Lead ECG

There are key features of a 12-lead ECG that identify and risk-stratify a patient with an ACS. Within 10 minutes of presentation to an emergency department with symptoms of ischemic chest discomfort, a 12-lead ECG should be obtained and interpreted. When possible, a 12-lead ECG should be performed by emergency medical system providers in order to reduce the delay until myocardial reperfusion. If available, a prior 12-lead ECG should be reviewed to identify whether or not the findings on the current ECG are new or old, with new findings being more indicative of an ACS. Key findings on review of a 12-lead ECG that indicate myocardial ischemia or infarction are STE, ST-segment depression, and T-wave inversion (Fig. 8—1). ST-seg-ment and/or T-wave changes in certain groupings of leads help to identify the location of the coronary artery that is the cause of the ischemia or infarction. In addition, the appearance of a new left bundle-branch block accompanied by chest discomfort is highly specific for acute MI. About one-half of patients diagnosed with MI present with STE on their ECG, with the remainder having ST-segment depression, T-wave inversion, or in some instances, no ECG changes. Some parts of the heart are more "electrically silent" than others, and myocardial ischemia may not be detected on a surface ECG. Therefore, it is important to review findings from the ECG in conjunction with biochemical markers of myocardial necrosis, such as troponin I or T, and other risk factors for CHD to determine the patient's risk for experiencing a new MI or having other complications.

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