricle flows toward the endomyocardial cells. The current flows from the more positive subepicardium to the less positive subendocardium during phase 2 of the fast fiber type depolarization, which on ECG occurs during ST segment. The positive electrodes on the anterior chest wall detect the movement of positive charge away from the electrode and record it as a downward deflection on the ECG paper.
- Subendocardial ischemia or even infarction. Subendocardial means non full thickness ischemia. In contrast, ST elevation is transmural (or full thickness) ischemia
- Non Q-wave myocardial infarction
- Reciprocal changes in acute Q-wave myocardial infarction (e.g., ST depression in leads I & aVL with acute inferior myocardial infarction)
- ST segment depression and T-wave changes may be seen in patients with unstable angina
Depressed but upsloping ST segment generally rules out ischemia as a cause.
Also, it can be a normal variant or artifacts, such as:
- Pseudo-ST-depression, which is a wandering baseline due to poor skin contact of the electrode
- Physiologic J-junctional depression with sinus tachycardia
Other, non-ischemic, causes include:
- Side effect of digoxin
- Right or left ventricular hypertrophy
- Intraventricular conduction abnormalities (e.g., right or left bundle branch block, WPW, etc.)
- Reciprocal ST elevation
- Mitral valve prolapse
- Central nervous system disease, such as stroke
A mnemonic can be used for some causes of ST depression, namely DEPRESSED ST:
D - Drooping valve (MV Prolapse) E - Enlargement of the left ventricle P - Potassium loss R - Reciprocal ST depression (e.g. inferior MI) E - Encephalon hemorrhage S - Subendocardial infarct S - Subendocardial ischemia E - Embolism (pulmonary) D - Dilated cardiomyopathy S - Shock T - Toxicity (digitalis/quinidine)
- Okin PM, Devereux RB, Kors JA, van Herpen G, Crow RS, Fabsitz RR, Howard BV (April 2001). "Computerized ST depression analysis improves prediction of all-cause and cardiovascular mortality: the strong heart study". Annals of Noninvasive Electrocardiology. 6 (2): 107–16. doi:10.1111/j.1542-474X.2001.tb00094.x. PMID 11333167.
- Okin PM, Roman MJ, Lee ET, Galloway JM, Howard BV, Devereux RB (April 2004). "Combined echocardiographic left ventricular hypertrophy and electrocardiographic ST depression improve prediction of mortality in American Indians: the Strong Heart Study". Hypertension. 43 (4): 769–74. doi:10.1161/01.HYP.0000118585.73688.c6. PMID 14769809.
- madscientist software > MicroEKG Manual Retrieved September 2010
- Yap LB, Arshad W, Jain A, Kurbaan AS, Garvie NW (2005). "Significance of ST depression during exercise treadmill stress and adenosine infusion myocardial perfusion imaging". The International Journal of Cardiovascular Imaging. 21 (2–3): 253–8, discussion 259-60. doi:10.1007/s10554-004-2458-y. PMID 16015437.
- X. ST Segment Abnormalities Frank G. Yanowitz, MD. University of Utah School of Medicine
- Togha M, Sharifpour A, Ashraf H, Moghadam M, Sahraian MA (January 2013). "Electrocardiographic abnormalities in acute cerebrovascular events in patients with/without cardiovascular disease". Annals of Indian Academy of Neurology. 16 (1): 66–71. doi:10.4103/0972-2327.107710. PMC 3644785. PMID 23661966.