<P> If heart failure ensues, elevated jugular venous pressure and hepatojugular reflux, or swelling of the legs due to peripheral edema may be found on inspection . Rarely, a cardiac bulge with a pace different from the pulse rhythm can be felt on precordial examination . Various abnormalities can be found on auscultation, such as a third and fourth heart sound, systolic murmurs, paradoxical splitting of the second heart sound, a pericardial friction rub and rales over the lung . </P> <P> The primary purpose of the electrocardiogram is to detect ischemia or acute coronary injury in broad, symptomatic emergency department populations . A serial ECG may be used to follow rapid changes in time . The standard 12 lead ECG does not directly examine the right ventricle, and is relatively poor at examining the posterior basal and lateral walls of the left ventricle . In particular, acute myocardial infarction in the distribution of the circumflex artery is likely to produce a nondiagnostic ECG . The use of additional ECG leads like right - sided leads V3R and V4R and posterior leads V7, V8, and V9 may improve sensitivity for right ventricular and posterior myocardial infarction . </P> <P> The 12 lead ECG is used to classify patients into one of three groups: </P> <Ol> <Li> those with ST segment elevation or new bundle branch block (suspicious for acute injury and a possible candidate for acute reperfusion therapy with thrombolytics or primary PCI), </Li> <Li> those with ST segment depression or T wave inversion (suspicious for ischemia), and </Li> <Li> those with a so - called non-diagnostic or normal ECG . </Li> </Ol>

Which of the following ecg changes makes the determination of acute mi the most difficult