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Day 4: Ischemia and Infarction

  1. The ST segment and T wave in ischemia

    1. There 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.

    2. 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.

    3. The specificity of the ST segment for ischemia is dependent on its morphology. (Day 4-01)

       

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    4. 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)

    5. The specificity of ST segment and T wave changes is decreased in patients with resting abnormalities, particularly LBBB and LVH. (Day 4-03)

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    DAY 4-01

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    DAY 4-02

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    DAY 4-03

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  3. Myocardial infarction(MI)

    1. ECG patterns in infarction

      1. A zone of ischemia typically produces ST segment depression.

      2. A zone of injury produces ST segment elevation.

      3. A zone of infarction produces a large Q wave in the QRS complex.

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    3. Genesis of the Q wave in infarction

      1. The normal situation

        1. For 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.

        2. The small Q wave is rapidly succeeded by forces directed inferiorly and laterally, resulting in a large R wave in Lead I.

      2. The infarct situation

        1. If there is a lateral myocardial infarction, however, the electrical vectors in the lateral direction are lost, the forces directed medially are unbalanced.

        2. A large Q wave results in Lead I.

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  5. The time course of myocardial and ECG changes during infarction

     

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  6. Anatomical and ECG locations of MI (Day 4-04) (Day 4-05)

     

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    DAY 4-04

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    DAY 4-05

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    DAY 4-06

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    DAY 4-07

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  7. Non-ST-segment elevation MI (NSTEMI) (Day 4-08)

    1. About half of the 750,000 MIs that occur annually in the U.S. do not develop new Q waves.

    2. 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.

    3. Anatomically, NSTEMIs are frequently associated with patchy subendocardial necrosis.

       

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DAY 4-08

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Sample Tracings

ECG 1

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ECG 2

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ECG 3

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ECG 4

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ECG 5

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ECG 6

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