An ST elevation is considered significant if the vertical distance inside the ECG trace and the baseline at a point 0.04 seconds after the J-point is at least 0.1 mV (usually representing 1 mm or 1 small square) in a limb lead or 0.2 mV (2 mm or 2 small squares) in a precordial lead. The baseline is either the PR interval or the TP interval. This measure has a false positive rate of 15-20% (which is slightly higher in women than men) and a false negative rate of 20-30%.
The ST segment corresponds to a period of ventricular contraction. Because of the complete depolarization of the ventricles, represented by the QRS complex, in theory there is no net movement of charge during the ST segment. Under physiological conditions the ST segment is isoelectric (i.e. same charge across the myocardium).
During transmural ischemia, the cells across endocardium to epicardium is damaged. ST segment elevation occurs because when the ventricle is at rest and therefore repolarized, the depolarized ischemic region generates electrical currents that are traveling away from the recording electrode; therefore, the baseline voltage prior to the QRS complex is depressed (red line before R wave). When the ventricle becomes depolarized, all the muscle is depolarized during the ST segment so that zero voltage is recorded by the electrode (red line after R wave). When the ventricle is completely repolarized after the T wave, the baseline is once again negative as in the resting state. Therefore, the net effect of the depressed baseline voltage is that the ST segment appears to be elevated relative to the baseline. Also see ST depression.
Repolarization of the ventricle normally occurs during the T wave, however one cause of ST segment elevation is the early repolarization of the heart wall. This is referred to as benign early repolarization.
The exact topology and distribution of the affected areas depend on the underlying condition. Thus, ST elevation may be present on all or some leads of ECG.
It can be associated with:
- Myocardial infarction (see also ECG in myocardial infarction). ST elevation in select leads is more common with MI. ST elevation only occurs in full thickness infarction
- Prinzmetal's angina
- Acute pericarditis ST elevation in all leads (diffuse ST elevation) is more common with acute pericarditis.
- Left ventricular aneurysm
- Blunt trauma to the chest resulting in a cardiac contusion
- Acute myocarditis
- Pulmonary embolism
- Brugada syndrome
- J-point elevation
- Early repolarization
- Subarachnoid Hemorrhage
- Family Practice Notebook > ST Elevation Retrieved Nov 2010
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