Sections View Full Chapter Figures Tables Videos Annotate Full Chapter Figures Tables Videos Supplementary Content +++ Day 4: Ischemia and Infarction ++ The ST segment and T wave in ischemia 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. 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 infarction A 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 infarction The normal situation 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. The small Q wave is rapidly succeeded by forces directed inferiorly and laterally, resulting in a large R wave in Lead I. The infarct situation If 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. ++ ++ DAY 4-08 ++ ++ ++ Sample Tracings ++ ECG 1 ++ ++ ++ ECG 2 ++ ++ ++ ECG 3 ++ ++ ++ ECG 4 ++ ++ ++ ECG 5 ++ ++ ++ ECG 6 +... Your Access profile is currently affiliated with [InstitutionA] and is in the process of switching affiliations to [InstitutionB]. Please select how you would like to proceed. Keep the current affiliation with [InstitutionA] and continue with the Access profile sign in process Switch affiliation to [InstitutionB] and continue with the Access profile sign in process Get Free Access Through Your Institution Learn how to see if your library subscribes to McGraw Hill Medical products. Subscribe: Institutional or Individual Sign In Error: Incorrect UserName or Password Username Error: Please enter User Name Password Error: Please enter Password Sign in Forgot Password? Forgot Username? Download the Access App: iOS | Android Sign in via OpenAthens Sign in via Shibboleth You already have access! Please proceed to your institution's subscription. Create a free profile for additional features.