23. Measurement of ST-Segment Deviation STEMI: 1 mm ST-segment elevation in 2 leads.* NSTEMI/UA: 0.5 mm ST-segment ischemic depression in 2 leads.* *Anatomically (regionally) contiguous leads.
24. Localizing Ischemia or Injury aVF inferior III inferior V 3 anterior V 6 lateral aVL lateral II inferior V 2 septal V 5 lateral aVR I lateral V 1 septal V 4 anterior
Key Concept: ST-segment deviation (elevation or depression) is measured 0.04 second after the juncture of the QRS complex and ST segment. The TP segment, not the PR interval, is used for baseline reference. Ask participants to point out the important segments and baseline for measurement of ST-segment deviation. How much ST-segment deviation is important for ST-segment elevation MI, ST-segment depression consistent with unstable angina or non – ST-segment MI? Discussion Points: The TP segment and end of PR interval are used as the baseline for measurement of ST-segment deviation. The PR interval is often abnormal in pericarditis, conditions that affect atrial repolarization, and atrial infarction. When possible, the TP segment is preferred in ACS. ST-segment deviation is measured between 0.04 and 0.08 second from the J point, the juncture between the QRS complex and the ST segment. Exercise stress testing uses 0.08 second after the J point for measurement, set by sensitivity and specificity patterns. For ACS, 0.04 second is used: one small box. ST-segment elevation MI qualifies for reperfusion therapy when 1 mm of ST-segment elevation is present in 2 contiguous leads. Early lytic trials used 2 mm in the precordial leads. ST-segment ischemic depression is considered significant if 1 mm at 0.04 second after the J point. But 0.5 mm is equally prognostic when the reader can identify ST-segment changes as ischemic.
Key Concept: Experienced providers should be proficient in ECG identification of the major ECG presentations of ACS. Use this slide and the following slide to review and assess their skills. Ask which coronary arteries supply the above distributions, ie, left anterior descending coronary artery = anterior; generally circumflex coronary artery = lateral (although a high large diagonal or intermediate ramus may supply this area); occlusion of right coronary artery = inferior wall MI. Teaching Point : The circumflex coronary artery may be electrically silent in 60% of patients with ACS. You are convinced a patient is experiencing ACS with persistent ischemic-type pain. What should you do? Extension of the anterior leads, eg, V 7 , V 8 , V 9 , may be helpful, but echocardiography will demonstrate a wall motion abnormality if significant ischemia/infarction is present. This noninvasive technique is also useful in assessing other acute cardiovascular problems, eg, pericardial effusion and aortic dissection involving the proximal aorta. Order it when in doubt. Hemodynamically unstable patients benefit from immediate diagnostic angiography. (See ACLS Provider Manual, Case 6, and ACLS: Principles and Practice, Chapter 17.)
Now review the anatomy discussed in the previous table. Note that the LAD supplies the largest amount of myocardium. The more proximal the occlusion the greater the change of cardiogenic shock and congestive heart failure. Ask what might be expected from an inferior wall MI. Emphasize the SA and AV nodal supply and discuss the possible development of bradyarrhythmias. Do not give too much away on the hemodynamic consequences or spend time discussing them because they are the key concepts for Case 2.
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