TY - CHAP M1 - Book, Section TI - Ischemia and Infarction A1 - Ferry, David R. PY - 2007 T2 - ECG in 10 Days, 2e AB - The ST segment and T wave in ischemiaThere are over 100 identified causes of ST segment and T wave changes, so the diagnosis of ischemia and infarction frequently requires comparison with previous ECGs and correlation with the clinical presentation and laboratory data.Myocardial ischemia produces a range of changes in the ST segment and T wave, depending on the severity of ischemia and the timing of the ECG.The specificity of the ST segment for ischemia is dependent on its morphology. (Day 4-01) In exercise stress testing, 1 mm or more of horizontal or downsloping ST segment depression 80 msec from the J point is considered an ischemic response. (Day 4-02)The specificity of ST segment and T wave changes is decreased in patients with resting abnormalities, particularly LBBB and LVH. (Day 4-03) DAY 4-01 DAY 4-02 DAY 4-03 Myocardial infarction(MI)ECG patterns in infarctionA zone of ischemia typically produces ST segment depression.A zone of injury produces ST segment elevation.A zone of infarction produces a large Q wave in the QRS complex. Genesis of the Q wave in infarctionThe normal situationFor example, in Lead I, the QRS complex begins with a small Q wave because left ventricular depolarization begins in the septum and the electrical forces are directed away from Lead I.The small Q wave is rapidly succeeded by forces directed inferiorly and laterally, resulting in a large R wave in Lead I.The infarct situationIf there is a lateral myocardial infarction, however, the electrical vectors in the lateral direction are lost, the forces directed medially are unbalanced.A large Q wave results in Lead I. The time course of myocardial and ECG changes during infarction Anatomical and ECG locations of MI (Day 4-04) (Day 4-05) DAY 4-04 DAY 4-05 DAY 4-06 DAY 4-07 Non-ST-segment elevation MI (NSTEMI) (Day 4-08)About half of the 750,000 MIs that occur annually in the U.S. do not develop new Q waves.Usually there are ST segment and T wave changes, but about 20% of NSTEMIs have no obvious ECG abnormalities and the diagnosis of MI is based on the clinical presentation and elevated cardiac markers.Anatomically, NSTEMIs are frequently associated with patchy subendocardial necrosis. SN - PB - McGraw-Hill Education CY - New York, NY Y2 - 2024/03/28 UR - accesscardiology.mhmedical.com/content.aspx?aid=1128942984 ER -