Acute Myocardial Infarction Signs

1. General

Patients usually appear anxious and are often sweating profusely. The heart rate may range from marked bradycardia (most commonly in inferior infarction) to tachycardia resulting from increased sympathetic nervous system activity, low cardiac output, or arrhythmia. The blood pressure may be high, especially in former hypertensives, or low in patients with shock. Respiratory distress usually indicates heart failure. Fever, usually low-grade, may appear after 12 hours and persist for several days.

2. Chest

Clear lung fields are a good prognostic sign, but basilar rales are common and do not necessarily indicate heart failure. More extensive rales or diffuse wheezing suggests pulmonary edema.

3. Heart

The cardiac examination may be unimpressive or very abnormal. An abnormally located ventricular impulse often represents the dyskinetic infarcted region. Jugular venous distention reflects right atrial hypertension, which may indicate right ventricular infarction or elevated left ventricular filling pressures. The absence of elevated central venous pressure, however, does not indicate normal left atrial or left ventricular diastolic pressures. Soft heart sounds may indicate left ventricular dysfunction. Atrial gallops (S4) are the rule, whereas ventricular gallops (S3) are less common and indicate significant left ventricular dysfunction. Mitral regurgitation murmurs are not uncommon and usually indicate papillary muscle dysfunction or, rarely, rupture. Pericardial friction rubs are uncommon in the first 24 hours but may appear later.

4. Extremities

Edema is usually not present. Cyanosis and cold temperature indicate low output. The peripheral pulses should be noted, since later shock or emboli may alter the examination.

Laboratory Findings

The most valuable laboratory tests are cardiac-specific markers of myocardial damage, including quantitative determinations of CK-MB, troponin I, and troponin T. All are relatively specific for necrosis (the troponins somewhat more so), though they may be elevated following severe ischemic episodes. Each of these tests may become positive as early as 4-6 hours after the onset of a myocardial infarction and should be abnormal by 8-12 hours. Circulating levels of troponins may remain elevated for 5-7 days or longer and should obviate the use of less specific LDH isoenzyme assays.

Electrocardiography

Most patients with acute infarction have ECG changes, and a normal tracing is rare. The extent of the electrocardiographic abnormalities provides only a crude estimate of the magnitude of infarction. The classic evolution of changes is from peaked ("hyperacute") T waves, to ST segment elevation, to Q wave development, to T wave inversion. This may occur over a few hours to several days. The evolution of new Q waves (> 30 ms in duration and 25% of the R wave amplitude) is diagnostic, but Q waves do not occur in 30-50% of acute infarctions (subendocardial or non-Q wave infarctions). If these patients have an appropriate clinical presentation, characteristic cardiac enzymes, and ST segment changes (usually depression) or T wave inversion lasting at least 48 hours, they are classified as having non-Q wave infarctions.