Anatomy - ST-segment Deviation

Anatomy - ST-segment Deviation

After an acute occlusion of a coronary artery and subsequent ischemia, the ST segment is the first ECG segment that manifests deviations or changes in its morphology.

In analyzing ST deviation in an ECG tracing, the clinician should assess the following characteristics:

  • Degree of ST deviation (expressed in millimeters)
  • Orientation of the ST deviation (look for presence of ST elevation and ST depression)
  • Location of the ST deviation (determine which leads manifest ST segment changes)

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These characteristics are essential in assessing the severity of ischemia and the extent of the myocardium affected.

Following occlusion of a coronary artery, persistent myocardial ischemia of the affected area can ultimately lead to myocardial infarction. It is therefore essential that the affected area is reperfused (i.e. blood flow should be restored to the affected region of the myocardium). If reperfusion does not occur, specific changes in the QRS complexes and T waves will appear in the ECG tracing.

By determining the leads where ST deviations are present, the clinician analyzing the ECG can localize the region of the heart affected and the possible coronary artery/arteries involved. In anterior wall myocardial infarction (where the culprit coronary artery is the LAD), it may also be possible to determine the probable location of the occlusion along the LAD. This concept is demonstrated in the ST segment deviation vector (or ischemia vector) diagram above and is further explained in the subsection "LAD occlusion."


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