The ST level is measured relative to the PR interval. While the U-P level theoretically would be preferred it cannot be used because it disappears with tachycardia. When there is a short PR interval, it can be particularly difficult for computer algorithms to find the isoelectric reference level. The direction of the ST vector and the relative position of the electrode measuring the vector determine whether the ST amplitude is positive (elevation) or negative (depression). For instance at rest, since the main ST vector projects along the long axis of the heart toward the apex, AVR will register a negative ST amplitude and V5 will register a positive ST amplitude (early repolarization). With tachycardia or ischemia the direction changes, as does the ST amplitude. As for any vector shift that is perpendicular to an electrode, no amplitude will be registered. Pathological processes that cause ST elevation shift the entire vector from it's long axis of the heart orientation and move it through the area of inflammation (pericarditis) or transmural ischemia (infarction or spasm). Thus, pathological ST depression does not localize but reflects a global subendocardial process while pathological elevation occurs directly over the involved area.
It is important to determine if the ST elevation was documented on previous ECGs and if the ST level is currently changing or is stable.
If the ST elevation has been present on prior ECGs:
If the ST elevation has been present on prior ECGs, the ECG does not exhibit diagnostic Q waves (no Q wave is wider than 35 millisec or 25% larger than the following R wave), and the elevation lessens with increases in heart rate then the benign finding of early repolarization is probable. The ECG with this finding usually will be obtained in a healthy, asymptomatic individual but it can also be obtained in a patient presenting with non-cardiac chest pain or with complaints due to another pathological process which makes the diagnosis more complicated. In this later circumstance, the early repolarization does not rule out a cardiac process but makes other diagnoses more likely.
If the ST elevation has been present on prior ECGs and occurs over diagnostic Q waves (Q wave wider than 35 millisec and 25% larger than the following R wave), a LV aneurysm or large wall motion abnormality is likely. In the case of an anterior location (i.e., V1-4), an abnormal precordial movement may be noted on examination. An echocardiogram would be the best way to evaluate LV function to see if an ACE inhibitor or other vasodilator was necessary to improve survival and lessen admissions for CHF.
When the ST elevation is a new finding:
The patient must be carefully questioned regarding ischemic symptoms including angina pectoris, chest pain, and shortness of breath, chest squeezing or pressure.
q If the symptoms are consistent with myocardial ischemia, an aspirin and nitrates should be given immediately. If due to coronary spasm, the symptoms and elevation should resolve. If they do not, then thrombolysis should be given unless there are contraindications (bleeding disorder, recent stroke, operation or bleeding, etc). Enzymes should be drawn but treatment need not be delayed for their analysis. The ECG often also exhibits increased R and T wave amplitudes and the elevation usually is in the area of the coronary artery with spasm or thrombus.
q If the patient's pain is pleuritic and positional, then consider pericarditis. A history of a URI or pneumonia, prior pericarditis, auto-immune disease and arthritis support the diagnosis of pericarditis. Physical exam may reveal a multi-component rub. ECG findings that also support pericarditis are PR depression, low amplitude T waves and ST elevation in multiple areas. If diagnosed, laboratory studies are indicated including renal function for uremia and patients should be closely followed for cardiac tamponade (distant heart sounds, narrow pulse pressure, pulsus paradoxus, hypotension). Echocardiography and cardiology consultation is usually indicated.
q If the patient is asymptomatic, then the most likely diagnosis is Early Repolarization. To confirm this, the ECG can be repeated to see if the pattern is stable. "Silent Ischemia" , ischemia occurring without symptoms, is a rare possibility that usually need not be considered.
View an ECG Example